20 TR News, November–December 1988

RESEARCH PAYS OFF

Laboratory tests at NYSDOTprove wick drainscheaper and easierto install

Wick Drains Gain Acceptance

One of the major concerns in geotech-nical engineering is the settlement thatoccurs over time when a highway em-bankment, bridge, or building is builton compressible soil. The magnitudeand rate of settlement are of concern forengineering as well as economic rea-sons. If 12 inches of embankment set-tlement occur over a period of 20 yearsat a pile-supported bridge abutment, itis not considered a major problem be-cause no early, frequent asphalticpatching would be required at thebridge approach. However, the reversewould be true if settlement took placewithin a few years after construction.

Several methods of treatment havebeen commonly used to control settle-ment, for example, waiting periods,surcharges, and increasing the rate offoundation settlement. Traditionallysand drains—vertical columns ofsand—have been used to increase therate of foundation settlement in prob-lem areas. With the introduction ofgeotextiles into geotechnical engineer-ing, prefabricated, or wick, drains havebeen developed as a substitute for sanddrains. Wick drains are porous geotex-tiles wrapped around a plastic drainage

core 3 to 4 inches wide and ¼ to 3/8inches thick. The geotextile allows wa-ter to enter the core and flow verticallythrough the core to a drainage blanket.The benefits of using wick drains ratherthan sand drains include ease and speedof installation, lower material cost, andlower overall cost, even though com-paratively more of them are needed.

Problem

In the early 1980s, as more wick drainswere being introduced on the market,there were no standard methods ofevaluating or comparing the effective-ness of various types of drains. Becauseof this, it became necessary for the NewYork State Department of Transporta-tion (NYSDOT) to develop such a testprogram. Following New York's lead,other states are conducting similar in-vestigations.

TR News, November–December 1988 21

Solution

In 1983, the NYSDOT Soil MechanicsBureau undertook an in-depth researchprogram to determine the effectivenessof wick drains. The program consistedof three different tests—a large diame-ter consolidation test, a crimp test, anda lateral pressure test—developed in theBureau's Soil Mechanics Laboratory.Following completion of the laboratorytest program, the Bureau completed anevaluation of the theoretical effect offriction on the flow capacity of thewick-drain cores.

The results of this testing programwere used in New York State to designwick drains on three different projectswith different soil conditions. Two ofthe projects were built, with consider-able savings over conventional alterna-tives. The third project was not built butwould have also resulted in consider-able savings.

The first phase of the testing programconsisted of performing consolidationtests in the laboratory using remoldedorganic silty clay from the proposedWestway Project in New York City.

The results of this testing programrevealed areas that needed further in-vestigation.

1. When the consolidation testingsystem was dismantled, one or morecrimps or bends were found in thestiffer wick drains. The crimps werecaused by consolidation of the soilsample. A program was developed toinvestigate the effects of crimps on theperformance of the wick drains underlaboratory and field conditions. Theinvestigation concluded that crimpingalone did not affect the performance ofthe wick drains.

2. Some of the wick drains have thickgeotextile core coverings and nonrigidcores. The test program showed thatlateral pressure alone seriously affectedthe performance of the nonrigid-corewick drains.

3. The effect of different soil types onthe performance of the wick drains wasalso investigated. The types of soiltested appeared to not significantly af-fect the rate at which the soil is drained.

Application

The two projects that were constructedusing wick drains were located in cen-tral New York State, one near Utica andthe other near Syracuse. The soil profilestudies in these two areas had indicatedsignificant thicknesses of water-saturated compressible soils. Given thehigh cost of conventional foundationtreatments, such as light weight fill,excavation and back fill, berms, orslope flattening, the use of wick drainswas investigated.

On the basis of the results of thelaboratory test program, the criteriaused for accepting wick drains werethat their net core flow capacity mustexceed the volumetric flow rate of wa-ter from the soil and that the wick drainmust drain the soil at the same rate as a38-mm-diameter sand drain at the samespacing. The contractor chose to use awick drain that showed an equivalentsand drain diameter of 50 mm and a netcore flow capacity of 18 cm3/s.

Benefit

An investment of about $25,000 in timeand equipment in the laboratory testprogram resulted in a specificationcontaining an approved list of wickdrains based on properties in which theNYSDOT had confidence. One conse-quence of the test program was the re-jection of nonrigid-core wick drains.

The savings realized in the Utica andSyracuse projects by using wick drainsas the drainage medium were approxi-mately $400,000. In contrast, conven-tional sand drains would effect savingsof only $100,000. Besides the dollarsavings, however, the use of wickdrains enabled construction of thesetwo projects within their establishedtime schedules.

Suggestions for "Research Pays Off ar-ticles are welcome.

Contact David K. Witheford, Trans-portation Research Board, 2101 Con-stitution Avenue, N.W., Washington,D.C. 20418 (telephone 202-334-2972).