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Preconsolidation
CHAPTER 9
Preconsolidation Preconsolidation was introduced in Chapter 6 as a method of deep densification, and then mentioned again in Chapter 7 as a method of hydraulic modification because it technically is a means of dewatering saturated fine-grained soils. While preconsolidation is both of these, it is for the most part a method to improve a site by reducing future settlements and increasing strength. Thus, it provides a direct benefit to improved foundation performance, allows more economical solutions to constructing projects on soft, compressible soils, and even permits economical construction where it may not otherwise be feasible. This chapter presents the current state of practice and methodologies to improving a site by preloading and draining prior to construction.
9.1 PRECONSOLIDATION CONCEPTS AND METHODOLOGIES When a load is applied from a new structure, embankment, or fill to a site underlain by soft saturated fine-grained soils, the load will initially be taken in part by the relatively incompressible water in the soil pores, transferring that load to excess pore water pressures. With time, the excess water pressure will dissipate as the load is transferred to the soil matrix, the soil consolidates, and settlement occurs. Long-term settlement can be the most critical parameter for many types of construction over soft compressible soils. The fundamental concept of preconsolidation is to load the soil prior to construction such that the soil can be compressed, thereby strengthening the soil and greatly reducing future settlement once the project is completed. Consolidation settlement is a stress- and time-dependent process based on applied load and soil compressibility parameters, as well as geometry and drainage conditions. Therefore, variation of each parameter will play an important role in the magnitude (amount) and rate (time) at which desired consolidation may be achieved. As consolidation is often difficult to accurately predict, it is imperative to closely monitor actual field progress of deformation and pore pressure generation/dissipation, and adjust prediction analyses accordingly. A very simple approach to preconsolidation is to apply a surcharge load approximately equal to the final design load anticipated for the completed Soil Improvement and Ground Modification Methods
© 2015 Elsevier Inc. All rights reserved.
209
210
Soil improvement and ground modification methods
project and allow the ground to naturally consolidate to the point where any predicted remaining settlement over the life expectancy of the project falls within tolerable limits. An important point that must be addressed is to ensure that the bearing capacity of the ground can safely handle the applied load. In some cases, where the bearing capacity of the foundation soil is too low, the surcharge may have to be applied in stages, allowing intermittent levels of consolidation (and associated strength gains) to be achieved prior to applying subsequent stage(s). Another practical design component is to ensure that adequate drainage is provided for discharge of the expelled water. Drainage is typically provided by placing a layer of free-draining material between the foundation soil and surcharge load. Alternatively, geocomposite drains may be utilized for this purpose. For smaller projects where a significant wait time is acceptable, this simple load application (with adequate drainage at the surface beneath the surcharge load) may be a feasible solution. If there is sufficient bearing capacity in the foundation soil, a larger load than the final design load (excess surcharge) may be applied to expedite preconsolidation. This is possible only as long as the load is not so great as to cause a bearing failure or excessive deformations in the subsurface soils. As exemplified in Figure 9.1, use of a surcharge larger than the final anticipated load (excess surcharge) will result in a settlement curve that is initially steeper, causing the required preconsolidation to take place in a much shorter period. When the required amount of settlement has occurred,
Excess surcharge load Surcharge load
Time
Time to achieve 90% with excess surcharge Time to achieve 90% with standard surcharge
90%
U%
Figure 9.1 Effect of excess surcharge on time rate (and amount) of consolidation.
Preconsolidation
211
the preload can be removed and construction initiated. As the time rate of consolidation is exponentially faster at the beginning of the application period, then even a relatively small amount of excess surcharge will greatly expedite the time it takes to reach the target level of settlement and shorten the time needed to initiate construction.
