This month’s blog is provided by Stephen W. Nickerson, PE with Earth Tech LLC. Stephen is the Area Manager for the Carolinas and is considered one of the industry’s leaders in earthquake drain installation.
Many people are unaware that the Charleston, SC area is considered one of the highest seismic regions in the world, with a 7.3 magnitude earthquake in August of 1886 causing at least 60 deaths. The shock was felt as far away as Boston and Milwaukee. Currently, South Carolina averages 10-15 earthquakes a year.
When most people think of damage caused by an earthquake, soil liquefaction is not the reason that comes to mind. However, it is one of the most dangerous phenomena caused by an earthquake. Soil liquefaction occurs when a saturated soil substantially loses strength and stiffness in response to earthquake shaking and behaves like a liquid. For example, watch what happens after shaking a bucket filled with water and sand. The sand will be disrupted as the bucket shakes, then re-settle in the bottom without the density it had formed previously. Not only does the soil lose strength, but after the earthquake has ended, the reconstituted soil settles. In some cases, the change in soil depth can be over 1 foot, which is extreme if a building is sitting on top. Soil liquefaction has been responsible for tremendous amounts of damage in historical earthquakes around the world and is also attributable to many deaths. In the 1964 Niigata, Japan and 2011 Christchurch earthquakes several large buildings completely collapsed due to the effects of soil liquefaction.
Earthquake Drains are a common method of mitigating liquefaction in the Lowcountry. They are fabricated using slotted plastic drainpipe tightly wrapped with geotextile filter fabric, allowing free access of pore water into the drain. They are installed by inserting the earthquake drain into a steel pipe attached to a drill rig and using vibration and crowd to advance the pipe down to treatment depth. A sacrificial plate at the bottom of the earthquake drain keeps it from coming back up to the ground surface when extracting the steel pipe. The earthquake drains are installed on a triangular grid pattern underneath the entire building footprint down to the depth of liquefiable soils. During an earthquake, excess pore water pressure buildup in the soil is limited by allowing the water to flow freely into the earthquake drain.
The relatively inexpensive cost of materials makes earthquake drains a very cost-effective solution versus the alternative of deep foundations such as driven piles or bored piles. The installation of earthquake drains allows the use of shallow foundations to support structures on liquefiable soils. Earthquake drains can be used in conjunction with other ground improvement techniques such as vibro stone columns or rigid inclusions when building loads would cause excessive static settlement. No spoils are created, and construction can start immediately after installation.