However, when encasing the full-length floating stone columns, the performance improvement decreases significantly. The combined application of basic geogrid and stone column casing is an additional advantage that further improves the load-bearing capacity of the composite foundation bed.
LIST OF TABLES
GLOSSARY
NOTATION
INTRODUCTION INTRODUCTION
- BACKGROUND
- GEOCELL REINFORCED SOIL
- STONE COLUMN REINFORCED SOIL
- OBJECTIVE OF THE PRESENT STUDY
- ORGANISATION OF THE THESIS
The purpose of this study is to develop an understanding of the behavior of geocell-sand mattress-stone column reinforced weak clay beds under monotonic circular loading. Summary of the reported research works and the scope of this study has been brought at the end of this chapter.
REVIEW OF LITERATURE AND SCOPE OF PRESENT STUDY
INTRODUCTION
STUDIES ON GEOCELL REINFORCED SOIL BEDS
- Sand beds
- Sand bed over weak clay bed
- Clay beds
In addition, it was found that geocell reinforcement significantly increased the ultimate bearing capacity of the system. A further improvement in performance is achieved by providing an additional planar geogrid layer at the bottom of the geocell mattress.
STONE COLUMN REINFORCED GROUND
The presence of stone columns greatly improved the bearing capacity of the soft clay bed. Load tests showed a clear improvement in the load bearing capacity of the stone column due to the wrapping.
GEOSYNTHETIC REINFORCED SOIL MATTRESS OVERLYING STONE COLUMN/ PILE REINFORCED CLAY STONE COLUMN/ PILE REINFORCED CLAY
The results of the loading tests indicated a clear improvement in the load-bearing capacity of the stone column as a result of the wrapping. The increase in axial load-bearing capacity strongly depends on the modulus of the casing and the diameter of the stone column. The effect of consolidation of the soft ground due to the inclusion of the stone columns was also included in the model.
The non-linearity in the behavior of the soft soil and the granular fill is reduced due to the use of geosynthetic reinforcement layer. The percentage of load carried by the granular pile increases with the increase in its stiffness and decreases with the increase in the relative size of the raft.
SCOPE OF PRESENT STUDY
It was observed that where a layer of geogrid was laid over the pile caps, its tension provided additional support and prevented lateral spreading of the embankment.
INTRODUCTION
The friction angle of the sand at different relative densities was determined using both the triaxial compression tests and direct shear tests. The preparation of the sand samples was done by a dry vibration method according to ASTM. Fig.3.10: Normal stress-shear stress response of sand in direct shear for different relative densities.
The load-strain behavior of the geogrid as obtained from the standard multi-rib tensile test (ASTM D 6637-01) is shown in figure. The load-strain behavior of the soft geomesh as determined using standard wide width tensile testing (ASTM D 6637). -01) is shown in Figure 3.27.
PLANNING OF EXPERIMENTS
The geometry of the composite foundation system and the parameters considered in the test program are illustrated in Fig. In these tests, the length of the stone column was varied for different heights of the geocell mattress. In test series 13-16, tests were performed on the composite foundation system by varying the stone column spacing for different heights of the geocell mat.
Under series 17-18, tests were conducted to study the influence of geocell pocket size on the overall performance of the composite foundation system (i.e. bed-stone column-geocell-mattress). The influence of a layer of geogrid at the base of the geocell mattress on the overall performance of a composite foundation system was studied in test series 21.
TEST DESCRIPTION .1 Test Set-up .1 Test Set-up
- Instrumentation of the Models
Therefore, the clear space between the base of the stone column and the bottom of the test tank is 345 mm [i.e. Mayerhof and Sastry (1978) noted that the failure zone beneath a pile extends to a depth of about 2 times the diameter of the pile. Foundation settlement and deformation on the fill surface were measured using a Linear Variable Differential Transducer (LVDT).
Foundation settlements were measured using two LVDTs, one at each end, diagonally opposite sides. The surface deformations were measured at locations 1D, 2D and 3D distance from the center of the foundation, on both sides of the foundation.
TEST BED PREPARATION .1 Preparation of the soft clay bed .1 Preparation of the soft clay bed
- Test procedure
The average density of the stone in the stone column was found to be 15.3 kN/m3. The stone pillar-reinforced clay bed thus produced was loaded with a seating pressure of 2.5 kN/m2 over the entire bed area for 4 hours to achieve uniformity in the test bed. By varying the height of free fall of dispersed sand particles, the placement density of the sand is varied.
Accuracy of sand placement and consistency of placement density were checked during rainfall by placing small aluminum cans of known volumes at various locations in the test tank. The dimensions and external shape of the Plaster of Paris column, which is the deformed shape of the stone column, was mapped.
STONE COLUMN REINFORCED CLAY BED
INTRODUCTION
UNREINFORCED CLAY BED
It is because of stress distribution from the footing that an additional mass of soil, other than that under the footing, settles with the footing. Since the soil is fully saturated and the rate of loading is faster, an undrained condition prevails, resulting in the uplift of clay surface outside the settlement zone (i.e. x > D).
