Behavior of Vertically Confined Square Footing on Reinforced Sand under Centric Inclined Loading

Geogrid reinforced vertically confined foundation system has been investigated in the present experimental study. Fig. 1 shows the schematic diagram of the experimental set up used in the laboratory. The model foundation bed was prepared in an iron tank. The test tank was rectangular having internal dimensions of 1.0 m × 1.0 m × 0.8 m (L′×B′×H′). To avoid lateral deformation, the tank walls were braced with iron section outside. The test tank was provided with a loading frame to facilitate load application through a manually operated hydraulic jack. A precalibrated proving ring of 30 kN capacity was used in between the hydraullic jack and footing to measure the magnitute of transferred load. A square footing of side B (20 cm) was used. The responses of the loading foundation were monitored at different loading stages by recording the footing setttlement with the help of four dial gauges having an accuracy of 0.01 mm placed at the four corners of the footing. Average of four dial gauge readings was taken as the settlement of the footing. The parameters D, d, L, y and Y are the top surface dimension of the confiner, depth of the confiner, length of the geo-grid, depth of the top layer of reinforcement below the footing and spacing between the reinforcement layers respectively. Medium dense sand was used for carrying out the experimental work.

Figure 1

A detailed investigation program was executed to find out the effect of confiner and confiner with horizontal reinforcement on the behavior of footing. Tests were performed in three series. In series A, only footing (surface footing) was used under centric inclined loading. In series B, footing with confiner was used, In series C, footing with confiner and horizontal reinforcements placed inside the confiner was tested under inclined loading. Parameters such as d, D, y, L and Y were normalized with respect to width of the footing (B). The top reinforcement layer (y) was kept at 0.1B (constant) below the base of the footing for all the tests. The consecutive horizontal reinforcement layer spacing was varied as 0.25B, 0.5B,0.75B and 1.0B. For each top surface dimension of the confiner (D) that is 1.0B, 1.5B and 2.0B the normalized confiner depth (d/B) was varied as 1.0, 1.5 and 2.0 respectively. The details of laboratory model tests are presented in Table 2. Schematic presentation of the confiner and confiner with reinforcement under centric inclined load with different geometric parameters has been shown in Fig. 4.

Figure 4

The sand used for the foundation bed in the experimental work was properly sun dried, cleaned and sieved through a 1-mm IS sieve. A predetermined amount of sand for a particular volume was filled in the test tank using the raining technique. For sand raining technique the required height of fall to achieve the desired relative density of 50% was determined through several trials. In this study, the sand was poured slowly with in the tank by a steel container from a fixed height of 25-cm and leveled by a wooden plate in each 10-cm height marked on the side of the tank to ensure that equal quantity of sand was being filled corresponding to 50% relative density. The quantity of sand was calculated multiplying the unit weight of sand with volume and for each lift, the amount of sand required to produce the desired unit weight (15.24 kN/m³) was weighed for the correctness of each layer (10 cm). The uniformity of sand bed was checked for unit weight determination by collecting the soil sample in small containers where as the thickness of sand bed was checked from the depth difference inside the tank for each lift with the help of a measuring tape.

Confiners were placed on the leveled sand bed, each time considering the desired depth of the confiner. The concentricity of confiner with the tank was throughly checked with the help of a plumb bob. After the confiner was placed in position, horizontal geogrid reinforcements were placed inside the confiner followed by the simultaneous filling and leveling of sand based on the required spacing. After the entire entity of confiner and reinforcement was placed, the remaining part of the sand layer was filled following the earlier procedure used for sand bed preparation. The same method was also used for individual placement of confiners on the sand bed for different footing configurations.

After the sand bed was prepared, the top surface of the sand bed was thoroughly leveled and the footing was placed exactly at the centre of the level surface with the help of a plumb bob. The footing was loaded with a hand operated hydraulic jack supported against reaction frame. Recess was made on the footing plate to accommodate ball bearing, through which centric inclined load was applied in small increments. A precalibrated proving ring was used to measure the transferred load. Each load increment was maintained constant until footing settlement was stabilized. Settlement of the footing was calculated as the average of four dial gauges arranged on each corner of the footing plate. The same procedure was followed till the complete collapse of the footing. Fig. 5 shows the experimental set up for centric inclined loading at 5° angle of inclinaion.