9.2 USE OF VERTICAL DRAINS The natural process of consolidation under a design construction load may take many years, all the while causing potential settlement problems for the constructed project. Drains can dramatically speed the time to reach a desired level of consolidation to increase strength and reduce future settlement. Vertical drains to aid in preconsolidation have been widely used for many decades. Initially constructed as predrilled sand drains, these were relatively expensive and had certain practical limitations (e.g., depth). For most preconsolidation applications, sand drains have been replaced by prefabricated (geosynthetic composite) vertical drains or strip drains. These are often referred to as “wick drains” in the United States, but that name is actually a misnomer, as the materials composing the drain are actually hydrophobic, and the drains do not wick water. The water is actually “pushed“ into the drains by differential pressure. The water then flows up (or in some cases, down) to where it can freely discharge. With the development and use of prefabricated vertical drains (PVDs) since the 1970s, economics of vertical drainage have been greatly improved and limitations minimized (i.e., depths of up to and exceeding 65 m can often be achieved with relative ease and efficient construction). The flow capacity of such drains is typically several times greater than that of most other types of vertical drains. Today’s geocomposite vertical drains usually consist of a relatively thin, rectangular, flexible polypropylene core, providing significant longitudinal flow capacity on both sides. Figure 9.2 depicts a typical PVD drain construction. The core is surrounded by a strong nonwoven (usually heat bonded polypropylene) geotextile, which acts as a filter and separator to keep surrounding soil from entering the drain core. Typical drains used for preconsolidation are approximately 10 cm (4 in) wide by 0.4 cm (0.15 in) thick, but thicker versions are available for increased flow capacity. Even with typical dimensions, the drains are capable of handling a significant discharge flow (Figure 9.3). The use of vertical drains greatly expedites the consolidation process by shortening the drainage path length, as well as allowing horizontal drainage, which is the preferential direction of flow with highest permeability in
212
Soil improvement and ground modification methods
Figure 9.2 Example of a strip drain used for consol.
Figure 9.3 Example of discharge from a prefabricated vertical drain (PVD) drain.
naturally horizontally deposited fine-grained sediments (Figure 9.4). The use of vertical drains in forced consolidation applications can speed the time to reach acceptable levels of bearing capacity (shear strength) and reduce expected future settlements beneath loads from decades to months or less, depending on project and site specifics (Figure 9.5). The following example provides a simple calculation of this. Example—Time rate of settlement with and without vertical drains
Preconsolidation
213
Drainage layer Constructed fill
max drainage path
Saturated clay
H = 15 m
(a)
Impervious bedrock
Drainage layer Constructed fill or preload
Vertical drains 3m H = 15 m Saturated clay
max drainage path
(b)
Impervious bedrock
Figure 9.4 Shortening of drainage path with vertical drains.
Given a 15-m thick layer of saturated clay over an impermeable bedrock (Figure 9.4a), the maximum drainage path (single drainage) is 15 m. Then the estimated time to achieve 90% consolidation can be calculated as t90 ¼
T 90 H 2dr 0:848ð15mÞ2 190:8m2 ¼ ¼ cv cv cv
214
Soil improvement and ground modification methods
Time
Settlement curve with drains Settlement curve without drains
90%
U%
Figure 9.5 Example of time-rate curves for preconsol with and without drains.
If vertical drains are installed in a triangular grid pattern so that the distance between drains is 3 m (Figure 9.4b), then the maximum drainage path is now 3/2 m ¼ 1.5 m. Now the estimated time to achieve 90% consolidation can be calculated as t 90 ¼
T 90 H 2dr 0:848ð1:5mÞ2 1:91m2 ¼ ¼ cv cv cv
So the calculated time to achieve 90% consolidation with vertical drains is 100 times faster! This is impressive enough, but in reality the consolidation time may be accelerated even more because the fluid flow is effectively horizontal rather than vertical, and horizontal permeability is typically about 1.5 times greater than vertical permeability in naturally undisturbed, horizontally layered soils. The prefabricated drains are installed by specialized equipment called “stitchers,” mounted on cranes or excavators, fitted with a mandrel to drive the drain from the surface to the desired depth, usually the bottom of the soft compressible strata (Figure 9.6). The installation is sometimes performed with the assistance of vibratory hammers (www.hbwickdrains.com), but most often is simply pushed into the ground hydraulically. The drains are usually laid out in a triangular or square pattern spaced at about 1-2 m (3-6 ft) apart. The designs are typically based on the most economical means of achieving a desired result projected from time-rate settlement curves and/or strength increases. These design curves are developed from radial consolidation theory (Barron, 1948; Hansbo, 1979), where time rate and estimated consolidation settlement are a function of horizontal coefficient of permeability (kh),
Preconsolidation
215
Figure 9.6 Photo of PVD installation. Courtesy of Hayward Baker/HB Wick Drains.
