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The deformation pattern of subgrade soil below the geocell layer was observed by placing thin horizontal layers of white colored sand at 50-mm vertical interval.On completion of each test,the deformed shape of the colored lines was recorded through the transparent Perspex walls of the test tank.Using the observed rupture surface in the sand subgrade, delineated through the discontinuity in the white colored sand layers, the load dispersion angle is calculated.
Two different series of model tests were carried out by varying the relative density of soil from loose to dense (i.e.,ID=30, 40, 50, 60, and 70%).In the first series,tests were carried out on unreinforced soil beds.In the second series,tests were carried out on geocell-reinforced soil beds.In order to understand the influence of relative density of soil on the overall performance,in all the tests,in second series, the geometry of the geocell layer was kept constant as, height h/B=1.6, width b/B=8, and depth of placement u/B=0.1.The pocket size that is considered as the equivalent circular diameter of the geocell pocket opening [shown through the hatched area in Fig.1(b)] was kept equal to 1.2B in all the tests.This is the optimum geometry giving maximum performance improvement (Dash et al.2008).
Results and Discussion
Bearing pressure versus settlement responses for unreinforced soil are shown in Fig. 2 and those for geocell-reinforced soil are shown in Fig. 3. The pressure-settlement responses, depicted in Fig.2,show that for ID=30 and 40% the unreinforced foundation beds have undergone local shear failure while for higher density (i.e.,ID=60 and 70%) a general shear failure has taken place.It could be observed that,while in general,the responses for unreinforced soil have a clear break point with increase in slope indicating shear failure in soil;The geocell-reinforced soil beds without showing any such failure continue to sustain increased footing loading until a settlement as high as 50% of the footing width.
The subgrade modulus ,which represents the stiffness of the geocell-reinforced foundation bed,is obtained as the secant modulus of the pressure-settlement responses (i.e.,slope of the line joining the point on the curve at a given settlement to the Origin), depicted in Fig.3. The variation of the subgrade modulus of the geocell-reinforced foundation bed with relative density of soil,at different settlement levels of the footing ,is presented in Fig.4.It shows that the subgrade modulus of the geocell-reinforced foundation bed increases with increase in relative density of soil.The value of subgrade modulus at 3% settlement has increased from about 10 MN/m3 with ID =30% to about 40 MN/m3 with ID=70%,indicating that the stiffness of the geocell-reinforced foundation bed has increased by fourfold with increase in relative density of soil from 30 to 70%. It is of interest to note that, for relative density of soil greater than 50%, the kr versus ID plots show a relatively steeper response,indicating that the rate of increase of is higher for Dense soil.
Fig. 3. Bearing pressure versus footing settlement for geocellreinforced
foundation bed
Fig. 4. Subgrade modulus versus relative density of soil for geocellreinforced foundation bed
Fig. 5. Bearing capacity improvement factor versus relative density
of soil
The improvement in bearing capacity due to the provision of geocell reinforcement is represented using a nondimensional improvement factor which is defined as the ratio of footing pressure with geocell at a given settlement to the pressure on unreinforced soil at the same settlement.If the footing has reached its ultimate capacity at a certain settlement,the bearing pressure qo is assumed to remain constant at its ultimate value for higher settlements.Variation of the bearing capacity improvement factor with relative density of soil ,at different settlement levels of the footing ,is presented in Fig.5. It could be observed that the value of the improvement factor increases with increase in relative density.The loose soil contracts under deformation,therefore,more strain is required before stress transfer to the geocell occurs,whereas soil with higher relative density, being a compact structure tends to expand (i.e.,dilation),under footing penetration thereby mobilizes higher strength of the geocell reinforcement leading to enhanced performance improvement.Besides,the dense soil when dilates induces higher frictional resistance at the geocell soil interface thereby increasing the resistance to downward penetration of sand and hence a higher improvement in load carrying capacity.