Figure 5

Effect of confiner on the behavior of square footing without reinforcement has been analyzed at different depths and top surface dimension of confiner. The load intensity-settlement curves for footing with confiner configurations under centric inclined loading (i=5°) are presented in Fig. 6. Load intensities corresponding to 25 mm settlement for all footing configurations are tabulated in Table 3. It can be observed from Fig. 6 that with increase in confiner depth load bearing capacity of footing increases and records the highest value at d/B = 2.0 for each increment in top confiner dimension. The optimum improvement in load intensity with increment in confiner depth is noticed for minimum top surface dimension of confiner D/B = 1.0 which gradually decreases with increment in top surface dimension of confiner. It is evident from Table 3 that ultimate load obtained at d/B = 2.0 are 89%, 61% and 42% compared to surface footing for top surface dimension of 1B,1.5B and 2B respectively at applied load of 5° angle of inclination. Same trend of increment in bearing capacity is also observed for 10° and 20° angle of inclination too, while it can be noticed that improvement becomes marginal for 20° angle of inclination due to sliding and heavy lateral displacement of sand particles. The BCR and SRF calculated using Eqs (1) and (2) are presented in Tables 4 and 5 respectively. It can be observed from both the tables that the BCR and SRF values increased with increase in depth of confiner and showed the optimum value at d/B = 2.0 for all variations of top surface of confiner. This is explained as follows. The installation of confiner resists the lateral movement of sand particles beneath the footing leading to increase in stiffness of the sand and modification of failure mode. As the d/B ratio increases the confining pressure also increases at the confiner tip level, Consequently elastic and plastic displacement of sand grains get constrained leading to a greater increase in load carrying capacity and considerable decrease in settlement.

Figure 6

The behavior of vertically confined square footing with reinforcement under centric inclined loading is investigated in this section. Typical load intensity-settlement plots for a particular confiner dimension at 5°, 10° and 20° angles of inclination are shown in Fig. 7. Ultimate load intensities corresponding to 25 mm settlement for all footing- confiner- reinforcement arrangements are presented in Table 3. Analysis of Table 3 shows that the bearing capacities of surface footing are 35.45 kN/m², 34.19kN/m² and 31.80 kN/m² respectively for inclined loading of 5°, 10° and 20° angle of inclinations. This bearing capacity value at d/B = 1.0, 1.5 and 2.0 increased to 79.93, 101.74 and 105.96 kN/m² respectively at 5° angle of inclination, for minimum spacing of 0.25B between horizonal reinforcements and top confiner dimension of 1B. It is seen that with increase in confiner depth and for minimum reinforcement spacing the bearing capacity increases significantly for each top confiner dimension and registered the optimum value at D/B=2.0. The bearing capacity for the confiner configuration of D/B = 2.0 ; d/B = 2.0 and minimum reinforcement spacing of 0.25B are found as 228.22, 188.57 and 134.39 kN/m² at 5°, 10°, 20° angles of inclination respectively showing a significant improvement compared to the load bearing capacity of their respective surface footing. It may be due to the fact that when horizontal reinforcements are placed at closer spacing like 0.25B inside the confiner, the interlocking between sand particles is increased, larger horizontal shear resistance is developed under footing due to interlocking between geogrids and sand particles, lateral displacement of soil is reduced considerably and the foundation soil system becomes stiffer due to strong bonding between soil particles. Further it can be observed that the BCR and SRF values as presented in Tables 4 and 5 respectively show a notable improvement with confiner and reinforcement. For the same confiner dimension (D/B =2.0; d/B = 2.0) and reinforcement spacing (0.25B) the reduction settlement is found to be 90%, 88% and 80% at 5°, 10° and 20° angles of inclination. From the above analysis it can be inferred that use of confiner and reinforcement together has significant effect on improving bearing capacity and reducing settlement even at 20° angle of inclination.

Figure 7

A series of model tests in series B and C were carried out to verify the effect of depth and top surface dimension of confiner on square footing under centric inclined loading. In both the series the top surface dimension of confiner (D) was kept as 1B, 1.5B and 2.0B for variation of the depth of confiner d/B= 1.0, 1.5, 2.0 respectively. It can be noticed from the test results of series B as mentioned in Table 3 that with increase in depth of confiner the bearing capacity of footing increases for each confiner dimension. The optimum value obtained at D/B = 1.0 and d/B = 2.0 further decreases as the D/B value increases to 1.5B and 2.0B. This behavior of confiner remains similar for all angles of inclination. The reason is attributed to a little larger aperture size of geogrid plates used as confiner which weakens down the lateral confinement with increase in top surface dimension. Variation of ultimate load intensity with normalized depth (d/B) and top surface dimension of confiner (D/B) for all applied angles of inclination is presented in Fig. 8. Further analysis of Table 3 reveals that test results obtained in series C (confiner with reinforcement) show that the bearing capacity of footing increases significantly with increase in both confiner depth and top surface dimension and shows the optimum value at highest increment in top surface dimension, that is, D/B = 2.0. This can be explained, as the top surface dimension increases the length of reinforcements placed horizontally inside the confiner increases simultaneously, provides a larger area to spread the applied load. This is the reason, increment in top surface dimension becomes notably beneficial with reinforcements where as its effect becomes marginal when used alone as confiner.

