Prefabricated Vertical Drains and Fill Preloading

Basic function: Prefabricated Vertical Drains (PVDs), a.k.a. wick drains, are used to accelerate the settlement and shear strength gain of saturated, soft foundation soils by reducing the drainage path length. Fill preloading consists of placing temporary fill on top of the embankment to speed settlement in the foundation soils.

Geologic Applicability

Saturated low strength, inorganic clays and silts. PVDs are routinely installed to depths of 100 feet (30.5 meters), and have been installed to more than 200 feet (61 meters) on some projects.

General Description

PVDs are band shaped (rectangular cross-section) products consisting of a geotextile filter material surrounding a plastic core.

Construction Methods

Installation of PVDs requires site preparation, construction of a drainage blanket and/or a working mat, and installation of the drains. Site preparation includes removal of vegetation and surface debris, and obstacles that would impede installation of the PVDs. It may be necessary to construct a working mat to support construction traffic and installation rig loads, which can later serve as the drainage blanket. There are many different ways of installing PVDs, but most methods employ a steel covering mandrel that protects the PVD material as it is installed. All methods employ some form of anchoring system to hold the drain in place while the mandrel is withdrawn following insertion to the desired depth. The mandrel is penetrated into the compressible soils using either static or vibratory force.

Additional Information

Design considerations include drain spacing, flow resistance and installation disturbance. Quality control tests usually relate to the material properties of the drain and the measurement of settlement and pore pressures during consolidation. Factors which affect the unit cost of installing PVDs include: the type, strength and depth of the soil, the specifications and requirements, the size of the project, material cost, and labor cost. The installed costs of PVDs are in the range of $2.50 to $3.25 per meter. Mobilization costs will typically range from $8,000 to $10,000 plus the cost of instrumentation and installation of a drainage blanket.

SHRP2 Applications

Embankment and roadway construction over unstable soils; roadway and embankment widening.

Example Successful Applications

Airport Runway and Taxiway Extension, Moline, IL.

Complementary Technologies

PVDs with a preload are typically not used in conjunction with other technologies.

Alternate Technologies

Deep foundation elements, sand drains, vacuum preloading, stone columns, deep dynamic compaction, grouting, deep soil mixing, excavation and replacement, and lightweight fill.

Key references for this fact sheet: Elias et al. (2006); Massarsch and Fellenius (2005); Rixner et al. (1986). (See the Bibliography note below regarding Massarsch and Fellenius 2005.)

The FHWA has design documents covering the preferred design procedure for this technology: Prefabricated Vertical Drains (Rixner et al. 1986, FHWA-RD-86-168) and Ground Improvement Methods, Volume 1 (Elias et al. 2006, FHWA NHI-06-019).

One Dimensional Model. Current FHWA reference(s): Elias et al. (2006); Rixner et al. (1986). Supporting reference(s): Arulrajah et al. (2004); Bergado and Patawaran (2000); Chu et al. (2002a); Chu et al. (2004); Hansbo (1979); Hansbo (1986); Hansbo (2004); Holtz et al. (1991); Indraratna et al. (2007); Lin et al. (2006); Wang and Jiao (2004); Welsh (1987).

The use of PVDs with a preload is an economical and beneficial solution when the time to reach desired consolidation without vertical drains is too great. The goal of PVD design is to reduce the length of the drainage path in order to accelerate consolidation. Design parameters include the selection of the drain type, drain spacing, drain length, and the amount of preload needed to achieve a specified consolidation within an allotted time. The design method begins with traditional settlement analyses without PVDs to determine the total magnitude and time rate of settlement under final project loads; the use of PVDs is then considered to reduce the time to reach the final settlement through reduction in the length of the drainage path. The conventional method follows the Terzaghi-Barron theory developed for sand drains in soft soils, wherein radial drainage and one-dimensional consolidation are assumed.

PVD spacing can be determined using the Barron-Hansbo relationship that relates the time to achieve a desired average degree of consolidation to drain diameter, drain spacing, and coefficient of consolidation:

The term F_s is a function of d_s, the radius of the smear zone, and accounts for soil disturbance in highly plastic and sensitive soils, where it is usually given the value of 2. If the value of c_h has been accurately determined or is given a highly conservative value, F_s can be ignored. Because of advances in PVD design, the well resistance (F_r) can usually be considered insignificant and can be ignored. Chu et al. (2004) provides guidance for when F_r and F_s should be considered in the design calculations. Methods to assess c_h include lab tests as detailed in Table 6 of Rixner et al. (1986); relationships between the coefficient of consolidation in the vertical direction (c_v), the liquid limit, and c_h are given in Elias et al. (2006).

