Analysis the Causes of the Slip Embankment Protecting the Slope of Lien Chieu Aviation Petroleum Depot

Abstract

The slope protection embankment of Lien Chieu aviation petroleum depot was built in 2001. The embankment is responsible for ensuring safety for the oil tank area and dispensing area, with the capacity of dispensing 13,800 m³ of fuel serving airports in the central region, from Hue to Binh Dinh provinces. In 2008, due to the occurrence of floods, a landslide has destroyed the entire 59.7 m middle segment of longitudinal embankment, leaving the foundation inert. The rest of the embankment body in the North was cracked on the entire surface; the embankment body fell inward; the weight relief plate was cut entirely from the embankment body and collapsed downwards. The collapse of the slope embankment led to the destruction of the oil tanks, causing the operation to stop. It was urgent at that time to investigate the causes of the slope failure and suggest the safe remedial design for the structure. The site investigation and numerical analyses show that the reasons for the failure could be due to the insufficiency of drainage holes to drain groundwater during the flood season characterized by heavy rain, which made the lateral pressure acting on the wall increase significantly, possibly causing the wall structure to be damaged. The proposed design which consists of a low retaining wall and upper slope embankment, both reinforced by geotextiles, could be stable according to numerical modelling with many loading cases. Attention must be paid to the stability of the excavation slope during construction and the difficulties of narrow space for the excavation.

1 Introduction

The aviation fuel depot construction project in Lien Chieu, Da Nang, is located in Hoa Hiep Bac ward, Lien Chieu district, Da Nang city (Fig. 1a). The construction area is situated on the southern slope of Hai Van Pass down to the sea. To the west, it borders National Road 1A, to the east is Kim Lien Bay in Da Nang Bay, located at 16⁰08^′00^″ north latitude and 108⁰09^′00^″ east longitude, in Hoa Hiep ward, Lien Chieu district, Da Nang city. The project was built in 2001 with a capacity of 13,800 m³ of various fuels to serve airports in the Central region, from Hue to Binh Dinh. The slope protection project has the task of ensuring the safety of the oil storage area and distribution area with a high difference of 18 m.

In 2008, during the period from October to November, due to the occurrence of floods landslides destroyed the entire 59.7-m-long central segment. The remaining embankment body in the south (about 4 m long) was still quite intact. The rest of the embankment body in the North (about 7 m) was cracked on the entire surface; embankment body fell inward; the load relief plate was severed from the embankment body and collapsed downwards. The collapse of the slope embankment led to the destruction of the oil tanks, causing the operation to stop (Fig. 1b).

To restore normal operations and long-term stability of the Lien Chieu oil storage tanks, it is necessary to rebuild the central segment of the slope embankment and to repair other damages to ensure the safety of the entire structure. Therefore, investment in the restoration of the tank foundation of the Lien Chieu aviation oil storage tank was very urgent and decisive in ensuring timely, effective and efficient supply of fuel to meet the increasing demand of the central region's airports.

The article analyzes the causes of the slope failure and suggests safety measures when reviewing some design proposals of relevant institutions as requested.

Fig. 1.

The aviation fuel depot in Lien Chieu, Da Nang.

2 Method of Study

In this study, we conducted a survey at the site to investigate the failure. Our subsequent task involved analyzing and numerically assessing designs proposed by various relevant institutions, as well as recommending safety measures. For the numerical simulations, we utilized both the finite element method (FEM) and the limit equilibrium method (LEM). We employed Plaxis and GeoStudio software to analyze the stress-strain relationship and the factor of safety of the retaining walls, utilizing plane problem models.

3 Results and Discussion

3.1 Project Site Investigation

According to the topographic survey [4], the project location is on the southern mountain slope of Hai Van Pass with a slope elevation of +57 m (fuel dispensing station platform) to +39 m (oil tank farm). It faces the straight direction of National Highway 1A, with a steep slope and a slope height of 70 ° compared to the flat surface. Along National Highway 1A, the slope direction is North-South, which is also the direction of water flow from the mountain slope and outflows through the road trench. The water in the fuel dispensing station platform area flows towards the foot of the slope. The foot of the slope is an existing oil tank farm, in which the foot-to-tank 1 distance is about 1.5 m and the foot-to-tank 2 distance is about 2.5 m, which are very narrow and fixed distances.

