Remedial Measures Impact on Slope Stability and Landslide Occurrence in Small-Scale Slope Physical Model in 1 g Conditions

Abstract

Physical modelling of landslides by analysing the behaviour of small-scale landslide models subjected to artificial rainfall can be divided into modelling under 1 g conditions and under increased acceleration (n times gravity) in a centrifuge. Physical modelling of landslide initiation began in 1970s in Japan on scaled natural slope models and after initial experiences with field and laboratory research, the small-scale landslide modelling has found a wide application around the world in different aspects of landslide investigations, analysing different types of landslides, different types of slope materials and landslide movements. The main task of landslide physical modelling is research of initiation, motion and accumulation of fast flow-like slides caused by infiltration of surface water or by shaking on a shaking table. Studies that have included landslide mitigation measures into small-scale physical models are rare and have not established correlations with a behaviour of on-site mitigation constructions. This paper discusses the behaviour of small-scale slope models supported by remedial measures under artificial rain in 1 g loading conditions. Models of slope built of different materials, with and without applied remedial measures (gravity retaining wall, gabion wall, pile wall) were exposed to identical intensities of artificial rainfall. The results of the simulations indicated that the slopes supported by remedial measures retained stability of the slope in the same conditions in which the sandy slope collapsed, as well as under significantly prolonged precipitations. At the end of the simulations, significantly higher rainfall intensities were applied to the supported slopes, exceeding the infiltration capacity of the slope material and affecting surface runout. The combination of surface erosion and saturation of superficial layer of a slope caused initiations of flow processes, while complete saturation of a slope when ground water level reached slope surface caused forming of a surface of rupture and consequently movements of the formed landslide body. The data obtained from the geodetic and geotechnical monitoring system enabled understanding of the overall process of rainfall infiltration and soil strength reduction to the development of the surface of rupture in a slope.

1 Introduction

Landslides are caused by exposure to hazardous motions of soil and rock down the slope (Cruden 1991) that threaten vulnerable human settlements in mountains, cities, along riverbanks, coasts and islands. An increase in the frequency and/or intensity of heavy rainfall caused by climate change, and shift in the location and periodicity of this rainfall due to changing climate significantly intensify the risk of landslides in some landslide prone areas. Although they are not frequent, strong earthquakes have potential to trigger rapid and long runout landslides. The combined effects of triggering factors including rainfall and earthquakes, can lead to greater impacts through disastrous landslides such as debris flows, rock falls and mega-slides. Landslides cause causalities and damage in the world every year: in the period between 2004 and 2016 landslides caused more than 4600 causalities and damage of more than 10 billion US$ per year over the world with the probable growth of these values in coming years (Froude and Petley 2018).

The answers of scientists, engineers, and all others included in landslide risk reduction is in significant developing of landslide science in different fields from landslide identification, mapping, and investigation; identification of landslide susceptibility, hazard, and risk; soil and rock testing; landslide modelling and simulation to landslide monitoring, mitigation, and remediation. Some of the answers are in different types of modelling of landslide related research. Landslide modelling for a long time was based on only numerical modelling techniques. Physical modelling of landslide analyzing small scale landslide models’ behavior was started in 1970s and 1980s in Japan (Oka 1972; Kutara and Ishizuka 1982; Yagi et al. 1985; Yamaguchi et al. 1989) in natural slopes exposed to artificial rainfall. The laboratory experiments of landslide behavior in a scaled physical model (also called flume or flume test) started in 1980s and 1990s in Canada (Hunger and Morgenstern 1984), Japan (Yagi et al. 1985) and Australia (Eckersley 1990) under 1 g conditions. On the other hand, small-scale landslide modelling under increased acceleration (n times the gravity) was successfully developed using a centrifuge (e.g. Kimura 1991; Take et al. 2004; Liang et al. 2017).

After initial experiences with field and laboratory research, the small-scale landslide modelling has found a wide application around the world in different aspects of landslide investigations, analyzing different types of landslides (e.g., flows, slides, falls and toppling) as well as different types of materials (rock mass, sandy, silty, and clayey materials) and landslide movements. The main task of landslide physical modelling was research of initiation, motion and accumulation of fast flow-like slides caused by infiltration of surface water in a slope and fluidification (Peranić et al. 2022c). Small-scale landslide physical modelling was also oriented to the second most important triggering factor, i.e., earthquakes after 2005 (Hong et al. 2005; Wartman et al. 2005; Lin and Wang 2006).

Although the small-scale landslide physical models, modern monitoring techniques and different monitoring equipment enable good insight into initiation and development of modelled landslide, crucial issue is to establish relationships between small-scale, brief, idealized and by artificial boundary restricted model with the complex natural landslide process (Iverson 2015). These relationships are known as scaling or scaling laws and play a crucial role in small-scale model designing and interpretation of results. Successful scaling (of material parameters, model dimensions, boundary conditions, rainfall intensity and shaking parameters as triggering factors, measured movements, velocities, accelerations etc.) is a necessary precondition for realization of successful small-scale landslide modelling (Peranić et al. 2022c). The use of physical modeling in landslide research has been significantly increasing in recent years.

The research Project Physical modelling of landslide remediation constructions’ behavior under static and seismic actions, funded by the Croatian Science Foundation, started in October 2018 at the Faculty of Civil Engineering, University of Rijeka, Croatia. The main Project aim is modelling behavior of landslide remedial constructions in physical models of scaled landslides in static and seismic conditions (Arbanas et al. 2019, 2020). In this paper, the behavior of small-scale slope models supported by remedial measures under artificial rain in 1 g loading conditions will be presented and discussed. Models of slope built of different soil materials, with and without of applied remedial measures (gravity retaining wall, gabion wall, pile wall) were exposed to identical intensities of artificial rainfall. The results of conducted measurements form installed geodetic and geotechnical monitoring system enabled understanding of overall process of rainfall infiltration and soil strength reduction to the development of the slip surface in a slope.

2 Material and Methods

2.1 Physical Model

The physical model of a scaled slope was designed to enable initiation of a landslide caused by controlled artificial rainfall and equipped with adequate photogrammetric equipment and complex sensor network with ability to measure displacements, soil moisture and pore water pressures within a slope (Fig. 1) (Arbanas et al. 2020; Jagodnik et al. 2020). The dimensions of the model (flume) are 1.0 m (width) × 2.3 m (length) × 0.5 m (height). The maximum depth of a soil material in the slope was adopted to be 30 cm. Slope inclination of the model can be adjusted from 20° to 45°. To prevent possible sliding of the soil mass at the contact with the slope base, the geogrid mesh is fixed to the slope base to increase friction. The series of tests were carried out on slope inclinations of 30°, 35° and 40° with three different types of soil as slope material without and with applied different types of remedial measures (gravity retaining wall, gabion wall, pile wall).

