Research Article  Open Access
In Situ Testing of Square Footing Resting on GeobeltReinforced Gravel Thin Cushion on Soft Silt
Abstract
A series of in situ static loading tests of square footing were carried out on the geobeltreinforced gravel cushion on soft silt. The reinforced gravel cushion was thin with the depthtowidth ratio of 0.2. A parameters study was conducted by considering the number of geobelt layers, the depth of the first geobelt layer beneath the footing, the vertical spacing between two geobelt layers, the linear density of reinforcement, and the material type of geobelt. The pressure distribution on bottom of the cushion was measured. The test results showed that the bearing capacity of reinforced gravel cushion was significantly larger than that of unreinforced gravel cushion, and the stress diffusion effect of reinforced gravel cushion was also more pronounced than that of the unreinforced cushion. The pressure distribution on bottom of reinforced gravel cushion was in a saddle shape. According to calculation and analysis, the stress diffusion angles of reinforced cushions were all larger than 20°.
1. Introduction
For using it as foundation, soft silt soil has to be treated due to its low bearing capacity and large settlement. An effective treatment is to replace a shallow depth of upper soft silt with geosyntheticreinforced gravel cushion, which is costeffective compared to other conventional methods [1, 2]. It has been demonstrated that geosynthetic reinforcement can improve the bearing capacity and reduce uneven settlement of the foundation [3–5]. By conducting 65 groups of model tests, Binquet and Lee [6, 7] first reported that, by reinforcing sand layer underneath the strip footing with strips of aluminum foil, the settlement and ultimate bearing capacity of the foundation was greatly improved. In literatures[8–14], similar results were reported by researchers. It should be noted that it is complicated to determine the ultimate bearing capacity of square footing on reinforced soil. In addition, there has been limited research investigating the stress diffusion effect of reinforced cushion.
For determining the bearing capacity and stress diffusion capacity, most studies have used smallscale models test, which would have size effect and limitations to reflect the actual behaviors of the deformation and bearing capacity of foundation [15–17]. Therefore, it is imperative to conduct fullscale in situ tests to investigate the effects of reinforcing geosynthetic materials. In the literature, geotextiles including geogrid [17–19], geonet [20], geocell [21, 22], and fiber [23, 24] are commonly used as reinforcing materials, while geobelt, a relatively new material, has been rarely utilized. Moreover, most of the previous studies have focused on the settlement and bearing capacity of foundation [1, 6, 9, 12, 15, 25], while little attention has been paid to the stress diffusion angle of reinforced cushion. In practical, due to presence of geobelt, the overall rigidity and stress diffusion capacity of the reinforced cushion can be greatly improved even for thin cushions. Therefore, based on the stress diffusion theory of doublelayered foundation, this study aims to investigate the bearing capacity and earth pressure distribution of square footing resting on the thin gravel cushion through a series of in situ static loading tests. The stress diffusion effect of geobelt on the reinforced gravel cushion was also studied.
2. Experiment
2.1. Test Design and Setup
The testing site, with a dimension of 30 m × 17 m, is located in southwest part of Taiyuan city, China. In the testing site, the natural soil beneath the gravel cushion was saturated soft silt soil. The properties of the silt soil are listed in Table 1.

Each static loading test was conducted on the field test pit with a length of 2.3 m and width of 2.3 m within the testing site. Figure 1 shows the layout of the test setup. A steel square rigid plate with a length of 1.5 m, width of 1.5 m, and depth of 0.3 m was used as a footing to apply the loading. A gravel cushion with a dimension of 2.3 m × 2.3 m × 0.3 m with or without geobelt reinforcement was prepared before each loading test. The loading was applied by a mechanical jackbeam loading system on the gravel cushion through the square footing.
Seven dial gauges were deployed in each test, as shown in Figure 2. Four gauges were attached on the four corners of the square rigid plate to measure the overall settlement of the footing. Other three gauges were attached on the bottom of the gravel cushion to monitor the settlement of silt soil. The deformation of the gravel cushion can then be calculated according to the measurements of seven gauges. As shown in Figure 3, twentytwo pressure cells, with a measuring capacity of 0.6 MPa, were deployed on the bottom of the gravel cushion to measure the pressure of the cushion resting on silt soil during loading.