drain spacing, and applied pressure (stress differential). Industry reports claim that typical applications of 8000 m per day can be accomplished with a single machine (www.cofra.co.uk) with installation rates of up to 1300 m (4000 l.f.) per hour (www.uswickdrain.com). Once vertical drains have been installed at a site, a horizontal drainage layer must be provided to discharge the drained water. This is often done with a free-draining granular material, combined with horizontal strip drains placed to intercept the flow from the vertical drains (Figure 9.7). Most commonly, vertical drains are used to strengthen subsurface soils prior to the application of loads such as highway embankments, bridge approaches, dams, buildings, and even airport runways. Vertical drains installed for the purpose of preconsolidation have even been used for underwater applications, port facilities, and near-shore marine construction (www.geotechnics.com; www.hbwickdrains.com) (Figure 9.8). One of the largest PVD projects to date was for the Virginia Port Authority, where over 4,000,000 m (12,700,000 l.f.) of drain was installed (www.uswickdrain.com). When considering projects such as these, each must be analyzed and designed individually, taking into account the many innovative ideas that have provided solutions for a wide variety of projects. For one case study, where an oil platform “island” was to be constructed over submerged soft clay river deposits in Alaska, drains were installed to depths of approximately 18 m (55 ft) through the compressible clay layers and into a permeable deposit of sand (www.hbwickdrains.com). The
216
Soil improvement and ground modification methods
Figure 9.7 Horizontal strip drain discharge system connected to PVDs. Courtesy of Hayward Baker/HB Wick Drains.
Figure 9.8 PVD install underwater (Port of Los Angeles). Courtesy of Hayward Baker/HB Wick Drains.
stitcher rig installed the drains from the frozen ice surface during the winter where the temperatures were near 40 C (Figure 9.9). The soft sediments were drained to the permeable sand below and so were not impeded by ice capping at the surface. Using the ice as a working platform, construction of the island was completed before the spring thaw. While PVDs have much less capacity than even a small-diameter sand drain, the low cost per length and ability to install to much greater depths often makes this a viable and economical choice. One reported difficulty with the use of PVDs is when the amount of consolidation settlement
Preconsolidation
217
Figure 9.9 PVD install through Arctic ice. Courtesy of Hayward Baker/HB Wick Drains.
exceeds about 5-10%. At that amount of deformation, the drains may “kink” and become ineffective (Holtz et al., 1988). Other problems that can produce inferior performance are smear of the drain fabric, which can occur during installation, and drain clogging resulting from ineffective filtering of fine-grained particles by the geotextile filter.
9.3 VACUUM-ASSISTED CONSOLIDATION Vacuum-assisted consolidation (commonly referred to as simply “vacuum consolidation”) is a method of preloading compressible fine-grained soils by applying vacuum pumps to the installed vertical and horizontal drainage system, either beneath an “airtight” membrane (Menard type; Figures 9.10 and 9.11) or through buried (embedded) horizontal pipes connected directly to the vertical drains (Cofra BeauDrain® type; www.cofra.co.uk). The Menard system requires significant care in creating an “airtight” seal, and the membrane often must be protected to ensure integrity of that seal. This type of system effectively applies a differential near-atmospheric pressure throughout the full depth of the drainage system, resulting in an isotropic consolidation and faster drainage at greater depths. In fact, use of vacuum systems has been shown to speed up improvements, allowing, in some cases, additional loads to be applied within 2 weeks after starting the application (www.menardusa.com). Duration of completed applications is often within 4-6 months, which is significantly faster than traditional methods utilizing surcharge loads and drains alone.
218
Soil improvement and ground modification methods
Vacuum gas phase booster Atmospheric pressure Vacuum air water pump
Impervious membrane Fill Draining layer
Air flow
Water treatment station
Dewatering
Horizontal drains Peripher trenches filled with bentonite and polyacrylate
Isotropic consolidation Vertical vacuum transmission pipes
Peripheral drain wall
Figure 9.10 Vac. assisted consol schematic. Courtesy of Menard USA.
Figure 9.11 Field application of vac consol. Courtesy of Menard USA.