Fig. 1

(a) Schematic view of the physical model with monitoring equipment: (1) tensiometers, (2) pore pressure transducers, (3) strain gauges, (4) accelerometers, (5) rainfall simulator (sprinkler system), (6) high speed cameras, (7) terrestrial laser scanner (TLS), (8) infrared camera; (b) photo of the physical model (Arbanas et al. 2020; Jagodnik et al. 2020)

2.2 Soil Material Properties

Three different soil materials with relatively simple behavior were selected to build the typical small-scale physical slope models: uniform sand (S) and sand-kaolin mixtures with 10% (SK10) and 15% (SK15) kaolin content. The fine-grained (0–1 mm) uniform Drava River sand was chosen as the base material to represent soil material in cohesionless slopes. The other materials were a mixture of the base sandy material with 10% and 15% by mass of industrial kaolin, representing the behavior of fine-grained, cohesive materials with low but stable present cohesion. Kaolin was chosen as a clay with low plasticity and a relatively well graded grain size distribution curve and is not too sensitive to changes in water content (Arbanas et al. 2019; Peranić et al. 2022c).

The sand-kaolin mixtures were prepared by adding a predetermined amount of dry kaolin to dry sand (gravimetrically) and then mixing thoroughly with a laboratory mixer. During or before mixing, the amount of clean water was also poured into the soil mixture to achieve the desired initial soil moisture content. Once a homogeneous mixture was achieved, the material was placed in plastic bags, sealed and left to rest until the model was built (Vivoda Prodan et al. 2023). Grain size distribution curves were determined using a combination of sieving and sedimentation (hydrometer) analysis and are presented in Fig. 2.

Fig. 2

Grain size distribution curves of sand (S) (in blue), sand-kaolin mixtures with 10% of kaolin content (SK10) (in orange) and sand-kaolin mixtures with 15% of kaolin content (SK15) (in green) (Vivoda Prodan et al. 2023)

The Mohr-Coulomb strength parameters of the described materials—friction angle and cohesion were determined in a direct shear device at low normal stresses like those in the small-scale slope model at the same relative density of 50%. Hydraulic conductivity was determined at the same conditions as in the slope model, using the falling head test in an oedometer and triaxial apparatus. The basic physical and mechanical properties of the described materials and the targeted initial conditions at the start of the tests are given in Table 1 (Arbanas et al. 2020; Jagodnik et al. 2020; Peranić et al. 2022c; Vivoda Prodan et al. 2023).

Table 1 The basic physical and mechanical properties of the sand and sand-kaolin mixtures built in the small-scale model and the initial conditions at the start of the tests

2.3 Monitoring Equipment

In all types of slope physical models, very important issue is use and installation of measuring equipment necessary to enable measuring of movement, deformation, oscillations, pressure, forces, moisture, and other parameters necessary to determine the behavior of the slope. The selection of the measuring techniques and equipment in the physical model follows the practice of monitoring systems established on real landslides. In this sense, the sensor systems used in physical models can be divided into geotechnical monitoring systems and geodetic monitoring systems (Mihalić Arbanas and Arbanas 2015), where geodetic systems are mostly based on modern photogrammetric techniques for multi-temporal landslide analysis (Zanutta et al. 2006; Scaioni et al. 2015; Cignetti et al. 2019). Modern photogrammetric techniques including Structure from Motion (SfM) techniques for multi-temporal landslide analysis represent the most important tools for measuring of movement (Bitelli et al. 2004), acceleration and velocity of soil mass during sliding along artificial slopes in the physical models. Modern high-speed cameras enabled to provide stereo image sequences during sliding process, which is very important for investigation of very fast landslide occurrences (e.g., Akca 2013; Feng et al. 2016). Geotechnical monitoring in physical models is based on application of miniature sensors equivalent to in field used geotechnical monitoring equipment (Wieczorek and Snyder 2009). Different researchers established different monitoring systems using available types of sensors with adequate precision for scaled slope models (de Dios et al. 2009; Ooi et al. 2010; Wang et al. 2013; Ooi and Wang 2014; Lu et al. 2015).

The monitoring system established in a physical model followed the principles used in the observation of real landslides and consisted of a geotechnical and a geodetic monitoring system. The geotechnical monitoring system comprised of a complex network of miniature sensors equivalent to the geotechnical monitoring devices used in the field (Mihalić Arbanas and Arbanas 2015).

The ARAMIS system is an optical, noncontact 3D measurement system (GOM mbH, Braunschweig, Germany) that provides the entire workflow from taking measurements to analyzing data and presentation of the results. A set of two 4-megapixel and two 12-megapixel high-speed cameras were used for multi-temporal landslide analysis from captured stereo image sequences (Čeh et al. 2022). The system enables continuous monitoring of the 3D coordinates of reference points through all stages of the activity of scaled landslide models. The FARO Focus 3D X 130 (FARO Technologies Inc., Lake Mary, FL, USA) terrestrial laser scanner was used to capture the 3D model surface at high resolution before the start and at the end of the test.

The scanner uses phase-shift technology to accurately determine the relative 3D position of the scanned points, while reference points are used to overlap the scanned areas and define the relative coordinate system. A Nikon D500 camera (Nikon Inc., Melville, NY, USA) with an ultra-wide-angle Tokina AT-X 11–20 lens was used for the SfM photogrammetry survey of the physical models. This technology allows the creation of 3D models from multiple overlapping images taken at different triangulation angles.

The geotechnical monitoring system consisted of a complex network of miniature sensors connected to data loggers, enabling continuous data collection on soil moisture, positive pore-water pressures and soil suction, electrical conductivity, temperature, pressures, displacements, and accelerations (Pajalić et al. 2021; Peranić et al. 2022a).

Accurate observation of the hydromechanical response of small-scale slopes under controlled initial and boundary conditions provides necessary information on variables controlling the instability phenomena of rainfall-induced landslides. The development of a suitable sensor network for the test conditions and the selection of appropriate measurement techniques is the crucial step in the development of small-scale physical landslide models. There were several requirements that were considered during the selection of the measuring devices. Information on soil moisture content and the change in pore-water pressure (both positive and negative values) during rainfall infiltration had to be available at appropriate time intervals, depending on the experimental conditions. This requirement necessitated the use of digitized sensors and pressure transducers in combination with suitable data loggers with programmable data collection intervals. The additional important requirement to be considered was that the project envisaged testing a wide range of soils, from sand to clay. Therefore, the selected measurement equipment had to be able to cover a wide measurement range occurring in different soil textures, while ensuring sufficient precision and resolution of the measured quantities, as well as the reasonable responsiveness and equilibration times depending on the tested soil type. Another consideration relates to data redundancy: data had to be collected simultaneously at several depths and along different profiles to obtain a complete picture of the temporal and spatial evolution of different variables during the experiment. Finally, the chosen monitoring equipment was intended to be used with both the small-scale physical landslide models for testing under static (rainfall) and seismic (earthquake) conditions (Peranić et al. 2022a). The detailed description of the selected and used sensors for monitoring the hydromechanical response of the slope model was provided by Peranić et al. (Peranić et al. 2022a) and Pajalić et al. (Pajalić et al. 2021).

