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Wick drain ground improvements

In the first of two parts, the authors explore the background of construction of a large mine waste stockpile at the Rainy River Mine in Ontario, Canada.

Features | August 1, 2022 | By: Michael Etezad, Ben Riddell, Ken Bocking, Dennis Becker, Travis Pastachak, Jason Bell, Erik Wimmelbacher and Steve Boileau

Editor’s note: This is part 1 of a two-part article. Part 2 will appear in the October/November issue of Geosynthetics.

FIGURE 1 Wick drain installation

Construction of a large mine waste stockpile was required at the Rainy River Mine in Ontario, Canada. The mine rock stockpile foundation has up to 131 feet (40 m) thick of low to high plastic, slickensided and lightly overconsolidated clay deposits. The stockpile required foundation ground improvement to support the planned ultimate height of more than 131 feet (40 m), to be raised over six years. Prefabricated vertical drains (wick drains) are used to accelerate the dissipation of excess pore water pressure due to stockpile loading, resulting in an increased rate of strength gain in the foundation clay. About 8,530,183 linear feet (2,600,000 linear meters) of wick drains over the area of 8,309,738 square feet (772,000 m2) were installed over the course of two field campaigns in 2019 and 2020. Wick drains were installed beneath the stockpile perimeter slopes and crest, over a width varying from 453 to 661 feet (138 to 203 m). The outer boundary of the wick drain ground improvement area was maintained at 89 feet (27 m) inside the toe of the stockpile. Overburden waste is placed at the interior of the stockpile and rockfill is placed at the exterior, buttressing the interior overburden fill. Only rockfill is placed above the wick drain zones.

Foundation conditions

The soil stratigraphy generally consists of (in order from ground surface to bedrock):

• A thin organic surficial layer

• Upper glaciolacustrine deposit

• Clay till

• Lower glaciolacustrine deposit

• Granular till

• Bedrock

The foundation conditions have been characterized based on geotechnical investigations, including boreholes, cone penetration tests (CPTs), bedrock probe holes and electrical resistivity imaging (ERI) geophysics survey. At selected locations, sonic drilling was carried out to obtain continuous cores to log the overburden units, collect samples, determine the thicknesses of the deposits, and check for the existence of pre-sheared (slickensided) layers. At other locations, mud rotary drilling was carried out to collect high-quality samples using a thin-wall piston tube sampler and to carry out field vane shear tests. An ERI geophysics survey was carried out to profile the depth to bedrock. The undrained shear strength was measured using an electrical shear vane. Index testing and a program of advanced laboratory testing were carried out to measure the foundation clay deposits index parameters and the total and effective stress shear strengths, and to determine the consolidation parameters.

The advanced laboratory tests carried out consist of consolidated isotropic undrained (CIU) triaxial compression, consolidated anisotropic undrained (CAU) triaxial compression, consolidated constant volume direct simple shear (DSS), oedometer, constant rate strain (CRS), direct shear and ring shear tests.

The three cohesive soil units (i.e., the upper and lower glaciolacustrine units and the clay till) control foundation geotechnical stability of the stockpile. 

Table 1 Geotechnical design parameters

Wick drain design

Geotechnical design parameters were selected based on a review of the results from field investigation and laboratory testing, and test fill observations. Selected design parameters are shown in Table 1. The overconsolidation ratio is generally more than 10 at the ground surface and reduces to 2 at 49 to 66 feet (15 to 20 m) below the ground surface.

The clay layers normally consolidated vertical coefficient of consolidation is 23 square feet/year (2 m2/year) and horizontal coefficient of consolidation is 108 square feet/year (10 m2/year). A triangular wick drain installation pattern was used in the design. Both radial drainage and natural (vertical) drainage were considered in the analyses. The spacing of wick drains was varied until the required rate of consolidation for the duration of stockpile construction was obtained. The required rate of consolidation at year six of the stockpile construction (end of construction) was 97%, resulting in 7-feet (2-m) wick drains spacing. A test fill was constructed to assess the performance of wick drains (Etezad et al. 2019). The stockpiles were designed to meet or exceed the required factor of safety of 1.3 for static undrained, 1.1 for slickensided and 1.0 for seismic conditions. The stockpile stability was checked for each year of the construction. The maximum calculated settlement is about 7 feet (2.2 m).

Construction

The construction was comprised of earthworks to prepare a graded foundation ground surface, placement of a geogrid, with crushed granular material, followed by the installation of the wick drains. The drainage blanket provided a drainage path for water released from the wick drains at the ground surface. It also provided a stable platform for the operation of the wick drain installation equipment. Monitoring instruments were installed to monitor the performance of wick drains during the stockpile operation.

