Fig. 1: Typical Geometry of the Slope Model
CSIR-CBRI undertook a project to provide suitable design and strengthening measures for safety and stability of building foundations in hilly regions.
All 13 EWS (Economically Weaker Section) building plans developed at CSIR-Central Building Research Institute (CBRI), Roorkee, for mass housing clusters have been analysed in Staad pro for earthquake zones IV and V (in which most of the hilly areas of India fall) for horizontal ground and sloping topography (Fig. 1).
From these structural analyses, the maximum footing reactions have been computed for different load combinations as per the IS codes. These load values will be utilised to carry out foundation proximity interaction problems for hilly areas where clusters of houses come up. Further, the bearing capacity values based upon various RMR ranges were used for the computation of required footing sizes. The findings for EWS Plan 1 are shown in Table1:
Table 1: Bearing Capacity of Rock Mass Based upon Various RMR Values
|
RMR of Rock Mass |
Net Safe Bearing Capacity (kN/m2) as per Table 3 of IS 12070: 2010 |
Earthquake Zone IV |
Earthquake Zone V |
||
|
Max. Design Load on Footing (kN) |
Footing Size Required |
Max. Design Load on Footing (kN) |
Footing Size Required |
||
|
100 to 81 |
4412 |
2720 for G + 3 |
0.78 x 0.78 |
1925 for G + 2 |
0.66 x 0.66 |
|
80 to 61 |
2843 |
0.97 x 0.97 |
0.82 x 0.82 |
||
|
60 to 41 |
1569 |
1.31 x 1.31 |
1.10 x 1.10 |
||
|
40 to 21 |
588 |
2.15 x 2.15 |
1.80 x 1.80 |
||
|
20 to 0 |
392 |
2.60 x 2.60 |
2.21 x 2.21 |
||
Similar results for other EWS plans have been computed.
In continuation of the 2D finite element analysis of conventional strip footings on slopes, further numerical analysis of strip footing under pseudo-dynamic loading has been performed.
The safety factor results have been compiled and shown in Table 2:
Table 2: Safety Factor of Slopes for Various Footing Sizes & their Locations w.r.t. Slope Crest
|
Foundation Depth (Df), m |
Width of Footing (Bf), m |
Footing Edge Distance (Xf) |
Safety Factor of Slopes |
||
|
Dry Static |
Wet Static |
Dry Pseudo-Static |
|||
|
0.2 |
0.2 |
5B = 1.0 m |
1.00 |
1.00 |
1.00 |
|
10B = 2.0 m |
1.25 |
1.01 |
1.02 |
||
|
0.3 |
0.3 |
5B = 1.5 m |
2.00 |
1.32 |
1.26 |
|
10B = 3.0 m |
2.08 |
1.37 |
1.29 |
||
|
0.4 |
0.4 |
5B = 2.0 m |
1.30 |
1.14 |
1.13 |
|
10B = 4.0 m |
1.34 |
1.21 |
1.18 |
||
2D and 3D jointed discrete element modelling for rock mass has been initiated. For this purpose, the Universal Distinct Element Code (UDEC) and three-dimensional Distinct Element Code (3DEC) have been utilised. Once the expertise is developed, these codes will be used for carrying out the necessary parametric analysis for creating design charts and tabular values for foundation design on jointed rock mass.
Safety analysis of a slope with joints dipping at 145° (i.e., daylight out of the slope at 35°) and joint spacing of 20 cm was performed using discontinuum modelling in 3DEC. The failure mechanism that develops combines sliding along joints near the slope toe with tensile failure of the blocks near the top of the slope. The calculated factor of safety is 1.31 for this case.
Numerical analysis for determining the maximum load-bearing capacity has been done for footing size of 1m x 1m on a horizontal intact rocky ground. The safe load capacity was obtained was 138 kPa corresponding to a settlement of 11 mm. Analysis for a single set of joints with varied spacing values is under progress.
Contributed by Mr Koushik Pandit & Dr Shantanu Sarkar, CSIR-CBRI, Roorkee
