IMPROVEMENT OF SOIL USING GEOGRIDS TO RESIST ECCENTRIC LOADS.

This paper presents the results of experimental investigations to predict the bearing capacity of square footing on geogrid-reinforced loose sand by performing model tests. The effects of several parameters were studied in order to study the general behavior of improving the soil by using the geogrid. These parameters include the eccentricity value, depth of first layer of reinforcement


INTRODUCTION
The reinforced earth has been most widely applied with something in excess of one million square meters of wall facing benign erected to the end of end of 1978, Mickittrick andDarbin (1979).The soil particles in direct contact with the reinforcements tend to slide over them under the effect of the load.The sliding is reduced by the frictional resistance between the particles and the surface of the reinforcement.Consequently this resistance will produce a tensile force along the reinforcing element which will acts as a tie between the particles surrounding it.The soil particles which are in direct contact with the reinforcement are bounded to other particles by the interlocking action.The frictional resistance will be transferred through the reinforced mass (Vidal, 1969).
The present study was undertaken to investigate the bearing capacity of square footings on geogrid-reinforced sand.The parameters investigated include

EXPERMENTAL TESTS AND TESTS PROCEDURE
A series of model loading tests were conducted inside steel box of 600 X 600mm in plane and 700mm in depth.The box was made of steel plate of 3mm thickness, stiffened with angle sections, as shown in Plate (1) .The internal faces of the box were covered with polyethylene sheets in order to reduce the slight friction which might be developed between the box surface and soil.Static vertical loads were applied using electrical hydraulic pump.Loads transferred from the pump to a hydraulic jack were carefully recorded by proving ring installed between the jack and the tested footing.
The footing was loaded at a constant loading rate to failure.The ultimate bearing capacity state was defined as the state at which either the load reached a maximum value where settlement continued without further increase in load or where there was an abrupt change in the load -settlement relationship.Settlement of the footing was measured using two dial gauges fixed in the middle and edge of footing.
The test footing was a square steel plate 60mm in plane and 5mm thick.The value of (Ø) was obtained from the result of triaxial test (UU.test) in accordance with ASTM(D2850-95).

REINFORCEMENT PROPERTIES.
The reinforcement used is polymer geomesh the general view for three types used in tests described, Plate (2) .The dimensions of the geogrid samples used in this study are listed in

Effect of Depth Ratio
The relative improvement for soil versus depth ratio for each value of eccentricity is shown in Fig ( 2).The optimum depth ratio (u/B= 0.75, 0.5, 0.25) show the maximum rate of strength improvement is define [{(Pr/P) -1}*100] where Pr and P is the max load for reinforced and un reinforced sand for eccentricity values (e= 0.05B, 0.135B, 0.22B) respectively.It is noted that for depth ratios (u/B=1.0),improvement values decreased and approach a constant for eccentricity values (e= 0.05B, 0.135B and 0.22B).The relative improvement increases with decreased the values of eccentricity.
It should be pointed out that there is no general consensus regarding the effect of depth ratio on the relative improvement of the soil .Singh (1988), based on the study of square footing on sands reinforced with mild steel grids (also called "welded mesh"), indicated that the effect of depth ratio on the bearing capacity was independent of the number of reinforcement layers and the optimum depth ratio was about 0.25.Selvadurai and Gnanendran, (1989) to improve the bearing capacity of footing located on slope fill using geogrids, showed that when (u<B), the failure path is penetrated below the reinforcement while when (u>B) the failure occurs at the soil geogrid interface (i.e. the failure path is limited in narrow zone) and the deep location of the geogrid layer at (u>2B) does not lead to any improvement in either the carrying load or the stiffness of the footing

Effect of Vertical Spacing of Reinforcement Layers :
Figure ( 4) shows the relative improvement (%) versus vertical spacing ratio(z/B=0.5,0.75, 1.0 and 1.5) for different eccentricity values (e= 0.05B, 0.135B and 0.22B).This figure illustrated the maximum improvement for eccentricity values (e= 0.05B, 0.135B and 0.22B) at z/b=0.5.It can be seen that the rate of strength improvement equals (0) for vertical spacing ratio (z/B= 1.5) for different eccentricity values thus the increase of (z/B) above 1.5 has no effect on the relative improvement for the soil.Similar to these findings were found by Fukuda et. al., (1987) who tested concentric load applied on footing with polymer grid reinforcement showed that the optimum vertical spacing between reinforcement is 2/3B.Guido et al (1987) indicated that the bearing capacity decreased with increasing vertical spacing ratio for Tensar SS1, SS and SS3 geogrid.
Figure (1) Geometric parameters of Reinforced Foundation.

Figure ( 4 )
Figure (4) The Relative Improvement Versus Vertical Spacing Ratio for (u/B=Optimum Value and Br/B=3)

Table ( 2
).The physical and chemical properties for sample used were listed in Table(3).The technical properties for sample used were listed in Table (4).