IGC. 50 th INDIAN GEOTECHNICAL CONFERENCE PSEUDOSTATIC SEISMIC ASSESMENT OF SLOPES AND ITS REMEDIATION

Size: px

Start display at page:

Download "IGC. 50 th INDIAN GEOTECHNICAL CONFERENCE PSEUDOSTATIC SEISMIC ASSESMENT OF SLOPES AND ITS REMEDIATION"

Valentine Shields
5 years ago
Views:

1 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India PSEUDOSTATIC SEISMIC ASSESMENT OF SLOPES AND ITS REMEDIATION Rupam Saikia, Post-Graduate Student, NIT Silchar, Priyanka Deka, Post-Graduate Student, Assam Engineering College, Sandhya Rani Kalita, Graduate Student, Assam Engineering College, ABSTRACT: Stability analysis of slopes mainly comprise of natural or anthropogenic slopes. Alongwith the increase in settlement in the hilly regions due to rapid urbanization, it has become necessary to access the remote parts in the hilly areas giving birth to the construction of new railways and road networks but the danger from landslide activities during earthquake and rainy condition has always prove to be a hindrance to this. The stability analyses of virgin slopes under seismic condition are of utmost importance in order to reduce hazard from landslides during earthquakes which is generally neglected in most cases in the North-Eastern region. This paper reports the slope stability analyses of several slopes having varying angle of slope and other parameters under seismic condition, considering the importance of such analyses in the highly active seismic zones such as North- Eastern part of India falling in Zone V as per Indian Standard Code of practice. The stability of the slopes is first analyzed under seismic loading by applying horizontal and vertical pseudostatic co-efficient considering Zone V using Slope/W module of Geo-Studio 2007 and the Factor of Safety (FOS) is computed for the respective slopes. The extent of vulnerability of such slopes to earthquake is thus shown under the pseudo-static condition with rise in angle of slope under varying c-φ parameter under both dry and submerged condition. Further the failed slopes are stabilized under this condition by using only soil nail, only retaining wall or a combination of both. Critical slip surface behavior was thus studied and it has been found that a combination of soil nails and retaining wall was found to be most effective. INTRODUCTION Slope stability analysis is an essential part of geotechnical investigation which is to be carried out primarily before construction in hilly regions, construction of embankments, excavated deep pits, etc. There are mainly two different approaches for slope stability analysis one is the total stress approach which mainly corresponds to clayey slopes or slopes with saturated sandy soils under short term loading where the pore pressure is not dissipated. The second approach corresponds to effective stress approach which is applicable for long term stability analysis which also considers drainage. Generally, for the analysis of natural slopes effective stress approach should be considered. Slope stability analysis can be considered for a general static analysis or for a pseudo-static analysis or can be analyzed for a complete dynamic analysis. Besides, this several remediation measures can be effectively applied to the slopes wherever it s necessary such as in-situ slope stabilization technique [1] [2], retaining wall [3], anchors [4] and geo-fabrics [5]. In this paper initially the stability of slope under dry static condition for varying strength parameters such as cohesion (c) and angle of internal friction (φ) is studied for three different heights of slopes of 5 m, 10 m and 15 m respectively. Then pseudostatic analysis was carried out for four selected slopes having angle of slopes (β) 30, 50, 60 and 80 respectively such that both mild to extreme slopes were studied, further they had same strength parameters for the three different height of slopes in each of the above four cases of slope angles. The Pseudo-static analysis for the above cases was further carried out under the presence of water table. Lastly, retaining wall, soil nails and if

