Geotechnical forensic investigation of a slope failure on silty clay soil—A case study

Abstract

Qila Bala Hisar is one of the noteworthy places of Peshawar, Khyber Pakhtunkhwa. The fort was constructed on a filled ground during the 18th century and it was renovated several times by the occupants ever since. Recently, due to an earthquake of magnitude 7.3, the upper part of the south-western wall of the fort collapsed. The collapse of the wall was attributed to the failure of the retained slope. This research was undertaken to characterize the slope material, study causal factors of failure and evaluate remedial strategy. The investigation involved conventional field and laboratory testing and geophysical investigation using electrical resistivity technique to evaluate the nature of stratum. Also, X-ray Diffraction and Scanning Electron Microscopy was used to study the slope material at a molecular level to evaluate the existence of swelling potential. The analysis has shown that excessive seepage of water caused by the poor maintenance of runoff and sewage drains is the causal factor triggered by the seismic event. A remedial strategy involving soil nails, micro piles and improvement of the surface drainage is recommended.

Keywords

forensic geotechnical investigation / slope failure / slope analysis / finite element method / numerical analysis / seismic loading / Qila Bala Hisar

Introduction

Landslides are natural phenomenon that has challenged scientist and researchers around the world. Every year a huge loss to infrastructures and human lives are attributed to the landslides [¹–³]. Unfortunately, this important issue has been given less attention in Pakistan. Slope failures have caused damage to many infrastructures including the heritage sites and monuments which are the most valuable imprints of cultural heritage and an important means of tracing back the roots of a civilization [⁴]. Qila Bala Hisar is situated in Peshawar, the provincial capital of Khyber Pakhtunkhwa. The fort is located in the north-western corner of Peshawar City. A total area of 10 acres (40000 m²) in the fort is covered by the inner wall and the outer wall houses a total area of 15 acres (61000 m²). The fort is 90 feet high with respect to the ground level. The outer walls are in two terraces each approximately 40 feet high. The fort was constructed on a filled ground and the soil mass was stabilized through retaining walls. Therefore, this slope is of prime importance and evaluation of slope stability is vital for structural integrity. The laboratory results show that the soil up to 35 ft is filling of medium brown, medium stiff, silty clay and below 35 ft is light brown, hard silty clay and is found up to maximum explored depth.

On October 27th, 2015, a major earthquake of magnitude 7.3 struck northern Pakistan. Due to this earthquake a part of upper wall of the fort on south-western side collapsed. The collapse caused the retained soil mass to move down by 25 feet. This, not only, affected the structural integrity of building inside the Fort but also damaged the aesthetic look of fort. The picture of the collapsed wall after the earthquake is shown in Fig. 1.

Geo-forensic investigation or forensic geotechnical investigations involves the application of earth sciences to receive reality [⁵] and integrating technology, creativity, and experience to evaluate the problems related to geotechnics. Forensic Geotechnical investigation deals with the analysis of distress and failures of geotechnical origin at micro and macro levels, inculcating the aspects of design, construction and maintenance. This characterizes forensic geotechnical investigation from the commonly adopted procedures of tests, analysis and design [⁶]. Experimental investigations at Nano level is difficult, expensive and unfeasible. Therefore, various numerical methods can be utilized to simulate and analyze their behavior [⁷]. The literatures presented by Refs. [⁸,⁹] on the numerical methods are the practical manifestation of application of these methods for similar problems.

The geotechnical analysis involves soil characterization, water levels, optimum depths of foundation, and soil bearing capacity to draw conclusions and present the solution for the failure in the structure [¹⁰,¹¹]. Forensic geotechnical investigations have grown popular in the recent years and literature on the topic has increased by manifolds [⁵]. The procedure has been widely adopted to bring improvements in the standards of geotechnical engineering.