2.4 Rainfall Simulator

The rainfall characteristics play a decisive role in rainfall-induced slope failures in small-scale physical slope models. The design of a rainfall simulator should meet specific requirements of the project and it was an important issue in the early stages of the platform’s development. In particular: the ability to apply a wide range of rain intensities depending on the soil material tested and the specific objectives of the experiment, sufficient spatial uniformity of the simulated rainfall and the ability to change rainfall patterns and characteristics, the prevention of excessive erosion due to raindrop impact forces (which are closely related to raindrop diameter and impact velocity), the portability of the rainfall simulator, due to its possible use not only with the platform for testing under static conditions, but also under dynamic conditions, were the main considerations in the design and construction of the simulator (Peranić et al. 2022a).

Rainfall simulators are widely used tools to study hydrological processes such as interaction of rainfall with soil, soil erosion, surface runoff and infiltration (Lora et al. 2016a). Although it was possible to use some of the already developed rainfall simulators (Iserloh et al. 2012, 2013; Lora et al. 2016b), a new rainfall simulator was constructed as part of the project due to the lack of standard solutions.

The rainfall simulator developed through the project consists of three independent branches that delivered water from the main control block to the spray nozzles each equipped with four different axial-flow full-cone nozzles with a spray angle of 45° or 60°. Each branch has been placed at such a height that the water can reach the edges of the plexiglass sides without creating too much water on the edges or water coming out of the model. This solution covers a wide range of rainfall intensities, from less than 30 l/h/m² to more than 140 l/h/m² at the reference pressure of 2 bars. A wide range of rainfall intensities and the possibility to change the location of rainfall by opening or closing valves on each of the branches allows the modelling of different rainfall patterns and different rainfall intensities applied to the small-scale slope model (Arbanas et al. 2020). The main control block was connected to the water supply network, and consisted of the control units that regulated the rainfall simulator’s work, such as water pressure regulators, manometers, flow measuring units, filters, etc. High-density polyethylene pipes conveyed the pressurized water from the control block to three sprinkler branches, which were adjustable in height and equipped with various spray nozzles, Figs. 2 and 3. The rainfall simulator was fully portable and could be easily dismantled and installed at any location with a water supply ensured (Peranić et al. 2022a).

Fig. 3

Photo of one branch with four axial-flow full-cone spraying nozzles

2.5 Remedial Structures

Remedial measures chosen to be tested in this research are mostly selected as constructions that are commonly used in remediation of relatively small landslides and are usually applied at the foot of the slope. These constructions belong to the group of retaining structures, but partly also to the groups of slope geometry modifications and drainage (Popescu 2001; Mihalić Arbanas and Arbanas 2015). Three types of retaining structures are selected as most effective: a gravity wall in sandy material, a gabion wall in silty material (sand-kaolin mixture with 10% of kaolin content, SK10) and a pile wall in clayey material (sand-kaolin mixture with 15% of kaolin content, SK15). The first step in construction of remedial measures was selection of scale factor. It was decided that the scale factor in geometrical scale low would be 1:20, and all dimensions of the selected remedial measure were scaled to 1:20.

Considering the gravity retaining wall and the pile wall construction as remedial measures in the foot of a slope, which usually behave as rigid structures without or with only very small internal deformations, 3D printing of elements of structures in scaled dimension using polylactic acid (PLA) material was selected to imitate remedial structures in real slopes. At the same time, the strength and deformability parameters of the material used for 3D printing are high enough to ensure rigid behavior of structures the without possibility of internal structure failure. Gabion wall structures are flexible structures that enable significant deformations of the overall structure, particular elements (filled gabion baskets) as well as along the contacts of the elements without losing the stability of the overall structure. Scaled gabion baskets are made of fiberglass netting, while the filling consists of gravel grains scaled in relation to the size of the stone blocks used in standard construction.

Gravity retaining model elements are 20.0 cm long and 26.0 cm high; the foundation is 6.0 deep and 9.0 cm wide, Fig. 4. The rear face of the wall is in the lower part inclined at 108⁰ while the upper part is vertical. The elements are connected with vertical key-pin elements enabling articular connection between the individual elements in one unique wall construction. In the lower part of the wall, above the foundation, two lines of the drains are derived to ensure adequate drainage and avoid forming hydrostatic pressure above the foundation behind the wall.

Fig. 4

Photo of gravity retaining model elements. (a) front view, (b) side view

The piles have 4.0 cm of outer and 3.0 cm of inner diameter, and a length of 32 cm (Fig. 5a). The piles are connected in elements of 3 piles, which are connected to the foundation beam (Fig. 5b) and head beam (Fig. 5c). The foundation beam is just an element that does not exist in field construction but is necessary to establish fixation in a base (e.g., bedrock in the field). The head beam is 6.0 × 6.0 cm and has 5.0 cm deep holes for the pile heads. The axial distance between piles is 6.5 cm. The base of the foundation beam is adjusted to the slope model inclination and 8.0 cm deep holes ensure the tightness of the pile bases. Head and foundation elements are connected with vertical key-pin elements enabling articular connection between the individual elements in one unique pile wall construction.

Fig. 5

Photo of pile wall elements. (a) piles, (b) foundation beam, (c) head beam

Scaled gabion baskets made of fiberglass netting have dimensions of 5.0 × 5.0 × 10.0 cm and the filling material is uniform gravel 0.4–0.8 mm, Fig. 6. The gabion baskets are tailored from one piece of net and fixed by hot gluing.

Fig. 6

Photo of gabion model elements

3 Construction of Slope Models

3.1 Construction of Slope Models without Remedial Measure

The previously described types of soil material (uniform sand (S) and sand-kaolin mixtures with 10% (SK10) and 15% (SK15) kaolin content) were built-in in the flume in five layers each 6 cm thick, up to a total height of 30 cm. To reach homogeneity of the material in the slope and relatively uniform conditions in built-up soil material, the model was built in three segments—lower (L), middle (M), and upper (U) part—building up the material from the foot to the top of the slope with compaction in each layer. To prevent possible sliding of the soil mass at the contact with the flume base, the geogrid mesh is fixed to the flume base to increase friction (Arbanas et al. 2020).