In total, 10 in situ tests were conducted on unreinforced and geobeltreinforced gravel cushions over saturated silt soil. The testing programs are summarized in Table 2.
 
^{∗}N represents the layers of geobelt set in the gravel cushion, U is the depth of the first geobelt layer beneath the footing, H refers to the vertical spacing between two layers of geobelt, and LDR is the linear density of reinforcement which means the ratio of the width of geobelt to the center spacing of two geobelts. 
2.2. Materials
2.2.1. Geobelts
Two types of geosynthetic materials used in this study are TG geobelt and CPE geobelt. TG geobelt, as shown in Figure 4(a), is mainly made of fiberglass coated with polyolefin. As shown in Figure 4(b), CPE geobelt is highstrength galvanized iron wires coated with chlorinated polyethylene. These two geobelts have high tensile strength, low expansion, and small creep and are resistant to bending fatigue, tension stress crack, bearing, and punching. Moreover, the geobelt has properties such as antiaging, acid/alkali resistance, and suitability for burying in soil. From construction perspective, the geobelts have the advantages of lightweight, simple construction, and shorter construction period. There are rough details and ribs on the surface of two types of geobelt materials for improving the bonding between geobelts and gravel. Their engineering properties are listed in Table 3.
(a)
(b)

In the test, in order to ensure strong bonding between geobelts and the gravel, the geobelts are bent at both ends by using a sand bag which is around 550∼600mm and then fixed by two clamps, as shown in Figure 5. Therefore, the effective length of the geobelt used in the cushion is around 3.5m, including the width of the cushion and the fixing length at both ends.
2.2.2. Gravel Cushion
The thin cushion consisted of gravels with diameter in the range of 10mm– 30mm. Particle size distribution of the gravels is shown in Figure 6. The physical parameters obtained in laboratory tests are listed in Table 4, in which the maximum dry density and optimum moisture content were determined by Standard Proctor Test. According to USCS and AASHTO classification, the gravel was classified as poorly graded gravel (GP).

2.3. Test Procedure
To ensure that each test was conducted under same conditions, the following preparations have to be done. First, the testing pits were dug, cleaned, and leveled to make sure their sizes meet the requirements. Secondly, the sorted fine sand was paved, compacted, and leveled on the bottom of the pits with a thickness of 10 mm∼15 mm for reducing stress concentration in the pressure cell. Then the pressure cells were put on the preset positions. The pressure cells should be waterproof and calibrated before each test. Six layers of gravel were paved on the pressure cell with each layer about 50 mm thick. Each layer of gravel in the loose state had the same weight and was then compacted to same compactness by using a wood hammer. Geobelts were laid on the specified position in the gravel cushion with the required linear density of reinforcement (LDR) in two dimensions. The geobelts should be tightened and straightened. A layer of fine sand with a thickness of 10 mm∼20 mm was paved above and beneath the geobelts for protecting them from not being punctured by gravel. Geobelts and gravel were laid alternately according to the requirements until the cushion height reached 300 mm. Then the loading plate was put on the top of the reinforced gravel cushion, leveled, and centered properly, to ensure that the load can be evenly distributed on the gravel cushion.
The static loading was applied by using a hydraulic jack. The loading method and stability criteria followed the Code for Design of Building Foundation (GB 500072011) [26]. The loading was applied with an increment of 20 kPa. If it was less than 0.1 mm/h in two hours, the settlement was recognized to meet the criteria and the next increment of loading can be applied. The test was terminated when the total settlement reached 0.06 B, that is, 90 mm in this program.
After one test was completed, a new test pit was dug in adjacent locations within the testing site. Identical testing procedures were used to perform each test.
3. Test Results and Analysis
3.1. Reinforcing Effect on Bearing Capacity of Foundation
Figure 7 shows pressuresettlement curves of foundations with and without geobelts. It can be seen that the settlement increased with the increase of pressure for both reinforced and unreinforced cushions. However, cushion reinforced with two geobelt layers showed the slowest increase rate of settlement, while unreinforced cushion showed the quickest increase rate. The results indicate that the bearing capacity can be effectively improved by geobelt reinforcement. The reason is that the lateral restraint of geobelt on gravel cushion can decrease the settlement of the gravel cushion, and thus improve the bearing capacity of the foundation compared to the unreinforced cushion. At early stage, the settlement increased linearly with pressure. As pressure increased, geobelt comes into play and decreases settlement effectively.
