A discrepancy between observed and predicted NATM tunnel behaviors and updating: a case study of the Sabzkuh tunnel

  • Majid Taromi
  • Abbas Eftekhari
  • Jafar Khademi Hamidi
  • Ali Aalianvari
Original Paper


The 11 km long Sabzkuh water conveyance tunnel is under construction using conventional and tunnel boring machine tunneling techniques in the Zagros Mountains of south west Iran. During excavation of the conventional section (i.e., 350 m), three daylighting collapses occurred in the tunnel, and large sinkholes developed at the ground surface, due to both poor ground conditions and inappropriate selection of excavation and support class (ESC). In this study, the influence of ESC on tunnel response and stability was investigated through three dimensional (3D) finite difference and finite element methods. The analysis results of numerical modeling indicated that the tunnel face would be stable if a single stage (full face) excavation and an advance step of 1.5 m (defined as ESC#1) were applied. However, the reality was different, and the tunnel collapsed soon after the face was only 35 m away from the portal. Having changed the ESC by taking into account new geological and geotechnical data obtained from face mapping, a field survey of surface cavity, and a trench study, the tunneling advance rate was satisfactory (1.5 m/day) despite the many challenges encountered. This paper presents a brief review of some of the key geological challenges faced in the tunnel design, including characterization of the ground conditions, selection of appropriate design parameters, and evaluation of the excavation and support installation sequence based on monitoring and analyzing ground behavior during construction. The discrepancy between the predicted and observed behavior of the tunnel, and some practical considerations, are also discussed.


Sequential excavation method Fault zone Daylighting collapse Sabzkuh tunnel Numerical method 


Tunneling in soft ground conditions is always challenging due to the various geotechnical and environmental constraints encountered. Some new findings and developments in the field of soft ground tunneling have emerged in recent decades to address these problems. Most of these challenges are related to geological and geotechnical conditions. Geotechnical aspects of soft ground tunneling have been addressed by many researchers (e.g., Sterpi et al. 2013; Elyasi et al. 2015). However, obtaining relevant information on tunnel ground conditions has been the key factor for successful construction of underground structures in difficult ground conditions.

Tunneling in the young Zagros Mountains in south-western Iran has always posed challenges due to existing geological structures such as faults and severe natural folds, as well as diverse stratigraphic and active tectonic zones (Shahidi and Vafaeian 2005). Hence, performing a detailed site investigation has become one of the most important prerequisites in the design and construction of tunnels in this region. The Sabzkuh–Choghakhor water conveyance tunnel is currently under construction in this region. Three daylight collapses have occurred during the construction of the tunnel, forcing changes to the excavation and support class (ESC) for the rest of the tunnel.

Here, we present the problems that occurred in the first 390 m of Sabzkuh tunnel during its construction, emphasizing the importance of an appropriate ESC, as well as monitoring and computational tools based on numerical analyses and practical considerations.

Project overview and geology

The Sabzkuh–Choghakhor water conveyance tunnel, at 10,617 m in length and with a 0.01 % gradient, is under construction in south-western Iran (Fig. 1). The tunnel is situated along the high Zagros, which is one of the important structural regions in the Zagros. This part of Zagros is known for its multiple thrust and transverse strike-slip faults. In under study area, the Solaghan thrust fault with a dip angle of 40°–65° disturbs the geological layers from the beginning of the tunnel.
Fig. 1

Geographical location of the Sabzkuh tunnel (Eftekhari et al. 2014)

The tunnel is going to be excavated using a double-shield tunnel-boring machine (DS TBM). Information obtained from the site, including field reconnaissance, geotechnical and geophysical studies, revealed that the tunnel, from the beginning to chainage 0 + 390 km (section T1), is located in alluvium. Taking into consideration the application range of DS TBM, and its limitation for working in alluvial soil, it was decided to excavate this part of the tunnel using conventional methods (Karami et al. 2014; Eftekhari et al. 2014).