The idea of increasing the productivity of preconsolidation methods by use of vacuum pumps is not altogether new. In fact, several early attempts were made as early as the 1950s. The literature indicates that much was learned through both research efforts and attempted field applications over the past few decades to the point where it is now a predictable and reliable
Preconsolidation
219
soil improvement method. Vacuum consolidation has been successfully applied for a variety of applications since the late 1980s, such as for power plants, sewage treatment plants, highway embankments, and airport runways (www.geopac.com). Most of the reported case studies have been outside of the United States, with some notable examples from France, Germany, the Netherlands, Australia, Japan, China, Thailand, Vietnam, and South Korea (www.menardusa.com; www.menardbachy.com.au; www.cofra.com; Bergado et al., 1998; Tang and Shang, 2000). In conventional preloading schemes, an embankment fill is typically placed on the area to be treated. This adds stresses to induce consolidation settlements as well as increased shear stresses in the ground. This, in turn, could manifest in stress-induced outward lateral deformations of the foundation soil beneath the embankment or worse—bearing capacity failure. When vacuum consolidation is used in conjunction with vertical drains, the surcharge may sometimes be omitted (or limited) as the typical design pressure differential of 80 kPa induced by the vacuum system is equivalent to more than 4 m of a soil surcharge load (www.menardusa.com). The added effective stress differential is also uniform (isotropic) throughout the depth of the drained material, as opposed to the added stress from surcharge loads, which will be reduced at depth as a function of stress distribution effects. In fact, there is greater stability added to the soil stratum being treated because the vacuum induces negative pore pressure (or increased effective stress), thereby allowing earlier application of increased surcharge or construction loads. Cofra’s BeauDrain® system (www.cofra.co.uk) uses a combination of vertical drains connected to horizontal vacuum pipe installed below the ground surface. This provides the same type of vacuum consolidation advantages as described above, but without the need for a sealed, airtight membrane. As a small compromise, these systems usually have a somewhat lower design pressure of 50 kPa. The drainage system is installed in one operation with specially designed equipment. This type of forced consolidation has been successfully applied in the Netherlands and Germany (www.cofra.co.uk).
9.4 INSTRUMENTATION AND PERFORMANCE MONITORING In all cases of preconsolidation applications, it is critical to ensure that actual conditions be continuously monitored. As discussed earlier, consolidation
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Soil improvement and ground modification methods
and especially the time rate of consolidation are some of the most difficult geotechnical processes to accurately predict. Because of this, a combination of relatively simple methods may be used to monitor progress of the consolidation and compare to predicted results. Regular collection and analysis of data can be extremely important in cases where construction or soil improvement applications may need to be altered. Instrumentation may include settlement markers either embedded at the original ground surface or at the top of the surcharge load (or constructed embankment), or both. It is important that settlement markers be measured from a static reference point not affected by the deformations occurring as a result of the forced consolidation. Piezometers are also important instruments that can be used to indicate the progress of the consolidation process. Piezometers may be located at various depths beneath the load to obtain a profile of how pore water pressures are changing within the subsurface profile as a function of time. As a conventional load is applied, pore pressures will rise. As consolidation continues, the dissipation of excess pore pressures above initial or hydrostatic conditions can be monitored to evaluate the percent consolidation completed at various depth horizons. Inclinometers may also be useful, as they can measure lateral deformations or displacements at depth, which in turn could indicate possible bearing or stability issues and/or account for some of the measured vertical settlement.
REFERENCES Barron, R.A., 1948. Consolidation of fine-grained soils by drain wells. In: Transactions ASCE, vol. 113. pp. 718–724, Paper 2346. Bergado, D.T., Chai, J.-C., Miura, N., Balasubramaniam, A.S., 1998. PVD improvement of soft Bangkok clay with combined vacuum and reduced sand embankment preloading. J. Geotech. Eng., Southeast Asian Geotechnical Society 29 (1), 95–121. Hansbo, S., 1979. Consolidation of clay by band-shaped prefabricated drains. Ground Eng. 12 (5), 16–25. Holtz, R.D., Jamiolkowski, M., Lancellotta, R., Pedroni, S., 1988. Behaviour of bent prefabricated vertical drains. In: Proceedings of the 12th ICSMFE, Rio De Janeiro, vol. 3, pp. 1657–1660. Tang, M., Shang, J.Q., 2000. Vacuum preloading consolidation of Yaoqiang airport runway. Geotechnique 50 (6), 613–623. http://www.cofra.co.uk (accessed 02.03.14.). http://www.geotechnics.com (accessed 08.25.13.). http://www.menardusa.com (accessed 08.30.13.).
