About Vacuum Consolidation
Vacuum consolidation method is a technique of applying vacuum suction to an isolated soil mass to reduce the atmospheric pressure in it, thus by the way of reducing the pore water pressure in the soil the effective stress is increased without changing the total stress.
The technology of vacuum consolidation provides an effective alternative solution to the conventional technique of vertical drains with fill preload or surcharge for the improvement of soft saturated soils (clays). This alternative solution proves to be efficient, enable a shorter consolidation period and enhance stability against lateral deformation and rotational circular failure
Comparison to Conventional Technique of Vertical Drains and Preloading & Surcharge
Stage construction of preload fill or surcharge fill is generally often required due to low initial strength of soft saturated soils (clays) even with vertical drain installation. Due to the repeated cost of handling large quantities of fill materials and the need for eventual disposal, often, only a
portion of the site is preloaded at one time using a limited amount of preload fill. Once consolidation of one phase is completed, the preload fill is moved to another location for the next phase of preloading or surcharging.
General Description of Vacuum Consolidation
The basic procedure of this technology consists of installing an airtight impervious membrane over the soft saturated clay deposit to be consolidated. Vacuum pressure is then created below the membrane using a dual venturi and well pumps system. The atmospheric pressure of 1 bar (100 kN/m2) is applied. A typical vacuum consolidation scheme is as shown in figure 1. For practical purpose, due to leakage and pressure lost,a conservative figure of 70%-80% efficiency is normally adopted for design purpose. Past records have indicated an average efficiency of 80%-90%. For design, the imposed pressure is then about 70-80kN/m2, which is equivalent to about 4 m of fill surcharge. Hence, instead of the 4m-fill surcharge we have a “hidden” 80kN/m2 equivalent surcharge effect below the membrane.
When Vacuum best to use?
- replace all fill preloading techniques (surcharging) which brings the risks of circular slip failure and excessive lateral deformation,
- Combine with water as additional preload when the source of water is nearby (e.g. coastal treatment area). The impermeable membrane will contain a pool of water which will act as a load; and
- When preload is used and the imposed load is exceeding the bearing capacity and run the danger of a slip failure, multiple stage construction is required. However, with the effect of vacuum consolidation it will maintain stability and a faster rate of construction with minimum stage construction is made possible.