STONE COLUMN REINFORCED CLAY BED
4.3 STONE COLUMN REINFORCED CLAY BED
4.3.1 Effect of length of stone column
Fig 4.6 depicts the bearing pressure versus footing settlement responses for different lengths of stone column, expressed in non dimensional form with respect to its diameter (L/dsc). It could be observed that even with stone columns having length as small as its diameter (L/dsc = 1), the performance of the clay bed, both in terms of increase in bearing capacity and reduction in settlement, can be increased substantially.
The performance improvement continues to increase with increase in length of stone columns. It is of interest to note that with the stone column length varying from 3dsc to
5dsc there is substantial improvement in bearing capacity and reduction in settlement of the foundation bed, beyond which further improvement is marginal. The variation of bearing capacity improvement factor IFsc with footing settlement for different lengths of stone column is depicted in Fig. 4.7. It could be observed that with the provision of stone columns the bearing capacity can be increased by 3.5 times that of the clay bed alone (i.e., IFsc = 3.5).
For all cases, initially the value of IFsc is high but it reduces with increased footing settlement. However, for settlement (s/D) beyond 3% it continues to increase with increase in settlement. The initially high improvement in bearing capacity is due to stiffening effect of the stone columns. This effect reduces as the stone column deforms under footing penetration. However, after a threshold limit of deformation, the shear resistance of the soil starts getting mobilized substantially, leading to increased load carrying capacity.
Fig. 4.8 presents the stone column induced reduction in footing settlement (PRS)sc, at different levels of footing pressure, (qr)sc/(qu)ult. Where, (qr)sc is the footing pressure on stone column-clay bed at a given settlement and (qu)ult is the ultimate pressure of the unreinforced clay bed. With stone column the footing settlement can be reduced to less than 5% that of unreinforced soil (PRS > 95%). The reduction in footing settlement increases with increase in stone column length (L) till 5 times of its diameter (dsc), beyond which further reduction is practically negligible. This is because further resistance of the stone columns brought by increased length remains unmobilised due to excessive bulging at upper layers of the stone columns.
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0 4 8 12 16 20 24 28
Footing settlement, s/D(%)
0 20 40 60 80 100
Bearing pressure (kPa)
Clay
Clay+SC(L/dsc=1) Clay+SC(L/dsc=3) Clay+SC(L/dsc=5) Clay+SC(L/dsc=7)
Fig.4.6: Variation of bearing pressure with footing settlement for different length of stone columns (S/dsc = 2.5) - Test series 2
0 4 8 12 16 20 24 28
Footing settlement , s/D(%) 0
1 2 3 4 5 6
Improvement factor, IFsc
Clay+SC(L/dsc= 1) Clay+SC(L/dsc= 3) Clay+SC(L/dsc= 5) Clay+SC(L/dsc= 7)
Fig.4.7: Variation of improvement factor with footing settlement for different length of stone columns (S/dsc = 2.5) - Test series 2
0 1 2 3 4 5 6 7 8 Normalised length of stone column, L/dsc
20 40 60 80 100
Reduction in footing settlement, PRSsc(%)
(qr)sc/(qu)ult
10%
20%
40%
60%
80%
100%
Fig.4.8: Variation of settlement reduction factor with length of stone columns (S/dsc = 2.5) – Test series 2
The surface deformation profiles are presented in Figs. A1-A4 (Appendix I). The corresponding variations of surface deformation with footing settlement, at x = D, 2D and 3D are shown in Fig. 4.9, 4.10 and 4.11 respectively. From Fig. 4.9 it could be observed that the settlement on fill surface, at x = D, with stone column of L/dsc=1 is comparable to that of the unreinforced clay bed. Being small in length and thereby having less peripherial area, the mobilised skin friction around the stone column is small. Besides, due to weak clay at base and shallow depth of embedment, the mobilized bearing capacity is very small. Therefore, the stone column, without offering much of resistance, just punches down under the footing loading leading to marginal reduction in surface settlement. Indeed the post test exhumed picture of stone column shape shows no bulging (Fig.4.12, L/dsc=1). With increased length of stone
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column the surface settlement (at x = D) is found to reduce indicating that the increased skin friction (due to increased peripherial area) and end bearing capacity (due to increased overburden) provides higher resistance against footing penetration leading to reduced settlement. The bulging in the stone columns (L ≥ 3dsc, Figs.4.12, 4.13, 4.14) indicates that the stone column, instead of getting punched down, has stood sustaining the footing pressure and hence has undergone radial volume expansion.