Preconsolidation Preconsolidation was introduced in Chapter 6 as a method of deep densification, and then mentioned again in Chapter 7 as a method of hydraulic modification because it technically is a means of dewatering saturated fine-grained soils. While preconsolidation is both of these, it is for the most part a method to improve a site by reducing future settlements and increasing strength. Thus, it provides a direct benefit to improved foundation performance, allows more economical solutions to constructing projects on soft, compressible soils, and even permits economical construction where it may not otherwise be feasible. This chapter presents the current state of practice and methodologies to improving a site by preloading and draining prior to construction.
9.1 PRECONSOLIDATION CONCEPTS AND METHODOLOGIES When a load is applied from a new structure, embankment, or fill to a site underlain by soft saturated fine-grained soils, the load will initially be taken in part by the relatively incompressible water in the soil pores, transferring that load to excess pore water pressures. With time, the excess water pressure will dissipate as the load is transferred to the soil matrix, the soil consolidates, and settlement occurs. Long-term settlement can be the most critical parameter for many types of construction over soft compressible soils. The fundamental concept of preconsolidation is to load the soil prior to construction such that the soil can be compressed, thereby strengthening the soil and greatly reducing future settlement once the project is completed. Consolidation settlement is a stress- and time-dependent process based on applied load and soil compressibility parameters, as well as geometry and drainage conditions. Therefore, variation of each parameter will play an important role in the magnitude (amount) and rate (time) at which desired consolidation may be achieved. As consolidation is often difficult to accurately predict, it is imperative to closely monitor actual field progress of deformation and pore pressure generation/dissipation, and adjust prediction analyses accordingly. A very simple approach to preconsolidation is to apply a surcharge load approximately equal to the final design load anticipated for the completed Soil Improvement and Ground Modification Methods
© 2015 Elsevier Inc. All rights reserved.
209
210
Soil improvement and ground modification methods
project and allow the ground to naturally consolidate to the point where any predicted remaining settlement over the life expectancy of the project falls within tolerable limits. An important point that must be addressed is to ensure that the bearing capacity of the ground can safely handle the applied load. In some cases, where the bearing capacity of the foundation soil is too low, the surcharge may have to be applied in stages, allowing intermittent levels of consolidation (and associated strength gains) to be achieved prior to applying subsequent stage(s). Another practical design component is to ensure that adequate drainage is provided for discharge of the expelled water. Drainage is typically provided by placing a layer of free-draining material between the foundation soil and surcharge load. Alternatively, geocomposite drains may be utilized for this purpose. For smaller projects where a significant wait time is acceptable, this simple load application (with adequate drainage at the surface beneath the surcharge load) may be a feasible solution. If there is sufficient bearing capacity in the foundation soil, a larger load than the final design load (excess surcharge) may be applied to expedite preconsolidation. This is possible only as long as the load is not so great as to cause a bearing failure or excessive deformations in the subsurface soils. As exemplified in Figure 9.1, use of a surcharge larger than the final anticipated load (excess surcharge) will result in a settlement curve that is initially steeper, causing the required preconsolidation to take place in a much shorter period. When the required amount of settlement has occurred,
Excess surcharge load Surcharge load
Time
Time to achieve 90% with excess surcharge Time to achieve 90% with standard surcharge
90%
U%
Figure 9.1 Effect of excess surcharge on time rate (and amount) of consolidation.
Preconsolidation
211
the preload can be removed and construction initiated. As the time rate of consolidation is exponentially faster at the beginning of the application period, then even a relatively small amount of excess surcharge will greatly expedite the time it takes to reach the target level of settlement and shorten the time needed to initiate construction.
