LIST OF PHOTOGRAPHS
GLOSSARY
NOTATION
INTRODUCTION INTRODUCTION
- BACKGROUND
- CONSTRUCTION OF GEOCELL FOUNDATION MATTRESS
- MECHANISM OF GEOCELL REINFORCEMENT
- ORGANISATION OF THE THESIS
When applying load, the foot causes pressure in each cell of the geocell. The influence of relative density and height of the sand layer is investigated and presented in this chapter.
REVIEW OF LITERATURE AND SCOPE OF THE PRESENT STUDY
INTRODUCTION
STUDIES WITH PLANAR REINFORCEMENT
- Sand bed reinforced with horizontal tensile reinforcement
- Sand bed with horizontal tensile reinforcement underlain by clay
- Clay bed with horizontal tensile reinforcement
The maximum improvement in load-bearing capacity was found to be 2.9 times the ultimate load-bearing capacity of the unreinforced sand. Increasing the length of reinforcement to more than seven times the width of the foundation resulted in only a marginal improvement in load-bearing capacity.
STUDIES WITH GEOCELL REINFORCEMENT
- Field Tests
- Laboratory Model Tests
- Triaxial Compression tests on Geocells
The optimum aspect ratio (ratio of height to diameter) of geocell pockets giving maximum improvement was found to be 1.67. An eightfold increase in subgrade modulus was observed by providing geocell reinforcement to the sand beds. The bearing pressure was found to increase with increasing height of the geocell.
The geocell-induced improvement in the bearing capacity of the weak soil was studied both experimentally and numerically. The dimensions of the geocell were varied and their influence on the overall performance was observed. The optimum width and height of the geocell mattress where maximum improvement was observed was found to be 4.9 and 1.8 times the foot diameter respectively.
Providing a geogrid layer below the geocell mat further enhances the performance of the foundation bed. They observed that the performance improvement increased as the width and height of the geocell mat increased. They observed that the peak axial stress of the structure decreased as the number of cells increased.
CONCLUDING REMARKS AND SCOPE OF THE PRESENT STUDY
INTRODUCTION
MATERIALS USED IN THE STUDY .1 Clay .1 Clay
- Sand
- Geocells
- Test Bed Preparation
The geometry of the reinforced foundation bed and the various parameters are illustrated in Fig. The influence of relative density and height of the sand layers was studied among these ranges. The height of the sand layer was kept equal to that of the geocell sand mattress, which was tested in the present study.
The pocket size that gives the maximum benefit and the critical depth of placement of the geocell mattress were obtained based on the test results. The parameters that are varied in these ranges are the relative density (ID) of infill sand, the pocket size (d) and the height (h) of the geocell mattress. The parameters investigated are the relative density of the backfill soil, pocket size and height of geocell reinforcement.
Variations in pocket size and relative density were investigated for different geocell mattress heights from Series I to Series K. A schematic diagram and a photograph of the test setup are shown in Fig. Vane shear tests were performed to determine the undrained shear strength of the test bed.
RESULTS AND DISCUSSIONS ON EXPERIMENTAL DATA BEHAVIOUR OF UNREINFORCED SAND LAYER
OVERLYING SOFT CLAY SUBGRADE
INTRODUCTION
SOFT CLAY SUBGRADE
Since the readings were recorded at incremental loads, these results cannot be presented at regular settlement intervals. At higher levels of subsidence (s/D = 14%), the base settled more on the right side than on the left side, indicating that it was undergoing rotation to the right. Corresponding variations of average (ie average of left and right side readings) strain on the fill surface ( ) at distance x = D, 2D and 3D from the center of the foundation; with foundation alignment are presented in fig.
The clay substrate being saturated and the tests being performed at a faster rate leads to an undrained condition. Therefore, there is practically no volume change leading to rise in the fill surface.
SAND LAYER OVERLYING SOFT CLAY SUBGRADE
- Influence of relative density (ID) of sand
- Influence of height (H) of sand
The changes of the average deformation of the surface ( ) in relation to the placement of the foundation at a distance (x) of D, 2D and 3D from the center of the foundation, are shown in Fig. At distance x = 2D and 3D the fill surface has only undergone uplift, which is the cumulative effect of the uplift of the soft clay substrate and the sand layer. The surface deformation graphs show greater elevation with ID = 80% indicating that the overall shearing tendency is more pronounced in this case while in the case with ID = 35% and 50%, the drilling mechanism is dominant.
In the case of sand layers of lower relative density and thickness, the pressure-settlement response shows a gradual increase in bearing pressure with settlement of the foundation, which is typical of shear breakthrough behavior.