2 Rupam Saikia, Priyanka Deka & Sandhya Rani Kalita necessary a combination of both retaining wall and soil nails was used as a remediation measure for the stabilization of the slopes. PSEUDOSTATIC ANALYSIS: GEOSTUDIO Pseudostatic analysis represents the effects of earthquake shaking by accelerations that create inertia forces, which act both in the horizontal and vertical directions at the centroid of each slice. The forces are defined as- F h = a h g F v = a v g W = k h W (1) W = k v W (2) Where, a h and a v = horizontal and vertical pseudostatic accelerations g = gravitational acceleration constant W = the slice weight In SLOPE/W, the inertia effect is specified as k h and k v coefficients. In SLOPE/W the horizontal inertia forces are applied as a horizontal inertia force on each slide. While the vertical inertia forces add up to the self weight of the body. Vertical coefficients can either be positive or negative. A positive coefficient means downward in direction of gravity while negative coefficient signifies upwards in direction i.e. opposite to the force of gravity. The application of vertical seismic coefficients often has little impact on the safety factor. This is because if the inertial force has the effect of increasing the slice weight, the base normal increases and then the base shear resistance increase. The added mobilized shear arising from the added weight tends to be offset by the increase in shear strength. Thus, it is mainly the horizontal seismic coefficient on which the stability of the slope is dependent. For this particular study, the pseudostatic co-efficient i.e. horizontal and vertical seismic co-efficient are selected as per Hynes- Griffin and Franklin [6]. As per their theory horizontal seismic co-efficient (k h ) was taken as 0.18 i.e PGA/g and vertical seismic coefficient (K v ) was taken as 0.09 i.e PGA/g. Here, Peak Ground Acceleration (PGA) was considered as 0.36g considering Zone V as per IS: [7]. The difficulty with the pseudostatic approach is that the seismic acceleration only acts for a very short moment in time during the earthquake shaking. The factor of safety may even momentarily fall below 1.0, but this does not mean the slope will necessarily totally collapse. NUMERICAL MODELLING For the purpose of the present study several numerical models was developed using SLOPE/W tool for the stability analysis with different parameters. SLOPE/W provides us the opportunity to model a slope with the various parameters and analyze the slope stability of earthen structures under static and pseudostatic condition. In this study, the Geo-Slope models were basically analyzed for three different heights (5 m, 10 m and 15 m). A plane strain 2-D model with uniform cross-section in the longitudinal direction is considered. The angle of inclinations of the slope (β) for the models was varied between 5 and 90. The type of analysis considered was Morgenstern- Price Method [8] under the presence of water table i.e. considering pore-water pressure; while for the rest of the analysis such as for dry static and dry pseudostatic Ordinary method by Fellenius [9] was considered. Out of all the models analyzed, four models were chosen which had sufficiently low FOS for the three different heights of slopes and were considered for the representative results in this report. The details of the chosen models are presented in Table 1. The foundation in each case was considered to be impenetrable forming a rigid boundary condition in the slope and foundation material interface. A typical model is shown in Figure 1. Table 1 Properties of the four selected models Models β( ) c(kpa) φ( )

3 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India Material Properties The materials used were namely back-fill soil, foundation, retaining wall and soil nails. The foundation material was considered to be bedrock (impenetrable) for all the models. Moreover, retaining wall material was considered to be the same wherever they were applied i.e. bedrock (impenetrable). While the back-fill material property was provided with different strength parameters (c and φ), though the unit weight in each case was considered to be 18 kn/m 3. The cohesion was varied between 0 (cohesionless soil), to 40 kpa. The angle of internal friction of soil was between 0 (cohesive) and 40, thus considering extremely loose to dense soil characteristics. Fig. 1 Geo- Slope model of slope height 5 m Retaining Wall In this study, the retaining wall was considered as a simple rectangular gravity retaining wall. The height of it was considered between 3 m to 5 m depending on the angle and height of slope as required for stabilization. Both the top width and base width of the retaining wall was considered as 0.5m. Soil Nail Soil nails were used in this study as a remedial measure in order to modify the critical slip surface and enhance the FOS against the slope stability failure. The soil nails used in this study had the following specifications as shown in Table 2. Table 2 Soil Nail Properties Property Bond Diameter Values 0.3 m Bond Safety Factor 1.5 Bond Skin Friction Nail Spacing Bar Capacity 100 kpa 0.5 to 1 m 300 kn Bar Safety Factor 1.5 Shear Capacity Shear Safety 100 kn 1 RESULTS AND DISCUSSIONS Under dry static condition for slope heights 5 m, 10 m and 15 m, the computed FOS decreased nonlinearly with increase in slope inclination for the same soil strength characteristics. A typical trend is revealed in Figure 2 for a slope height of 5 m having c as 10 kpa and φ as 5. The figure illustrates the increase in vulnerability of slopes having greater slope angles. It has been found that slopes of height 5m with all possible angles of inclination (flat slopes to vertical cuts) and having cohesion above 20 kpa were found to be stable. Thus, for such slopes, there are substantially less possibility of failure, and the requirement of reinforcement is very less for any intermittent destabilization incidents. It was further observed that for the slopes of height 10 m for angle of inclination up to 50 were found to be stable with cohesion 25 kpa or above and φ as 10 or above. While, for height 15 m cohesion required for stability was found to be more than that. Thus, for slopes with height above 10 m, substantially higher cohesion is required for stability and can be figure out from Table 3. These slopes were found to be more prone to failure and remediation measures for such slopes have to be implemented.