FIGURE 2 Executing the cut and fill grading plan

Earthworks construction

The first step of wick drain construction was the stripping of organics and surficial debris to expose the competent subgrade.

Subgrade preparation included cut and fill grading and compaction of the subgrade to develop a uniformly graded ground surface (Figure 2). The earthworks were designed to provide a ground surface that was suitably graded for wick drain installation while at the same time reducing the required amounts of cut and fill.  

A layer of either geosynthetic geogrid and geotextile composite or reinforced woven geotextile material was placed over the prepared ground, mainly to increase the foundation strength to support the wick drain installation rigs (Figure 3). For this project, due to the high cost of the drainage blanket material, the use of geosynthetic reinforcement was more cost efficient than simply increasing the thickness of the crushed granular drainage blanket. Due to the large quantities required, both Titan TE-BXC30 biaxial geogrid composite and Mirafi HP370 reinforced woven geotextile were used at different locations. The first noted geosynthetic is made of biaxial polypropylene geogrid heat bonded to nonwoven polyester geotextile. 

A minimum 1 foot (0.30 m) thick granular drainage blanket was installed over the geosynthetic reinforcement layer. The drainage blanket primarily consists of sand and gravel-sized particles and has less than 5% fines. Loading, hauling, spreading and nominal compaction of drainage blanket material was carried out and the final surface of the drainage blanket was checked to be consistent with the required design grades.

Some construction activities were carried out during the winter. These activities included foundation soil cut, geosynthetic placement and drainage blanket placement (Figure 4). During the winter, the earthworks design was altered to use cut only; no fine-grained foundation fill placement was allowed over this period.

FIGURE 3 Geotextile-geogrid installed over approved subgrade

Wick drains installation

Wick drains were installed through the drainage blanket in a triangular pattern with 7-foot (2-m) spacing. The selected spacing was verified by means of a test fill (Etezad et al. 2019). The wick drains were generally pushed through the cohesive soil units to refusal in the lower granular till layer above the bedrock, thus providing double drainage for wick drains.

The installed wick drains were Mebra-Drain MD-88, which consist of a 4-inch (100 mm) wide fishbone profile polymer core wrapped in a nonwoven filter jacket. The installed wick drains were anchored at their base with a steel plate. The wick drains were installed using six hydraulic wick drain rigs mounted to tracked excavators of varying sizes and installation depth capacities. Wick drain installation data were collected through rig-mounted data acquisition systems (Figure 1, above). The Ontario Provincial Standard Specification (OPSS 220 2014) was generally followed for the wick drains installation. Water was used to lubricate the wick drain rig mandrels, as required to penetrate through the cohesive soil units to the required depths. Use of water was not practicable in freezing temperatures.  As a result, the wick drain installation was not carried out during the winter.  

FIGURE 4 Dozer grading drainage blanket surface

Wick drain trials

Prior to utilizing a wick drain rig for production wick installation, the wick drain contractor installed at least 10 trial wick drains for each wick drain rig at designated locations, where geotechnical borehole and/or CPT information was available to verify installation equipment and methods (Figure 5). Results of wick drain trials were reviewed for each wick drain rig and approval was provided for the rig to commence the program. 

Monitoring instruments

Geotechnical instrumentations were installed to monitor the geotechnical performance of the foundation clay within the wick drain area as well as the overall stability of the waste stockpile during operation. The installed instrumentation consists of:

  • Vibrating wire piezometers (VWPs)
  • Settlement plates (SPs)
  • Slope inclinometers (SIs) 

VWPs and inclinometers were in-stalled in boreholes and grouted in place using a Portland cement-water-bentonite grout backfill selected to be appropriate for the surrounding soil stiffness. Inclinometers were anchored a minimum of 10 feet (3 m) into bedrock to provide a stable anchor point.

The VWPs were installed to measure the pore water pressure dissipation following the loading of the stockpile and to monitor the performance of the wick drains. Settlement plates were installed to measure the foundation settlement during and following the construction of the waste stockpile. Slope inclinometers were installed to monitor the foundation clay deformation and the stability of the stockpile. Monitoring threshold levels and the appropriate responses were provided. 

VWPs were installed at several depths at locations centered between the adjacent wick drains. The VWP cables were mainly placed in trenches within the top of the clay foundation or within the drainage blanket and brought to the toe of the stockpile, where they were connected to data loggers. The VWP cables were snaked along the base of the trench allowing for ground settlement that is expected to occur during stockpiling. Inclinometers were installed close to the stockpile toe and incorporated telescoping sections to accommodate anticipated settlements.