STONE COLUMN REINFORCED CLAY BED
- Effect of length of stone column
- Effect of spacing of stone columns
Fig.4.6: Variation of bearing pressure with footing for different length of stone columns (S/dsc = 2.5) - Test series 2. Fig.4.7: Variation of improvement factor with footing for different lengths of stone columns (S/dsc) = 2.5) - Test series 2 Fig.4.16: Variation of improvement factor with footing for different spacing of stone columns (L/dsc = 5) - Test series 3.
Fig.4.18: Variation of surface deformation, at stone columns (L/dsc = 5) – Test series 3.
GEOCELL SAND MATTRESS REINFORCED CLAY BED
INTRODUCTION
Where su is settling of the unreinforced clay base and (sr)gc is settling of the geocell-reinforced clay base, both taken at the same bearing pressure (Fig. 5.2). The effect of various parameters of the geocell mattress on the overall response of the foundation system is presented and discussed in the following sections.
EFFECT OF HEIGHT OF GEOCELL MATTRESS
The geocell mattress by mobilizing anchoring through mobilizing interfacial friction and soil-passive resistance stands against the foot penetration. This is attributed to local yielding of the geocell walls underfoot, due to high contact pressure. This is because at higher bearing pressure the soil yields and the geocell reinforcement, which is relatively TH.
This is attributed to the increased stiffness of the geocell mattress as discussed earlier. The higher uplift at x = 2D and 3D, observed in the case of geocell mattress with relatively low height (h/D = 0.53), is attributed to the uplift of the geocell mattress due to its centrally loaded beam-like deflection.
EFFECT OF RELATIVE DENSITY OF INFILL SOIL
The reduction factor for settlement due to variation in the relative density of the soil, with a geocell mattress with a height of 0.53D, is shown in Figure 5.15. With greater backfill soil density, the geocell mattress behaves more coherently and deflects more like a centrally loaded beam going up. Moreover, with a higher density of the fill soil, the relatively stiff geocell mattress effectively limits the heaving of the underlying clay mass, leading to a greater settlement on the surface.
With geocell mattress of relatively higher height (h/D = 0.9) similar trends have also been observed with regard to the relative density of backfill soil (Fig. This is because with increased height, large anchoring resistance develops at both ends of the geocell mattress.
EFFECT OF POCKET SIZE OF GEOCELLS
This shape is caused by the anchorage at the ends, which tries to restrain the geocell mattress from downward deformations caused by subgrade settlement. With a very large geocell pocket size (ie dgc = 1.33D), the overall geocell reinforcement area is significantly reduced. This reduces the mobilized interfacial frictional resistance and thus the final anchorage to a limiting value where the geocell reinforcement is pulled down below the foundation penetration, reducing the efficiency improvement (Figure 5.25).
This is because with a smaller pocket size, as the geocell mattress settles over a larger area, it correspondingly displaces a larger volume of clay mass beneath it, which due to accumulation in the adjacent area has caused a higher swell. Therefore, it can be said that the overall behavior of the geocell mattress, with respect to the change in pocket size of the geocells, remains almost the same regardless of the height of the geocell mattress, i.e.
GEOCELL MATTRESS-STONE COLUMN REINFORCED CLAY BED
INTRODUCTION
Where, qu and (qr)gcsc are the bearing pressure of the clay bed reinforced with unreinforced mats and with reinforced geocell-columns, respectively, both taken at a given foundation settlement (Fig. 6.1). The influence of different parameters such as the length and spacing of the stone columns, the height and size of the geocell pocket, the density of the soil in the geocell; on the overall response of the composite foundation system are presented and discussed in the following sections.
EFFECT OF LENGTH OF STONE COLUMN
Similarly, the factor IFgcsc/IFgc (ie qgcsc/qgc) is the contribution of stone columns in the composite foundation system. The contribution from geocell reinforcement decreases with the increase in the length of stone columns and becomes almost constant for L/dsc ≥ 5. 6.21, it could be observed that when the contribution from stone columns (IFgcsc/IFgc) increases with the increase in the length of stone columns, corresponding the contribution from geocells (IFgcsc/IFsc) is reduced.
However, the contribution of stone columns to the performance improvement is relatively smaller compared to that of geocell mattresses with a height of 0.53D. It can be observed that the increase in load-bearing capacity due to the stone columns is relatively less prominent.
EFFECT OF SPACING OF STONE COLUMNS
This observation once again establishes that when the distance is reduced from 3.5 dsc to 2.5 dsc, there is a significant change in the behavior of the stone columns, that it changes from almost isolated response to an interacting response. Similarly, the geocell mattress carried the maximum load for large distances between stone columns (S = 3.5dsc, Fig. 6.51), while the contribution of geocell mattresses with reduced stone column spacing (S ≤ 2.5dsc) is much smaller. When the distance is large, the stone columns under and around the footing are subjected to relatively higher loads and are therefore prone to yielding.
By releasing the stone piers under and around the footing, the geocell mat is bridged leading to increased contribution to load sharing. In such a case, the geocell mattress behaves more like a load-transmitting element, while for larger stone column spans (S = 3.5dsc) it behaves like a load-bearing member, similar to a plate with central load resting on columns.
