Study Of Combined Vacuum-Surcharge Preloading Mechanism For Ground Treatment

Preloading is a popular soil improvement technique for soft soils sometimes named pre-compression. The most common and oldest form of preloading used practically by heaping fill materials of a sand fill inducing mechanical surcharging. Recently, one of the common solutions for soft soil improvement is the use of vacuum preloading as a new method technique.Vacuum preloading is another form of preloading, does not involve a fill material since the load applied to the soil surface is in the form of a vacuum through a sealed membrane system. The vacuum causes the pore-water drain out, and creates negative pore-water pressures in the soil. Vacuum preloading is a technique used to improve the strength of soft clayey soils.In vacuum preloading method, beside of increasing the effective stresses in the soil mass by increasing the total stresses, vacuum preloading relies on increasing the effective stresses by decreasing the pore-water pressure. Thus, vacuum preloading combined with surcharging can shorten the consolidation period, especially the long-term consolidation considerably without endangering the stability of the test embankment. Preloading using a fill surcharge was not feasible as it is difficult to build a fill embankment several meters high on soft soil. Sand drains and more recently prefabricated vertical drains, PVDs are used with vacuum loading to distribute the vacuum and dissipate the pore-water pressure.Predictions of secondary consolidation are important for the maintenance of the earth structures such as highway embankments constructed on soft grounds because they lead to differential settlement. There are many publications on secondary consolidation under one, two, and three- dimensional conditions, but all those studies did not conduct creep consolidation under conditions of vacuum preloading method.This study is concerning on the behaviour of creep and microstructure of fined-grained soils subjected to consolidation by using the vacuum preloading technique. A new triaxial consolidation apparatus has been designed, manufactured, and conducted for experimental investigation of the time-dependent behaviour of a soft clayey soil samples collected from Wen Zhou, Zhe Jiang Province - China.This study is considered a new analytical solution of the vertical consolidation due to the vertical flow of water referring to Terzaghi's theory of vertical consolidation, and reviewing the Barron's theory of horizontal consolidation due to the radial flow of water with changing the boundary and initial conditions to accommodate the vacuum, surcharge, and combined vacuum-surcharge preloading methods. The solution is confirmed the validity of the triaxial apparatus to be used in designing vacuum preloading projects prevails the same three-dimensional conditions.This study is also considered the Theological behaviour of bodies possessing rheological properties, namely elasticity, plasticity, and viscousity. When clay is subjected to a constant load, within time interval, the clay will be deformed with viscous behaviour. The magnitude of the time-dependent deformation differs of course according to the properties of the clay and also according to the preloading mechanism. The time-dependent deformation of a real body depends on all the properties of elasticity, plasticity, and viscousity. Just as the properties may appear in diverse combination, description of the deformation of real body often becomes very difficult to deal with. Hence, deformation of real bodies is, generally described by models considering simplified characteristics or the characteristics of ideal bodies. This kind of models is known as rheological models.There is mainly phenomena related to the viscousity of soils considered in this study, namely creep. The development of deformation in the course of time under a constant load is called creep. In soils, two types of time-dependent phenomena can be distinguished. One is due to the dissipation of excess pore-water pressure. The other is caused by the inherent viscousity of the skeleton of soils, such as c...