Each type of soil material was previously tested using standard Proctor test according to the American Society for Testing and Materials (ASTM) standards and the American Association of State Highway and Transportation Officials (AASHTO) standards to determine optimal moisture content and adequate dry density of the material. The used materials were pre-prepared at optimal water content (Table 1) and build-in by manual compaction using the undercompaction method (Ladd 1978). For each segment of the slope and each layer, the necessary mass of a soil material was previously calculated and weighed to ensure that the required initial compaction has been achieved. Before placement of the next layer, the surface of the previous layer was raked and sprayed with water to maintain the initial moisture and achieve the best possible connection between soil material of each layer.

All sensors used in the tests inside the slope material were installed parallel to the soil layers after compaction of a layer was completed. The water-related sensors that were used in this research were TEROS 10 and TEROS 12 frequency-domain reflectometry-based soil moisture sensors which provided an indirect measurement of the volumetric water content of porous materials installed in all three tests, while the TEROS 31 mini tensiometers and the TEROS 21 maintenance-free matric potential sensor for measurement of soil water potential were used only in the tests with sand-kaolin mixtures. All sensors were manufactured by the METER Group AG (Munich, Germany) (Peranić et al. 2022b). The locations of the sensors were chosen as critical points for monitoring of the hydro-mechanical response of soil material in a slope model. The sensors were installed in the lower (L), middle (M) and upper (H) part of the slope, at different depths (6, 12, 18 and 24 cm) along the same profile to provide data at the same cross-section that enables validation and numerical analysis of the observed landslide initiation (Fig. 8) (Vivoda Prodan et al. 2023a).

3.2 Construction of Slope Model with Installation of Gravity Wall

In the test after construction of the sand slope in same conditions as in the test without remedial measures, installation of gravity retaining wall was completed following the sequence of works that is commonly carried out in a real slope. The gravity retaining wall was constructed from five 20.0 cm long 3D-printed elements described previously, interconnected by an articular connection. The height and other dimensions of the wall were based on geometrical similarity of 1:20. The material used for the backfill of the wall was gravel, the grain size distribution of which was determined from geometrical similarity with coarse crushed stone fill material 1:20 that is usually used as backfill material behind walls. According to the position in the slope, the strength parameters of this material have no importance, while the hydraulic conductivity is significantly higher than for sand in the slope and has no influence on the infiltration process (Arbanas et al. 2022a) (Fig. 7).

Fig. 7

Positions of the embedded water related sensors in the small-scale landslide model

Additional impact of buttressing material is in increasing of vertical forces in the foot of the slope with consequently rise of soil strength in the zone as well as general decreasing of slope inclination.

When the model slope of sandy material (S) was built at the inclination of 35°, the foundation pit in the foot of the slope was excavated and additionally compacted, and the retaining gravity wall was lined (Fig. 8a, b, c). After installation of sensors at the rear face of the wall and laying geotextile, backfilling with gravel material was started (Fig. 8d, e). The slope was stepwise excavated to ensure adequate contact between the sandy slope and the gravel buttressing embankment (Fig. 8f, g). The gravel material was separated from the sandy material by a geotextile and compacted with a manual compactor (Fig. 8d to g). The buttressing material was built-in to approximately two thirds of the height of the slope with an inclination of the surface of 1:1.5 (Fig. 8h).

Fig. 8

Installation of the gravity wall and buttressing embankment behind the wall into the prepared sandy slope: (a) excavated foundation pit and installed element of the gravity wall, (b) first wall element installed in the foundation pit, (c) a view at the partially installed wall in the foot of the slope, (d) excavation for backfill behind the wall with installed sensors; geotextile is laid, (e) filling the gravel material behind the wall, (f) filled buttressing embankment behind the gravity wall along the slope, (g) a view at buttressing embankment behind the gravity wall, (h) model prepared for testing

3.3 Construction of Slope Model with Installation of Gabion Wall

In the test after construction of the silty slope (SK10) under the same conditions as in the test without remedial measures, installation of gabion retaining wall was completed following the order of works that is usually performed on a real slope. The gravity retaining wall was constructed from five previously described gabion boxes in 4 rows. The height and other dimensions of the wall were based on geometrical similarity of 1:20. The material used for the backfill of the wall was gravel with the grain size distribution determined from geometrical similarity with coarse crushed stone fill material 1:20 that is usually used as backfill material behind walls. According to the position in the slope, the strength parameters of this material are of no significance, while the hydraulic conductivity is significantly higher than for sand, allowing infiltration and vertical drainage to the level of the gabion wall foundation. As with the gravity wall, an additional effect of the buttressing material is in increasing vertical forces in the slope’s foot area and the consequent increase in soil strength in the zone, as well as general decreasing of slope inclination.

When the 35° model slope from silty material (SK10) was built, the foundation pit in the foot of the slope was excavated, filled with 2.0 cm of gravelly material and additionally compacted (Fig. 9a), and the gabion boxes were built-in rows with regular overlapping (Fig. 9b to f). After installation of sensors and laying the geotextile, backfilling with gravel material was started (Fig. 9d to f). The slope was stepwise excavated to ensure adequate contact between the silty slope and gravel buttressing embankment (Fig. 9d to g). Gravel and silty material were separated by geotextile and compacted with a manual compactor (Fig. 9d to g). The buttressing material with a slope of 1:2 was built-in to approximately two thirds of the height of the slope (Fig. 9h).

Fig. 9

Installation of the gabion wall and buttressing embankment behind the wall in a prepared silty slope: (a) excavated foundation pit with compacted layer of gravel, (b) first row of gabion boxes installed in foundation pit, (c) second row of gabion boxes installed, (d) filled buttressing embankment behind the gravity wall separated from the slope material with geotextile, (e) installed second row of gabion boxes, (f) filled buttressing embankment behind the gravity wall along the slope, (g) a view at the buttressing embankment behind the gabion wall, (h) model prepared for testing

3.4 Construction of Slope Model with Installation of Pile Wall

The installation of a pile wall in the slope model differs significantly from the installation of a gravity and gabion wall. While the installation of gravity and gabion wall models completely follows the order of works corresponding to those of a real slope, the installation of the pile wall and establishing of a small-scale slope model are carried out in parallel (Vivoda Prodan et al. 2023b). The first step is setting and fixing the foundation beam to the base of the model platform. The role of the foundation beam is to ensure the tightness of pile’s bases for the future retaining the load of the sliding body due to the movements down the slope. The foundation beam was assembled from five previously described 3D-printed elements, Fig. 10a. The pile wall consists of 15 piles with 4 cm of outer diameter and at 6.5 cm of axial distance. The height (40 cm) and other dimensions of the pile wall and its elements were based on geometrical similarity of 1:20. After setting and fixing foundation beam, the first layer soil material was built in and compacted in all parts of the slope (L, M, H) and the model slope of clayey material (SK15) was built at the inclination of 35°. The following step was the installation of the piles into the foundation beam was, Fig. 10b, after which placement and compaction of the other four layers of slope clayey soil material was conducted, until the planned slope model surface, Fig. 10c, d. After the clay slope model was built, the buttressing counterfort of gravelly material is constructed in the front of the pile wall. The embankment is 20.0 cm tall, with a 10.0 cm wide crown and 1:1.5 inclined front slope, Fig. 10e. The material used for the backfill of the wall was gravel with granulometric composition determined from geometrical similarity with coarse crushed stone fill material 1:20 that is usually used as embankment fill materials. The gravel material in the counterfort was separated from the clayey material on the slope by geotextile and compacted with a manual compactor. This buttressing counterfort has two roles: to hold on to the pile wall and to ensure faster drainage of water behind the pile wall. In real site, an additional role of this embankment is to provide a working plateau for heavy equipment for installation of piles.