This research was carried out in the backdrop of recent slope failure at Qila Bala Hisar which raised concerns on the possible loss to the public life and property due to increased occupancy level. Research has established that the soil continuously moves downward on slopes due to gravity. The factors that influence this movement are slope angle, water, atmosphere, and slope material [¹²]. To evaluate these factors, this paper presents the forensic geotechnical investigation and the geophysical investigation using Electrical Resistivity Technique (ERT) to characterize the nature of stratum and explore the surface variation in both horizontal and vertical extents. It presents the use of X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) techniques to characterize the mineralogy and shape of the soil particles and numerical modeling to analyze the slope failure. The tests were performed using ASTM standards, and the results are presented and interpreted in order to highlight the significance of the results, reach the dependable conclusions and analyze the problem presented in the study. Remedial measures are also presented with their detailed analysis.

Geology of the region and soil strata

Geology of the region

The Peshawar valley is secured with unconsolidated stores of sand residue and rock. The geology of the region is fundamentally that of steep low-beaten mountains going from 1000 to 4000 feet in tallness. The mountains increment in height and roughness toward the west and particularly north-west, while toward the east they drop to low slopes. At some places, the low, level slopes and valleys are secured with adjusted rock and clayey matter which may have been deposited in lake beds, or all the more presumably, by waterway or surges.

Soil strata

Standard penetration tests (SPT) using a split spoon sampler was done in the cohesive and granular deposits using ASTM D1586 standards [¹³]. The results are tabulated on borehole logs as N-values at the respective depths. From the sample recovered from the boring of the two boreholes the bore log was generated, which showed the type of soil strata encountered during the boring the boring depth was 40 feet and the bore log are shown in Fig. 2.

Soil profile

The soil profile of the site was prepared using the data collected from the two boreholes and then using the SPT correlation graphs [¹⁴]. The data was collected at 5 feet intervals and the SPT value used were the average SPT blow count value at the specified heights. The soil strength parameters at the site at different depths below the ground are given in Table 1.

Forensic geotechnical investigation technique

According to Ref. [¹⁵], the tests necessary for a Forensic geotechnical investigation are XRD, Scanning Electron Microscope, Transmission Electron Microscope, Chemical Tests (sulfate test, chloride test) and Electrical Resistivity Tomography. The Forensic geotechnical investigation of Qila Bala Hisar was carried out using conventional and unconventional testing techniques. Tests performed included SPT, Atterberg Limit, ERT and Activity soil. The results are presented and interpreted in order to highlight the significance of the results, reach the dependable conclusions and to analyze the problem presented in the study.

SPT boreholes

The percussion drilling was done in the two boreholes as specified in the Fig. 3. The split spoon sampler was used, and the depth of the boreholes was kept at 40 feet for each.

From the sample collected from these two boreholes the soil profile of the site was made which showed that the soil found is silty clay in the extends of boring. Moreover, through it was found that there was no water table in the boreholes. It was observed that with increase in the depth of bore hole, the SPT blow counts increased considerably as shown in Fig. 4. This could possibly happen because of the seepage of water, which had reached only to the depth of 30 feet. However, if care is not taken in this regard then it could increase to further depths.

In future the surface drainage must be improved, moreover, the leakage from the water supply pipes and sewerage supply pipes must be controlled in order to prevent water from seeping into the site.

Atterberg limits

The Atterberg limits of the samples collected from the two boreholes is shown in Table 2. The tests were performed according to ASTM D 4318 standards [¹⁶]. The results have shown that the plasticity of soil decreases with depth which means the moisture content in the depth is less. High value of plasticity can induce a failure even on almost flat hill slopes.

From Table 2 it can be interpreted that there is decrease in the Atterberg limits after 30 feet depth and above this depth the chances of failure of slope are more pronounce. The effect of moisture content which in turns show the effective depth to which the water has seeped is shown in Fig. 5 below.

From Fig. 5 it can be inferred that the effective depth to which water has seeped is 30 feet and future care has to be taken to avoid the further exaggeration of the situation.

