Practical aspects of vertical drainage
In cases where external loading of low-permeability soils, such as clay, gyttja2), decomposed peat etc., causes a stress increase exceeding the pre-consolidation pressure of the soil, excess pore water pressure will be induced, followed by a consolidation process in which pore water is squeezed out of the soil. The volume decrease of the soil caused thereby is accompanied by a gradual increase in effective stress and a corresponding decrease in excess pore water pressure. The consolidation process will continue until the excess pore water pressure has completely dissipated and the load is carried by effective stresses, a process whose duration depends on the consolidation characteristics of the soil and the drainage paths (the longer the drainage paths, the longer the consolidation process). The aim of vertical drain installation is to shorten the drainage paths and the time required for the excess pore water pressure, induced by the loading operation, to dissipate. The time of excess pore water pressure dissipation (the consolidation time) will be shorter the closer the drains are installed.
A.2 Fields of application
As mentioned in A.1 the installation of vertical drains is carried out as a means of speeding up long-term consolidation settlements caused by loading. Another objective is to improve stability conditions by an overall increase in shear strength. In seismic regions vertical drainage can also be used for the purpose of mitigating liquefaction phenomena.
Examples of areas where this technique has generally been applied are:
- embankments for roads and railroads;
- construction and reinforcements of dikes;
- embankments for construction sites of housing estates, industrial estates, terminals etc.;
- preloading for landfills;
- marine constructions and near-shore applications;
- land reclamation, ports and airports.
Vertical drains have also been used as a means of electro-osmotic dewatering. In this case electrodes are inserted into the prefabricated band drains and connected to a voltage gradient  and . The rate of consolidation thus achieved will be influenced by the voltage gradient and the electro-osmotic permeability coefficient.
A growing area of application is in the environmental field, remediation of contaminated ground. Contaminated water squeezed out through the drains may have to be treated before disposal.
The required life span of vertical drains is normally limited to a maximum of about 5 years, with the exception of drains used for liquefaction mitigation where the lifetime needs to be significantly longer.
A.3 Execution of vertical drainage
The functional requirements of the project form the basis for the geotechnical design of vertical drainage. The execution of a vertical drainage system is shown in Figure A.1. It includes the creation of a working platform, the placement of a drainage blanket, positioning of the drain pattern and installation of the drains, followed by the loading operation and monitoring.
Prefabricated drain types have gradually replaced sand drains, which previously were frequently used. The installation of vertical drains may detrimentally affect the original properties of the soil (e.g. decrease the shear strength and coefficient of consolidation). A possible decrease in shear strength has to be taken into account in cases where stability under loading conditions may be threatened. Vertical drainage and preloading are illustrated in Figure A.2. Due to the excess pore water pressure created by loading, pore water is squeezed out of the soil in the horizontal direction towards the drains and thereafter in the vertical direction through the drains. A generally smaller amount of water is also squeezed out of the soil in the vertical direction between the drains (contributory effect of one-dimensional consolidation).
- 1 surcharge load
- 2 drainage blanket
- 3 vertical drains
- 4 clay layer
- 5 pore water flow
Depending upon the installation method and procedure used, the installation of vertical drains may affect the original properties of the soil (e.g. decrease the shear strength and coefficient of consolidation). This should be considered in the design.