Willits bypass update: On the use of wick drains; Why they are being used

A sample piece of wick drain similar to the type planned for the north segment of the Highway 101 bypass around Willits. (Linda Williams/TWN)

CalTrans is installing about 55,000 wick drains to a depth of as much as 80 feet in the northern segment of the new Highway 101 bypass around Willits. Wick drains were invented in the 1930s and have been in widespread use for certain types of construction around the world since the 1970s.

CalTrans uses them extensively in construction projects built on heavy clay soils to speed up the time it takes for the road base to settle, according to CalTrans Transportation Engineer Ben Barnes. "They are used quite a bit in the Bay Area. They were used on the Bay Bridge in the eastern approach. There has been no effect on the ground water level where they have been used."

Wick drains are more appropriately named prefabricated vertical drains because they are not real wicks.

Engineers in the 1920s found they could shorten the time it took for the ground to stabilize under a new structure, especially when building on clay soils, by drilling a series of holes and packing the holes with sand. These holes were drilled into the earth spaced every few feet and were connected with French drains. They were then covered with a calculated amount of fill. The weight of the fill squeezed some of the water out of the soil under the fill area and into the adjacent dirt. A close geometrical pattern of sand drains meant water being squeezed from the soil only had to travel a few feet through the dense clay to escape rather than through many yards of clay.

In the 1930s engineers discovered small strips of corrugated cardboard had a similar effect and could be installed more cheaply than sand drains and the "wick drain" was born.

The "wick drain" or PVDs proposed for the Willits project are 80 feet long, four inches wide and 1/8 inches thick. At its heart is a 3.75 inch wide and 1/8 inch thick ribbed plastic sheet with the ribs forming ? inch wide channels running the full 80 feet length of the drain. It is covered with geotechnical fabric sleeve to prevent dirt from clogging the drain channels. Nearly 90 years later wick drains still resemble a long cardboard strip.

The Willits drains will be driven about 80 feet deep into the earth five feet apart, says Barnes. The thin drain sheets are pushed into the soft clay by a long metal wedge or mandrel driven into the soil to the depth needed. The mandrel is removed from the hole leaving the drain in place. The tops of the drain sheets are cut off a few feet above grade and the machine moves over five feet and drives the next one.

After all the drains are installed in an area the drain tops are usually tied into a horizontal drain which acts as a French drain. The horizontal drain will mainly prevent the drains from developing a vacuum, says Barnes. The fill material is then piled on top of the drain grid to the desired depth.

In the north segment of the Willits bypass the maximum fill height is 30 feet. The fill will generate pressure of about 25 pounds per square inch. Engineers calculate, based on soil borings in this area it could take between two and 10 years for the soil to settle and stabilize naturally under the weight of the new fill. Without the drains, water in the middle of the 40 foot wide roadbed would be forced to travel as much as 20 feet when being squeezed under the new fill.

By using the proposed grid of wick drains the settling time can be reduced to six months, according to Barnes. Depending upon soil density and other characteristics engineers can increase the drain spacing or decrease it to achieve the desired settling time.

Some area residents have expressed concerns to area politicians about the amount of water that will be flowing from the drains. "I have seldom seen any water draining out the top of the drains," says Barnes, "not much water at all."

When the soils are well draining this is not a problem, in heavy clay soils this can be problematic. The drains reduce the travel to 2.5 feet. Within the soils at the north end of the bypass, the soil is not a solid mass but is thick layers of heavy clay interspersed with thin layers of sand. The drains help connect the sand layers underneath the new highway and allow the water to move under the pressure of the new fill and be redistributed to an area outside the fill zone.

No drains are proposed in the southern sector because soil borings there indicate the natural settling time ranges from one to nine months, says Barnes.

The one to nine month range is within normal construction parameters and no vertical drains are going to be used in that sector and the ground will be allowed to settle naturally.

Short video showing schematically how wick drains are installed

Video showing a contractor installing a wick drain.