Study Of Replacement Combining With Reinforcement To Treat Soft Ground Under Widened Subgrade

In recent years, high-grade highway is growing with unexampled speed. Some of them, which were constructed in early days, have been or are to be widened because of too much traffic. These widened or need to be widened roads mostly lie on the area with much soft ground in flourished regions along the rivers or coast. How to control the different settlement between the widened and the old part of the subgrade is a difficult problem, but it has to be solved.When the thickness of the soft ground is small, placing a geosynthetics under the subgrade will solve the problem noted above very well. If the thickness of soft ground exceeds a critical value, this measure will not get an expected result. In the latter situation, applying replacement combined with reinforcement may get a good effect, which is to be proved an economical, practical, effective and safe method. The effect and the mechanism of such method are focused, in order to provide the base of practical plan.The FEM is used to analyzed the system of widened subgrade with different improving methods: (1)no improvement;(2)with reinforcement but without replacement;(3)with both reinforcement and replacement. For every system mentioned here, the effecting rules to the different settlement between the widened and the old part of the subgrade by such factors as the thickness of the soft ground, the depth of the replacement, the height of the subgrade and the modulus of the geosynthetics are studied. The following is the conclusions.(1) If the thickness of the soft layer and the height of the subgrade are both bigger, the effect can hardly obtained to reduce the different settlement only with reinforcement. But this different settlement may be under control by reinforcing and replacing a certain depth of the soft soil with sand and gravel. The larger the replacement depth is, the smaller the different settlement is.(2) When any other subgrade parameters are given, the larger the modulus of geogrid is, the better the function of decreasing lateral deformation is, and thus thesmaller the different settlement is. The detail of this conclusion proposed in this paper may guide to choose a geogrid with proper modulus to achieve an effective and economical purpose.(3) When the different settlement is given, the modulus of geogrid and the depth of replacement may be matched best if the thickness of the soft ground, the height and the width of the subgrade are certain. Generally, if a modulus of geogrid or a replacement depth of the soft soil is assumed, then a matching replacement depth of the soft soil or a matching modulus of geogrid can be determined by the method described in this paper.