Since no abrupt increase of settlement was observed in all tests, the ultimate bearing capacity of foundation was determined when the settlement of foundation reached 0.06 B (B is the length of the square footing).
To account for the size effect of footing, the bearing capacity ratio (BCR) recommended by Binquet and Lee was calculated as following:in which and are the bearing capacity for reinforced and unreinforced soil, respectively. For convenience, the test results were analyzed according to the BCRs calculated at different settlement ratios (s/B). The settlement ratio is calculated by dividing settlement of footing (s) with the width of footing (B). The values of BCR at settlement ratios (s/B) of 0.01, 0.02, 0.03, 0.04, 0.05, and 0.06 are listed in Table 5. The BCR for onelayer geobelt reinforcement was between 1.22 and 1.37, and gradually decreased with the increase of the settlement ratio; the BCR of twolayer geobelt reinforcement was between 1.34 and 1.70 and gradually increased with the increase of the settlement ratio. In current study, the values of BCR are lower than that proposed by Adams and Collin [27], and Chen et al.[28]. This may be due to the relatively small thickness of the cushion used in this study.

Figures 8(a) and 8(b) show the relationship between the number of reinforced layers (N) and BCR. As can be seen, twolayer geobelt reinforcement was much better than onelayer geobelt reinforcement, especially at the late stage of loading. At s/B of 0.06, BCRs of onelayer reinforcement were from 1.22 to 1.26, while for twolayer reinforcement, they were from 1.62 to 1.70, which indicates that the bearing capacity of soil reinforced with twolayer geobelt can be increased more at the limit state.
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(b)
Figure 9 shows the relationship between BCR and the depth of the first geobelt layer beneath the footing (U). At the initial stage of loading (s/B ≤ 0.2), BCR reached the maximum at 50 mm (U), and gradually decreased as U increased. At the later stage of loading (s/B ≥ 0.3), BCR slightly increased and the bearing capacity of cushion was improved. This can be explained that the geobelt comes into play after certain amount of settlement happened. In addition, the geobelt could take the effect earlier if it is closer to the footing.
Figure 10 shows the relationship between BCR and the linear density of the reinforcement (LDR). As can be seen, BCR increased slightly with the increase of LDR. When LDR increased from 25% to 50%, the value of BCR increased from (1.22∼1.29) to (1.26∼1.34). This can be ascribed to the increase of shear strength since the interactions between the geobelt and gravel is improved when LDR is increased which provides more lateral restraint to the gravel cushion.
Figures 11(a) and 11(b) demonstrate the relationship between BCR and vertical spacing (H) for twolayer geobeltreinforced cushion. At the initial stage of loading (s/B ≤ 0.2), BCR decreased with the increase of H, while at the later stage (s/B ≥ 0.3), BCR increased with the increase of H. The results also indicate that the geobelt takes effect when certain amount of settlement happened. At the later stage of loading, reinforcing effect of the geobelt became more pronounced as there was a continuous increase of deformation of the cushion.
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(b)
Figures 12(a) and 12(b) show the bearing capacity ratio against the settlement ratio for the cushion reinforced with the TG geobelt and CPE geobelt to compare their reinforcing effects. At the initial stage of loading, in terms of bearing capacity, the TGtype geobelt performed better than the CPEtype geobelt. However, the difference between them became unnoticeable with the increase of loading.
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(b)
3.2. Stress Distribution at the Bottom of Reinforced Cushions
Figure 13(a) shows the stress distribution on bottom of the unreinforced gravel cushion. Figures 13(b) and 13(c) show stress distribution on bottom of the reinforced gravel cushion under different pressures. As can be seen in Figure 13(a), for the unreinforced gravel cushion, the stress distribution on bottom of the cushion was parabolic with maximum net earth pressure reached at the center of the cushion. The stress distribution curves on bottom of the reinforced cushion were both in saddle shape and the maximum earth pressures were found to be about 370 mm (onelayer geobelt) and 750 mm (twolayer geobelt) away from the center with the minimum pressure at the center. For the unreinforced cushion, surrounding soft silt soil could not provide strong constrain. As loading increases, the gravels on bottom edges of the footing could be easily pushed out laterally since the pressure on bottom could not be adjusted by the unreinforced cushion itself. While for reinforced cushions, the geobelt restricted the lateral displacement of the cushion owing to frictions between the geobelt and gravels. Therefore, it was not easy to push out the soils located on bottom edges of the footing. The results indicate that, due to using the geobelt, the stress distribution was improved. The central reaction force was decreased, and the edge reaction force was increased. For onelayer and twolayer reinforced cushions (Figures 13(b) and 13(c)), it can be found that, compared to the onelayer cushion, the pressure on the edges of the twolayer reinforced cushion dramatically increased and the pressure distribution became smoother. It is clear that in terms of the reinforcing effect, the twolayer reinforcement is much better than the onelayer reinforcement.
