The characteristics of vacuum consolidation and the advantages of preloading soft clayey deposits by combining vacuum pressure treatment with embankment loading are discussed . Laboratory oedometer test results show that vacuum pressure induces less or about the same settlement compared to an applied surcharge load of the same magnitude. If the applied vacuum pressure is larger than the lateral stress required to maintain a K。condition (no horizontal strain), there will be inward lateral displacement and the vacuum pressure will induce less sttlement. In cases where the bottom of the soft clayey deposit is deposit is drained (e.g. where the clay overlies a sand or gravel layer),application of a vacuum pressure will cause less consolidation than an equivalent surcharge load because than at the bottom drainage boundary effectively the vacuum pressure can not be applied. For this type of subsoil condition it is suggested than if vacuum consolidation is combined with the use of perfabricated vertical drains (PVDs) for ground improvement, the PVDs should not penetrate the entire clayey layer. An equation for calculating the optimum penetration depth has been derived for this case. The advantages of combining vacuum pressure with embankment loading are discussed in terms of increasing the effective surcharge loading,reducing construction time in the case of road construction,and reducing the preloading-induced lateral displacement of the subsoil.
For many engineering constructions on soft some form on ground improvement is often required. Preloading is a common method used to provide ground improvement. The surcharge pressure required for preloading can be either due to the weight of imposed fill material (e.g.embankment) and/or the application of a vacuum pressure to the pore fluid contained within the soil. Consolidating soft clayey deposits by applying a vacuum pressure has several advantages over embankment loading e.g. no fill material is requied,construction periods are generally shorter and there is no need for heavy machinery. In addition,the vacuum perssure method does not put any chemical admixtures into the ground and consequently it is an environmentally friendly ground improvement method. However, vacuum consolidation also has shortcmings,e.g the applied vacuum is limited by atmospheric pressure and it may cause cracks in the surrounding surface area due to consolidation-induced inward lateral displacement of the ground. Therefore, in some cases, the combination of vacuum pressure and embankment loading may provide overall ground improvement.
In this paper, the characteristics of vacuum consolidation are first discussed and illustrated with some laboratory test data, and the advantages of combining vacuum pressure with embankment loading are explained in general terms.
Characteristics of Vacuum Consolidation
Magnitude of vacuum pressure induced settlement. Laboratory oedometer tests under surcharge load and vacuum pressure were conducted and compared to Maruto Multiple Oedometer Apparatus with a sample size of 60 mm in diameter and typicall 20 mm in height. Soil samples used were reconstituted Ariake clay (Ariake clay) and Ariake clay and sand mixture (mixed soil).some of the physical and mechanical properties of the samples are lested in Table 1.
Table 1. Some physical and mechanical properties of the soil samples
\
Under one-way drainage condition and using Ariake clay samples, the comparisons of the settlement versus time curves are given in Figs 1(a) and (b) for 0 and 80 kPa initial effective vertical stress in the samples, respectively (after Chai et al.2005). It can be seen that when the initial vertical effective stress is 0, the vacuum pressure-induced settlement is less than observed under the corresponding surcharge load (Fig.1(a)). For the case where the initial vertical effective stress ia 80 kPa, the setttlements induced by vacuum pressure and surcharge load are almost the same (Fig.1(b)). Generally,where the vacuum pressure can result the same settlement as a corresponding surcharge load under oedometer conditions depends on whether a K。condition (no horizontal strain) can be maintained Under oedometer conditions and with an incermental vacuum pressure loading,if there is any lateral displacement in the sample, there will eventually be no confining stress applied by the oedometer constraining ring, and the only horizontal stress will be due to the vacuum pressure, Therefore, if the vacuum pressure is larger than the stress required to maintain a K。condition, there will be inward lateral displacement and the vacuum pressure will induce less settlement than the surcharge load. Otherwise there will be no lateral deformation and the vacuum pressure will induce the same settlement as an equivalent load. The condition for inward lateral displacement to occur can be given as follows:
Where K。is the at-rest horizntal earth pressure coefficient, is initial (in situ) vertical effective stress, and is the incremental vacuum pressure.