FIGURE 5 Wick drain rig completing a wick drain trial

Construction quality control and quality assurance

Construction monitoring consisted of construction quality control (CQC) and construction quality assurance (CQA) programs. CQC refers to measures implemented by the contractors to verify that materials used in construction and their workmanship meet the project requirements. CQA refers to measures implemented by owner or owner’s representative to ensure the contractor’s conformity to the CQA plan, the construction drawings, the technical specifications and any field changes implemented during construction. CQA includes reviewing the CQC plan, monitoring construction activities, verifying the suitability of construction material properties both visually and through laboratory testing, documenting on-site and off-site testing procedures and results, and ensuring quality workmanship.

Earthworks visual inspections were routinely performed to verify compliance with the construction drawings, the technical specifications and design changes implemented in the field. Earthworks inspections focused on confirming the use of suitable equipment, materials, lift thickness, compaction effort, and geosynthetic material coverage, overlap and quality. Additionally, inspections of the stripped subgrade, the prepared subgrade and the completed drainage blanket surfaces were carried out. Laboratory testing was carried out on representative samples of geosynthetic and drainage blanket materials used for construction. 

CQC data were provided by the wick drain contractor through rig-mounted data acquisition systems. The data systems recorded information on the rig ID, wick ID, installation location, depth, inclination, heading, penetration rate with depth and crowd force with depth. 

Wick drain installation data were reviewed as a CQA measure to confirm that wick drain installation was consistent with the intent of the construction drawings and technical specifications requirements and changes implemented in the field, as required and appropriate. The review included verifying the 7-feet (2-m) wick spacing, refusal depth, missing installations, wick drain inclination and crowd force data. Data were analyzed using CAD Civil 3D software. CQA visual inspections of the wick drain installation was performed daily to confirm the use of suitable wick drain material, wick drain rig inclination and wick drain installation depth. Post-installation CQA inspections were completed, including final walk-through inspections and spot-checks of potential non-conformances identified following the review of the CQC data (such as wick drain spacing issues or missing installations). 

Daily CQA reports were prepared summarizing the construction activities, weather conditions, identified potential issues and non-conformances. Photographs were taken daily to document construction activity performed by the earthworks and wick drain contractors.

Conclusions

A literature review of wick drain ground improvement papers indicates that most of the documents are related to the theoretical aspects of wick drain design, with little to no consideration of aspects associated with field conditions. A practical document summarizing construction considerations is not readily available.  

Part 1 of this paper provides details on the construction of a very large wick drain project carried out in North America. Special attention has been provided on the construction considerations of the wick drain installation and construction quality control and construction quality assurance to ensure that the wick drains performance would be in accordance with the design. Instrumentation monitoring was also carried out to check the performance of the wick drains during and following the stockpile construction.

References

Etezad, M., Riddell, B., Bocking, K., Becker, D, Hamdani, A., Buchanan, P., and DaSilva, F. (2019). “Interpretation and analysis of a test embankment in soft clay.” The 72nd Canadian Geotechnical Conf., St. John’s, Canada.

Leps, T. M. (1970). “Review of the shearing strength of rockfill.” Journal of the Soil Mechanics and Foundations Division, ASCE, 96(4), 1159–1170.

Ontario Provincial Standard Specification (OPSS) 220. (2014). “Construction specification for wick drain installation,” OPSS, Ont., Canada. 

Michael Etezad, P.Eng., is a principal geotechnical engineer at WSP Golder in Mississauga, Ont., Canada.

Ben Riddell, P.Eng., is a lead geotechnical engineer at WSP Golder in Mississauga, Ont., Canada.

Ken Bocking, P.Eng., is a fellow geotechnical engineer at WSP Golder in Mississauga, Ont., Canada.

Dennis Becker, P.Eng., is a fellow geotechnical engineer at WSP Golder in Mississauga, Ont., Canada.

Travis Pastachak, PMP, is capital projects manager at New Gold Rainy River Mine.

Jason Bell is projects technical supervisor at New Gold Rainy River Mine.

Erik Wimmelbacher, P.Eng., was project coordinator at New Gold Rainy River Mine for the work and is now underground engineer.

Steve Boileau, P.Eng., P.Geo., is geotechnical engineer at New Gold Rainy River Mine.

All figures courtesy of the authors.


Project Highlights

Rainy River Mine

OWNER: New Gold Inc.

LOCATION: Ontario, Canada

ENGINEERS: WSP Golder

GEOtextile PRODUCTs: Titan TE-BXC30 biaxial geogrid composite; TenCate Mirafi HP370 reinforced woven geotextile

Wick drains: Mebra-Drain MD-88

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