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(b)
(c)
3.3. Stress Diffusion Angle of Cushion
As shown in Figure 13, the earth pressure measured on bottom of the cushions was not evenly distributed. So in this section, the average pressure on the bottom of cushion was calculated according to the measured settlement results which are more accurate and uniform. In same stress path, there exists a onetoone relationship between settlement and pressure of soil layers. Therefore, the average pressure (P_{z}) under the cushion can be obtained by using the pressuresettlement curves of natural ground without cushion and with upper cushion, as shown in Figure 14. When the settlement of natural ground under the cushion was equivalent to that of natural ground without the cushion, the corresponding relationship between the pressure P_{0} under the cushion and the load P_{y} on the top of natural ground was obtained; that is, P_{y} is equal to the mean earth pressure of cushions P_{z}. Based on the principle of stress diffusion (Figure 15), the stress diffusion angle of the reinforced cushion can be calculated by the following formula:
To investigate the effect of geobelt on stress diffusion, a ratio SDR was defined, in this study, by dividing the stress diffusion angle of the geobelt reinforced cushion with that of the unreinforced cushion. Table 6 lists stress diffusion angles of the cushion and the corresponding SDR values at the proportional limit state and stable state. As can be seen in the table, the stress diffusion angle of reinforced cushions was obviously larger than that of unreinforced cushions. At the proportional limit state, SDR values spanned from 1.46 to 1.60, which means the stress diffusion angle was improved by 1.46– 1.60 times with geobelt reinforcement. At the limit state, SDR values spanned from 1.76 to 2.63, which means the stress diffusion angle was improved by 1.76–2.63 times. Current results are consistent with the result of Gabr et al. [29, 30] that the stress diffusion angle at the stable state was smaller than that at the proportional limit state.

Figures 16–19 show the relationships between different parameters and the stress diffusion angle ratio. As can be seen in Figures 16(a) and 16(b), under same reinforcement conditions, the twolayer geobeltreinforced cushion showed larger stress diffusion angle compared to the onelayer geobelt reinforced one. At the proportional limit state, SDR of the twolayer reinforced cushion was similar to that of the onelayer reinforced cushion, while at the stable state, there was a big difference between them. The reason is that, for twolayer geobelt reinforcement, first geobelt closer to the footing takes effect when the loading is small. As the loading increases, the second geobelt would then take its effect. Therefore, at the later stage of loading, the reinforcing effect of twolayer reinforcement became more pronounced. As can be seen in Table 6, at the stable state, SDRs spanned from 1.76 to 1.84 for the onelayer geobelt reinforced cushion, while for the twolayer geobelt reinforced cushion, the ratios were ranging from 2.46 to 2.63.
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(b)
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Figure 17 shows the relationship between the depth of the first geobelt layer beneath the footing (U) and SDR. Figure 18 shows the relationship between the linear density of reinforcement (LDR) and SDR. As can be seen, U and LDR had little impact on stress diffusion of the cushion. The possible reason is that the depth of the cushion adopted in this study was small. As can be seen in Table 6, at the proportional limit state, the SDR decreased from 1.50 to 1.46 as U increased from 50 mm to 200 mm, while at the stable state, the SDR increased from 1.76 to 1.84 as U increased from 50 mm to 200 mm. It should be noted that the relationship between SDR and U is consistent with that between BCR and U. This also indicates that the geobelt only takes its effect efficiently after certain amount of deformation of cushion happens. The SDR was found to increase from 1.47 to 1.48 at the proportional limit state and increase from 1.77 to 1.84 at the stable state as the LDR increased from 25% to 50%. Therefore, the LDR showed unnoticeable effect on the stress diffusion of the geobelt reinforced cushion.