In the design phase of the project, consultant engineers proposed the first excavation and support class (ESC#1) for the soil (T1) section of the tunnel, taking into consideration the preliminary site investigation data. Actually, this section was designed based on insufficient information. This was why, after full face excavation of about 35 m in length from the tunnel inlet, an unexpected ground condition was encountered and the tunnel collapsed when it came across an unexpected running ground, low-viscosity layer, and groundwater inflow. Running ground is defined when the soil (usually clean medium to coarse sands and gravels) invades the tunnel until a stable slope is formed at the face. As a result of this, after the first daylight collapse, and taking only 8 h, about 350 m3 soil flowed into and filled 30 m of the tunnel. A progressive chimney-like collapse then led to formation of a big crater at the trench portal of the tunnel. The IPE Arc Support Technique (IAST) was suggested, and duly employed to pass the collapse zone (Eftekhari et al. 2015). A little farther into the tunnel, two more daylight collapses occurred, and were followed by ground settlement and cavities where these developed in the ground surface. Figure 2 shows the location of the collapses, along with the geological settings in this section of the tunnel.
Fig. 2

Plan and longitudinal geological profile of the tunnel site, and the location of three daylight collapses in the Sabzkuh tunnel

According to the results of surface geological survey and geotechnical investigations conducted on samples of three boreholes in the vicinity of the collapses, the geology of the area consists of alluvium and debris units combined with alternative saturated layers of clay, silt and sand. Location of the boreholes, along with typical sections of borehole logs and soil characterization are shown in Fig. 3.
Fig. 3

Borehole logs in the T1 section of Sabzkuh tunnel

Using the supplementary geotechnical data, the consultant engineers were able to review the tunnel construction process and update the design by using additional data collected during the construction process. However, in the rest of the tunnel, various challenges, including active tectonics, existence of fault zones, poorly consolidated and cohesionless ground conditions, and confined and non-confined underground aquifers, were expected.

Stability analysis and design of the tunnel support system

For stability analysis, and with the aim of achieving an appropriate tunnel model, several analytical, observational and empirical methods are often used in parallel. In recent years, numerical modeling using the finite element method (FEM) (Hajjar et al. 2014; Sharifzadeh et al. 2013), the boundary element method (BEM), the distinct element method (DEM) and the finite difference method (FDM) (Vlachopoulos and Diederichs 2014; Elyasi et al. 2015) has been applied widely in rock engineering problems (Jing and Hudson 2002; Fairhurst and Carranza-Torres 2002; Hoek et al. 2008; Carranza-Torres et al. 2013). By using these methods, modeling of complicated geometry under different loading conditions and employing appropriate constitutive models is possible.

Estimation of loads on support system

In his book Theoretical soil mechanics, Terzaghi (1943) presented one of the first models for computing the stability of sections of shallow tunnels excavated in soils, based on the principle of soil arching. This model is based on the condition that the soil extending laterally and above the tunnel is in a state of limit equilibrium. A method for computing support requirements for (both shallow and deep) tunnels in both soils and rock was later developed from the Terzaghi original model. This became to be known as the Terzaghi’s ‘rock load’ method (Proctor and White 1946) and has been a popular method for designing tunnel supports in many countries since its introduction (Carranza-Torres et al. 2013).

The support load estimated from various methods in section T1 of the Sabzkuh tunnel is summarized in Table 1. The width and height of the tunnel are 4.9 and 5.4 m, respectively. The vertical and horizontal loads on the support system estimated from various methods are given in the table.
Table 1

Estimation of support load in Sabzkuh tunnel, section T1



Value (MPa)


Vertical load



\( \frac{{B_{1} \left( {\gamma - \frac{2c}{{B_{1} }}} \right)}}{k\tan \phi }\left[ {1 - e^{{\frac{KD\tan \phi }{{B_{1} }}}} } \right] \)

Bierbäumer’s theory


\( \gamma H\left[ {1 - \frac{{\tan \phi \cdot \tan^{2} (45 - \phi /2)H}}{b + 2m \cdot \tan (45 - \phi /2)}} \right] \)

Balla’s theory


\( \gamma H\left[ {F_{H} + \frac{b}{H}F_{B} - \frac{c}{\gamma H}F_{c} } \right] \)




Horizontal load



0.3γ(0.5 m + hp)


\( \frac{{B_{1} \left( {\gamma - \frac{2c}{{B_{1} }}} \right)}}{k\tan \phi }\left[ {1 - e^{{\frac{KD\tan \phi }{{B_{1} }}}} } \right] \)

Rankin theory


\( \gamma H\tan^{2} (45 - \phi /2) \)




Meanwhile, for safety reasons, the lining horizontal and vertical pressures in Sabzkuh tunnel were set at 0.09 MPa and 0.2 MPa, respectively.