Fig. 10

Installation of pile wall construction and buttressing counterfort in the front of the wall in clayey slope: (a) setting and fixing foundation beam to the model base, (b) installation of piles in foundation beam, (c) built in the soil layers in the slope, (d) base of buttressing counterfort in the front of piles, (e) complete buttressing counterfort and excavated trenches (f) filled trenches and installed head beam, (g) a view at pile wall with embankment behind the head beam, (h) model prepared for testing

Three trenches, 8.0 cm wide, 10.0 cm deep and 65.0 cm long (from the pile wall along the slope), were excavated behind the pile wall and filled with gravel material Fig. 10e, f. Gravel material in trenches was separated from clayey material in the slope by geotextile and compacted by a manual compactor. The main role of these trenches is to collect part of the surface water on the slope and enable faster drainage to the toe of the slope. The second role is to increase the resistance of the slope surface to erosion by surface outflow.

The next stage in slope construction is the installation of a head beam at the top of the piles and connecting the individual piles into a single construction, Fig. 10f, g. After the installation of the head beam, the space between it and the slope was filled and compacted with a manual compactor, Fig. 10g, h.

4 Testing and Results

4.1 Testing of Slope Models without Remedial Measures

The previous chapters have described the materials and methods used in small-scale landslide modelling in this research. Series of tests at the slopes of different materials (i.e., uniform sand (S) and sand-kaolin mixtures with 10% (SK10) and 15% (SK15) kaolin content) and different slope inclinations (30°, 35°, 40°) and different rainfall scenarios, but without installed remedial measures are conducted.

A very large volume of result data was collected from various sensors installed inside a slope (tensiometers, pore pressure sensors, accelerometers, MEMS, pressure cells), as well as surface displacements captured by the ARAMIS system, terrestrial LiDAR scanning and digital camera that enables in-depth analysis of the sliding triggering and initiation process inside the slope, and its evolution under different initial and rainfall conditions. A significant part of research and analyses were carried out considering and comparing the conditions present in the model at the moment of landslide initiation and the modes of further landslide development. The following analyses were carried out and published: initiation and development of a landslide in a slope of clear uniform sand (S) (Arbanas et al. 2020; Jagodnik et al. 2020; Pajalić et al. 2021); initiation of a sandy slope due to the rise of the groundwater level and hydraulic pathways and a reduction in the shear strength of the soil due to infiltration in small-scale slope model built in sandy (S) and silty (SK10) materials (Peranić et al. 2022a); comparison of the mechanism of rainfall induced landslides in small-scale models built of different materials (sandy (S), silty (SK10) and clayey (SK15)) (Vivoda Prodan et al. 2023a, b); comparison of landslide initiation in small-scale sandy (S) and clayey (SK15) slopes (Peranić et al. 2022c); comparison of the behavior of sandy (S) and clayey (SK15) slopes during rainfall in small-scale model (Arbanas et al. 2022b); and rainfall loss and erosion process in a small -scale slope (Bezak et al. 2022). In this chapter, these results will not be presented separately. Instead, results are used to highlight the impact of remedial measure constructions exposed to the same conditions as slopes built from the same material and exposed to same rainfall events without present remedial measure constructions.

4.2 Testing of Slope Model with Gravity Retaining Wall

After the slope models were built and the monitoring equipment installed in the slope, the slope models were exposed to artificial rainfall from three nozzles, one nozzle in each part of the slope—the upper (U), middle (M) and lower (L) part. The results of two tests are presented here: two models of a sandy (S) slope, with and without applied gravity retaining wall, were exposed to identical intensities of artificial rainfall. The course of the test developments is shown in Fig. 11. In both tests, the slopes were exposed to an initial rainfall intensity of 81.4 mm/h. The selection of rainfall intensity was based on infiltration conditions in a slope and the main requirement was that all rainwater at the point of contact on the model surface infiltrates without forming surface runoff. The applied intensities were at the upper precipitation values that can be infiltrated in a soil (Arbanas et al. 2022a).

Fig. 11

Simulated rainfall during the test on the sandy slope without gravity wall (blue) and the test on the sandy slope with gravity wall applied in the foot of the slope (black). Vertical lines mark the times of the first crack evidence

After the initial establishment of a constant rainfall intensity and a stable infiltration process, both slope models were exposed to rainfall with a constant intensity for both slopes to allow similar rainfall conditions in a slope with and without a gravity retaining wall applied. The intensity of 81.4 mm/h was maintained for 56 min when the first crack occurred in the slope model without remedial measures applied (Test no. 2) and after the ground water level reached soil surface in the lower (L) part of the model, Fig. 12a. The constant groundwater level in the lower (L) part of the model was maintained until the end of the test and the nozzle above the lover part of the model was closed. During this period, the landslide in the slope without gravity wall applied developed retrogressively to the top of the slope (Fig. 12a, c, e), while in the slope with gravity wall applied there were no visible signs of landslide initiation in the form of development of cracks during this entire period.

Fig. 12

Development of landslides in small-scale sandy slope without and with installed gravity retaining wall: (a) initial instability in the foot of the unsupported slope (62 min), (b) slope after the first cracks in the slope with retaining wall (129 min), (c) development of the landslide to the top of the unsupported slope (90 min), (d) development of the landslide to the top and advancement of the debris flow on the slope with retaining wall (134 min), (e) landslide in the unsupported slope at the end of the test (134 min), (f) landslide and debris flow in the slope with retaining wall at the end of the test (155 min)

When the retrogressive development of the landslide in the slope without gravity wall reached the top of the slope, the rainfall intensity was increased to 157.2 mm/h in the next 10 min, and finally increased to 229.9 mm/h until the final failure of the slope 128 min after the start of the test. A similar succession of rainfall was applied to the slope without a gravity wall, but there was no initiation of a landslide in a similar time period and with similar rainfall intensities (Fig. 11).

