A shaking table model test was carried out to develop an understanding of the performance improvement of saturated silty soil ground using stone column composite foundation as reinforcement. It is found that at less than 0.161 g loading acceleration, soil between piles has not yet been liquefied, the response acceleration scarcely enlarges, and the shear displacement almost does not appear in silty soil. At 0.252 g loading acceleration, as a result of liquefaction of soil between piles, the response acceleration increases rapidly and reaches its peak, and the shear displacement of silty soil increases significantly. At 0.325 g loading acceleration, the integral rigidity of foundation decreases greatly, which reduces its capability of vibration transmission and result in the response acceleration amplification coefficient is less than that at the former loading acceleration, but the shear displacement of silty soil further increases. The stone column composite foundation can greatly reduce both the shear displacement and the settlement of ground compared with untreated foundation. Under the condition of 7-degree seismic fortification, the design meets seismic resistance requirements.
Beijing-Shanghai high speed railway is the first line of over 300 km/h in China, which is built on large acreage of silty soil ground that is mainly located in Haihe river basin, alluvial, and deposit zone of Yellow River, Yellow River old channel, Yangtze River deposit zone, and alluvial and deposit zone of other rivers. These areas belong to earthquake zones of 7, 8, and 9 seismic intensities, and the saturated silty soil ground is of high liquefaction potential under seismic load.
Construction of embankments on silty soil is a very challenging task due to possible bearing failure, excessive settlement, and local and global instability under dynamic load [
Diagram of stone column composite foundation.
It has been observed that many analytical or numerical studies have been carried out to study the effect of unreinforced and geogrid-reinforced granular bed on settlement and bearing capacity of stone column-improved soft soil. Very limited experimental investigations have been conducted on this topic, especially for liquefied soil foundation reinforced with stone column composite foundation.
In recent years, different forms of filling soil devices, such as rigid sand box [
The ground of line between DK719 + 525 and DK720 + 057 is typical saturated silty soil of Beijing-Shanghai high speed railway, which is liquefied soil under 7-degree seismic fortification. So the line is considered to be the prototype of shaking table model test. Along the line, the average height of silty soil layer is about 8 m, and the subjacent bed is stiff-plastic clay layer. Stone column composite foundation is used to improve the liquefied soil ground, and its design parameters are as follows: pile diameter is 0.5 m, pile spacing is 1.2 m, pile length is 9.5 m, and pile depth penetrating into stiff-plastic clay layer is 1.5 m. Height of gravel cushion is 0.6 m, middle of which lays a layer of geogrid with tensile strength no less than 50 kN/m. Table
Physical and mechanical parameters of the in situ soil.
Soil type | Natural water content/% | Natural bulk density/kN·m−3 | Relative density | Natural porosity ratio | Cohesive strength/kPa | Internal friction angle/° | Clay particle content/% |
---|---|---|---|---|---|---|---|
Silty | 30.4 | 19.1 | 2.7 | 0.9 | 7.0 | 34.8 | 6.9 |
Clay | 27.2 | 19.8 | 2.7 | 0.8 | 27 | 14 | — |
A large-scale laminar shear box with inner size of 4.0 m × 1.5 m × 2.5 m is designed, as shown in Figure
Specific technical characteristic parameters of shaking table.
Size of shaking table board/m2 | Range of frequency/Hz | Maximum payload capacity/T | Maximum displacement/mm | Maximum acceleration/g | Wave form | Driving way |
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5 × 5 | 0.1–120 | 20 | Horizontal ±40 | Horizontal 1.0 | Regular wave | Electrohydraulic servo |
Vertical ±30 | Vertical 0.7 | Seismic wave |
Design of large-scale laminar shear box.
Considering the limit of laminar shear box space and payload capacity of shaking table, geometrical proportion of 1 : 10 is first fixed. Since gravitational acceleration simulation must be considered in shaking table test, similarity coefficient of mass density fixed for the test is 1.0. Due to large reduction of boundary effect by using laminar shear box, similarity coefficients of damping and Poisson ratio are also fixed as 1.0. According to Bockingham
Similitude coefficients of the shaking table model test.
