ABSTRACT
A new method of soil improvement by using semi-flexible vertical reinforcing elements was investigated in the laboratory and shows promise for future work. Load tests were conducted on two model footing in a sand box using unreinforced sand subgrades and also after reinforcing the sand with vertical reinforcing elements. The ultimate bearing capacities for the unreinforced and reinforced sand cases have been compared. The effect of length, spacing, lateral extent of the reinforcement, and initial relatives density of sand on the ultimate bearing capacity is discussed. The effect of the roughness of reinforcing elements has also been investigated. Based on the results of these tests, it appears that significant improvement in the ultimate bearing capacity of loose and medium sands can be achieved by reinforcing it with flexible vertical elements.
Keywords: reinforcement, footings, bearing capacity, and soil improvement.
INTRODUCTION
The concept of improving week and difficult subsoils for safe and economical construction has been in use for more than a century. However, the techniques for soil improvement have been changing and, during the last three decades, the concept of soil improvement by reinforcing it with tension-resistant elements in the form of sheets, strips, metal nets, woven or resin fibers, polymers, and plastics has received the attention of researchers and field engineers alike. The applications of reinforced earth technology to date show that most of the work has been done with reinforcement laid horizontally. Several studies relating to the evolution of bearing capacity of shallow foundations supported on soil with horizontal layers of reinforcement have been published.
In recent years major research effort has been applied on use of geotextiles and geogrids soil improvements. Hence it appears possible to use semi-flexible non-horizontal reinforcement in soil to improve its load-bearing capacity for supporting shallow foundation. Vertical reinforcement may be easier to install than the horizontal reinforcement since no soil excavation or recompaction may be needed. A preliminary study for determination of the beneficial effects of vertical reinforcement on the load-bearing capacity of a model footing resting on the surface of a sand layer reinforced with vertical semi-flexible reinforcement (i.e., metal rods) has been reported by Verma and Char (1986).
The present study deals with the evaluation of beneficial effects of vertical reinforcement in sand in relation to the improvement in ultimate bearing capacity of shallow strip foundations. A laboratory investigation was conducted to study the important parameters influencing the effectiveness of the vertical reinforcement in improving the load-settlement characteristics of sand subgrades by conducting a series of model footing tests. The details of these tests and the results are presented here.
MODEL TEST DETAILS
Test Set-Up
Model footing tests under plane strain conditions were conducted in a sand box measuring 914.4 mm x 152.4 mm x 609.6mm (length x width x height). The larger side of the box was made of thick plexiglass to observe the deposition of sand in the box during sample preparation and the development of the failure surface in the sand under the foundation during the model tests. The sooth surface of the plexiglass also helped to minimize the effect of side resistance on the rupture surface in the soil resulting in plane strain condition. The walls of the box were also restrained against lateral deformation by stiffing them with angle irons. A schematic diagram of the test arrangement is shown in Figure 1.
The load on the model footing was applied with the help of a hand-operated screw jack and measured with a proving ring. The vertical settlement of the footing was observed with a pair of dial gauges fixed to extension links on either side of the model footing.
Test Parameters
The model footing used in this study measured 50.8 mm x 152.4 mm x50.8 mm (thickness) and 101.6 mm x 152.4 mm x 50.8 mm (thickness) and were cut from hard wood. The base of the footings was made rough by gluing sand grains to the base.
The soil used for this study was medium silica sand with a unified soil classified of SP. The effective size of the sand and the uniformity coefficient were 0.398 mm and 1.2 mm, respectively. The tests were conducted by depositing sand at initial relative densities of 45, 60, and 70 percent.
Figure 1. Schematic of the load set-up
Two types of reinforcing elements were used in the investigation: (a) plain reinforcement that consisted of 1.58 mm diameter steel rods and (b) rough (or ribbed) reinforcement which consisted of steel rods with a single grain layer of very fine sand bonded onto its surface using epoxy glue. The over all diameter of the steel rod with glued sand was also kept as 1.58 mm. tests were conducted using several combinations of length (L), spacing (S), and extent of the reinforcement (R) as shown in Table 1. The parameters L, S, and R are defined in Figure 2a, b.
