Wick Drains Research Articles

Experiences with prefabricated vertical (wick) drains at Vernon, B.C.

This case record describes the performance of two test embankments constructed over a soft compressible clay, one with prefabricated vertical drains and one without. The measured settlements were significantly different from the predicted settlements, and the pore-water pressures were inconsistent with the observed consolidation. The observed performance of the vertical drains is compared with theoretical predictions to assess the sensitivity of the predicted rate of consolidation to input parameters. Key words : case record, consolidation, field observations, prefabricated vertical drains, wick drains, preloading, settlements, test embankments.

Use of drainage wicks for an embankment on slickensided clay

The 650-ha cooling pond for the Genesee thermal power plant near Edmonton required more than 15 km of earth-containment dyke up to 25 m high to be constructed on a high plastic clay foundation. Other embankment construction activity at the Genesee site indicated that there was very little dissipation of construction pore pressure in the foundation clay and that embankment heights of more than 6 m would require stabilization measures. The high portions of the cooling-pond dyke were built in three stages between 1982 and 1986. An extensive instrumentation monitoring program was established to control the rate of fill placement during construction. Counterbalancing earth berms were used to stabilize most of the high sections of dyke. During the later stages of dyke construction, lack of dissipation of high excess pore pressure in the foundation clay indicated that the sole use of counterbalancing berms might not provide sufficient stabilization. Prefabricated drainage wicks were used as an additional measure to lower the foundation pore pressures under the counterbalancing berms in these critical areas. Spacing of the wick drains was determined by applying Hansbo's wick-drain theory and from data collected from a wick-drain test section. Additional piezometers were installed prior to wick installation to assess the effectiveness of the wicks. Over 14 000 wick drains were installed at spacings of 2.5–2.75 m to a maximum depth of 26 m below the top of the berm. The installation procedures are discussed, along with difficulties of installation. The wicks were very effective in lowering the foundation pore pressures and improving dyke stability. Instrument monitoring data used to assess the wick-drain performance and to control the rate of construction during dyke completion are presented and discussed. Key words: drainage wick, high plastic clay, pore pressure, dissipation, deep, fill, dyke, instrumentation, monitoring.

Postoperative conjunctival chemosis in cataract surgery caused by subconjunctival gentamicin injection.

A prospective study of 75 patients undergoing cataract surgery was conducted to identify the cause of postoperative conjunctival chemosis seen in patients with cataracts in this department. The patients were split in three groups of 25 each. Group G patients were given a subconjunctival injection of gentamicin at the end of the cataract procedure; group W patients did not receive subconjunctival gentamicin, but a cellulose wick drain was used during the procedure; and Group GW patients received a subconjunctival injection of gentamicin and a wick drain was also used. The incidence, severity, and the duration of chemosis were observed in the three groups. Chemosis was seen only in groups G and GW, and the difference between the number of patients who received gentamicin and those who did not receive gentamicin was significant (p less than 0.001).

A259 littlehampton bypass embankment construction with vertical drains : Anandan, J; Piercy, J Highways TranspV38, N2, Feb 1991, P10–15

The bypass was constructed over soft alluvial soils next to the River Arun where undrained shear strengths were too low to permit construction of the six metre high embankment without consolidation. Comparative estimates of using methods of removing pore water showed that the least costly solution was to install Mebra wick drains at close centres of one metre. A total length of 150000m of wick drains were used and the embankment was built as an advanced earthworks contract, to half its full height at a rate of not more than 500mm per week. Pore pressure dissipation was closely monitored by nine piezometers and five magnetic settlement gauges; eight inclinometers were also used to monitor lateral movement of the embankment. The computer programs, SLOPE and ONEDIM were used to calculate slope stability and predict settlement. A granular drainage blanket was first laid on a layer of Terram 1000 geotextile taking care that no heavy loads travelled directly on the fabric. Wick drains were then installed with an average length of seven metres to penetrate the underlying soft chalk to about one metre. A smooth-wheeled vibratory roller was used to compact fill material with tipper lorries only being allowed to run on previously compacted material. It was necessary to raise instrumentation at about two week intervals. (TRRL)

Use of penetration tests to predict wick drain performance in a soft clay : Lutenegger, A J; Kabir, M G; Saye, S R Proc 1st International Symposium on Penetration Testing, ISOPT-1, Orlando, 20–24 March 1988V2, P843–848. Publ Rotterdam: A A Balkema, 1988

Use of penetration tests to predict wick drain performance in a soft clay : Lutenegger, A J; Kabir, M G; Saye, S R Proc 1st International Symposium on Penetration Testing, ISOPT-1, Orlando, 20–24 March 1988V2, P843–848. Publ Rotterdam: A A Balkema, 1988

Prediction of wick drain performance using piezometer cone data

The technique of installing prefabricated wick drains is well established in Europe and Japan, but is relatively new to North America. This paper describes the use of piezometer cone data to predict the performance of wick drains in a soft clayey silt in Burnaby, British Columbia. A description of the testing procedures and the theory to calculate the rate of consolidation are briefly presented. The predicted rate of consolidation using the piezometer cone data and the measured performance is compared and discussed; good agreement between the two was found. Key words: wick drains, piezocone, prediction, preload, settlement.