4 Rupam Saikia, Priyanka Deka & Sandhya Rani Kalita Table 3 Higher cohesion requirement with increased height of slope Models Height β( ) c(kpa) φ( ) of slope (m) FOS under static condition (Ordinary Method) After the static analysis under dry condition, the selected four representative models corresponding to the three different slope heights namely 5 m, 10 m and 15 m respectively were analyzed for pseudostatic condition. The vulnerability of the slopes to earthquake forces can be reflected from the computed FOS after the pseudostatic analysis. Table 3 shows the differences in vulnerability in terms of FOS computed for both static and pseudostatic cases under dry condition for the four selected models for height 5 m as an example. Thus, it will not be efficient to consider just the static condition for the stability analysis of slopes as it gives quite conservative results as compared to pseudostatic analysis which can be observed clearly from Table 4. Fig. 2 Decreasing trend in FOS with angle of slope for height 5 m, c 5 and φ 5 Table 4 Comparison of FOS for static and pseudostatic analysis for height 5 m Models β( ) c(kpa) φ( ) FOS under static condition (Ordinary Method) FOS under pseudostatic condition (Ordinary Method) Further the models were now analyzed under the presence of water table for pseudostatic condition. It was seen that the stability condition of the models further degraded which can be seen by further reduction in the FOS values. Considering, slope height of 5 m as an example having β as 80, c as 10 kpa and φ as 5 initially for dry static condition it has been found that the FOS of the virgin slope was found to be very low as which indicates the failure of the slope. Figure 3 shows the virgin slope. Further, under the presence of water table and on application of pseudostatic forces the FOS was seen to be further decreased to which is illustrated in Figure 4. This, particular slope was then stabilized by a combination of retaining wall and soil nails for the worst condition i.e. under the presence of water table and considering the pseudostatic condition and the FOS was increased well above. It can be illustrated from Figure 5. Further from the comparison of Figure 5 and Figure 6, it can be concluded that a slope reinforced for pseudostatic condition is ultimately safe for static condition as for the same detailing of reinforcement the FOS was seen to be increased from under pseudostatic condition inclusive of presence of water table to under dry static condition. So design of stabilization as per pseudo-static consideration should be implemented rather than simple static consideration.

50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India Fig.

5 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India Fig. 3 FOS of virgin slope of height 5 m (β=80, c=10, φ=5 ) Fig. 4 FOS of virgin slope of height 5 m under pseudostatic condition (β=80, c=10, φ=5 ) Fig. 6 FOS of reinforced slope of height 5 m under static condition (β=80, c=10, φ=5 ) Lastly, an overall comparison of the vulnerability of the slopes for the three respective heights chosen for this study i.e. 5 m, 10 m and 15 m under dry static condition and under pseudostatic condition inclusive of the presence of water table is shown in Figure 7. In that particular figure, H5D and H5P represented the dry static and pseudostatic condition with the water table presence for height 5 m respectively, while H10D and H10P represented the dry static and pseudostatic condition with the water table presence for height of slope 10 m respectively and similarly H15D and H15P represented the dry static and pseudostatic condition with the water table presence for height of slope 15 m respectively. Fig. 5 FOS of reinforced slope of height 5 m under pseudostatic condition (β=80, c=10, φ=5 ) Fig. 7 Overall comparison of FOS for the three slope heights for dry static and pseudostatic condition with the presence of water table

6 Rupam Saikia, Priyanka Deka & Sandhya Rani Kalita CONCLUSION Slope stability analysis is of utmost importance in geotechnical investigation and it has been found that static analysis gives a conservative result as compared to pseudostatic analysis. Also, combination of retaining wall and soil nails were found to be comparatively efficient in terms of stabilization of slopes as compared to use of sole soil nails or retaining walls, though in some cases only retaining wall was found to be enough to stabilize the slope. Besides, a slope reinforced for pseudostatic analysis was found to be extremely safe for static conditions. A proper dynamic analysis would remain as part of future study to be carried out. U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, Mississippi. 7. IS: , Indian Standard Criteria for Earthquake Resistant Design of Structures. 8. Morgenstern, N.R. and Price, V.E. (1965), The analysis of the stability of general slips surfaces, Geotechnique, 15(1), Fellenius, W. (1940), Earth static calculation with friction and cohesion and use of circular sliding surfaces, Wilhelm Ernst and Sohn, Berlin, 1927, II Ed. REFERENCES 1. Lazarte, C.A., Elias, V., Espinoza, R.D and Sabatini, P.J. (2003), Geotechnical Engineering Circular, No. 7, Soil Nail Walls, FHWA Technical Manual. 2. Babu, G.L.S. (2009), Case studies in soil nailing, In proceedings of Indian geotechnical Conference , Guntur, India. 3. Loupasakis, C., Spanou, N. and Rozos, D. (2010), Reinforced soil retaining walls restoration of an extended failure on a soft rock formation, Geologically Active, Williams et al. (Eds), , ISBN Yeh, H.n-S., Wang, C.-S., Wei, C.-Y., Lee, S.- M., Ho, T.-Y., Hsiao, C.-A., and Tsai, L.-S. (2013), Inspection and capacity assessment of anchored slopes, In proceedings of 18 th International Conference on Soil Mechanics and Geotechnical Engineering, Paris, Choudhury, P.K. and Sanyal, T. (2010), Embankment slope stabilization with jute geotextiles a case study in NH-2 Allahabad bypass, In proceedings of Indian Geotechnical Conference 2010, IGS Mumbai Chapter and IIT Bombay. 6. Hynes-Griffin ME, Franklin AG. (1984), Rationalizing the seismic coefficient method,

Seismic Slope Stability

ISSN (e): 2250 3005 Volume, 06 Issue, 04 April 2016 International Journal of Computational Engineering Research (IJCER) Seismic Slope Stability Mohammad Anis 1, S. M. Ali Jawaid 2 1 Civil Engineering,