ERT

ERT is an advanced geophysical study technique to measure the sub surface’s resistivity circulation by making estimations on the surface of ground [¹⁷]. ERT was performed at the two specified location on the site in order to get picture of the sub-surface soil and to see any saturated and lose pockets. For ERT diploe-diploe electrode arrangement was used in order to get more vertical coverage of the soil in revealing the saturated and lose pockets. The two line on which ERT was performed are shown in the Fig. 6. The two lines were selected after the carefully observing the site so that the maximum sub-surface information can be obtained.

The result of the line-1 and line-2 are shown in the Figs. 7 and 8, respectively. The result has highlighted that there is presence of saturated clay pockets.

The blue packets in the figures above shows the possible saturated pockets of soil.

Activity clay soil

Clayey soils may not contain 100% clay in them. In a fine soil there is always a presence of small quantities of mineral flakes (<2 mm size) which has some effect on the characteristics of the soil, particularly the swelling and shrinkage with the change in the moisture content [¹⁸]. Activity of the soil is the degree of plasticity of soil with respect to the clay content in the soil [¹⁹]. From the activity of the test sample of soil the swelling potential of clay can be found. The Table 3 given below the shows the classification based on the activity of soil.

The activity soil can also be used to find the type of mineral the clay contains. The Table 4 shows the general range of activity for different type of clay minerals.

The Activity of different soil samples collected from the site is given below in the Table 5.

The results have shown that the average value of activity at site is 0.34 and clay contains kaolinite mineral which is stable and does not have swelling potential.

Mineralogy

The mineralogical classification was carried out to identify the type of the clay and its structure and to see if it contains any traces of bentonite because bentonite or the Montmorillonite have the swelling potential. Also, the properties of clay are affected by its mineralogical structure.

For this purpose, the XRD and SEM test were carried out to check the micro structure of the clay. The results of XRD and SEM are shown in Figs. 9 and 10, respectively.

The SEM and XRD results have shown that the clay does not contain the traces of bentonite and its structure shows that its particles are flat and elongated in shape and it also contains some other minerals like Quartz, Dolomite, Calcite, etc. The result further shows that the contained clay belongs to the Kaolinite, which is a stable clay mineral with low activity. Soils containing Montmorillonite generally execute large volume changes depending on available water.

Analysis of results

Causal and triggering factors

The results of the testing have been used to establish some causal factors which have led to the failure of the slope. These factors are discussed below.

Surcharging on the site

With the passage of time the surcharge load has increased with the addition of the new buildings at the site. The number of occupants has also increased and the fort which was once designed for few numbers of people is now being utilized by huge number of occupants. The rate of surcharging can be visually seen in the Fig. 11 below, which clearly depicts the increase in the number of buildings in year 2016 compared to year 2000.

Earthquake

The wall failed on the October 27th, 2015, at the time of the earthquake and this can be the one of the factors which may have triggered the failure of the wall along with the soil mass. The failed soil mass can be seen in the Fig. 12.

Seepage of water

The ERT of the site have shown the presence of the saturated pockets under the ground. These possible saturated pockets show the presence of water which has seeped into the ground the possible reason for the seepage of water can be

1) seepage of water from nearby lawn;

2) broken pipes of water supply and sewerage system in the near vicinity;

3) poor drainage, blockage of drain or poor gradient of ground surface;

4) seepage of rainwater.

Figures 13–15 show the possible seepage causes at the site.

These factors are modeled in the SLIDE 2018 for analysis and to check their contribution toward failure. The necessary data and parameters to model the slope are derived from the test results.

Numerical analysis

Slope failures and fracture induced failures in rocks are phenomenon of major concern for researchers [²¹]. Numerical methods to solve these types of a problem are presented by [²²–²⁴]. The aim of this analysis is to provide a quantitative measure of the stability of the slope which is normally expressed as a factor of safety against failure of that slope. The factor of safety is defined as the ratio of the restoring force to the disturbing force. If the factor of safety is greater than one, it shows the slope is stable and factor of safety less than one is considered unstable. The slope stability analysis was carried out on SLIDE 2018 and the factor of safety was calculated after the model was subjected to the surcharge, earthquake and seepage conditions in order to find the real factors contributing toward the slope failure. The geometry of the slope shown in Fig. 16 was measured at the site and modeled accordingly in the software. The soil parameters were taken from the laboratory and field tests results discussed in Table 1.