Analysis of a composite liner with the ‘equivalent section’ approach

Use of shotcrete as a primary support is a common practice in tunnel design and construction (American Society of Civil Engineering 1984). Steel arches can also be used with or without additional support or reinforcement to stabilize blocky or deformable ground. If the magnitude of loads transmitted by the ground to the support is large enough to preclude shotcrete alone, or if squeezing or raveling behavior requires complete surface coverage, steel sets are commonly used in combination with shotcrete. This combination can be in the form of a complete composite annulus or may be a semi-circular or partial arch configuration (Table 2).
Table 2

Input data for the analysis of a semi-circular liner comprised of shotcrete and steel sets according to the ‘equivalent section’ approach from Carranza-Torres and Diederichs (2009)

Excavation and support class#1

A stability analysis of the Sabzkuh tunnel was performed by using FDM with the input data given in Table 3. In Fig. 4, the support system for the excavation step of 1.5 m simulated in a 3D finite difference program (using FLAC3D software; Itasca Consulting Group 1997) is shown. The Mohr–Coulomb constitutive model was used in the model. Four history points were considered for evaluating the vertical displacement in the tunnel face. Four history points are enough with regard to the tunnel cross sectional area. The model geometry is 30 m long (y direction), 50 m deep (z direction) and 30 m wide (x direction). The lateral and bottom surface of the model are displacement boundaries. The model is fixed in the horizontal direction at lateral surface, and the vertical displacement is limited at the bottom surface. The top surface of the model is the ground surface and is free in all directions. Accordingly, the maximum displacement of 5.5 cm was determined as shown in Fig. 5.
Table 3

Soil properties as input data in the finite difference method (FDM) model




Type of material behaviour


Total unit weight

kN m−3


Young’s modulus



Poisson’s ratio



kN m−2


Friction angle



Fig. 4

Tunnel excavation and support model; excavation step and four monitoring points

Fig. 5

Displacement histories of four monitoring points at the tunnel face

Displacement vectors surrounding the tunnel circumference at a plane strain cross section are shown in Fig. 6. The deformation pattern is dominated by high downward displacement at the tunnel crown, small upward displacements at the invert, and small horizontal displacements at the face. The magnitude and direction of displacement vectors are consistent with those observed at the tunnel.
Fig. 6

Displacement vectors at the tunnel face

The tunnel excavation process may cause loosening and plastic deformation in the area surrounding the tunnel. Figure 7 shows the potential plastic zone developed in the vicinity of the tunnel. As seen in the figure, parts of the plastic zone are developed in the vicinity of the tunnel due to excavation. The plastic zone is expected to propagate from the tunnel boundary to a maximum depth of about 5–7.5 m. Also, as it can be followed from the figure, configuration of the plastic zone above the tunnel crown and below the tunnel invert may be attributed to weakness of the tunnel face.
Fig. 7

Plastic zone around the tunnel

As illustrated in Fig. 8, the shear strain reaches a maximum level at the center of the tunnel face. Figure 9 shows the displacement contour around the tunnel. It is, therefore, necessary to reduce the peripheral displacement by closing the tunnel invert at the right time (with a maximum distance of 15 m). This reduces the deformation potential of the soil surrounding the tunnel. Moreover, a load-bearing ring is formed in the tunnel, to carry the imposed load.
Fig. 8

Contour of shear strain around the tunnel

Fig. 9

Contour of displacement around the tunnel

The results obtained are presented in Figs. 6, 7, 8 and 9. It can be revealed that the tunnel is stable, with a 1.5 m excavation step and 5.5 cm vertical displacement. Accordingly, full face excavation of the tunnel section was chosen, referring to the results of the design model (Table 4).
Table 4

Excavation and support class#1 (ESC#1) for the tunnel

Tunnel excavation started and continued by using a single-stage (full face) technique until the tunnel face reached about 35 m from the portal. A critical zone (landslide) was created in front of the face, the ground was disturbed, and the initial state of stress was changed. Due to non-uniform distribution of stresses, and the creation of an extensive plastic zone as well as high axial displacements around the tunnel, it became unstable, and finally resulted in a tunnel daylight collapse (Fig. 10).
Fig. 10

Tunnel collapse and cavity created in the portal of Sabzkuh tunnel

Analysis of the collapse factors and designing the construction after the tunnel daylight collapse

Taking into account the inefficiency of the existing excavation method, a database was created using experiences from similar projects and data obtained from field studies and studies conducted during construction in this region. Moreover, it was decided to conduct a complementary site investigation to determine the link between tunnel collapse and the geological settings in this section of the tunnel.

