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CHAPTERS EFFECT OF COIR GEOTEXTILES ON BEARING CAPACITY 8.1 GENERAL One of the important functions of geotextiles is to increase thc bearing capacity of the soil. The reinforcement effect is achieved either by micro reinforcement or by macro reinforcement. In the former, the reinforcement of the soil is achieved by mixing it with randomly oriented fibres. This aspect has been discussed in detail in Chapter 12. Macro reinforcement on the other hand, co
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  CH PTERS EFFECT OF COIR GEOTEXTILES ON BE RING C P CITY 8 1 GENER L One of the important functions of geotextiles is to increase thc bearing capacity of the soil The reinforcement effect is achieved either by micro reinforcement or by macro reinforcement. In the former, the reinforcement of the soil is achieved by mixing it with randomly oriented fibres. This aspect has been discussed in detail in Chapter 12 Macro reinforcement on the other hand, consists of placing reinforcing elements such as strips, bars, sheets, grids, cells, etc. in the soil. This can be placed in single layer or in multiple layers. In many cases roads and embankments are subjected to submergence and the soil will be in saturated conditions. To know the effectiveness of coir geotextiles in such situations it is necessary to carry out experimental studies in saturated conditions also. In the present investigation the possibility of utilizing coir geotextiles as reinforcement is explored by conducting studies on square footings resting on coir geotextile reinforced sand beds, both in dry and saturated conditions. 8 2 B CKGROUND Consider a square footing of size B x B resting on coir geotexrile reinforced sand bed, which is subjected to an intensity of loading q as shown in Fig. X. . The depth of the sand bed is Z and the coif geotextile is placed at a depth z below the footing. The ultimate bearing capacity of foundation can be givcn by the following equation Vesic, 1973)  ----------------------(8.1) where y is the unit weight of soil, is the bearing capacity factor and Si is the shape factor which may be expressed as ---------------------(8.2) Typical nature of settlement curves for unreinforced and reinforced cases is shown in Fig. 8.2 The improvement in ultimate bearing capacity of a foundation due to soil reinforcement is generally expressed in a non-dimensional tonn called Bearing Capacity Ratio, BCR u (Binquet and Lee, 1975) defined as ---------------------(8.3) where qu R is the ultimate bearing capacity with soil reinforcement and qu is the ultimate bearing capacity without reinforcement. In practice, most of the shallow foundations are designed for limited settlement. Hence it is essential to determine the BeR at various levels of settlement. Referring to Fig.8.2, the CR at a settlement level S S; Su can be defined as ------------------- 8.4) where q R) is the load per unit area of foundation for reinforced case and q is the load per unit area for unreinforced soil at a settlement level S. Eight series of experiments comprising of two plate load tests on unreinforced soil and 6 plate load tests on coir reinforced soils were carried out. From the observed 103  data applied pressure versus settlement curves were plotted and presented for each test Variations of eR for different coir geotextiles placed at different levels under dry and saturated conditions were studied. q o in~ r 1 Fig. 8.1 Square footing supported by coir geotextile reinforced saod bed s Su Rl Load Intensity q ) q u q R ã ã \ \ Vnreinforced / soil q u\R , - Coir Geotextile Reinforced so il Fig. 8.2 Typical pressure settlement curves or unrein orccd and coir geotextiJe reinforced sand supporting a square rooting 104  83 EXPERIMENT L SET -UP A series of model footing tests were conducted in a cubical steel tank measuring Im x Im x Im made up of mild steel sheets and angles one side of which was constructed with perspex sheet. The metallic sidewalls were braced with stiffeners to a vo id any possible lateral yielding during the placement of soil and loading of the model footing. Inside of the tank was painted and graduated. Control v al ves were provided at the bottom of the tank to facilitate saturation of the so il sample. A standpipe was fixed o ut s id e the tank wall to observe the water lev el in the soil Fig. R.3 shows the schematic diagram for the test set -up and th e sequence of the test procedure is show n in Fig. 8.4. ~~~~ j r t 7:Z=:~ ==== Hydraulic J ack Loadille Fmmc >.. ; ;; . LVDT ___ Plate 20cm X 20cm Sand ___ Co ir Geotextile ~ Sand Tank ImX ImX Irn  Fig. 8.3 Sc h ematic test set - up Model footing used was 25 .4 mm thick mild steel plate measuring 200mm x 200mm so that there is minimum dimensional effect. Static loads were applied using 200 kN capacity hydraulic jack jacked against a reaction frame fabricated using mild steel 1 sections. Settlements were measured at four corners of the footing using LVDTs. 10 5
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