The concept of mechanically stabilized earth has been widely used in various geotechnical applications such construction of embankments, pavements, bridge abutments, soft ground improvement and so on. Addition of reinforcements to soil have been performed by either incorporating continuous reinforcement inclusions such as sheet, bar or strip within a soil mass in a well defined pattern, or by randomly mixing discrete fibres with a soil fill. The effect of conventional reinforcements on soil has been extensively investigated by Fleming et al. (2006), Iizuka et al. (2004), Katarzyna (2006), Latha and Murthy (2006), Park and Tan (2005), Patra, Das and Atalar (2005), Varuso, Grieshaber and Nataraj (2005), Yetimoglu, Inanir and Inanir (2005). The concept of three dimensional reinforcement was first introduced by Lawton et al. (1993), who performed laboratory investigations on sand reinforced with geo-jacks. They also observed that use of geo-jacks on top of the geogrid substantially improved the performance of the soil foundation and that the combination of geogrid and geo-jacks performed better than a combination of geogrid and gravel. Zhang et al.(2008) investigated the use of three dimensional reinforcements in the form of rings with varying heights of vertical elements.
In this investigation, results from laboratory plate load tests conducted on square footing on sand bed reinforced with single and multiple layers of multi-directional reinforcements are presented, in order to determine the feasibility of using multi-directional reinforcements to improve the bearing capacity of soil and to investigate the significance of parameters such as volume ratio of reinforcements, depth to first layer, spacing between reinforcements in a layer, spacing between layers and number of layers.
Locally available clean river sand obtained from the premises of NIT Calicut, Kerala, India was oven dried and was used for the present study. Reinforcing elements were manufactured from injection moulding of ABS plastic granules. ABS is derived from acrylonitrile, butadiene, and styrene. Acrylonitrile is a synthetic monomer produced from propylene and ammonia; butadiene is a petroleum hydrocarbon obtained from the C4 fraction of steam cracking; styrene monomer is made by dehydrogenation of ethyl benzene, which is a hydrocarbon. ABS combines the strength and rigidity of acrylonitrile and styrene polymers with the toughness of polybutadiene rubber. ABS has superior properties in terms of hardness, gloss, toughness, and electrical insulation. The properties of sand and reinforcements are mentioned in Table 1 and Table 2 respectively. The reinforcements consisted of four legs or protrusions in a single plane (x–y) and two protrusions in plane perpendicular to this plane (z), with an average length of 30 mm and a diameter of 5 mm were used for the study, as shown in Fig.1. The effect of parameters such as volume ratio of reinforcements, depth to first layer, spacing between reinforcements in a layer, spacing between layers and number of layers are investigated vide plate load test performed on a 150 mm square MS plate placed in an MS tank of size 75cm x 75 cm x 75 cm. The test setup is shown in Fig.2.
The general nomenclature adopted for the study are as follows:
b= width of reinforcing element
B= width of plate
u=depth to first layer
d= spacing between layers
s= spacing between reinforcing elements in a single layer
N= number of layers.
Depth to first layer of reinforcement
Typical pressure vs. settlement curves for the model footing on unreinforced and reinforced sand bed is shown Fig.3. The reinforcing elements were placed, close to each other, such that the spacing between them, s/b=0.This corresponds to a volume ratio of 0.3%. Each test was performed twice and the average of the settlement values was taken into consideration, to account for accuracy. It can be seen that placement of a single layer of reinforcement, too close to the surface (u/B=0.1), does not improve the settlement response significantly. As the depth to first layer increases beyond 0.1B, the response improved drastically. This is due to the fact that at shallow depths of placement, the magnitude of mobilized frictional resistance at the sand-reinforcement interface is relatively less, due to the smaller overburden pressure. Placing the first reinforcement layer at a depth greater than 0.7B, had an adverse effect on bearing capacity and settlement values, since the settlements began to increase, although better than the unreinforced case. This behaviour is due to increased thickness of sand layer over the reinforcements, resulting in higher settlement. Surface heave reduces with increasing depth of placement of reinforcement upto u/B=0.5, and thereafter decreases.
The improvements in bearing capacity and settlement behaviour are quantified using three factors viz. Bearing capacity ratio (BCR), Settlement reduction factor (SRF) and Heave ratio (dx100/B). BCR is calculated as the ratio between the ultimate bearing capacity of reinforced sand to that of unreinforced sand. The ultimate bearing capacity in all cases, in the study is calculated, corresponding to a settlement of 25 mm.
s0 is the settlement of unreinforced sand bed at a given pressure and sr represents settlement of reinforced sand bed at the same pressure. Heave ratio is defined as the ratio between the maximum heave, observed at a distance of 1B from the edge of the plate for unreinforced sand bed to the maximum heave observed in reinforced sand bed.
Fig.4 shows the variation in BCR corresponding to 25 mm settlement. For a single layer of reinforcement, the bearing capacity improved by as much as 1.3 times, corresponding at a depth of placement, 0.5B. Settlement reduction factor was calculated for various depths to first layer of reinforcement, as shown in Fig.5.