According the original design [9], the length along the ridge is 101 m. The slope structure is made of reinforced concrete with a grade of 250, 13 m high. The thickness changes from 40 cm (at the foot of the roof) to 30 cm (at the top of the slope). The slope angle is 70 °. The foundation is 50 cm thick and 1.5 m wide, placed on natural ground. The top of the ridge is at elevation +48 m, while the foot of the ridge is at elevation +35 m. The load-bearing horizontal part is at elevation +44 m. The top of the ridge features a horizontal flange 1m wide and 25 cm thick for locking the roof. The inside of the ridge is filled with sand and gravel material with a compaction ratio (K value) of 0.9. From elevation +48 m to the distribution platform (elevation +53.6 m) is the soil slope, which is 70 °, reinforced by 11 layers of watered TT060, 3.5 m wide, with a spacing of 0.5 m.

Da Nang is situated in an area influenced by monsoon activity, thus falling within the monsoon climate zone. The Truong Son Mountain range to the west significantly influences the type and characteristics of the climate in the region. This area experiences a 2-season climate pattern: a dry season extending from January to September and a rainy season from September to December. The interplay of the monsoon regime and the Truong Son range generates distinct variations between the rainy and dry seasons across the project area.

The soil strata of the surveyed area (Fig. 2), from top to bottom, are as follows [10]:

Layer 1a: The yard consists of concrete, sand, crushed stone, and clay, with a poorly structured composition. This layer is distributed across the surface and encompasses most of the surveyed area. It has been identified in boreholes HK09, HK05, and HK03, with thicknesses ranging from 0.20 m (HK05) to 1.40 m (HK01). The average thickness of this layer is 0.67 m.

Layer 1b: consists of filling soil characterized by mixed clay with yellow-brown, yellow-gray, gray-brown, red-brown, mixed with grit, weathered rock in hard plastic state. This layer is distributed throughout the survey range, found in all boreholes (from HK01 to HK09), the thickness varies from 0.40 m (HK08) to 5.90 m (HK05).

Layer 2: comprises clay with yellow-brown, yellow-gray, and gray-brown hues, mixed with gravel and weathered rock, presenting a semi-hard to hard state. This layer is partially distributed across the survey area and has been encountered at boreholes HK02, HK04, and HK08. The thickness of this layer varies from 2.60 m (HK08) to 7.80 m (HK04).

Fig. 2.

Typical geotechnical cross section [10].

Layer 3: consists of intensely weathered granite, clay mixed with yellow-brown, gray-brown, and gray-white tones, combined with grit and soft weathered seams, presenting a semi-hard to hard state. This layer is identified in boreholes HK01, HK02, HK04, HK05, and HK06. The thickness of this layer varies from 0.50 m (HK06) to 13.80 m (HK02).

Layer 4: is characterized by strongly weathered granite, displaying yellow-grey and white-gray hues, with cracks and strong fragmentation, often exhibiting significant weathering. This layer is distributed and identified in boreholes HK02, HK03, HK04, HK05, HK07, and HK09. The thickness of this layer within the survey range is unknown, reaching depths of up to 25.00 m, as boreholes HK02 and HK05 terminate within this layer.

Layer 5 consists of yellow-grey, white-gray, and green-gray granite exhibiting moderate to light weathering, characterized by a block structure and cracks. This layer is partially distributed within the survey range and is only identified in boreholes HK01, HK04, HK06, HK07, HK08, and HK09. The thickness of this layer is unknown within the survey depth range, varying from 5.50 m (HK07) to 25.00 m (HK04), as all boreholes terminate within this layer.

Fig. 3.

Existing condition of the work at site: (a) General status of damaged slope embankment, (b) Cracked face plate and remaining broken offload plate (CD segment), (c) Slope protection embankment before 2008, (d) Layout of boreholes of this study.

In 2008, floods triggered landslides, resulting in the destruction of the entire section spanning 59.7 m along the embankment (from point D to point E, as shown in Figs. 3a, b, c). The remaining structure is now exposed, revealing a foundation that is 50 cm thick and 1.5 m wide. The southern section of the embankment (approximately 4 m long in the EF segment) remains largely intact. However, the northern portion of the embankment (around 7 m in the CD segment) is extensively cracked across its surface, causing the embankment body to collapse inward. The offload plate has been separated from the embankment body and has collapsed downwards. Consequently, the collapse of the slope embankment resulted in the destruction of the oil tanks, prompting the depot to cease all operations.