9.2 USE OF VERTICAL DRAINS The natural process of consolidation under a design construction load may take many years, all the while causing potential settlement problems for the constructed project. Drains can dramatically speed the time to reach a desired level of consolidation to increase strength and reduce future settlement. Vertical drains to aid in preconsolidation have been widely used for many decades. Initially constructed as predrilled sand drains, these were relatively expensive and had certain practical limitations (e.g., depth). For most preconsolidation applications, sand drains have been replaced by prefabricated (geosynthetic composite) vertical drains or strip drains. These are often referred to as “wick drains” in the United States, but that name is actually a misnomer, as the materials composing the drain are actually hydrophobic, and the drains do not wick water. The water is actually “pushed“ into the drains by differential pressure. The water then flows up (or in some cases, down) to where it can freely discharge. With the development and use of prefabricated vertical drains (PVDs) since the 1970s, economics of vertical drainage have been greatly improved and limitations minimized (i.e., depths of up to and exceeding 65 m can often be achieved with relative ease and efficient construction). The flow capacity of such drains is typically several times greater than that of most other types of vertical drains. Today’s geocomposite vertical drains usually consist of a relatively thin, rectangular, flexible polypropylene core, providing significant longitudinal flow capacity on both sides. Figure 9.2 depicts a typical PVD drain construction. The core is surrounded by a strong nonwoven (usually heat bonded polypropylene) geotextile, which acts as a filter and separator to keep surrounding soil from entering the drain core. Typical drains used for preconsolidation are approximately 10 cm (4 in) wide by 0.4 cm (0.15 in) thick, but thicker versions are available for increased flow capacity. Even with typical dimensions, the drains are capable of handling a significant discharge flow (Figure 9.3). The use of vertical drains greatly expedites the consolidation process by shortening the drainage path length, as well as allowing horizontal drainage, which is the preferential direction of flow with highest permeability in
212
Soil improvement and ground modification methods
Figure 9.2 Example of a strip drain used for consol.
Figure 9.3 Example of discharge from a prefabricated vertical drain (PVD) drain.
naturally horizontally deposited fine-grained sediments (Figure 9.4). The use of vertical drains in forced consolidation applications can speed the time to reach acceptable levels of bearing capacity (shear strength) and reduce expected future settlements beneath loads from decades to months or less, depending on project and site specifics (Figure 9.5). The following example provides a simple calculation of this. Example—Time rate of settlement with and without vertical drains
Preconsolidation
213
Drainage layer Constructed fill
max drainage path
Saturated clay
H = 15 m
(a)
Impervious bedrock
Drainage layer Constructed fill or preload
Vertical drains 3m H = 15 m Saturated clay
max drainage path
(b)
Impervious bedrock
Figure 9.4 Shortening of drainage path with vertical drains.
Given a 15-m thick layer of saturated clay over an impermeable bedrock (Figure 9.4a), the maximum drainage path (single drainage) is 15 m. Then the estimated time to achieve 90% consolidation can be calculated as t90 ¼
T 90 H 2dr 0:848ð15mÞ2 190:8m2 ¼ ¼ cv cv cv
214
Soil improvement and ground modification methods
Time
Settlement curve with drains Settlement curve without drains
90%
U%
Figure 9.5 Example of time-rate curves for preconsol with and without drains.
If vertical drains are installed in a triangular grid pattern so that the distance between drains is 3 m (Figure 9.4b), then the maximum drainage path is now 3/2 m ¼ 1.5 m. Now the estimated time to achieve 90% consolidation can be calculated as t 90 ¼
T 90 H 2dr 0:848ð1:5mÞ2 1:91m2 ¼ ¼ cv cv cv
So the calculated time to achieve 90% consolidation with vertical drains is 100 times faster! This is impressive enough, but in reality the consolidation time may be accelerated even more because the fluid flow is effectively horizontal rather than vertical, and horizontal permeability is typically about 1.5 times greater than vertical permeability in naturally undisturbed, horizontally layered soils. The prefabricated drains are installed by specialized equipment called “stitchers,” mounted on cranes or excavators, fitted with a mandrel to drive the drain from the surface to the desired depth, usually the bottom of the soft compressible strata (Figure 9.6). The installation is sometimes performed with the assistance of vibratory hammers (www.hbwickdrains.com), but most often is simply pushed into the ground hydraulically. The drains are usually laid out in a triangular or square pattern spaced at about 1-2 m (3-6 ft) apart. The designs are typically based on the most economical means of achieving a desired result projected from time-rate settlement curves and/or strength increases. These design curves are developed from radial consolidation theory (Barron, 1948; Hansbo, 1979), where time rate and estimated consolidation settlement are a function of horizontal coefficient of permeability (kh),
Preconsolidation
215
Figure 9.6 Photo of PVD installation. Courtesy of Hayward Baker/HB Wick Drains.