Figures 19(a) and 19(b) show the effect of vertical spacing between two geobelt layers (H) on the stress diffusion angle. For different types of geobelt, the values of SDR showed a similar trend, which is the stress diffusion angle increased slightly with the increase of H. At the proportional limit state, the SDRs of the TGreinforced cushion were 1.57 and 1.60, while for the CPEreinforced cushion, the values were 1.51 and 1.52, which is slightly smaller than that of the TGreinforced cushion. At the stable state, the SDRs of the TGreinforced cushion were 2.54 and 2.63, while for the CPEreinforced cushion, the values were 2.46 and 2.60, which was also slightly smaller than that of the TGreinforced cushion. It can be concluded that the reinforcing effect of TG geobelt was slightly better than that of CPE geobelt. The results are also consistent with BCR results discussed in Section 3.1.
4. Engineering Applications
In this section, a practical project included in Technical code for ground treatment of buildings [31] was presented to demonstrate the effect of geobelt.
The site of the project is located on a saturated silt soil with a bearing capacity of 80 kPa. The footing has a length and width of 60.8 m and 14.9 m, respectively. The thickness of the footing is 2.73 m. An average pressure beneath the footing is 130 kPa. Two possible treatment solutions are (1) unreinforced gravel cushion with a density of 19.5 kN/m^{3}, and the thickness of the cushion is taken as 3.73 m by calculation and (2) twolayer TG geobeltreinforced thin gravel cushion. The density of the cushion was 19.5 kN/m^{3}. The width (B) was 17 m and thickness (Z) was 1.5 m with a Z/B ratio of 0.088 (<0.2). The project adopted the second treatment solution by using the geobeltreinforced cushion. The first layer geobelt beneath the cushion was 0.6 m, and the vertical spacing between two layers was 0.4 m. The linear density reinforcement is 17%. According to the test results, the stress diffusion angle takes 35° considering a certain safety factor. After a comparative analysis, the reinforced cushion used in the second solution reduced the cushion thickness by 60% compared to the unreinforced cushion. The deformation after engineering settlement was 3.9 mm which meets the requirements.
According to this project, the stress diffusion angle of the reinforced gravel cushion was larger than that of the unreinforced one, which satisfies the bearing capacity and deformation requirements. In addition, the thickness of the cushion and construction cost can be reduced by using geobelt reinforcement.
5. Conclusions
By conducting the in situ square footing loading test on the geobeltreinforced thin gravel cushion, this study investigated the reinforcing effect of geobelt on bearing capacity of the foundation and stress diffusion of cushions. The following conclusions can be made:(1)Compared to unreinforced gravel cushions, geobelt reinforced cushions can improve the bearing capacity of foundation. The bearing capacity of onelayer geobeltreinforced foundation was increased by 1.22–1.37 times while that of twolayer geobelt reinforced ones can be improved by 1.34–1.7 times.(2)The earth pressure distribution on bottom of reinforced cushions was in a saddle shape, which was smoother than that of unreinforced cushions. When loaded, geobelt can restrict the movement of soils due to interfacial frictions between it and crushed gravels; therefore, the stress distribution of reinforced cushions becomes less varied compared to that of unreinforced cushions.(3)The deformation and stress distribution of underlying soil layers under reinforced cushions were much different from that under unreinforced cushions. Especially, the effect of geobelt on stress diffusion was pronounced. The stress diffusion angles of reinforced cushions were all larger than 20°.(4)The reinforcing effect of twolayer geobelt reinforcement is obviously better than that of onelayer geobelt reinforcement. At the stable state, the stress diffusion angles of onelayer reinforced cushions were increased by 1.76–1.84 times while twolayer reinforcement was improved by 2.46–2.63 times, compared to unreinforced cushions.(5)At the stable state, for the onelayer geobelt reinforced cushion, the stress diffusion and bearing capacity can be improved by increase of distance between the first geobelt and footing. A higher linear density of reinforcement can get a better bearing capacity and stress diffusion.(6)At the stable state, for the twolayer geobelt reinforced cushion, the greater the vertical spacing between two layers, the better the stress diffusion effect and the larger the bearing capacity of the reinforced cushion. And the performance of the reinforced cushion with TG geobelt was better than that of the reinforced cushion with CPE geobelt.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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Copyright
Copyright © 2018 Wei Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.