With the Barron-Hansbo equation, the soil properties (c_h), time (t), and design requirements (Ū_h) are held constant while the drain spacing factor (F(n)) is varied. It is assumed that F(n) = ln(n) − 0.75, where n is the spacing influence factor — the ratio of the equivalent well diameter (d_e) to the well diameter (d_w). The equivalent well diameter is 1.05 times the spacing between each drain for an equilateral triangular spacing and 1.13 times the spacing between drains for a square spacing pattern. The well diameter is the equivalent circular drain size for a PVD and is typically approximated as:

The spacing influence factor (n) has a significant effect on the consolidation rate and should be carefully considered during design. The radius of the smear zone, d_s, is difficult to quantify, as it is affected by the shape and size of the mandrel, the type of soil, and the sensitivity of the soil. Chu et al. (2004) present multiple theories to approximate d_s, ranging from d_s=2d_w to 5–8 times d_w. Lin et al. (2006) suggest approximating d_s as twice the diameter of the mandrel, and propose an equivalent horizontal permeability of surrounding soils (k_e) that incorporates the radius of the mandrel, d_s, d_e, and the horizontal and smear zone permeability (k_h and k_s). Wang and Jiao (2004) present an equation for an equivalent horizontal permeability as a function of radial distance, based on the assumption that soil permeability changes continuously (more representative of actual conditions), typically breaking the permeability into two zones (smear zone and undisturbed). Once consolidation is underway, the Asaoka method can be used to give updated values of c_h based on settlements as they occur, using Figure 8 in Elias et al. (2006).

Increasing the surcharge loading or decreasing the PVD spacing will increase the rate of consolidation. If spacing is kept constant but the surcharge is increased, the time period needed to attain the desired settlement will decrease; however, decreasing PVD spacing is not always a viable method for increasing the rate of consolidation. Arulrajah and Bo (2008) demonstrate through back-analysis that the value of c_h decreases for smaller PVD spacing — spacing drains too closely can be counterproductive, as close spacing increases the ratio of the smear zone to the undisturbed zone, which can decrease the rate of pore water pressure dissipation. If the time available for preloading needs to be shortened, increasing the surcharge is recommended instead.

The PVDs do not provide any reinforcement and serve only to increase the rate of consolidation. Stability failures along the perimeter of the preloaded area must be prevented throughout the filling and consolidation process, which requires that the shear strength of the underlying soft soil be adequate to support the overlying fill slopes. On many projects this is only possible either by controlling the rate of filling so soft-soil strength gain from consolidation can keep up with the rising demand for shear strength as fill height rises, or by construction of stability berms around the fill. Continuous monitoring of the actual rates of pore pressure dissipation and settlement is essential; monitoring techniques include measuring the displacement ratio and reading inclinometers (see the QC/QA Procedures section).

A concern during design is that if the engineer does not account for stress-dependent variations in compressibility and permeability, performance predictions are unlikely to coincide with field drainage processes. Since compressibility and permeability vary in the same direction, the ratio k/m_v (and the coefficient of vertical consolidation, c_v) may remain reasonably constant; more common sources of discrepancy between predictions and actual field performance are non-linearity in stress-strain behavior, large strains, and secondary compression, along with uncertainty in approximating soil parameters rather than oversimplifications in the theory itself.

The FHWA's Prefabricated Vertical Drains document (Rixner et al. 1986), pages 68–71, addresses construction monitoring and is considered the best available QC/QA guidance for PVD and fill preloading in support of the SHRP2 objectives. Quality Control (QC) refers to procedures, measurements, and observations used by the contractor to monitor and control construction quality; Quality Assurance (QA) refers to measurements and observations by the owner or the owner's engineer to provide assurance that the facility has been constructed in accordance with the plans and specifications.