Figure 8

Analysis of Table 3 reveals that number of reinforcements and spacing between reinforcement layers play a vital role in enhancing load bearing capacity of footing. The spacing between the reinforcement layers was varied as 0.25B, 0.5B, 0.75B and 1B. It can be observed that with increase in confiner depth, the number of reinforcement layers increases and for minimum spacing between the layers it shows optimum improvement in bearing capacity as well as reduction in settlement for all top surface dimensions of the confiner. As the spacing between the geogrid layers increases, bearing capacity starts decreasing gradually and the minimum improvement is observed at 1B spacing. It also shows that the increment in bearing capacity of footing for spacing of 0.75B and 1.0B remains marginal for all individual normalized confiner depths as the number of reinforcements placed inside the confiner is almost equal for both the spacing ratios which only show improvement with increase in confiner depth. This behavior of spacing remains the same for all angles of inclination of applied load even though a decrement in magnitude of bearing capacity is observed with increase in angles of inclination. Effect of spacing between reinforcement layers on the BCR and SRF values is shown in Fig. 9 for different angles of inclination. It is noticed from the figures that both BCR and SRF values decrease with increase in spacing and the lowest value is obtained at a spacing of 1B. From Table 3 it can also be seen that optimum increment in bearing capacity is observed at the inclusion of maximum number of reinforcements for a minimum spacing(0.25B) inside the confiner at d/B = 2.0 (N = 8) for all top surface dimensions which enables the footing to carry more load and reduce settlement even at 20° angle of inclination. This can be explained as follows: with the increase in number of geogrid the contact area between the geogrid layers and soil particles increases which strengthens the interlocking effect. Consequently larger horizontal shear resistance is developed under footing and lateral displacement of soil is reduced considerably due to confinement provided by the confiners. The load is transferred by geogrid layers to a larger soil mass. Therefore the failure wedge becomes larger and frictional resistance on the failure plane increases. This may be the reason due to which soil reinforcement enables the foundation to carry more load which gradually weakens down with increment in spacing between subsequent reinforcement layers.

Figure 9

The above study also showed that the optimum number of horizontal reinforcements is much dependent on the vertical spacing between the horizontal geogrid layers and placement of first layer of reinforcement. With increase in spacing between horizonal reinforcements, the number of reinforcement layers decreases in side the confiner for all confiner depths and shows the highest increment in bearing capacity and reduction in settlement at minimum spacing (0.25B) by transfer superimposed load to a deeper depth where the confining and overburden pressure are higher. It is also recommended to arrange the reinforcement in the effective zone when placed under the footing for better results.

Effect of angle of inclination on vertically confined square footing on multi-layered geogrid-reinforced sand has been investigated through model test analysis. Ultimate bearing capacities of footing with different foundation configurations are presented in Table 3. These are 35.9, 35.4, 34.19, 31.8 kN/m² for surface footing at 0°, 5°, 10°, 20° angles of inclination which shows a decrease in load bearing capacities with increase in angle of inclination. Similar trend has been observed for bearing capacity at other footing configurations as evident from Table 3. A more detailed analysis of both the test series (B and C) shows that at 20° angle of inclination the effect of confiner underneath the footing shows almost marginal increment in bearing capacity, whereas confiner with reinforcements shows considerable improvement at the same angle of inclination. It can be observed from Table 3 that as the angle of inclination of the applied load increases the footing becomes more prone to sliding failure and loses contact with the supporting soil due to unequal pressure distribution, which is the primary reason for reduction in bearing capacity. It is found that this effect is more prominent on surface footing which gradually gets subsided to a greater extent with the use of confiner and reinforcements. This is attributed to the reason that with increase in top surface dimension of the confiner, the length of reinforcements increases which provides larger contact area to distribute the load and for minimum spacing shows optimum effect to counter against the failure due to incremented angle of inclination. It can be inferred from the above discussion that although the footing behavior gets affected due to increase in angle of inclination in case of surface footing and footing with confiner, the effect later counteracts well due to the combined effect of confiner and reinforcements and provides a significant improvement in footing bearing capacity and reduction in settlement.