At the same time, under the same rainfall conditions, no sliding process in the slope with the retaining gravity wall applied occurred, indicating that the remedial measures applied with the gravity wall and the coarser backfill material behind the wall confirmed the intended role in maintaining the stability of the sandy slope (Test no. 10). The first cracks in the test on the slope with the gravity wall occurred in the 123rd min after the increase of rainfall intensity to 157.2 mm/h and it was completed after the 134th min with the opening of the nozzles in the upper part of the slope with an intensity of 229.9 mm/h, which was above the infiltration capacity of the sandy material.

After the application of a high rainfall intensity in the upper part of the slope (229.9 mm/h), higher than the infiltration capacity of the sandy material, there was a sudden development of the erosion process of the sandy material and a global instability of the slope occurred, which resulted in exceeding the bearing capacity of the soil under the retaining wall foundation, Fig. 12b, d, e. Although the retaining wall did not completely collapse and overturn, the horizontal displacement caused by the local failure below the retaining wall foundation and the horizontal soil compaction in the foot of the slope enabled a global collapse of the slope. The applied rainfall intensities that caused this global collapse are significantly high and, considering scaling relative to real slope sizes, represent rare extreme rainfall events.

The superficial signs of landslide initiation that occurred and the reasons for their occurrence in the case of a slope without applied gravity wall can be explained by the results of continuous volumetric water content monitoring (Fig. 13). The process of landslide initiation and development was described by the volumetric water content increasing due to infiltration until saturation in the deepest part of the slope, forming the water table. As it becomes established, a flow down the slope begins, causing a rapid rise in the groundwater level in the lower part of the slope. This leads to a decrease in the soil shear strength with complete loss of matric suction, failure and further retrogressive landslide development to the top of the slope (Peranić et al. 2022c), Fig. 12a, c, e.

Fig. 13

Volumetric water content and pore water pressure measurements in tests without applied gravity retaining wall (a) at 6 cm below the surface; (c) 12 cm below the surface; (e) 18 cm below the surface; (g) 24 cm below the surface and in the test with applied gravity retaining wall; (b) at 6 cm below the surface, (d) 12 cm below the surface; (f) 18 cm below the surface; (h) 24 cm below the surface in upper (H), middle (M), and lower (L) part of the slope

In both tests, almost the same rainfall exposure conditions were achieved, and the monitoring devices were installed in the same positions on the slope to allow measurement of changes in volumetric water content. The sensors for measuring volumetric water content were installed at four different depths (6, 12, 18 and 24 cm below the slope surface) in the lower (L), middle (M) and upper (H) parts of the slope (Fig. 13). When analyzing the development of rainfall infiltration and the formation of a ground water level along the slope in both models, no significant deviations in the measurement results were found. Neglecting the differences in water contents at the beginning and during the tests caused by initial differences and heterogeneity in soil density (primarily) and water content (which is also related to density), the plots showing the change in volumetric water content in the model within the tests show similar trends over a similar time period of applied rainfall. This suggests that the main role of the gravity wall and the coarser backfill material behind the wall is to increase soil strength in the slope foot and associated erosion prevention that has occurred on the slope without remedial measures (Arbanas et al. 2022a).

After the rainfall intensity in the upper part of the slope (157.2 mm/h) exceeded the infiltration capacity of the sandy material, there was a sudden development of the erosion process of the sandy material and a global instability of the slope occurred, which resulted in exceeding the bearing capacity of the soil under the foundation of the retaining wall. The retaining wall did not completely collapse and overturn, but the horizontal displacement allowed a global collapse of the slope. The applied rainfall intensities that caused this global collapse are significantly high and, when scaled relative to real slope sizes, represent rare extreme rainfall events. Despite this fact, in the following tests, the applied remedial measures should be improved to avoid this type of collapse even under such extreme circumstances.

4.3 Testing of Slope Model with Gabion Wall

As described earlier, the slope model with a gabion wall applied in the foot of the slope as remedial measure was constructed in a similar way to the slope model with a gravity retaining wall. The results of two tests are presented: models of a silty (SK10) slope with and without a gabion retaining wall applied, exposed to identical intensities of artificial rainfall. The course of the test developments is shown in Fig. 14. In both tests, the slopes were exposed to an initial rainfall intensity of 32.8 mm/h. Again, the selection of rainfall intensity was based on infiltration conditions in a slope and the main requirement was that all rainfall at the point of contact on the model surface infiltrates without forming surface runoff. The intensities applied were at the upper precipitation values that can be infiltrated in a soil.

Fig. 14

Simulated rainfall in the test on the silty slope without applied gabion wall (blue) and test on the silty slope with applied gabion wall in the foot of the slope (black). Vertical lines mark times of the first crack evidence

After the initial establishment of a constant rainfall intensity and a stable infiltration process, the models were exposed to rainfall with constant intensities for both slopes to enable similar rainfall conditions in both models. The intensity of 32.8 mm/h was maintained throughout the tests. The constant groundwater level in the lower (L) part of the model was maintained until the end of the tests and the nozzle above the lover part of the model was closed after the groundwater reached surface in this part of the model.

The first crack on the slope without gabion wall applied (Test no. 5) occurred in the 22nd min in the upper part of the slope. Thereafter, the landslide progressively developed to the foot of the slope (Fig. 15a, c). In the 74th min, the ground water level reached the bottom of the slope (Fig. 15a) and started to flow to the surface (Fig. 15a, c). As the test progressed, the ground water in the middle part of the slope (M) was rising, reaching the slope surface (Fig. 16a, c, e, g) in increasingly higher parts of the slope, while the amount of outflow water was gradually increasing (Fig. 15a, c, e). This outflow and the resulting surface runoff as a consequence have two additional processes: washing out of silty (kaolin) particles from the soil in the slope and surface erosion (Fig. 15a, c, e). Surface flow was enhanced by rainfall water that could no longer infiltrate into the slope. Parallel to this process, retrogressive development of the landslide to the top of the slope started (Fig. 15c) and after 135 min the landslide had spread to the top of the slope. A steady state flow conditions were established (Fig. 16a, c, e, g) and no further development of the landslide occurred. Only the erosion process caused by the surface runoff continued and the test was terminated after 155 min.