A typical comprehensive QC/QA program for PVD and fill preloading projects varies based on size, complexity, and application of the improved site. Simpler projects not involving stability concerns or strict time constraints only require settlement monitoring and materials and construction inspection; more sensitive projects will utilize additional testing such as inclinometers, piezometers, determination of the Displacement Ratio, and/or surveying. An advanced program for large and high-risk projects should include materials testing, field vane shear tests or other soil-strength methods, piezometers, inclinometers, and, in some cases, field test sections. End result specifications are not usually used for PVD and preload projects, as they would require an extensive soil exploration (Elias et al. 2006).

Settlement magnitudes can be determined using surveying, settlement plates, and piezometers; data from these measurements verifies design settlement predictions and monitors project progress. A large number of case histories have shown a significant discrepancy between the rate of settlement and the rate of pore pressure dissipation — when this occurs, settlement data is typically given precedence. Piezometer readings can be influenced by pore gas, poor readings, collapse of soil structure, secondary compression, and structural viscosity. Settlement and pore water pressure readings should be taken for a period of time after the fill preload has been removed to monitor rebound.

For projects where stability is a concern, inclinometers and piezometers can be installed and measured; Elias et al. (2006) recommends installing inclinometers at the toe of the embankment or in front of retaining walls, with settlement plates and piezometers beyond the crest of the embankment and/or near the centerline. For sites where stability is especially a concern, Elias et al. (2006) provide a method to determine the Displacement Ratio — based on case history experience, a Displacement Ratio of 0.2 generally corresponds to a site factor of safety of 1.3, while higher ratios may indicate an unstable site.

Prior to installation, the quality of the PVD material and conformance to design specifications should be verified, and trial sections should be used to generate the best QC and installation standards. Field vane shear tests work well in assessing shear strength changes in the soil (time-dependent, increasing with time of consolidation); ASTM D2573 can be referred to for guidance.

Review of existing specifications: most specifications reviewed were written by state DOTs, primarily based on the guide specification presented in FHWA's Ground Improvement Methods (Elias et al. 2006). Certain DOT specifications were particularly well written and included sections not present in the FHWA guide specification. Regardless of which PVD specification is used, it is recommended that the Bo et al. (2005), Holtz and Christopher (1987), Rixner et al. (1986), and Hansbo (1986) references be reviewed to ensure that all PVD material and installation requirements are included.

Preferred specification: the Standard Method Approach Specification for Prefabricated Vertical Drains and Fill Preloading is intended as a complete and fair specification, easily adaptable by the user for a specific project. It was generated by combining information from several recent DOT specifications and the Rixner et al. (1986) guideline specification, along with input from the SHRP2 R02 research team and advisory board. This is a fill-in-the-blank template (project-specific values such as durations, dimensions, and quantities are left for the user to specify); the summary below reproduces its organization and technical content rather than its blank fields.

This specification has been created as a basic template to aid state DOTs and other parties in creating complete, project-specific specifications; text in the source document marked as drafting guidance (and blank fields for project-specific values) has been summarized here in descriptive form rather than reproduced as fill-in-the-blank text.

Commentary: the cost of PVDs on a highway project is typically captured in a contract bid item measured by the linear foot (LF), which usually includes incidental costs such as additional excavation and embankment to provide a level working surface or for temporary access routes, instrumentation for monitoring settlement, and augering through stiff upper soil layers (unless a separate pay item is used). Other costs that may be measured and paid for separately include mobilization, the drainage layer (granular material or horizontal strip drains), and surcharge embankment placement and removal.

Definitions: Preload — embankment material placed within the PVD installation area that is part of the final roadway cross-section. Surcharge — additional embankment placed above the design profile grade line, used to accelerate settlement and/or minimize secondary consolidation.

An average production rate for PVD installation ranges from 10,000 to 15,000 LF per day. Cost ranges below are based on data from 2005 through 2010; project-specific characteristics and constraints should be carefully examined, as they can significantly influence actual costs.

Conceptual Cost Estimating Tool: a cost-estimating spreadsheet is provided for producing a preliminary project scoping estimate: www.geotechtools.org/documents/pvdfp_costtool_r1.xlsx. The tool can be based on the typical construction cost item values in the Cost Information product above or on user-specified cost values when known; users should read and understand the Cost Information product before using the estimating tool.

Location: the Quad City Airport, Moline, Illinois. Owner / Contractor / Engineer: not recorded in the source case history. Year constructed: 2006.

Project summary/scope: a 1,000-foot runway extension was constructed to increase capacity. With land-use constraints on one end of the runway, construction required up to 15 feet of fill placement over a bluff. With concerns of post-construction settlement under the added fill, Prefabricated Vertical Drains were selected to accelerate consolidation.