Based on the survey, the failure of the slope embankment wall could be attributed to the following reasons [3]:

• 1. Due to the absence of drainage holes on the slope embankment wall, heavy rainfall saturated the backfill soil, leading to increased lateral pressure on the wall. This heightened pressure likely contributed to structural damage within the wall. The failure primarily occurred at the base of the wall, where the highest shear stress was experienced. Consequently, the wall underwent significant displacement, ultimately collapsing under its own weight and the pressure from the sliding soil. As a result, the embankment wall collapsed, and the embankment behind it slipped, exacerbating damage to concrete structures and adjacent components.

• 2. The placement of the relief plate on the embankment surface resulted in its transformation into a large-span cantilever (3 and 4 m) when the embankment subsided. The maximum shear stress occurred at the connection point between the relief plate and the wall panel. Due to the considerable weight of the relief plate and the soil above it, the plate was severed at the joint and collapsed. This phenomenon is evident in the northern section, specifically the beginning of the CD section, where the remaining part of the embankment wall became fractured (Fig. 3).

• 3. The destruction of the embankment wall originated in the middle near the beginning of the CD segment (Fig. 3), where the backfill was most extensive, resulting in the collapse of the entire segment. Subsequently, the backfill soil mass at the outset of the CD segment slid along the embankment, creating a significant gap between the excavated slope and the embankment wall surface. This displacement caused the embankment wall panel to move inward, in the opposite direction to its original position when the wall body was destroyed, resulting in surface-wide cracking. Consequently, the load relief plate was severed from the wall panel and collapsed. Although this segment of the embankment wall did not collapse entirely, its ability to support force was compromised, even though the reinforcements were still capable of maintaining its current state.

• 4. The damage to the embankment wall resulted in the landslide of the upper reinforced embankment from elevation +48 m to +54 m. The embankment block situated atop the embankment slipped, causing settlement and fracturing of the reinforced concrete layer of the fuel dispensing station platform, which subsequently collapsed.

• 5. In the original design [9], two types of embankment wall sections were utilized. Essentially, these two types are identical, differing only in the size of the load relief plate. However, observations from the actual site post-failure indicate that the efficacy, if any, of load reduction is minimal. For instance, at the termination of the longitudinal line (from position Sect. 15 to E; Fig. 3), the foundation consisted primarily of solid rock, visibly close to the embankment wall surface. Consequently, one end of the relief plate (3 m wide) likely rested on a stable stone foundation. However, during instances of increased lateral pressure on the wall due to soil saturation from groundwater, this load reduction plate did not perform adequately, potentially resulting in the undermining of the wall base.

• 6. The remnants of the bottom footing of the embankment wall were nearly intact, indicating that the embankment wall did not experience overall sliding, and the bottom foundation was situated on a solid rock foundation.

3.2 Numerical Simulations of the Structure at Site

To assess the impact of groundwater on the stability of the retaining wall numerically, we conducted simulations under various loading scenarios, including construction, basic, and special cases such as water drain pipe clogging (Fig. 4). The selected cross-section was at station 13 (KM: 0 + 94.37), corresponding to the geological cross-section passing through boreholes LK2 and LK5, associated with Sect. 2 of the embankment (Figs. 3c and 3d). At this location, the natural ground elevation is −1.95 m. The top elevation of the wall is +57.0 m, with a wall height of 18 m from the base. The ground soil comprises layers 1, 3, 4, 5A, and 5. For simplicity, the backfill and ground soil were simulated using the Mohr-Coulomb model [7], with parameter values as shown in Table 1.

Table 1. The Mohr-Coulomb material model parameters for ground and backfill soils.

The construction process can be divided into three main stages: excavating the foundation pit and constructing the wall bottom slab; constructing the retaining wall; and backfilling soil behind the wall.

In the basic case, the groundwater level was assumed to be at the bottom of the foundation (elevation +37 m). The distributed load on the top surface was assumed to have values of q = 0 and 60 kPa. However, in the special case, we considered the scenario of a clogged water drain pipe, causing the groundwater level behind the wall to rise to elevation +45 m, while the water level in front of the wall was assumed to be at elevation +39.0 m. In this case, the distributed load on the top surface was assumed to be q = 55 kPa. It is important to note that the earthquake effect was not considered in these analyses.

The Plaxis software, version 8.6 [7], was utilized to analyze the stress-strain behavior using the Finite Element Method (FEM). The Factor of Safety (FOS) was calculated using the shear strength reduction technique [1]. Table 2 presents the results of deformation calculations for both the basic and special cases.