drain spacing, and applied pressure (stress differential). Industry reports claim that typical applications of 8000 m per day can be accomplished with a single machine (www.cofra.co.uk) with installation rates of up to 1300 m (4000 l.f.) per hour (www.uswickdrain.com). Once vertical drains have been installed at a site, a horizontal drainage layer must be provided to discharge the drained water. This is often done with a free-draining granular material, combined with horizontal strip drains placed to intercept the flow from the vertical drains (Figure 9.7). Most commonly, vertical drains are used to strengthen subsurface soils prior to the application of loads such as highway embankments, bridge approaches, dams, buildings, and even airport runways. Vertical drains installed for the purpose of preconsolidation have even been used for underwater applications, port facilities, and near-shore marine construction (www.geotechnics.com; www.hbwickdrains.com) (Figure 9.8). One of the largest PVD projects to date was for the Virginia Port Authority, where over 4,000,000 m (12,700,000 l.f.) of drain was installed (www.uswickdrain.com). When considering projects such as these, each must be analyzed and designed individually, taking into account the many innovative ideas that have provided solutions for a wide variety of projects. For one case study, where an oil platform “island” was to be constructed over submerged soft clay river deposits in Alaska, drains were installed to depths of approximately 18 m (55 ft) through the compressible clay layers and into a permeable deposit of sand (www.hbwickdrains.com). The
216
Soil improvement and ground modification methods
Figure 9.7 Horizontal strip drain discharge system connected to PVDs. Courtesy of Hayward Baker/HB Wick Drains.
Figure 9.8 PVD install underwater (Port of Los Angeles). Courtesy of Hayward Baker/HB Wick Drains.
stitcher rig installed the drains from the frozen ice surface during the winter where the temperatures were near 40 C (Figure 9.9). The soft sediments were drained to the permeable sand below and so were not impeded by ice capping at the surface. Using the ice as a working platform, construction of the island was completed before the spring thaw. While PVDs have much less capacity than even a small-diameter sand drain, the low cost per length and ability to install to much greater depths often makes this a viable and economical choice. One reported difficulty with the use of PVDs is when the amount of consolidation settlement
Preconsolidation
217
Figure 9.9 PVD install through Arctic ice. Courtesy of Hayward Baker/HB Wick Drains.
exceeds about 5-10%. At that amount of deformation, the drains may “kink” and become ineffective (Holtz et al., 1988). Other problems that can produce inferior performance are smear of the drain fabric, which can occur during installation, and drain clogging resulting from ineffective filtering of fine-grained particles by the geotextile filter.
9.3 VACUUM-ASSISTED CONSOLIDATION Vacuum-assisted consolidation (commonly referred to as simply “vacuum consolidation”) is a method of preloading compressible fine-grained soils by applying vacuum pumps to the installed vertical and horizontal drainage system, either beneath an “airtight” membrane (Menard type; Figures 9.10 and 9.11) or through buried (embedded) horizontal pipes connected directly to the vertical drains (Cofra BeauDrain® type; www.cofra.co.uk). The Menard system requires significant care in creating an “airtight” seal, and the membrane often must be protected to ensure integrity of that seal. This type of system effectively applies a differential near-atmospheric pressure throughout the full depth of the drainage system, resulting in an isotropic consolidation and faster drainage at greater depths. In fact, use of vacuum systems has been shown to speed up improvements, allowing, in some cases, additional loads to be applied within 2 weeks after starting the application (www.menardusa.com). Duration of completed applications is often within 4-6 months, which is significantly faster than traditional methods utilizing surcharge loads and drains alone.
218
Soil improvement and ground modification methods
Vacuum gas phase booster Atmospheric pressure Vacuum air water pump
Impervious membrane Fill Draining layer
Air flow
Water treatment station
Dewatering
Horizontal drains Peripher trenches filled with bentonite and polyacrylate
Isotropic consolidation Vertical vacuum transmission pipes
Peripheral drain wall
Figure 9.10 Vac. assisted consol schematic. Courtesy of Menard USA.
Figure 9.11 Field application of vac consol. Courtesy of Menard USA.