GEOCELL REINFORCED SAND LAYER OVERLYING CLAY SUBGRADE
INTRODUCTION
The performance improvement due to geocell reinforcement is quantified using a non-dimensional improvement factor, IFgc, which is defined as the ratio of bearing pressure (qgc) on geocell-reinforced soil to bearing pressure on unreinforced soil (qs), both taken at equal settlement (s) of the pedestal (Fig. 5.1). The bearing pressure improvement due to the sand layer, IFs, has already been defined in the previous chapter. It could be observed that while for the clay subsoil, at a settlement of approximately 10% of the footing diameter, the bearing pressure is 18 kPa, for the clay subsoil covered by sand, it is approximately 60 kPa.
The bearing pressure increases to about 110 kPa with the inclusion of the geocell-sand mat over the clay substrate. Thus, due to the sand layer, a three-fold increase (IFs 3) in the bearing pressure can be achieved. Therefore, by providing the geocell-sand mat, the load-bearing capacity of the clay substrate can be increased up to six times (IFgc × IFs = 6).
However, with the provision of the geocell mattress, the geocell-soil structure behaves as a composite unit and deflects as a centrally loaded slab under the footing, thus transmitting the pressure over a wider area creating a much wider area of residence. The influences of individual parameters on the pressure settlement and surface deformation behavior of the foundation system are presented and discussed in the following sections. Based on the analysis of the test data, the critical values of the different geocells, the soil parameters that give maximum performance improvement, are derived.
DEPTH OF PLACEMENT
As the applied load increases, the sand layer above the geocell mattress begins to shear off and the resistance to downward penetration of the footing is reduced. Therefore, both heaving of the clay and bending of the geocell mattress are reduced, which leads to reduced heaving on the fill surface (Figs. 5.8, 5.9 and 5.10). However, for smaller pocket sizes (Figs. 5.17 and 5.18), a prominent elevation of the surface is observed when the geocell mattress is placed immediately under the bottom of the foot.
Typical variations of surface deformation with soil settlement for pocket geocell mattresses d = 1.6D and 0.4D, at a distance of x = D from the center of the foundation, are shown in Fig. The comparison of the improvement factor (IFgc) obtained with geocell mattresses of two different heights (i.e. h = 0.27D and 0.8D) is shown in Table 5.3. The consistency in the results obtained from tests performed at two different heights of the geocell mattress.
It is found that regardless of the pocket size of the geocells, the value of the critical placement depth (ucr) of the geocell mattress remains constant (i.e. ucr = 0.1D). Based on the above discussions, it can be said that the critical placement depth (ucr) of the geocell mattress, which provides maximum performance improvement, is approximately 0.1D from the base of the foundation. The formation pattern, pocket size and height of the geocell mattress have little influence on the critical installation depth of the geocell mattress.
PATTERN OF FORMATION
- Influence of pocket size of geocells
- Influence of relative density of infill sand
It could be seen that the chevron pattern generally gives a higher improvement in bearing capacity and this is independent of the pocket size of the geocells. It could also be observed from Table 5.4 that the difference in performance between the two patterns becomes more apparent with the decrease in the pocket size of the geocells. The pocket size (d) of the geocells and the relative density (ID) of the backfill sand were kept constant at 0.8D and 80%, respectively.
This is true regardless of the relative density of the backfill soil, pocket size, and geocell mattress height. 5.41, it can be observed that the performance improvement increases with decreasing geocell pocket size. Since the gain of reinforcement remains negligible, the improvement factor does not increase with increasing footing settlement.
On the contrary, geocells with smaller pocket size offer greater resistance to foundation punching by mobilizing greater reinforcement strength, leading to increased performance improvement. Another factor to consider is the relative size of the foundation to the size of the geocell pocket openings. Therefore, it can be concluded that, in general, the improvement of the bearing pressure capacity increases with the decrease of the pocket size (d) of the geocells.
HEIGHT OF GEOCELL MATTRESS
- Influence of relative density of infill sand
- Influence of pocket size of geocells
With increasing the height of the geocell mattress, the bending stiffness of the reinforced bed increases. In addition, the overall frictional resistance that limits the downward punching of the encapsulated sand is also increased due to the increase in the surface area of the geocell reinforcement. Therefore, only a marginal increase in performance improvement is achieved with further increase in the height of the geocell mattress.
As height increases, the geocell reinforcement shares a higher percentage of the foundation load, leading to greater performance improvement. The heaving on the surface is caused by the yielding of the geocell mattress under the load of the ground, as explained earlier. For the case of lower relative density (ID = 35%) of sand, it could be observed that, at a distance of x = 2D and x = 3D, the swell on the filling surface decreases with the increase in geocell reinforcement height. .
This is because at smaller heights of the geosel mattress, the sand inside the geosel bags overcomes the. For a higher relative density of sand (ID = 80%, Table 5.8), the critical height (hcr) is approximately 0.53D, regardless of the pocket size of the geocells. However, for a given relative density of infill sand, the critical height (hcr) is independent of the pocket size (d) of the geocells.
RELATIVE DENSITY OF INFILL SAND