In the basic case, the wall demonstrated stability under two different surface load values: FOS = 1.42 (q = 0) and FOS = 1.24 (q = 60 kPa). However, in the special case of water drain pipe clogging, the rising groundwater level increased the horizontal pressure acting on the wall due to the water pressure. Consequently, the reduction of FOS to 1.11 (q = 55 kPa) did not meet the allowable FOS = 1.13 specified by TCXDVN 285–2002 [8]. Hence, there is a necessity to revise the remedial design plan to enhance the stability of the slope protection embankment.

Fig. 4.

Slope analysis of the retaining wall in the special case of clogged water drain pipes.

Table 2. Calculation results of deformation in the basic and special cases.

3.3 Checking Proposed Remedial Designs

In consideration of the actual topographical conditions, constructing a wall higher than 10 m would require extensive excavation, posing challenges due to limited terrain availability. Therefore, a design featuring a lower wall height of 6.0 m (from elevations +39.80 m to +45.80 m) and an upper slope embankment (from elevations +45.80 m to +57.00 m) was proposed [2].

Given the steepness of the excavated slope (Fig. 5a), which fails to meet slope stability Factor of Safety (FOS) standards, an adjusted design [6] was proposed for the protection of the northern segment, as depicted in Fig. 5b. This structure comprises two main components:

Lower Part: A reinforced concrete (RC) wall, 6.5 m high, with a bottom plate 600 cm wide and 80 cm thick, featuring a vertical wall plate 80–50 cm wide. Triangular middle counterforts, 50 cm thick, and rectangular side counterforts, 50 cm thick, are included. Two rows of plastic pipes for drainage are arranged on the wall surface.

Upper Part: A 13.5 m high soil embankment with a slope factor of 1:1.5. Two berms are included: the slope leg berm, 250 cm wide with a drainage ditch (40-40 cm) made of 20 cm thick RC #200, and the middle berm placed at an elevation of +47 m, 700 cm from the leg berm, also with a drainage ditch (40-40 cm) made of RC #200, 20 cm thick. Geogrid reinforcement, covered with concrete mortar #200 or employing a reinforced concrete frame system inside the panel of protective stones, is utilized. Additionally, the embankment is reinforced with geotextile reinforcement, approximately 30 cm high.

The new design underwent rigorous slope stability analysis, considering various loading cases such as during construction, normal operation, and special cases. The GeoStudio software, version 6.02 [5], was utilized for analysis. A grid of centers of potential slip surface (13 × 13) and radii were implemented to determine the minimum FOS. The limit equilibrium Morgenstern-Price method was employed in the analysis, resulting in a stable excavation slope with an FOS of 1.19, as depicted in Fig. 6a.

In the case of the end of construction, it was assumed that the backfill soil is dry, and the groundwater level appeared at the wall footing elevation. In the special case, it was assumed that the water drain filter in the wall is clogged, causing the discharge water to overflow to the top of the wall. This scenario presents a particularly dangerous case.

All results satisfied the relevant standards when the wall was reinforced with geotextiles, with FOS values of 1.707, 1.858, and 1.591 for the construction, normal operation, and special cases, respectively (Fig. 6). It's noteworthy that for the design without slope reinforcement, the FOS value was 1.175, which did not meet the required standard in the case of construction. Clearly, the geotextile reinforcement proved effective in increasing the embankment stability.

Fig. 5.

Proposed remedial designs: (a) steep excavation, (b) gentle excavation.

Fig. 6.

Slope stability analyses of the new design.

4 Conclusion

The failure of the slope protection embankment at Lien Chieu aviation petroleum depot underwent thorough investigation. The reasons for the failure could be attributed to the insufficient drainage holes, which failed to effectively drain groundwater during the flood season with heavy rain. This resulted in a significant increase in lateral pressure acting on the wall, potentially causing structural damage.

To address these issues, a proposed design comprising a low retaining wall and upper slope embankment, both reinforced by geotextiles, was developed. Numerical modeling with various loading cases suggests that this design could ensure stability. However, particular attention must be given to the stability of the excavation slope during construction.

Considering the challenges posed by limited traffic space, a combination of soil nailing and geotextiles could prove effective for slope embankment protection in narrow excavation areas. This approach enhances stability while accommodating space constraints.