Surcharge modeling

The buildings shown in Fig. 17 were modeled to consider the effect of the surcharge because these buildings are close to the site and can have possible effect on the slope. There average distance from the crest of the slope is 70 feet.

The building load that was used for analysis was 300 psf (average). The result of the slope analysis before and after applying surcharge load are shown in Figs. 18 and 19 below.

The analysis shows that the slope is out of the influence are of building loads so, the surcharge factor can be neglected as the cause of slope failure. However, it can be established from the results that if the buildings are constructed within 40 feet of the crest then it can influence the stability of the slope.

Earthquake

The slope is analyzed in the software for earthquake loading to see its effect on the FOS of slope. The earthquake can have major effect on the FOS of slope and can trigger the failure. As Peshawar is in the Zone 2B according to the Building code of Pakistan so, the value of horizontal acceleration is taken as 0.24 for the analysis purpose. Figure 20 shows the analysis result for the earthquake loading.

The analysis have shown that the FOS has reduced to 0.717 and confirms that earthquake is one of the factors which has contributed toward the failure of the slope.

Seepage

The seepage can be a contributing factor toward the failure of the slope. The seepage normally decreases the strength parameter of the soil that is cohesion and friction angle. The seepage in the slope is modeled in the software with the help of ERT results.

The ERT result is used to get the depth till water is seeped and properties till that depth are lowered to simulate seepage conditions. Figure 21 shows the result for the seepage analysis.

The analysis shows that the FOS reduced to 0.873, which means that seepage of water does affect the FOS of the slope. The results show that saturated clay is below 8 feet of ground which extends nearly 25 feet more toward the toe of the slope. The causative factors were established through the numerical analysis which actually contributed toward the failure of the slope are earthquake and seepage.

Summary and discussions

A detailed site investigation and survey was carried out for site characterization and to assess the reasons for the damage. The valley of Peshawar is filled with unconsolidated pockets of sand and residue that is more or less rock type. The geotechnical investigation enabled us to draw required parameters through various laboratory and field tests. Two boreholes were bored to a depth of 40 feet from the present ground surface to execute SPT and the collection of Shelby tube samples of undisturbed soil. The laboratory test included size distribution of grains, liquid and plastic limit and other index properties. Also, ERT tests were performed as ERT information is quickly converged with an apparatus called terra meter. This test was conducted using dipole-dipole electrode arrangement. The determination of soil quality through ERT is fast economic ad efficient as compared to the in situ tests, therefore it is valuable in geotechnical investigation. The SPT and grain size results were analyzed in conjunction with the ERT results. Direct relationship has been exhibited between transverse resistance obtained from the ERT and N-values acquired from geotechnical tests at these locations.

The slope of the Qila Bala Hisar fort was analyzed using different field techniques and numerical analysis. The field investigation and laboratory results indicate three probable causes of the slope failure which are; surcharge load, earthquake and seepage. However, the numerical analysis shows that surcharge load has no effect on the FOS of the slope. Hence the established causal factors for the slope failure are earthquake and seepage.

Remediation measures

Various options for slope remediations are: soil nailing, grouting, micro piles, retaining wall, anchors, geo textile and gabions [²⁵]. Each method has certain advantages and disadvantages for its application, but all these methods aim at stabilizing the slope in order to avoid the failure and increase the FOS of slope. Keeping in view the condition and strata, the most suitable remedial measure for this problem is the provision of soil nailing. Soil nailing is a soil retention technique that uses grouted steel nails to hold the soil together and can provide temporary or permanent support.