Fig. 15

Development of landslides in small-scale silty slopes without and with installed gabion wall: (a) initial instability in the unsupported slope (79 min), (b) slope before the first cracks in the slope with gabion wall (24 min), (c) progressive development of landslide and flow out of ground water to the surface (90 min), (d) occurrence of landslide to the top on the slope with gabion wall (135 min), (e) landslide and erosion in the unsupported slope (118 min), (f) a view of the slope with gabion wall at the end of the test (210 min)

Fig. 16

Volumetric water content and pore water pressure measurements in tests without applied gabion wall (a) at 6 cm below the surface; (c) 12 cm below the surface; (e) 18 cm below the surface; (g) 24 cm below the surface and in the test with applied gravity retaining wall; (b) at 6 cm below the surface, (d) 12 cm below the surface; (f) 18 cm below the surface; (h) 24 cm below the surface in upper (H), middle (M), and lower (L) part of the slope

Under the same rainfall conditions, the behavior of the slope with gabion wall installed differs significantly (Test no. 11). The first crack of the landslide in the slope with the gabion wall applied occurred in the 35th min in the upper part of the slope, but no significant sliding development was observed in the period that followed the landslide initiation. This indicates that the applied remedial measures with gabion wall and coarser backfill material behind the gabion wall confirmed the intended role in retaining the stability of the silty slope (Fig. 15b, d, e). The other differences from the slope without remedial measures are the absence of ground water outflow from the slope to the slope surface and, secondarily, washing out of silty particles. It is obvious that the coarser backfill material behind the gabion wall plays an important role in slope surface erosion protection as well as in faster drainage of ground water than the basic silty material in the slope. The test continued with the same rainfall intensity until the 210th min. However, after steady state flow conditions were achieved (Fig. 16b, d, f, h), there was no further landslide development and practically without erosion in the upper part of the slope (Fig. 15f).

Although the stability of overall slope was not completely retained and the main crack at the top of the slope indicated the formation of a sliding surface, the overall displacements are generally small and no signs of instability in the foot of the slope were visually observed. The displacements measured in the central part of the crown of the gabion wall were less than 2.5 cm, but no visible signs of loss of stability of the gabion wall itself were detected.

The superficial signs of sliding initiation and the causes of their occurrence in the case of the slope with and without applied gabion wall can be explained by analyzing the results of continuous volumetric water content monitoring (Fig. 16a to h). The process of landslide initiation and development was described by the increase in volumetric water content due to infiltration until saturation in the deepest part of the slope, forming the water table. As it becomes established, a flow down the slope begins, causing a rapid rise in the groundwater level in the lower part of the slope. This leads to a decrease in the soil shear strength with complete loss of matric suction, failure, and landslide development to the top of the slope, Fig. 15a to f.

4.4 Testing of Slope Model with Pile Wall

The slope model with installed pile wall as a remedial measure in the foot of the slope was constructed significantly differently from the slope models with gravity retaining wall and gabion wall as described in the previous chapters. The results of two tests are presented here: two models of a clayey (SK15) slope, with and without applied gabion pile wall, which were exposed to identical intensities of artificial rainfall. The course of the test developments is shown in Fig. 17. In both tests, the slopes were exposed to an initial rainfall intensity of 32.8 mm/h. The selection of rainfall intensity was again based on infiltration conditions in a slope and the main requirement was that all rainfall water at the point of contact on the model surface is infiltrated, i.e., no surface runoff is generated. The applied intensities were at the upper precipitation values that can be infiltrated in a soil.

Fig. 17

Simulated rainfall in the test on the clayey slope without applied pile wall (blue) and test on the clayey slope with applied pile wall in the foot of the slope (black). Vertical lines mark times of the first crack evidence

After the initial establishment of a constant rainfall intensity and a stable infiltration process, the models for both slopes were exposed to constant rainfall intensities to enable similar rainfall conditions. The intensity of 32.8 mm/h was maintained throughout the tests. The constant groundwater level in the lower (L) part of the model was maintained until the end of the tests and the nozzle above the lover part of the model was closed after the groundwater reached the surface in this part of the model. The intensity in the upper parts of the slopes did not change.

The first crack of the landslide in the slope without applied pile wall (Test no. 8) occurred in the middle part of the slope after 35 min. After that, the landslide retrogressively developed to the top of the slope, but without significant displacements. In the 48th min, the ground water level reached the surface in the middle part of the slope (Fig. 18a) and started to flow to the surface (Fig. 15a, c). As the test progressed, the ground water was rising and reached the surface along the slope (Fig. 16a, c, e, g), while the amount of outflow water was gradually increasing (Fig. 18a, c, e). This outflow and the resulting surface runoff as a consequence have two additional processes: the washing out of silty (kaolin) particles from the soil in the slope and surface erosion (Fig. 18a, c, e). Surface runoff was increased by rainfall water that could no longer infiltrate into the slope. During this increasing erosion process, no further landslide development occurred. Once a steady state flow condition was established (Fig. 19a, c, e, g), only the erosion process due to surface runoff continued and the test was terminated after 195 min. Even in the situation when the rainfall intensity was significantly increased (to 82.5 mm/h from the 175th min to the end of the test), no further landslide development occurred.

Fig. 18

Development of landslides in small-scale clayey slope without and with installed pile wall: (a) erosion process after initial instability in the unsupported slope (85 min), (b) slope after the first cracks in slope with pile wall (107 min), (c) development of erosion process on unsupported slope (159 min), (d) start of erosion process in the upper part of the slope with retaining wall (175 min), (e) overall erosion at the unsupported slope (181 min), (f) erosion process in the upper part of the slope with pile wall (194 min)

Fig. 19

Volumetric water content and pore water pressure measurements in tests without applied pile wall (a) at 6 cm below the surface; (c) 12 cm below the surface; (e) 18 cm below the surface; (g) 24 cm below the surface and in the test with applied gravity retaining wall; (b) at 6 cm below the surface, (d) 12 cm below the surface; (f) 18 cm below the surface; (h) 24 cm below the surface in upper (H), middle (M), and lower (L) part of the slope

Under the same rainfall conditions, the behavior of the slope with installed pile wall differs considerably (Test no. 12). The first crack of the landslide in the slope with applied pile wall occurred in the 81st min in the upper part of the slope. Thereafter, no significant sliding displacements occurred in the slope with the pile wall applied, indicating that the applied remedial measures with pile wall and coarser backfill material behind the head beam of the pile wall confirmed the intended role in maintaining the stability of a clayey slope (Fig. 18b, d). The other differences related to the slope without remedial measures are absence of ground water outflow from the slope to the slope surface and washing out of silty particles. The drainage tranches behind the pile wall have an important role in collecting surface water to prevent slope surface erosion. The test was conducted at the same rainfall intensity until the 108th min, when the erosion process started in the upper part of the slope (Fig. 18d). The steady state of the flow (Fig. 19b, d, f, h) was achieved and there was no further development of the landslide until the 230th min of the test. At that moment, the rainfall intensity was significantly increased to 82.5 mm/h (from 230th min to the end of the test), which significantly exceeded the infiltration capacity, intensifying the erosion process in the entire upper part of the slope. Additional displacement of the landslide body was also observed, but the pile wall and buttressing construction in the toe of the slope remained stable (Fig. 18f).