Subsurface conditions: alluvial soils consisting of silty clay overlying fine sand. The upper 3.3 to 6.6 feet of silty clay had N-values ranging from 5 to 10 blows per foot and moisture contents ranging from 22 to 30 percent; the lower portion of the silty clay had N-values ranging from 1 to 4 blows per foot and moisture content ranging from 30 to 45 percent.

A small surcharge of 5.2 feet, along with PVDs, was selected to accelerate consolidation. Horizontal strip drains were used instead of a drainage blanket due to the limited availability and high cost of clean sand. The 12 by 1 inch strip drains connected to four rows of PVDs spaced in a 5-foot triangular pattern. These design parameters were selected to achieve a maximum total settlement of 14 inches within 6 months. Since the taxiway was not as critical as the runway extension, the project was split into two phases. Approximately 252,600 linear feet of PVDs were installed to an average depth of 23 feet, with nearly 10,000 PVDs installed in three weeks.

Alternate technologies considered: construction without PVDs, or excavation and replacement combined with surcharging.

Performance monitoring: settlement plates were used to determine the time rate of consolidation. Actual settlements ranged from 4 to 16 inches in 3 to 4 months.

Cost information: the project was bid on a per-drain basis at $11.50 per drain. Mobilization costs were $10,000. Horizontal strip drains were installed at $1.50 per foot.

Summary source: Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J. G., and Berg, R. R. (2006). Ground Improvement Methods, Federal Highway Administration, NHI-06-019 Vol I and NHI-06-020 Vol II, Washington, D.C.

Date case history prepared: November 2012. Unlike most case histories in this library, the Owner, Contractor, and Engineer fields were all left blank in the source document; every other field — subsurface conditions, design parameters, quantities, monitoring results, and cost data — is populated, making this one of the more complete case histories on file despite that gap.

The references below were identified and used to complete the technology summaries, assessments, and website documents on file for this technique. A reference matrix follows the reference list in the source document, cross-referencing each reference against topics such as design procedure, construction methods, QA/QC, and case history — as with every other technique page in this library, that matrix extracts from the source PDF as visually mangled, unparseable table fragments and is skipped here. Of the 51 references on file, the 21 below are cited by name in this page's own Design Guidance, QC/QA, and Specifications summaries and are listed in full; the remaining references cover additional authors including Asaoka, Atkinson and Eldred, Bergado (with Anderson, Miura, and Balasubramaniam), Calabria, Cedergren, the Canadian Geotechnical Society, Chai (with Shen, Miura, and Bergado; and with Miura alone), Chen and Tan, Chu (2009 solo), Chu (with Varaksin, Klotz, and Menge), Goughnour, Hansbo (1981 solo; and with Jamiolkowski and Kok), Indraratna and Redana, Koerner, Ladd, Lin (with Masud, Karim, Sriwiroj, and Chantawong, 2000), Liu and Chu, McGown and Hughes, Mitchell (with Cooke and Schaeffer), Nogami and Li, Saye, Schaefer et al., Smith and Rollins, Stark et al., Terashi and Juran, USACE, Ye (with Xu and Gao), and Yildiz.