A maximum reduction of 0.72 was obtained corresponding to u/B=0.5 with zero spacing between reinforcing elements. The variation of heave ratio with depth of first reinforcement layer is shown in Fig.6. Similar to the trends observed, the maximum reduction in heave corresponds to a depth of 0.5B.
The general ground displacement profile in terms of settlement and heave with variation in depth of first reinforcement layer is described in Fig.7.
The optimum depth of placement of the first layer of reinforcement was determined as 0.5B. The reinforcing elements were placed close to one another, so that there was zero spacing between them (s/b=0; b being the width of the reinforcing element).
The next phase involved the determination of optimum spacing between the reinforcing elements. Two different configurations of reinforcement arrangement, viz. s/b=0.5 and s/b=1. were tested. The corresponding volume ratios are 0.135% and 0.084%. As expected, placing the reinforcing elements close to one another, with zero spacing between them, accounts for maximum improvement in the strength parameters, on account of increased volume ratio of reinforcement. However, it is to be noted that the reduction in strength improvement, when the reinforcements are arranged at a spacing of s/b=0.5 and s/b=1 is only marginal. Considering the practical difficulties in placing the reinforcing elements one by one, at zero spacing between them in field applications, and the economical aspects of the project, the placement of reinforcements at a spacing of s/b=1, proves to be feasible. Thus, considering a balance between the strength improvement and the overall economic benefits, an intra-layer spacing of s/b=1 is adopted for the study.
Number of layers and spacing between layers
The next phase of tests involved determining the influence of number of reinforcing layers on the bearing capacity and settlement. In all tests conducted in this phase, the depth to first reinforcement layer is fixed at 0.5B. The number of reinforcement layers is varied from one to four, the final layer being kept at a depth of 2B. The reinforcement layers were placed in such a way that they were spaced evenly between 0.5B and 2B depths. The thickness of the reinforcing elements was around 0.2B. Hence, it was not possible to place the reinforcements at spacing less than 0.5B, due to practical difficulties in ensuring the correct placement of reinforcing elements and compaction of the sand above. Placing a layer above 0.5B depth was not considered in this phase, as the optimum depth for a single reinforcement layer was obtained as 0.5B. Also, if a reinforcement layer was placed above this level, providing due allowance for the thickness of the reinforcing element, the depth of placement would be less than 0.3B and would not contribute significantly to the improvement in strength parameters, as discussed in the previous sections. The bearing capacity was found to increase with an increase in number of reinforcing layers, coupled with a decrease in settlements. This is due to the ability of reinforcements to spread the superimposed load to larger depths, where the confining and overburden pressures are higher. It was observed that as the number of layers increased from 2 to 3, a steep increase of about 45% was observed in BCR and SRF, whereas this improvement reduced to about 14% and 3% for BCR and SRF respectively, when the number of layers increased beyond 3.Similarly, the reduction in heave observed was about 10% when the number of layers increased from 2 to 3, and only 0.6% when the number of layers increased from 3 to 4. Since the tests are conducted on laboratory scale models, the effect of confinement and scaling play a significant role. Further large scale field studies are required to be performed in this regard to determine the exact behaviour of multi-directional reinforcements.
A comparison of the improvements in bearing capacities imparted by conventional reinforcing elements such as geogrids and the multi-directional elements developed in this study are provided in Table 3. It can be seen that the multi-directional reinforcements perform on par with the conventional reinforcing systems. Additionally for a given aerial coverage, the cost of these elements is about 50% less. compared to the existing methods of conventional reinforcements.
Based on the results from experimental investigations on the behaviour of model footing, resting on sand bed reinforced with plastic multi-directional reinforcements, the following conclusions can be drawn:
a. An appreciable increase in bearing capacity was observed as the depth to the first layer of reinforcement increased beyond 0.1B. The optimum depth of placement of the first layer was 0.5B. Placing reinforcements beyond 0.5B depth, in a single layer, resulted in a reduction in increase of bearing capacity. The bearing capacity increased by 1.3 times and the settlements reduced by almost 72%.
b. Within a single layer of reinforcement, the maximum improvement in BCR was obtained corresponding to zero spacing between the inclusions. However, considering a balance between the strength improvement and economical aspects, an optimum spacing of 1b was adopted, where b is the width of the reinforcing element.
c. BCR increased with increase in number of reinforcement layers. As the number of layers increased from 2 to 3, a steep increase of about 45% was observed in BCR, whereas this improvement reduced to about 14 % when the number of layers increased beyond 3. SRR showed similar trends, the improvements being 43% and 3% respectively. A similar trend was observed in case of Heave factor also.
d. Owing to the size of the reinforcing elements and the practical difficulties in laying and compacting sand layers between the reinforcing elements, a minimum spacing of 0.2B was required to be maintained between the reinforcement layers. Hence, considering these factors, the optimum layer spacing was maintained as 0.5B.
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