Although the stability of overall slope with the installed pile wall and buttressing embankment in the toe was not completely retained and the main crack at the top of the slope indicated slip surface formation, the overall displacements were relatively small and there were no visible signs of instability in the foot of the slope. The measured displacements in the central part of the head beam of the pile wall were less than 5 cm, but no visible signs of loss of overall stability of the pile wall structure were observed (Vivoda Prodan et al. 2023b).

The superficial signs of sliding initiations that occurred and the causes for their occurrence on slopes with and without applied pile wall and other remedial elements (buttressing, drainage tranches) can be explained by the results of continuous volumetric water content monitoring inside the slope (Fig. 19a to h). The process of landslide initiation and development was described by the volumetric water content increase due to infiltration, until the complete saturation in the deepest part of the slope, forming the water table. Once it becomes established, a flow down the slope begins, causing the groundwater level to rise rapidly in the lower part of the slope. This leads to a decrease in the soil shear strength with complete loss of matric suction, an increase in soil weight, failure and landslide development to the top of the slope, Fig. 18a to f.

5 Discussion and Conclusions

Physical modelling of landslides by analyzing small scale landslide models’ behavior was started in 1970s and 1980s in Japan on natural slopes exposed to artificial rainfall. Laboratory experiments on landslide behavior in a scaled physical model under 1 g loading conditions started in the 1980s and 1990s in Canada, Japan and Australia.

The small-scale landslide modelling has found a wide application around the world in different aspects of landslide investigations, analyzing different types of landslides (e.g., flows, slides, falls and toppling) as well as different types of materials (rock mass, sandy, silty, and clayey materials) and landslide movements. The main purpose of landslide physical modelling was to study the initiation, motion and accumulation of fast flow-like slides caused by infiltration of surface water into a slope and fluidification, but also to study earthquake-induced landslides. The use of physical modeling in landslide research has been significantly increasing in recent years, as evidenced by the increasing number of articles published in journals and at conferences.

Although the small-scale landslide physical models, modern monitoring techniques and different monitoring equipment enable good insight into initiation and development of modelled landslide, one crucial issue is to establish relationships between small-scale, brief, idealized and by artificial boundary restricted model with the complex natural landslide process. These relationships are known as scaling or scaling laws and play a crucial role in small-scale model designing and interpretation of results. Successful scaling (of material parameters, model dimensions, boundary conditions, rainfall intensity and shaking parameters as triggering factors, measured movements, velocities, accelerations etc.) is a necessary precondition for realization of successful small-scale landslide modelling.

In this paper we presented partial results of the research Project Physical modelling of landslide remediation constructions’ behavior under static and seismic actions, funded by the Croatian Science Foundation, started in October 2018 at the Faculty of Civil Engineering, University of Rijeka, Croatia. The main Project aim was modelling and researching the behavior of landslide remedial constructions in physical models of scaled landslides in static and seismic conditions. In this paper, the behavior of small-scale slope models supported by remedial measures under artificial rain in 1 g loading conditions are presented and discussed. Models of slope were built of different soil materials, with and without applied remedial measures (gravity retaining wall, gabion wall, pile wall) and exposed to identical intensities of artificial rainfall. The measurements results collected from the geodetic and geotechnical monitoring systems enabled understanding of overall process of rainfall infiltration and soil strength reduction to the development of the slip surface in a slope.

In the previous chapter, the results of two series of tests carried out on slope inclinations of 35° degrees built of different soil materials, uniform sand (S) and sand-kaolin mixtures with 10% (SK10) and 15% (SK15) kaolin content, with and without applied remedial measures were presented. The first series of tests was carried out on basic slope models, while in the second series of tests the gravity retaining wall, gabion wall or pile wall were installed as adopted remedial measures and then exposed to artificial rainfall intensities identical to that of the first series of tests. The aforementioned constructions were selected as appropriate remedial measures against instability that occurred on the slope models in the first series of tests without remedial measures. The models, materials and procedures that were applied during the models’ construction were presented in detail. Both series of models with and without applied remedial measures were exposed to almost completely identical rainfall scenarios to enable a comparison of the slope behavior results obtained and the effects of the applied remediation measures to slope stability.

As it was described above, in the same time exposed to the same rainfall scenarios, almost no significant sliding processes and displacements in the slope with applied remedial measure occurred that indicate that the applied remedial measures with remedial measure and coarser backfill material behind the wall constructions confirmed the intended role to retain slope stability. However, it should be emphasized that after applying of the rainfall intensity at the upper part of the slopes higher than infiltration capacity of slope materials, sudden development of erosion process of slope materials and occurrence of global instability of slope appeared. The retaining constructions did not collapse but horizontal displacement pointed on global possible global collapse of the slopes in ace of prolonged rainfall of high intensity.

The analyses of the rainfall infiltration and the formation of a ground water level along the slope in both model series with and without remedial measure constructions applied do not reveal any significant deviations in the measurement results, if the differences in water contents at the beginning and during the tests, caused by initial differences and heterogeneity in soil density and water content, are neglected. The graphs representing the development of volumetric water content in the model during the tests show similar trends over similar time periods of applied rainfall in both series of tests, suggesting the conclusion that the main role of the applied remedial constructions and coarser backfill material behind the constructions is in increasing soil strength in the slope foot and the associated erosion prevention in the slopes. As one of the most important findings which came out from this research is that the matric suction in a slope at low confining stresses, such are those present in slope models, has a governing role in retaining stability of slopes. In almost all cases, the failures, and evidences of sliding process were observed in time period when ground water was formed in a slope causing full saturation and total loss of matric suction. The impact of pore pressure to reduction of effective stresses and strength at the slip surface in small-scale models is too low and has no significant role in instability occurrences in 1 g conditions. This fact can be applied to shallow landslides in general and would be very easy to confirm, despite all the difficulties in applying small-scale model results to real slopes.

The results presented were mostly based on observations from high-speed cameras that enabled to provide stereo image sequences during the sliding process, and on a geotechnical monitoring network of miniature sensors connected to data loggers that provided continuous data collection on soil moisture, positive pore-water pressures and soil suction, electrical conductivity, temperature, and pressures. During the tests, some other equipment was used as part of the slope monitoring, e.g., photogrammetric equipment for multi-temporal landslide analysis of image sequences obtained from a pair of high-speed stereo cameras, terrestrial laser scanning and SfM photogrammetry surveys, the results of which were not included in this paper. These data were recorded and collected during the conducted tests, and these results will enable detailed kinematic analyses of the points at the surface of the slope model determined in the pre- and post-slide phases. Although the results currently analyzed already give a relatively good insight into the impact of the applied remedial constructions on increasing the overall stability of the slope in the models, a comprehensive analysis of all collected data will allow a more detailed and precise description of all parts of the models and especially of the remediation structures behavior.