Physical quantity | Similarity coefficient |
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Geometry |
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Mass density |
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Dynamic elastic modulus |
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Dynamic Poisson ratio |
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Frequency |
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Damping coefficient |
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Subgrade deadweight |
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Effective overlying stress |
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Gravitational acceleration |
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Input acceleration |
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Dynamic response stress |
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Dynamic response angular displacement |
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Dynamic response linear displacement |
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Dynamic response strain |
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Dynamic response acceleration |
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Excess pore pressure |
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Model foundation soil is acquired from the near prototype section, which dries in the air under the natural state. Before model subgrade filling, the water content of silty and clay is tested, which are converted into mass density of the filling soil. Model foundation is evenly compacted according to the standard of the weight of per 5 cm height to control its density and saturated 48 hours by discharging water from the model bottom after filling finished.
Model pile diameter is 50 mm, model pile spacing is 120 mm, and model pile length is 950 mm, which means that the depth of model pile penetrating into stiff-plastic clay layer is 150 mm. The height of model net cushion is 60 mm, which is filled with coarse sand. In the middle of the cushion, phosphor-bronze belt net is laid to simulate geogrid and its width and numbers of per 1 m width are fixed according to the tensile strength and deformation property of the geogrid, which are deduced from prototype in accordance with similarity coefficients.
Graded gravel, of which grain size is less than 20 mm, is used for subgrade filling and compacted by layers. Phosphor-bronze belt nets lay in the two sides of subgrade slope to simulate the reinforced geogrid. Cast iron shot evenly spreads on the subgrade surface to simulate the track dead load, which is derived from similarity coefficient. Figure
Model and arrangement of instruments (unit: mm).
Generally, natural frequency of foundation soil is 1 to 2 Hz, so it takes 1 Hz as loading frequency in the test. Due to the limit of input acceleration of shaking table, loading frequency needs to increase appropriately under high loading acceleration, so it turns into 2 Hz when loading acceleration is more than 0.161 g. In addition the maximum loading acceleration is higher than 7-degree seismic fortification value (peaking acceleration is 1.5 g). The input wave form of the test is sine wave with load direction along the cross-section of embankment. It is loaded step by step from low to high, and the next step loading is carried on after the excess pore water pressure caused by the previous step loading is dissipated. The load time lasts 10 seconds and the collection time is 100 seconds. Table
Input acceleration of the model test.
Loading frequency | 1 Hz | 2 Hz | |||
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Loading order | 1 | 2 | 3 | 4 | 5 |
Loading acceleration/g | 0.030 | 0.097 | 0.161 | 0.252 | 0.325 |
At less than 0.161 g loading acceleration, there is no visible settlement on subgrade. While at 0.252 g loading acceleration, there is obvious uplift on both sides of ground surface, and the subgrade is damaged at 0.325 g loading acceleration. The horizontal shear displacement of ground is not obvious when loading acceleration is less and gradually increases with the increase of loading acceleration. At 0.252 g loading acceleration, sandboils and waterspouts begin to appear on ground surface. At 0.325 g loading acceleration, the water of ground surface further increases. Dredging the upper embankment after test finished, the deformation of ground surface is midst concave and both sides convex along the cross-sectional direction of subgrade. During excavation foundation, there is no obvious dislocation pile.
Figure
The relation between excess pore water pressure and loading acceleration.
As shown in Figure
Compared with untreated foundation model test, excess pore water pressure of stone column composite foundation is less than that of untreated foundation model at the same loading acceleration. It shows that stone column composite foundation can effectively restrain the increase of excess pore water pressure to improve the antiliquefaction ability of ground.
