Soil improvement and ground modification methods chapter 9 preconsolidation

Soil improvement and ground modification methods chapter 9 preconsolidation Soil improvement and ground modification methods chapter 9 preconsolidation Soil improvement and ground modification methods chapter 9 preconsolidation Soil improvement and ground modification methods chapter 9 preconsolidation Soil improvement and ground modification methods chapter 9 preconsolidation Soil improvement and ground modification methods chapter 9 preconsolidation Soil improvement and ground modification methods chapter 9 preconsolidation Soil improvement and ground modification methods chapter 9 preconsolidation

Preconsolidation was introduced inChapter 6as a method of deep densifica-tion, and then mentioned again inChapter 7as a method of hydraulic mod-ification 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 compress-ibility 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 gener-ation/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

209

Soil Improvement and Ground Modification

Methods

© 2015 Elsevier Inc.

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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 ade-quate 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 sur-face 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 pre-consolidation This is possibleonly as long as the load is not so great as to cause a bearing failure or excessive deformations in the subsurface soils

As exemplified inFigure 9.1, use of a surcharge larger than the final antic-ipated 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,

Surcharge load Excess surcharge load

U%

Time

90%

Time to achieve 90%

with excess surcharge

Time to achieve 90%

with standard surcharge

Figure 9.1 Effect of excess surcharge on time rate (and amount) of consolidation.

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 settle-ment Vertical drains to aid in preconsolidation have been widely used for many decades Initially constructed as predrilled sand drains, these were rel-atively expensive and had certain practical limitations (e.g., depth) For most preconsolidation applications, sand drains have been replaced by prefabri-cated (geosynthetic composite) vertical drains orstrip 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 geo-composite vertical drains usually consist of a relatively thin, rectangular, flex-ible polypropylene core, providing significant longitudinal flow capacity on both sides.Figure 9.2depicts 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 approx-imately 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

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

Figure 9.3 Example of discharge from a prefabricated vertical drain (PVD) drain Figure 9.2 Example of a strip drain used for consol.

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

t90¼T90H2dr

cv

¼0:848 15mð Þ

2

cv

¼190:8m2

cv

(a)

(b)

H = 15 m

Constructed fill

Drainage layer

Saturated clay

Impervious bedrock

max drainage

path

H = 15 m

Drainage layer Constructed fill or preload

3 m

Vertical drains

Saturated clay

Impervious bedrock

max drainage path

Figure 9.4 Shortening of drainage path with vertical drains.

If vertical drains are installed in a triangular grid pattern so that the dis-tance between drains is 3 m (Figure 9.4b), then the maximum drainage path

is now 3/2 m¼1.5 m Now the estimated time to achieve 90% consolida-tion can be calculated as

t90¼T90H2

dr

cv ¼0:848 1:5mð Þ

2

cv ¼1:91m2

cv

So the calculated time to achieve 90% consolidation with vertical drains

is100 times faster! This is impressive enough, but in reality the consolidation time may be accelerated even more because the fluid flow is effectively hor-izontal rather than vertical, and horhor-izontal permeability is typically about 1.5 times greater than vertical permeability in naturally undisturbed, horizon-tally 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 consolida-tion settlement are a funcconsolida-tion of horizontal coefficient of permeability (k ),

U%

Time

90%

Settlement curve

with drains

Settlement curve without drains

Figure 9.5 Example of time-rate curves for preconsol with and without 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 construc-tion (www.geotechnics.com; www.hbwickdrains.com) (Figure 9.8) One of the largest PVD projects to date was for the Virginia Port Author-ity, 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

Figure 9.6 Photo of PVD installation Courtesy of Hayward Baker/HB Wick Drains.

stitcher rig installed the drains from the frozen ice surface during the winter where the temperatures were near40C (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

Figure 9.8 PVD install underwater (Port of Los Angeles) Courtesy of Hayward Baker/HB Wick Drains.

Figure 9.7 Horizontal strip drain discharge system connected to PVDs 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 pro-duce inferior performance aresmear of the drain fabric, which can occur dur-ing installation, and drain cloggdur-ing resultdur-ing from ineffective filterdur-ing of fine-grained particles by the geotextile filter

9.3 VACUUM-ASSISTED CONSOLIDATION

Vacuum-assisted consolidation (commonly referred to as simply “vacuum consol-idation”) 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

Figure 9.9 PVD install through Arctic ice Courtesy of Hayward Baker/HB Wick Drains.

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

Figure 9.11 Field application of vac consol Courtesy of Menard USA.

Atmospheric pressure

Vacuum gas phase booster

Vacuum air water pump

Water treatment station

Impervious membrane

Isotropic consolidation

Air flow Fill

Draining

layer

Dewatering

Horizontal

drains

Peripheral drain wall

Peripher trenches filled with bentonite and polyacrylate

Vertical

vacuum

transmission

pipes

Figure 9.10 Vac assisted consol schematic Courtesy of Menard USA.