The idea of increasing the productivity of preconsolidation methods by use of vacuum pumps is not altogether new. In fact, several early attempts were made as early as the 1950s. The literature indicates that much was learned through both research efforts and attempted field applications over the past few decades to the point where it is now a predictable and reliable
Preconsolidation
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soil improvement method. Vacuum consolidation has been successfully applied for a variety of applications since the late 1980s, such as for power plants, sewage treatment plants, highway embankments, and airport runways (www.geopac.com). Most of the reported case studies have been outside of the United States, with some notable examples from France, Germany, the Netherlands, Australia, Japan, China, Thailand, Vietnam, and South Korea (www.menardusa.com; www.menardbachy.com.au; www.cofra.com; Bergado et al., 1998; Tang and Shang, 2000). In conventional preloading schemes, an embankment fill is typically placed on the area to be treated. This adds stresses to induce consolidation settlements as well as increased shear stresses in the ground. This, in turn, could manifest in stress-induced outward lateral deformations of the foundation soil beneath the embankment or worse—bearing capacity failure. When vacuum consolidation is used in conjunction with vertical drains, the surcharge may sometimes be omitted (or limited) as the typical design pressure differential of 80 kPa induced by the vacuum system is equivalent to more than 4 m of a soil surcharge load (www.menardusa.com). The added effective stress differential is also uniform (isotropic) throughout the depth of the drained material, as opposed to the added stress from surcharge loads, which will be reduced at depth as a function of stress distribution effects. In fact, there is greater stability added to the soil stratum being treated because the vacuum induces negative pore pressure (or increased effective stress), thereby allowing earlier application of increased surcharge or construction loads. Cofra’s BeauDrain® system (www.cofra.co.uk) uses a combination of vertical drains connected to horizontal vacuum pipe installed below the ground surface. This provides the same type of vacuum consolidation advantages as described above, but without the need for a sealed, airtight membrane. As a small compromise, these systems usually have a somewhat lower design pressure of 50 kPa. The drainage system is installed in one operation with specially designed equipment. This type of forced consolidation has been successfully applied in the Netherlands and Germany (www.cofra.co.uk).
9.4 INSTRUMENTATION AND PERFORMANCE MONITORING In all cases of preconsolidation applications, it is critical to ensure that actual conditions be continuously monitored. As discussed earlier, consolidation
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and especially the time rate of consolidation are some of the most difficult geotechnical processes to accurately predict. Because of this, a combination of relatively simple methods may be used to monitor progress of the consolidation and compare to predicted results. Regular collection and analysis of data can be extremely important in cases where construction or soil improvement applications may need to be altered. Instrumentation may include settlement markers either embedded at the original ground surface or at the top of the surcharge load (or constructed embankment), or both. It is important that settlement markers be measured from a static reference point not affected by the deformations occurring as a result of the forced consolidation. Piezometers are also important instruments that can be used to indicate the progress of the consolidation process. Piezometers may be located at various depths beneath the load to obtain a profile of how pore water pressures are changing within the subsurface profile as a function of time. As a conventional load is applied, pore pressures will rise. As consolidation continues, the dissipation of excess pore pressures above initial or hydrostatic conditions can be monitored to evaluate the percent consolidation completed at various depth horizons. Inclinometers may also be useful, as they can measure lateral deformations or displacements at depth, which in turn could indicate possible bearing or stability issues and/or account for some of the measured vertical settlement.
REFERENCES Barron, R.A., 1948. Consolidation of fine-grained soils by drain wells. In: Transactions ASCE, vol. 113. pp. 718–724, Paper 2346. Bergado, D.T., Chai, J.-C., Miura, N., Balasubramaniam, A.S., 1998. PVD improvement of soft Bangkok clay with combined vacuum and reduced sand embankment preloading. J. Geotech. Eng., Southeast Asian Geotechnical Society 29 (1), 95–121. Hansbo, S., 1979. Consolidation of clay by band-shaped prefabricated drains. Ground Eng. 12 (5), 16–25. Holtz, R.D., Jamiolkowski, M., Lancellotta, R., Pedroni, S., 1988. Behaviour of bent prefabricated vertical drains. In: Proceedings of the 12th ICSMFE, Rio De Janeiro, vol. 3, pp. 1657–1660. Tang, M., Shang, J.Q., 2000. Vacuum preloading consolidation of Yaoqiang airport runway. Geotechnique 50 (6), 613–623. http://www.cofra.co.uk (accessed 02.03.14.). http://www.geotechnics.com (accessed 08.25.13.). http://www.menardusa.com (accessed 08.30.13.).