When loading acceleration is less than 0.161 g, the foundation is not liquefied and response acceleration amplitude is almost unchanged at each measuring point, so the average amplitude of response wave on the time-history curve is taken as response acceleration amplitude. While when loading acceleration is 0.252 g and 0.325 g, the response acceleration amplitude gradually enlarges due to liquefaction of soil between piles, and relatively decreases with continuous load after reaching its maximum value, so the maximum amplitude of response wave on the time-history curve is taken as response acceleration amplitude. Although they are not the values of response acceleration at the same time, enough to reflect the largest destructive power at each point of subgrade during load. Figure
Values of response acceleration amplitude.
Using acceleration record of each model measuring points under all input loads, the contour of response acceleration amplification coefficient relative to table board acceleration is drawn, as shown in Figures The response acceleration of subgrade increases with the increase of the distance away from the table board during the course of loading acceleration. At less than 0.161 g loading acceleration, the response acceleration of subgrade is close to the corresponding input acceleration of table board, and its amplification coefficient is about 1.0. It illustrates that the saturated silty soil layer has not yet been liquefied and the integral rigidity of the foundation has almost no change. When loading acceleration increases to 0.252 g, the response acceleration amplification coefficient increases obviously and reaches its peak, the change of which ranges from 1.259 to 3.0. The main reason is that the saturated silty soil between piles is liquefied, which leads to response acceleration amplification coefficient to increase rapidly. At 0.325 g loading acceleration, the response acceleration amplification coefficient reduces relatively and ranges from 1.121 to 1.849. Due to the stress of pile and soil distributed after soil between piles liquefaction, most stress is shared by piles, the integral rigidity of foundation decreases greatly, although the amplitude of response acceleration is still larger, the capacity of vibration transmission for the whole foundation is reduced, and result in the response acceleration amplification coefficient is less than that at the former loading acceleration. The response acceleration amplification coefficient of clay layer is close to 1.0 at less than 0.161 g loading acceleration, and it is slightly more than 1.0 at 0.252 g and 0.325 g loading accelerations. It illustrates that during the course of loading acceleration, amplification effect of response acceleration is obvious in silty soil, but not in clay layer.
At 0.030 g loading acceleration.
At 0.097 g loading acceleration.
At 0.161 g loading acceleration.
At 0.252 g loading acceleration.
At 0.325 g loading acceleration.
Figure
The vertical distribution of foundation shear displacement.
The shear displacement of different measuring points increases with the increase of input load and the distance away from the table board. Compared with untreated foundation, under the same load condition, the shear displacement of stone column composite foundation is less than that of untreated foundation. It illustrates that stone column composite foundation can effectively improve the resistance ability of shear deformation of foundation and enhance the whole earthquake resistance ability.
Figure
The accumulated settlement distribution of ground surface along subgrade transverse section direction.
Compared with untreated foundation model, the settlement of stone column model is very small, and the settlement of ground surface and subgrade surface is relatively uniform. The results indicate that the settlement of foundation and subgrade can be greatly reduced by stone column composite foundation.
The response acceleration amplification coefficient of subgrade is about 1.0 at less than 0.161 g loading acceleration, while increases rapidly at 0.252 g loading acceleration as a result of silty soil between piles liquefied. Due to the integral rigidity of foundation great decreasing, the response acceleration amplification coefficient at 0.325 g loading acceleration is less than that at the former loading acceleration. The response acceleration of subgrade increases with the increasing of the distance away from the table board, and it is larger in silty soil, but not in clay layer. At less than 0.161 g loading acceleration, there is almost no shear displacement, while at 0.252 g and 0.325 g loading acceleration, the shear displacement significantly increases in silty soil. Under the same loading acceleration condition, the shear displacement of stone column composite foundation is less than that of untreated foundation. The accumulated settlement and nonuniform settlement of ground can be greatly reduced by stone column composite foundation. The test results show that the design meets seismic resistant requirements of Beijing-Shanghai high speed railway under the condition of 7-degree seismic fortifications.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research is supported by the National Basic Research Program of China (no. 2013CB036405), the Key Research Program of the Chinese Academy of Sciences (no. KZZD-EW-05), and the Natural Science Foundation of China (nos. 51209201 and 51279198). These financial supports are gratefully acknowledged.