The results of the review indicate that a complex geological model of the region requires the continuous identification, analysis and monitoring of the movement of the surrounding soil mass to achieve the most economic and stable excavation, and to support the methods chosen in different conditions of the tunnel.

Figure 11 shows the construction process flowchart suggested for the Sabzkuh project after the tunnel daylight collapse. This process includes analyzing the data gathered from geological engineering practices, determination of ground design parameters, preparation of a basic scheme for excavation and maintenance, evaluation of hazards, revision. and correction of studies and risk management.
Fig. 11

Flowchart of tunnel construction process

In order to review and increase the scrutiny of the geotechnical investigations, and to reanalyze the tunnel stability with the updated real data, after the event, a field survey was conducted around and inside the ground surface cavity. The field reconnaissance revealed that the wall is composed principally of alternating layers of alluvial deposits with dip angles varying from 75° to 80° (Fig. 12). Meanwhile, a series of channels were excavated on the both sides in order to investigate the dip and alternation of the layers. Figure 13 shows a view of a layered soil outcrop in the trench at the tunnel portal.
Fig. 12

Field survey of surface cavity created at the tunnel portal

Fig. 13

Layered soil outcrop in the trench at the tunnel portal

Geophysical investigations were carried out with different geophysical methods such as common reflection point (CPR) and reflection seismology in the first 3 km of the tunnel route. Several faults were identified in the portal area and alluvial section. In addition, multiple soil samples were taken from different soil layers at the portal and the geological profile for the area under study was modified (see Fig. 2).

The results obtained showed an increase in soil layers and changes in some geotechnical parameters (Table 5).
Table 5

Geotechnical properties of soil layers at the tunnel portal


Soil unit

γ (g/cm3)

φ (°)

C (kg cm−2)
































Slide surface




Accordingly, a series of 3D numerical modeling using the FEMs were performed in order to investigate the influence of active span and partial-face excavation on tunnel stability.

Using the developed database, numerical modeling and continuous monitoring of the tunnel behavior, ESCs were selected for different geological units along the tunnel alignment and remedial solution in collapse-prone zones.

Excavation and support class#2

Taking into consideration the ineffectiveness of the full face excavation method as a construction solution, the ground behavior and classification were re-examined to define a suitable excavation method and construction process. Accordingly, an excavation scheme was adopted considering different factors such as the tunnel section, soil type, ground behavior and classification, and underground water table (Sharifzadeh et al. 2013). Taking all these factors into consideration, the sequential excavation method (SEM) was selected as the most appropriate method with which to continue the construction.

FEM analyses, in conjunction with empirical methods, were used to evaluate potential ground behavior upon tunnel excavation, and to determine the required excavation sequence and support measures.

Stability analysis for sequential excavation method

In this study, 3D finite element models with the capability of simulating sequential excavation were adopted in order to simulate the construction process in the Sabzkuh tunnel; each construction stage may involve soil excavation and/or support installation.

The tunnel diameter and depth are 5 m and 30 m, respectively. With regard to the symmetric geometry of the tunnel, one half of the tunnel can be modeled. In this study, the left half of the tunnel was selected. The model has dimensions of 25 m × 50 m × 50 m in the X, Y and Z directions, respectively (Fig. 14).
Fig. 14

a Finite element model. b Stages of excavation and support system

In modeling material behavior, the constitutive model of Mohr–Coulomb was used. The elastic–plastic Mohr–Coulomb model involves five input parameters, i.e., elasticity modulus (E) and Poisson’s ration (ν) for soil elasticity; friction angle (φ) and cohesion (c) for soil plasticity (Table 6). This model represents a ‘first-order’ approximation of soil or rock behavior. It is recommended to use this model for a first analysis of the problem considered.
Table 6

