Improved consolidation times with wick drains or preformed vertical drains (PVD).
Consolidation of deep, soft, clayey soils is routinely undertaken to improve the strength and stiffness of the soil strata. Application of increased vertical stress initially elevates the pore water pressure in the soil and over time this excess PWP dissipates as water flows from the soil and the particles can move into a more closely packed configuration. In deep, low permeability clay soils the consolidation process can take months or years to achieve adequate improvement. Most construction projects require consolidation to be carried out in much shorter timeframes, typically weeks or at most a few months, and, in these circumstances, wick drains or PVD’s can be used.
The major factor influencing the time required for adequate consolidation of deep, low permeability, soils is the length of the flow path. In deep natural clayey soils, that are extensive and continuous on plan, the water flowing from the soil must flow vertically upwards or, where a high permeability layer such as sand or gravel lies beneath the clayey soils, both upwards and downwards. These situations can result in very long flow path lengths, measured in tens of metres, for very deep soil strata.
The most effective way of reducing the time required to achieve the required consolidation is to install wick drains, or PVD’s, vertically down through the soil strata of concern. The drains form a vertical, high permeability, flow path to the top and, if present, under-drainage layer at bottom of the clayey strata. In this way the flow path length through the low permeability soil is reduced to around 50% of the drain spacing. Typically, the drains are installed on a triangular or square grid with a spacing in the range of 2m to 6m centres. A further advantage is that, in some soil deposits the horizontal permeability is much higher than the vertical, and so a horizontal flow path, to intersect the vertical drain, is more effective than simply reducing the length of the flow path.
Vertical drains were first constructed with clean sand, placed into a borehole, to form a sand drain. Subsequent early development of cardboard drains was investigated to address the common difficulty of obtaining suitable sand and to reduce drain installation time and costs. Later development of geofabric wrapped, ribbed plastic, wick or band drains, and high capacity dedicated mandrel installation rigs, has significant improved the performance, potential depth of installation and cost effectiveness of vertical drains.
The CWM team has significant experience in the selection, design, specification and monitoring of consolidation projects incorporating wick drains. This capability enables us to efficiently and effectively help project teams identify and resolve project geotechnical risks associated with ground improvement, construction works and infrastructure developments located over areas of deep, soft clayey material.