Earthquake Drains are geosynthetic drains installed vertically through a cohesionless (sandy) soil deposit to mitigate liquefaction. The drains provide a shortened drainage path for the rapid dissipation of earthquake-generated pore pressures during a seismic event. By maintaining the excess pore pressures below sixty-five percent (65%) of the soil’s effective stress (strength), bearing capacity failures and intolerable differential settlements can be avoided.
Earthquake Drains are typically installed on an equilateral triangle pattern across the desired treatment area. During a seismic event, water flows radially to the nearest drain, keeping the pore pressures at safe levels. The drain spacing must be sufficiently close to ensure this rapid dissipation of earthquake-generated pressures. Typical spacings range from four to six feet (4.0’ – 6.0’) depending on the density of the liquefiable deposit and the magnitude of the design earthquake. This spacing is highly dependent on the permeability of the treated soil and rarely exceeds six feet (6.0’) center-to-center.
Earthquake Drains consist of corrugated, perforated plastic pipes (HDPE) wrapped in a specifically-designed, non-woven filter fabric. They measure roughly four inches (4.0’) in nominal diameter. The flow capacity of an Earthquake Drain is governed by open pipe flow equations (Manning’s Equation). The capacity of a four-inch drain far exceeds the flow rate needed to effectively drain a liquefiable deposit during an earthquake. Because the final design of an Earthquake Drain system is primarily governed by the permeability and compressibility of the treated soil deposit, not the drain capacity, any drain measuring more than four inches in diameter is unnecessarily expensive to obtain, fabricate and install.
For new construction, Earthquake Drains are installed using vibratory methods and specially-purposed equipment. The vibratory installation method densifies loose cohesionless soils during the penetration and withdrawal of the Earthquake Drain mandrel. Seismic Solutions has performed numerous tests in the field using proprietary in-situ vibration monitoring equipment to measure the ground accelerations during installation. It is not uncommon for us to measure accelerations exceeding 2.0g at depth within three feet of the mandrel. Our installation techniques subject the site to accelerations exceeding those of the design earthquake while effectively draining the liquefiable soils with the surrounding recently installed drains.
For existing structures and areas with restricted access and low overhead concerns, Earthquake Drains may be installed using drilling methods. Although these methods do not have the added benefit of densifying the underlying soils, they provide a liquefaction mitigation option in areas where other ground improvement techniques simply cannot be employed. Seismic Solutions has successfully used these methods on multiple projects in and around significant historical structures in Charleston, South Carolina.
During an earthquake, water flows radially out of the soil deposit to the closest drain and then vertically to a natural or man-made reservoir. Because each Earthquake Drain is serving primarily as a pressure relief valve, the volume of water that flows into each drain is relatively small. In most cases, the overlying soil or drain itself serves as a reservoir for flow from the soil deposit. However, there are cases where it is necessary to provide additional reservoir space in the form of a gravel blanket or pipe system. The need
As a side note:
Historically, ground improvement contractors have relied on densification to mitigate liquefaction. The goal was to densify the sand deposit to a point where no further densification could occur during the design earthquake. In some of these cases, these contractors have also suggested that the stone columns or aggregate piers elements served as drainage elements. This simply isn’t the case. Liquefaction mitigation by drainage techniques relies on the dissipation of pore pressures rapidly during a short duration earthquake. Drainage design is governed by the permeability of the treated soil and typical stone column spacing is too great to be effective. In addition, the flow through a stone column or aggregate pier is governed by Darcy’s Law (flow through a porous media). The flow capacity of a three-foot diameter stone column is only a fraction of the flow through an Earthquake Drain. Stone columns and aggregate piers cannot effectively mitigate liquefaction by drainage unless they are installed on the same center-to-center spacings as Earthquake Drains.