• Arulrajah, A., Nikraz, H., and M.W. Bo (2004). “Factors affecting field instrumentation assessment of marine clay treated with prefabricated vertical drains.” Geotextiles and Geomembranes, vol. 22, no. 5, pp. 415–437.
• Arulrajah, A, and Bo, M.W. (2008). “Factors Affecting Consolidation Related Prediction of Singapore Marine Clay by Observational Methods.” Geotechnical and Geological Engineering, vol. 26, no. 4, p. 417.
• Bergado, D.T., and Patawaran, M.A.B. (2000). “Recent developments of ground improvement with pvd on soft Bangkok clay.” Proc. Intl. Seminar on Geotechnics in Kochi 2000, Kochi, Japan, October, 2000.
• Bo, M.W., Chu, J. and Choa, V. (2005). “Use of prefabricated vertical drains for the improvement of ultra-soft soil.” Proc. 6th International Conference on Ground Improvement Techniques, July 18–19, Coimbra, Portugal.
• Chu, J., Yan, S.W., and Yang, H. (2002a). “Closure on discussion on ‘Soil improvement by vacuum preloading for and oil storage system’.” Geotechnique, Vol. 52, No. 2, p. 151.
• Chu, J., Bo, M.W., and Choa, V. (2004). “Practical considerations for using vertical drains in soil improvements projects.” Geotextiles and Geomembranes, Vol. 22, 101–117.
• Chu, J. and Choa, V. (1995). “Quality control tests of vertical drains for a land reclamation project.” Compression and Consolidation of Clayey Soils, Proc. of an Int. Symp., Hiroshima, Japan, May 10–12, H. Yoshikuni and O. Kusakabe (Eds), A.A. Balkema, 43–48.
• Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J. G., and Berg, R. R. (2006). Ground Improvement Methods, Federal Highway Administration, NHI-06-019 Vol I and NHI-06-020 Vol II, Washington, D.C., pp. 536 (Vol I) and 520 (Vol II).
• Hansbo, S. (1979). “Consolidation of Clay by Band-Shaped Prefabricated Drains.” Ground Engineering, 12(5), pp. 16–25.
• Hansbo, S. (1986). “Preconsolidation of Soft Compressible Subsoil by the use of Prefabricated Vertical Drains.” Annales des Travaux Publics de Belgique, Issue 6, p. 553.
• Hansbo, S. (2004). “Band drains.” Chapter 1 in Moseley, M.P. and Kirsch, K. (Editors), Ground Improvement, 2nd Edition, Spon Press, New York, pp. 4–56.
• Holtz, R.D. (1987). “Preloading with Prefabricated Vertical Strip Drains.” Geotextiles and Geomembranes, Vol. 6, Issue 1, p. 109.
• Holtz, R.D. and Christopher, B.R. (1987). “Characteristics of prefabricated drains for acceleration of consolidation.” Proceedings of the 9th European Conference on Soil Mechanics and Foundation Engineering, Vol. 2, Dublin, Ireland, pp. 903–916.
• Holtz, R.D., Jamiolkowski, M.B., Lancellotta, R., and Pedroni, R. (1991). “Prefabricated Vertical Drains: Design and Performance.” Construction Industry Research and Information Association.
• Indraratna, Sathananthan, Iyathurai, Buddhima, Rujikiatkamjorn, and Cholachat (2007). “Evaluation of Smear Zone Extent Surrounding Mandrel Driven Vertical Drains Using the Cavity Expansion Theory.” International Journal of Geomechanics, Vol. 8, Issue 6.
• Lin, D.G., Liu, W.T., and Lin, P. C. (2006). “Numerical Analyses of PVD Improved Ground at Reference Section of Second Bangkok International Airport.” Geotechnical Engineering, Vol. 37, No. 3, pp. 157–170.
• Rixner, J.J., Kraemer, S.R. and Smith, A.D. (1986). “Prefabricated Vertical Drains.” U.S. Department of Transportation, Federal Highway Administration, Research, Development and Technology, Vol. I: Engineering Guidelines, Report No. FHWA/RD-86/168.
• Wang, X-S. and Jiao, J.J. (2004). “Analysis of soil consolidation by vertical drains with double porosity model.” International Journal for Numerical and Analytical Methods in Geomechanics, 28, 1385–1400.
• Welsh, J.P. (1987). “Soil improvement – A Ten Year Update.” Geotechnical Special Publication No. 12, ASCE, New York.
• Yan, S. and Chu, J. (2003a). “Experience gained from two vacuum preloading projects.” Proceedings 12th Asian Regional Conf. on Soil Mechanics and Geotechnical Engineering, August 4–8, Singapore, 195–198.
• Yan, S. and Chu, J. (2003b). “Soil improvement for a road using the vacuum preloading method.” Ground Improvement, UK, Vol. 7, No. 4, 165–172.

Two references are cited by name elsewhere on this page but do not appear in this technique's own Bibliography document on file. Massarsch, K.R. and Fellenius, B.H. (2005), “Deep vibratory compaction of granular soils,” Chapter 19 in Ground Improvement – Case Histories, Elsevier, is listed as a Key Reference in the Technology Fact Sheet. The “SHRP2 Phase I Technology Assessment (2008)” is cited as a reference for the Piezometer, Settlement Plates, and Surveying QC/QA methods. Both are reproduced here as they appear in their respective source documents, which are likely drawing on foundational or shared references not separately tracked in this particular technique's bibliography.