Material properties of the soil





Material model


Type of material behavior


Soil weight above phr. level


kN m−3


Soil weight below phr. level


kN m−3


Young’s modulus (constant)


kN m−2

5 × 104

Poisson’s ratio



Cohesion (constant)


kN m−2


Friction angle




Dilatancy angle




The results of the analyses indicate that the tunnel is quite stable, and that the initiated deformations are relatively low and can be controlled in an acceptable range (Fig. 15).
Fig. 15

Extreme displacements in the sequential excavation method (SEM) tunnel face

As seen in the figure, the displacement is maximal in the vertical (Z) direction. Hence, the tunnel will probably collapse when it comes across situations where the ground conditions are poor, as, for instance, in water-bearing sandy lenses. One way to prevent this might be to install one layer of flashcrete at the right time, or fibreglass nailing and conservative protective intervention. Also, the maximum displacement in the X direction is seen at the tunnel toes, necessitating an early invert closure. The ground water pressure may increase the pressure imposed on support systems, and subsequently lead to failure of temporary support. The easiest way to overcome this is to leave systematic drainage pipes in the tunnel walls. As for the Y direction, in case of soils with low cohesion properties, instability of side walls and roof fall are likely to occur. Furthermore, the possibility of invert uplift may be reduced by early invert closure. The excavation sequence, round lengths and support measures for excavation class B are given in Table 7.
Table 7

Excavation and support class#2 (ESC#2)


Ground deformation monitoring in tunnelling is a common means of selecting and controlling the excavation and support methods among those predicted in design, ensuring safety during tunnel excavation (including the safety of personnel inside the tunnel and structures at the ground surface) and construction quality (Kavvadas 2005).

In the Sabzkuh tunnel, a continuous geodetic monitoring began using two targets mounted on the tunnel wall, and one target in the tunnel crown at about 20-m intervals. In this tunnel, a tape extensometer (model Convex HR) was used to determine tunnel convergence. Technical specifications of the instrumentation system are listed in Table 8.
Table 8

Tape extensometer technical specifications




mm (in.)

Available ranges

20, 30 m and 66, 100 ft

Measurement range

50 mm


0.01 mm


0.1 mm


2.2 kg

The targets were mounted at least 1 m behind the tunnel face. Recordings followed at regular and predetermined time periods (Fig. 16).
Fig. 16

Tunnel convergence at chainage 0 + 040

The monitoring observations were in a good agreement with the results obtained from numerical analyses. However, on the right wall of the tunnel, due to high groundwater condition, larger deformations were observed in comparison with the left side. This problem was resolved by installing drainage facilities in this area.

In one case, long advance steps of top heading and bench resulted in late invert closure and high stress concentration in the side walls. Accordingly, this could lead to crack formation in shotcrete (Fig. 17). Hence, it is worth mentioning that all factors of excavation stages, invert closure duration, advance length, and the time of support installation are systematically employed in control of stress redistribution and tunnel stability, and any change to any of these factors must be made with caution and using information gained through the monitoring process in the tunnel.
Fig. 17

Crack formation in tunnel side wall due to long advance step and non-completion of lining ring

Excavation and support class#3

According to the experiences gained from excavation in the first section of the Sabzkuh tunnel and surface and the subterranean studies conducted, the ground conditions in this section indicate a complex geological model, along with numerous uncertainties largely pertaining to soil characteristics, fault zones, pore water pressure, sliding surfaces, alternation and dip angle of soil layers.

These studies confirm the presence of groundwater-bearing fault zones in tunnel alignment. After the face entered the fault zone at chainage 0 + 150, some engineering geological problems, such as poor soil quality, high groundwater seepage into crushed zones, and permeable layers were encountered. Moreover, the fault caused an increase of displacement and sliding potential in weak layers with low cohesion and sand lenses. Also, the presence of groundwater in the tunnel face caused an increase in the pore pressure and decrease in effective stress and material cohesion. These conditions resulted in the tunnel face changing rapidly from a stable to unstable state, and the tunnel collapsed for the third time. Figure 18 shows the sinkhole created at the ground surface, and face mapping in the collapsed zone.
Fig. 18

Sinkhole created at the ground surface (left) and face mapping in collapsed zone (right)

Therefore, it was essential to use ground improvement techniques such as pre-grouting, and also predict specific measures to achieve a safe construction process. The collapse occurred at a distance of 150 m and ground adverse conditions (i.e., a fault zone with a length of 50 m) made the rest of the excavation problematic without pre-reinforcement systems.