Geogrid reinforcement for cochrane bridge embankment

Construction of a 6·62 m (27·7 ft) high bridge approach embankment over weak marsh deposits in Mobile, Alabama was accomplished through the use of geogrid reinforcing and wick drains, resulting in estimated savings of US$600 000. Lightweight polypropylene geogrids and a nonwoven polyester geotextile were used in tandem to permit the construction of a 0·61 m (2 ft) sand blanket across the marsh. The sand blanket served as a working platform for the installation of wick drains which were used to reduce the time required for settlement of the embankment to less than two months. High density polyethylene geogrids were employed to reinforce the embankment so that full embankment height could be constructed without incurring the time delays of stage construction. The advantages of geogrid reinforcement and the construction procedure are discussed.

Great yarmouth by-pass - earthworks and pavement design : Boyce, J C Highways TranspV33, N1, Jan 1986, P3–8

The paper outlines the geological conditions, the design concept which evolved from the combination of geological conditions and the stated requirements of the client; construction of a trial embankment to confirm design parameters; and the special measures which were incorporated in the contract to allow construction to be controlled. The design which evolved and which was partially tested in the trial embankment brought together a number of techniques: the use of wick drains, geomesh reinforcing layers and a geomesh enclosed drainage blanket, surcharging of embankments, and, as a final measure to minimise the appearance of differential settlement at the road surface, continuously reinforced concrete road base. The embankments are heavily instrumented with settlement gauges, inclinometers and piezometers. Monitoring of the instruments was considered to be such a vital element in the control of construction that they were directly read onto magnetic tape which was then analysed on computer in the site office. This permitted the recording and analysis of results to be brought within an acceptable time scale and ensured adequate control of the operations on site. The paper includes comments on the performance of the construction to date, although the final assessment must of necessity wait until the road has been open to traffic for several years. (Author/TRRL)

Stability of Steel Oil Storage Tanks

Oil storage tanks built on weak and compressible foundations have suffered catastrophic failures due to foundation instability. These experiences are reviewed and used to establish simple procedures for evaluating factors of safety against base shear failure and edge shear failure. Methods for averaging foundation shear strength values for these two modes of failure are described, which make it possible to apply simple bearing capacity formulas to conditions where the shear strength of the foundation varies with depth, and where the foundation contains or is overlain by layers of sand or gravel. Recommendations concerning minimum values of factor of safety against undrained foundation failure are provided. In cases where the first loading of the foundation is very slow, or where drainage is accelerated by sand drains, wick drains, or natural sand lenses, appreciable drainage and strength gain occurs during water load tests. Methods of rehabilitating tanks after failure of the foundation are reviewed.

Successful highway construction on very soft soils : Vidmar, S; Gaberc, A; Kardelj, E Proc 10th International Conference on Soil Mechanics and Foundation Engineering, Stockholm, 15–19 June 1981, V1, P263–268. Publ Rotterdam: A. A. Balkema, 1981

The Ljubljana-Postojna highway crosses the northern boundary part of the Ljubljana marsh. Due to high compressibility of the upper peaty, silty and clayey layers of the subsoil with total thickness of about 12 M settlements up to 160 cm were expected. In order to reduce the post-construction settlements to tolerable values, vertical wick drains, preloading, light fill material and partially the excavation of superficial peat were used. The applicability of simplified consolidation and stability analyses was checked by observations on a test embankment and the efficacy of polypropylene covers of wick drains by laboratory and field tests. Settlements observed during the construction of the highway and two years afterwards agree satisfactorily with forecast values. (TRRL)

Booming business in wick drains : Morrison, A Civ Engng, NY, V53, N3, March 1982, P47–51

Prefabricated vertical wick drains, used to improve weak, saturated soils, have been used in Japan and Europe for 20 years but were introduced in the U.S. only four years ago. Since then they have replaced sand drains on most soil consolidation projects. Compared to sand drains, the plastic wicks offer quicker and less labor-intensive installation, higher reliability, equally or more rapid drainage, less environmental impact, and lower cost. Differences abound among the five wick drain systems currently available in this country. Some distributors claim the superiority of their products on the basis of such factors as installation method, filter strength and permeability, and discharge capacity. The article discusses these differences and their varying significance depending on site conditions. It also presents a brief history of vertical drainage, photos and diagrams of typical installations, and a list of drain suppliers.

Wick drains employed on malton by-pass : Beyer, M G; Falconer, J S Ground Engng, V11, N1, Jan 1978, P16–18

The purpose of this new type of vertical drainage system is to accelerate the process of consolidation of soft alluvial deposits under embankment loading. An economical method of accelerating the consolidation process is achieved by shortening the distance between the drainage paths by means of highly permeable vertical drains inserted into the ground. The wick drain is a high quality special non-woven polyester fabric which is resistant to all acids and basic chemicals in soils, to bacteria, frost, and UV radiation. Its pore volume in excess of 80% accounts for the drain's high water transmissibility. The wick drain is 300 mm wide and 4 mm thick and is supplied in roles of up to 300 m in length. The route of the bypass crosses a flood plain before rejoining the original truck road. Problems were anticipated with settlement and stability of the embankments. Because of potential flooding, the embankment was formed above the flooding level with rock fill arising from the cutting. To reduce the drainage path within the aluvium and accelerate the consolidation settlement and increase shear strength, vertical drains were installed to an average depth of 5 m below ground level. A specially designed tubular steel lance was vibrated into the ground through which the wick drain was passed through and into the ground. The drain was then cut off approximately 0.5 m above ground level. Instrumentation was provided to monitor the construction of the embankments as well as both the drained and undrained areas of the special embankment.