Table 1. Test Parameters
Figure 2. Geometry of reinforcement in the soil box (a) Section view (b) Top view
Test Procedure
The sand test beds were prepared by depositing sand in layers through a long –stemmed funnel. The height of free fall of sand to ensure a deposit of uniform density was decided by conducting trail tests. The uniformity of deposit was checked by placing small containers before depositing any sand layer and taking out and weighing them after the layer was deposited.
The vertical reinforcement was gently pushed into the sand bed at predetermined spacings. The sand bed was leveled after the placement of the reinforcement and before placing the model footing on it.
The load to the foundation was applied in increments. Each load was maintained constant until no further settlement of the footing occurred. The process of loading was continued until ultimate load was reached or the settlement became excessive (25 mm).
TEST RESULTS AND DISCUSSION
The effect of providing vertical reinforcement in the soil was to increase its ultimate bearing capacity compared to the case where no reinforcement was provided. Figure 3 shows typical plots of load intensity versus settlement for footing of width 50.8 mm for the cases of unreinforced sand and for reinforced sand (for L = B, 1.5B and 2B, and S/B=0.4 ). These plots are sand at an initial relative density of 45%. It can be seen from these plots that, with the introduction of reinforcement, the ultimate bearing capacity increases and the load intensity for any given value of settlement is higher for a reinforced soil than for the unreinforced soil. The improvement in the load-settlement characteristics of the soil reinforced with vertical elements may be due to several factors, such as increase in density of sand (due to installation of reinforcing elements), the change in stress distribution within the soil and the change in the mode of failure.
The effect of reinforcement parameters such as length (L), spacing (S), and extent (R) was first investigated and an optimum combination of these parameters was determined. The effects of initial soil density and roughness of reinforcement were then determined.
Effect of the Length of Reinforcement (L)
Typical plots showing the effect of length of reinforcing elements on the load settlement characteristics of the footing with a width B = 50.8 mm tested on a sand bed at an initial relative density of 45% are been shown in Figure 3. In these tests the spacing (S) and extent of reinforcement (R) were maintained constant at 0.4B and B, respectively, and the tests were conducted by using reinforcing elements of length, L = B, 1.5B and 2B. As can be seen from this figure, both the ultimate bearing capacity and the load-settlement characteristics improve with increase in length of reinforcing elements. Similar effects of length of reinforcing elements on bearing capacity were observed in all other tests as well.
Figure 3. Settlement versus load intensity for footing, B = 50.8 mm, R = B, and S/B = 0.4
Effect of Spacing on Reinforcement (S)
Typical plots showing the effect of spacing of vertical reinforcing elements on the bearing capacity and load-settlement characteristics of the footing with B = 50.8 mm are shown in Figure 4. For these tests, the length of reinforcement (L) and its extent (R) were maintained constant, and spacings S = 0.2B, 0.3B, and 0.4B were used. It may be observed in Figure 4 that the ultimate bearing capacity and the soil pressure for a given value of settlement increase as the spacing of reinforcing elements is decreased. The result of other tests conducted for investigating the effect of spacing also gave similar results.
Figure 4. Settlement versus load intensity for footing, B = 50.8 mm, R = B, and L = B
Effect of Extent (R)
A typical plot showing the effect of the extent of reinforcement (R) is shown in Figure 5. The general effect of increasing the extent from R = B to R = 2B is to increase the ultimate bearing capacity (with parameters L and S kept constant).
Figure 5. Settlement versus load intensity for footing, B = 50.8 mm, L = B, and S/B = 0.4
Optimum Combination of Length, Spacing, and Extent of Reinforcement
Based upon the results of load tests conducted on the model footing with B = 50.8 mm and 101.6 mm and within the range of variables L, R, and S used in this study, it was observed that for the best improvement in ultimate bearing capacity, the combination of reinforcement parameters would be as shown in Table 2 for any given initial relative density of sand. Spacing of vertical reinforcement for best results (i.e., maximum improvements in this case) is in the range of 0.15B to 0.2B.