Correspondingly, the surface deformation responses depicted in Figs. 4.10 and 4.11 show that the heaving is maximum for L/dsc = 1 and continues to reduce with increase in length of the stone columns.
0 4 8 12 16 20 24 28
Footing settlement, s/D(%) 0.0
0.5
1.0
Average surface deformation ,δ/D(%)
Clay
Clay+SC(L/dsc= 1) Clay+SC(L/dsc= 3) Clay+SC(L/dsc= 5) Clay+SC(L/dsc= 7)
Fig.4.9: Variation of surface deformation, at x = D, with footing settlement for different lengths of stone columns (S/dsc = 2.5) - Test series 2
.
0 4 8 12 16 20 24 28 Footing settlement, s/D(%)
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Average surface deformation ,δ/D(%)
Clay
Clay+SC(L/dsc=1) Clay+SC(L/dsc=3) Clay+SC(L/dsc=5) Clay+SC(L/dsc=7)
Fig.4.10: Variation of surface deformation, at x = 2D, with footing settlement for different lengths of stone columns (S/dsc = 2.5) - Test series 2
0 4 8 12 16 20 24 28
Footing settlement, s/D(%) -1.0
-0.8
-0.6
-0.4
-0.2
0.0
Average surface deformation ,δ/D(%)
Clay
Clay+SC(L/dsc= 1) Clay+SC(L/dsc= 3) Clay+SC(L/dsc= 5) Clay+SC(L/dsc= 7)
Fig.4.11: Variation of surface deformation, at x = 3D, with footing settlement for different lengths of stone columns (S/dsc = 2.5) - Test series 2
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The deformed shape of the stone columns depicted in Figs.4.13 and 4.14 indicate that bulging dies down to a practically negligible value for length of stone column greater than 4dsc. Similar observations have been reported by Rao et.al (1997) and Sharma et.
al. (2004).
With increased length of stone column, it mobilizes higher end bearing due to high overburden pressure and greater skin friction due to increased peripheral area, therefore punches less and hence bulges more. A bulged stone column mobilises increased magnitude of soil passive resistance leading to increased load carrying capacity. However, beyond certain length (i.e. L = 5dsc) though the resistance against punching continues to increase but the stone column reaches its maximum bulging capacity and hence further improvement in performance is marginal (Fig. 4.7). Hence, it can be concluded that the length of stone column (L) giving maximum performance improvement is about 5 times its diameter (dsc).
L/dsc = 1 L/dsc = 3 L/dsc = 5
Fig. 4.12: Photograph showing the post-test deformed shape of central stone column (S/dsc = 2.5) - Test series 2
-200 -150 -100 -50 0 50 100 150 200 Diameter of stone column after test (mm)
0 1 2 3 4 5 6 7 Length of stone column (L/dsc)
L/dsc= 1 L/dsc= 3 L/dsc= 5 L/dsc= 7 original deformed
Fig. 4.13: Post-test longitudinal section of the central stone columns (S/dsc = 2.5) - Test series 2
0 1 2 3 4 5 6 7 Length of stone column (L/dsc)
-50 -40 -30 -20 -10 0 10 20 30 40 50
Radial strain in stone column [(rd-ro)/ro] (%)
original deformed L/dsc= 1 L/dsc= 3 L/dsc= 5 L/dsc= 7
Fig. 4.14: Post-test radial strain in central stone columns (S/dsc = 2.5) - Test series 2