Design specification

The soil nails for the failed slope of Qila Bala Hisar will be designed as a remedial measure for its stability. Soil nails are designed according to the requirement of the active earth pressure of the wall which is calculated using Rankin lateral earth pressure theory [²⁶]. The active earth pressure is calculated both manually and using software GEO 5. The value of the active earth pressure comes out to be 64350.72 lb/ft.

Grout specifications

The specification of the grout selected for soil nailing is shown in Table 6.

The Concrete selected should have the f_c′ as 4000 psi and rest of the specifications are listed above in table.

Soil nail spacing

The soil nails will have the same horizontal and vertical spacing. The S_H of soil nails is 5 feet and S_v of soil nail is also kept as 5 feet which more than the minimum spacing of 3.5 feet.

Soil nail inclination

The soil nails are normally inclined at angle of 10 to 20 degree from the horizontal and most commonly at angle of 15 degree. At these inclinations the grout can easily flow from the bottom to the head without leaving any air pockets. For our design the angle of inclination is kept as 10 degree from the horizontal.

The soil nail length

The length can be estimated to be about 0.7H, where H stands for height of wall. Nail lengths are not kept less than 0.6H and more than 1.2H. In our case the length of soil nails is kept as 0.73H which is 35 feet in length.

Soil nail pattern on face of wall

Soil Nails are installed on the excavated portion’s face in square staggered pattern. For our design the soil nail pattern on the wall is staggered.

Estimated bond strength of soil nails in cohesive soil

The estimated bond strength of soil nail in the cohesive clays is given below in Table 7.

The bond strength for our design is taken as 50 kPa from the above table and #7 solid threaded bar of grade 75 will be used for nail reinforcement and the welded wire mesh of grade 75 with 4″×4″ spacing will be used. In the analysis on the software the value bond strength will be input in the lb/ft instead of kPa.

Initial and final facing

The initial and final facing will be of 4 inch and 8-inch thickness respectively and will be reinforced with WWM of grade 75. Shotcrete used for the facing should have f_c′ as 4 ksi.

Bearing plate

The bearing plate used for soil nail should be 8″×8″ and 2″ thick and of grade 75.

Sketch of design

The sketch of the final design is shown below in the Figs. 22(a) and 22(b), respectively.

Numerical analysis

The causative factors that were established on the basis of analysis of the results are also considered in the design of soil nails. The factors which contributed toward failure are:

1) earthquake;

2) seepage of water.

After the analysis in the software it was found that these two factors led to slope failure. The soil nails were designed, and the model was analyzed in the software to find the global FOS of the slope after reinforcement and before that. The result of analysis is shown in the Figs. 23(a)–23(c), respectively.

The analysis shows that the FOS of slope increases after the provision of retaining wall and soil nails. The provision of masonry wall stabilized the slope considerably, however, to further stabilize it, soil nails were used. The length of the soil nails are 35 feet, which are placed an inclination of 10 degree from horizontal with a spacing of 5 feet. The FOS of the slope increases to 1.554 after provision of the soil nails, which is desirable.

Conclusions

In the light of all the issues discussed and the result of the tests shows that there were three possible saturated pockets and the reason for these saturated pockets were; seepage of water from nearby lawn, broken pipes of water supply and sewerage system in the near vicinity, poor drainage or blockage of drainage, rat hole leading to the affected area, vibrations from generating sets placed directly near the damaged portion and the surcharge loads. Out of the above factors the main causal factor was seepage of water and triggering factor was earthquake. Both these factors contributed toward the collapse of wall and failure of slope. Keeping in view the condition and strata remedial measure was suggested for the reconstruction of the destroyed wall and its adjacent failed slope. The most suitable remedial measure for this problem is the Provision of Soil Nailing with grouting. Soil nailing is a soil retention technique that uses grouted steel nails to hold the soil together and can provide temporary or permanent support. The authors believe that the research will enhance the understanding of the behavior of forts built on filled grounds and findings of the research would contribute toward the perseverance practices of heritage sites in Pakistan.

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