As a technique of ground reinforcement installed before tunnel excavation, the umbrella method aims to increase face stability, restrict and decrease the ground surface subsidence, and prevent slope failure and landslides. The present authors developed IAST for the first time as an umbrella method to pass through collapsed zones in the Sabzkuh tunnel (Eftekhari et al. 2015). The practical and theoretical research conducted showed that the selection and implementation of a pre-reinforcement system generally depends on the geological conditions around the tunnel, as well as overburden height, excavation and support methods, tunnel height, soil characteristics, advance length and stiffness of the support system. Taking into consideration the implementation constraints and the risk acceptance level, as well as experiences obtained from two previous daylighting collapses and the use of IAST, a forepoling method was selected, with steel pipes having a diameter of 63.5 mm and length of 4 m (Oke et al. 2014). Table 9 gives a brief introduction to the use of the ESC#3.
Table 9

Excavation and support class#3 (ESC#3)

The results proved to be highly satisfactory from both technical and economic points of view. Given the unfavorable ground conditions, the presence of groundwater, and an advance rate of 1 m/day (contrary to 1.5 m/day for ESC#2), a good and regular plan for construction process and an acceptable level of safety for tunnel operation have been achieved. In addition, analysis of results obtained from the monitoring process showed that, with implementation of the new pre-support system, the ground strength surrounding the tunnel increased, and displacement and pressure imposed on support systems decreased compared to ESC#2.

Concluding remarks

Due to the complicated geological conditions, tunneling in the Zagros Mountains has been always challenging, with consequent time delays and cost overruns in project completion. In the construction process of the Sabzkuh tunnel, a geotechnical model was planned, implemented and analyzed independently and regardless of the geological model. The lack of a logical relationship between these two models, as well as the complex ground conditions, together with other uncertainties, provided inappropriate information to the designers, affecting the analytical model as well as the excavation method. This led to an early daylighting collapse when the tunnel face reached an advance of 35 m from the tunnel portal. Unstable ground and collapsing zones, and the required techniques for passing through were the on-going challenges during construction of the tunnel. The ground conditions and scale of the collapse revealed that the tunnel goes through a complicated geological model, which makes the reconnaissance, analysis and continuous monitoring of ground movement inevitable. Accordingly, the single stage (full face) excavation was shifted to a NATM/SEM method due to its applicability and flexibility when confronted with the actual tunnel site conditions. A data server was established in order to gather data from field studies and as-built findings, regarding previous experiences. This server helps engineers review the plan and improve the design for the remaining tunnel sections.

Experiences obtained during the construction of the Sabzkuh tunnel in the soil (T1) section revealed that the complicated geological model requires frequent exploration, analysis and behavior checking of the surrounding soil mass in order to find the most economical and stable excavation model and support system for different conditions encountered along the tunnel alignment. Hence, the project owners should take into consideration preventive measures and the most practical and feasible solutions with an appropriate factor of safety according to the changing ground conditions encountered during the construction process of this tunnel.

This case study demonstrates once again the importance of early detection or prediction of potentially problematic zones, for instance, through probe drilling and monitoring in tunneling, especially in changing or difficult ground conditions characterized by alternating layers, faulting and localized zones of high water pressure.



Many thanks are due to IMN Consulting Engineers and the project manager of the Sabzkuh tunnel, Mr. Ali Asghar Izadi for sharing their experience and guidance with the authors.


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Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  • Majid Taromi
    • 1
  • Abbas Eftekhari
    • 2
  • Jafar Khademi Hamidi
    • 3
  • Ali Aalianvari
    • 2
  1. 1.Department of Geotechnical EngineeringIslamic Azad UniversityTehranIran
  2. 2.Mining Engineering Department, Faculty of EngineeringKashan UniversityKashanIran
  3. 3.Mining Engineering Department, Faculty of EngineeringTarbiat Modares UniversityTehranIran

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