Table 2. Optimum Combination of Reinforcement Parameters
In order to make a quantitative assessment of the beneficial effects of vertical reinforcing elements in improving the ultimate bearing capacity of the reinforced soil compared to unreinforced soil at different placement densities of sand, a nondimensional term, bearing capacity ratio (BCR) can be defined as
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in which qu(u) = ultimate bearing capacity of unreinforced soil and qu(r) = ultimate bearing capacity of reinforced soil.
Plots were then made of the ‘BCR’ versus initial relative density of sand ‘Dr’ for footing with B = 50.8 mm and 101.6 mm at optimum combinations of reinforcement parameters as shown in Table 2. It can be observed from Figure 6 that the value of BCR for all initial relative densities of soil used in the tests is greater than one. The beneficial effect of the vertical reinforcement therefore occurred at all placement densities used in this study. The magnitude of the increase in BCR is, however, a function of the initial relative density of sand. The value of BCR is seen to increase with the increase in initial relative density Dr up to a maximum value and decreases thereafter (Figure 6). This may be due to the fact that when the vertical reinforcement (elements) are installed in sand at relatively lower densities, it gets somewhat compacted and the combined effect of this increase in density and presence of reinforcing elements results in a substantial increase in the value of the BCR. When the reinforcements are installed in a relatively dense sand, the upper sand layer get somewhat loosened and the increase in BCR diminishes somewhat. The maximum value od BCR will thus be achieved at some intermediate density. For the present tests, the maximum value was observed at a relative densities of compaction of about 50 to 60%. It is also seen from Figure 6 that the plots of BCR vs. Dr for the footings of width 50.8 mm and 101.6 mm used in this study, are rather close and may be represented by an average curve and thus may be considered to be practically independent of the width of the foundation.
Figure 6. Plot for BCR versus Dr for optimum combination of reinforcement parameters
Effects of Roughness of Reinforcement
The tests conducted by using rough (ribbed) reinforcement showed that rough reinforcing elements are more effective in improving the load settlement behavior of sand as compared to the case of plain reinforcement. Figure 7 shows typical pressure versus settlement plots for the footing with B= 101.6 mm for sand reinforced with plain and ribbed reinforcement at an initial relative density of 60%, L = 1.5B, R = 2B, and S = 0.4B. Benefits of using ribbed reinforcement as compared to plain reinforcement are obvious from these plots (Figure 7). A similar trend was observed in all tests where rough reinforcement was used. Other parameters remaining constant, the ultimate bearing capacity was generally 40-50% higher with ribbed reinforcement as compared to the value for the case of plain reinforcement. It must be mentioned here that, even though rough reinforcement is more effective in improving the load settlement behavior of sand deposits compared to plain reinforcement, its installation poses some problems.
Figure 7. Plot for settlement versus load intensity for footing
B =101.6 mm, L/B = 1.5, R/B = 2, and S/B = 0.2
CONCLUSIONS
The beneficial effects of using vertical reinforcing elements in improving the load settlement behavior of sand subgrades have been demonstrated through a series of small scale footing tests in the laboratory. The improvement in the ultimate bearing capacity of reinforced sand subgrades depends upon the spacing, length, and extent of the reinforcing elements. Based on the results of these tests the best results can be obtained by using the reinforcing elements as suggested below:
Spacing of the vertical reinforcement used should be about 0.15 to 0.2 times the width of the footing.
The length of the reinforcement used should be at least equal to the width of the footing and preferably should be 1.5 times the footing width. Reinforcing elements longer than about 1.5 times the footing width may pose problems during placement and may not enhance the bearing capacity further.
The extent of the reinforcement used should be at least 1 to 1.5B. The rough reinforcement was found more effective in improving the ultimate bearing capacity as compared to plain reinforcement.
For the sand used in this study, the beneficial effects of using vertical reinforcement are more prominent for soils at lower initial densities. The bearing capacity ratio increases as the initial density of sand increases, becomes maximum at a certain optimum density, and decreases thereafter. Further studies are necessary to quantify parameters for actual design conditions.
REFERENCES
Verma, B.P. and A.N.R. Char (1986) “Bearing Capacity Test on Reinforced Sand Subgrades,” Journal of the Geotechnical Engineering Division, ASCE, Vol.112, No. GT7, pp. 701-706.
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