Earthquake drains are a ground improvement technique used in granular soils in earthquake-prone regions where there is potential for liquefaction. The ground shaking associated with an earthquake will drive up water pressure in granular soils, causing the soils to lose strength. This can lead to large ground movements and the collapse of structures on these soils.
Earthquakes can lead to soil liquefaction, in which soil takes on water and turns into a thick liquid—like quicksand. This can lead to landslides and major structural damage to bridges, buildings, and dams.
To mitigate liquefaction potential, geotechnical engineers can install earthquake drains to give the water a path to escape during a seismic event, preventing the buildup of water pressure in the soil matrix.
Helitech CCD can install earthquake drains before or after a building’s construction.
Earthquake drains are prefabricated vertical drains made of a perforated pipe wrapped with a geotextile filter fabric; these drains have a high flow capacity that helps water dissipate, reducing excess pore pressure on the soil below.
The earthquake drain’s filter fabric is fine enough to stop loose sand and soil from entering the drain; ground water can still enter the drain during and after an earthquake.
The drains, which range from 3 to 8 inches in diameter, are installed in a grid pattern across the area to be treated. The depth of the drains is dependent upon the depths to which liquefiable soils have been identified, with depths typically ranging from 20 to 50 feet.
Some earthquake engineering projects require thousands of these drains.
When an earthquake occurs, the rock beneath the earth shakes, which moves each layer of soil above it as well. In areas where the ground is made up of granular soils like loose sand, the soil will condense because of gravity.
But if the earthquake zone has a shallow water table, the loose sand cannot condense because the water will fill the spaces in the adjacent soil.
As the amount of water increases, so does the excess pore water pressure; this is referred to as soil liquefaction. The water pressure will eventually equal the strength of the soil, which reduces the shear strength of the soil, pushing it apart.
Liquefaction can turn this silty sand into quicksand, which can have devastating effects on the ground above, affecting its load-bearing capacity and the structural stability of nearby structures.
Earthquake drains are installed to provide a route for water to escape during an earthquake event, preventing water pressure from building up and the soil liquifying.
This can improve the IBC design site class and eliminate the potential for a failure associated with liquefaction of the soils.
In addition to mitigating liquefaction hazards, earthquake drains also:
Earthquake drains can be used to prevent damage to many major structures, including:
Existing structures can be retrofitted with earthquake drains as well.
To install an earthquake drain, engineers drive a vibro probe, also known as an insertion mandrel, made of steel pipe into the ground. The mandrel has a steel anchor plate at the bottom of it. The drain is placed into the mandrel and driven down to the correct depth, vibrating the soil as it goes.
Once the drain is installed, the steel probe is then pulled back out. The anchor plate stays in the soil so that the earthquake drain has a place to rest.
The mandrel needs to vibrate during installation and extraction to aid in soil densification, which leaves less space for ground water to enter during an earthquake.
There are several ground improvement techniques for geotechnical engineering projects, including vibro replacement, vibro compaction, dynamic compaction, wick drains, wet soil mixing, stone columns.
Geotechnical engineers often combine one or more of these techniques with earthquake drains. Earthquake drains are particularly useful because: