prefabricated vertical drains (pvd)

8/10/2019 Prefabricated Vertical Drains (Pvd)

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PREFABRICATED VERTICAL

DRAINS (PVD)


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GENERAL

The consolidation settlement of soft clay subsoil creates a lot problems in foundation and infrastructure engineering. Becauvery low clay permeability, the primary consolidation takes a to complete. To shorten this consolidation time, vertical draininstalled together with preloading by surcharge embankmentvacuum pressure. Vertical drains are artificially-created draina

which can be installed by one of several methods and which cvariety of physical characteristics


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GENERAL

The purpose of vertical drain installation is twofold. Firstly, toaccelerate the consolidation process of the clay subsoil, and, sto gain rapid strength increase to improve the stability of struweak clay foundation. Vertical drains can be classified into 3 gtypes, namely: sand drains, fabric encased sand drains, andprefabricated sand drains


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GENERAL


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GENERAL

Figure below illustrate a typical vertical drain installation for hembankments. In this method, pore water squeezed out duriconsolidation of the clay due to the hydraulic gradients createpreloading, can flow a lot faster in the horizontal direction towdrain and then flow freely along the drains vertically towards permeable drainage layers. Thus, the installation of the vertic

in the clay reduces the length of the drainage paths and, therreducing the time to complete the consolidation process.Consequently, the higher horizontal permeability of the clay itaken advantage


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GENERAL


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GENERAL

Applications of sand drains for improvement of soft ground inSoutheast Asian region have been reported by Tominaga et alManila Bay Reclamation Area, Philippines; by Choa et al. (197Changi Airport, Singapore; by Chou et al. (1980) in Taiwan; by(1981), Balasubramaniam et al. (1980), Brenner and Prebahar(1983), Moh and Woo (1987), and Woo et al. (1989) in Bangk

Thailand. Recent sand drain applications in Japan were reportTakai et al. (1989) and Suzuki and Yamada (1990) in the KansaInternational Airport Project and by Tanimoto et al. (1979) in Japan.


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GENERAL

In Southeast Asia, various applications have been recently repwith regards to prefabricated vertical drains by Choa et al. (19et al. (1989), and Woo et al. (1988) in Singapore, by Nicholls (Indonesia; by Volders (1984) and Rahman et al. (1990) in Malby Belloni et al. (1979) in the Philippines. In the soft Bangkok Thailand, prefabricated vertical band drains have been succes

applied and tested by full scale test embankments by Bergado(1988, 1990a,b, 1991).


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PRELOADING

Preloading refers to the process of compressing foundation soapplied vertical stress prior to placement of the final permaneconstruction load. If the temporary applied load exceeds the floading, the amount in excess is referred to as surcharge loadpreload is rapidly applied to a saturated, soft clay deposit, thesettlement can be divided into three idealized components, n

immediate, primary consolidation, and secondary consolidatiactual condition, the settlement behavior is more complex


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PRELOADING

Figure below illustrates a general relationship of the three idecomponents. The relative importance and magnitude of eachsettlement depends on many factors such as: the soil type ancompressibility characteristics, the stress history, the magniturate of loading, and the relationship between the area of loadthe thickness of the compressible soil. Generally, the primaryconsolidation settlement predominates and, for many preload

projects, is the only one considered in the preload design. Pretechniques have been discussed in detail elsewhere (Jamiolko1983; Pilot, 1981). One very important key point is that the ampreloading should provide surcharge stresses that exceed themaximum past pressure in the clay subsoil


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PRELOADING


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PRELOADING

Figure below shows the initial (

v0) and final (

vf) effective strunder the centerline of the test embankment compared with maximum past pressure obtained by Casagrande method (Beal. 1991). The Poulos (1976) method assuming finite elastic larigid base was found to be approximately 35 % higher than thpredictions of Janbu et al. (1956) assuming semi-infinite elast

the soil mass.


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PRELOADING


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SAND DRAINS

Early applications of vertical drains to accelerate consolidatioclay subsoils utilized sand drains. These are formed by infillinga hole in the soft ground. There are two categories of installatmethods, namely: displacement and non-displacement typesdisplacement type, a closed end mandrel is driven or pushed soft ground with resulting displacements in both vertical and

directions. The non-displacement type installation requires drhole by means of power auger or water jets and is consideredless disturbing effects on soft clay


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SAND DRAINS

Casagrande and Poulos (1969) concluded that driven sand draharmful in soft and sensitive clays due to the disturbance in ddrains causing the reduction of shear strength and horizontalpermeability. However, Akagi (1979) asserted that the mere inof the sand drains alone results in the consolidation of the sofbecause of the large stresses induced during the installation.

excess pore pressure is generated (Brenner et al. 1979) and, asubsequent dissipation, a gain in strength is achieved (Akagi,


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CHARACTERISTICS OF PREFABRICATED D

A prefabricated vertical drain can be defined as any prefabricamaterial or product consisting of synthetic filter jacket surrouplastic core having the following characteristics: a) ability to pporewater in the soil to seep into the drain; b) a means by whcollected porewater can be transmitted along the length of th


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The jacket material consists of non-woven polyester or polyprgeotextiles or synthetic paper that function as physical barrieseparating the flow channel from the surrounding soft clay sofilter to limit the passage of fine particles into the core to prevclogging. The plastic core serves two vital functions, namely: tthe filter jacket and to provide longitudinal flow paths along t

even at large lateral pressures. Some details of various drain cthe configuration of different types of prefabricated vertical d(PVD) are illustrated in Fig. below. The PVD core can be classifmain categories, namely: grooved core, studded core, and filacore.


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CONSOLIDATION WITH VERTICAL DRAIN

Barron (1948) presented the first exhaustive solution to the pconsolidation of a soil cylinder containing a central sand draintheory was based on the simplifying assumptions of one-dimconsolidation theory (Terzaghi, 1943). Barron's theory enablesolve the problem of consolidation under two conditions, namfree vertical strain assuming that the vertical surface stress re

constant and the surface displacements are non-uniform duriconsolidation process; (ii) equal vertical strain assuming that tvertical surface stress is non-uniform.


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In the case of equal strain, the differential equation governinconsolidation process

where u is the average excess pore pressure at any point and

given time; r is the radial distance of the considered point frocenter of the drained soil cylinder; t is the time after an instaincrease of the total vertical stress, and C, is the horizontal cof consolidation.

r

U

rr

UC

t

U

h

1

2

2


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For the case of radial drainage only, the solution of Barron (1ideal conditions (no smear and no well resistance) is as follow

Where:

and De is the diameter of the equivalent soil cylinder, dw is thequivalent diameter of the drain, and n (n = De/dw) is the spa

)(

8exp1

nF

TU

h

h

e

hh

D

tCT

2

2

143)(

)1()(

nnn

n

nnF


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Hansbo (1979) modified the equations developed by Barron (194prefabricated drain applications. The modifications dealt mainly wsimplifying assumptions due to the physical dimensions, charactethe prefabricated drains, and effect of PVD installation. The modiexpression for average degree of consolidation is given as:

F = F(n) + Fs + Fr

where F is the factor which expresses the additive effect due to thof the drains; F(n); smear effect, Fs; and well-resistance, Fr

)(

8exp1

nF

TU

h

h


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For typical values of the spacing ratio, n, of 20 or more, the spacisimplifies to:

To account for the effects of soil disturbance during installation, adisturbance with a reduced permeability is assumed around the vthe drain, as shown in Fig. below. The smear effect factor is given

where ds is the diameter of the disturbed zone around the drain; coefficient of permeability in the horizontal direction in the distu

4

3ln)(

w

e

d

DnF

w

s

s

h

s

d

d

k

kF ln1


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Since the prefabricated vertical drains have limited dischargecapacities, Hansbo (1979) developed a drain resistance factorassuming that Darcy's law can be applied for flow along the vof the drain. The well-resistance factor is given as:

where z is the distance from the drainage end of the drain; L ithe length of the drain when drainage occurs at one end onlyto the length of the drain when drainage occurs at both ends;coefficient of permeability in the horizontal direction in theundisturbed soil; and qw is the discharge capacity of the drainhydraulic gradient 1.

w

hr

q

kzLzF )(


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Incorporating the effects of smear and well-resistance, the timobtain a given degree of consolidation at an assumed spacinggiven as follows:

For convenience on the part of users in designing vertical draa design graph devised by Bergado et al. (1993a) is given in FiThis is the first design graph that incorporates both the effectand well- resistance.

h

rs

h

e

UFFnF

C

Dt

1

1ln))((

8

2


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DRAIN PROPERTIES

The theory of consolidation with radial drainage assumes thatis drained by vertical drain with circular cross section. The equdiameter of a band-shaped drain is defined as the diameter ocircular drain which has the same theoretical radial drainageperformance as the band-shaped drain. Subsequent finite elestudies performed by Rixner et al. (1986) and supported by H

(1987) suggested that the equivalent diameter preferable for practice can be obtained as:

2

)( bad

w


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DRAIN PROPERTIES

The relative sizes of these equivalent diameters are comparedband shaped cross-section of the prefabricated drain by Rixne(1986).

The discharge capacity of prefabricated drains is required to a

the drain resistance factor and is usually obtained from publisresults reported by manufacturers. Rixner et al. (1986) reportof vertical discharge capacity tests and those obtained by othshown in Fig. below. The results demonstrate the major influeconfining pressure.

DRAIN PROPERTIES


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DRAIN PROPERTIES


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DRAIN INFLUENCE ZONE

The time to achieve a given percent consolidation is a function of tof the equivalent diameter of soil cylinder, De. This variable is contsince it is a function of drain spacing and pattern. Vertical drains ainstalled in square or triangular patterns as shown in Fig. below. Tbetween drains establishes De through the following relationships

De

= 1. 13S (square pattern)

De = 1.05S (triangular pattern)

The square pattern has the advantage for easier layout and contropattern is usually preferred. However, the triangular pattern proviuniform consolidation between drains.


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DRAIN INFLUENCE ZONE


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WELL RESISTANCE

Hansbo (1979, 1981) presented, for equal-strain conditions, aform solution which allows for ready computation of the effewell-resistance on drain performance. The finite drain perme(well-resistance) was considered by imposing on the continuitequation of flow toward the drain. In this assumption, the flothe considered section of the drain is equal to the maximum f

which can be discharged through the drain.


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WELL RESISTANCE

The introduction of the well-resistance concept affects the vadegree of consolidation, U, which is no longer constant with dshown in Fig. below.

WELL RESISTANCE


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WELL RESISTANCE


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WELL RESISTANCE

Taking well-resistance into consideration, the rate of radialconsolidation is controlled not only by Ch and D. but also by tqw/kh as shown in Fig. below. This factor may play a very improle when prefabricated band drains of great lengths are usedtypical values of qw/kh less than 500 m

2 where the time necesachieve a specific degree of consolidation is increased (Jamio

al. 1983).


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WELL RESISTANCE


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WELL RESISTANCE

The influence of well-resistance on the consolidation rate incrthe drain length increases. This is illustrated in Fig. below for aband-shaped drain (qw/kh = 400 m

2).

WELL RESISTANCE


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WELL RESISTANCE


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SMEAR EFFECTS AND DISTURBANCES

Although there are numerous variations in installation equipmvertical drains, most of the equipment has fairly common featsome of which can directly influence the drain performance. Tinstallation rigs are usually track-mounted boom cranes. The protects the drain during installation and creates the space fodrain by displacing the soil during the penetration. The mand

penetrated into the subsoil using either static or vibratory fordrain installation results in shear strains and displacement of surrounding the drain.


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An example of soil movements produced in Bangkok clay as athe installation of displacement sand drains is given in Fig. beshearing is accompanied by increases in total stress and pore



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The installation results in disturbance to the soil around the ddisturbance is most dependent on the mandrel size and shapmacrofabric, and installation procedure. The mandrel cross-seshould be minimized, while at the same time, adequate stiffnmandrel is required. Bergado et al. (1991), from a full scale teembankment performance, obtained faster settlement rate in

mandrel area than in the large mandrel area indicating lesser zone in the former than the latter.


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For design purposes, it has been evaluated by Jamiolkowski e(1981) that the diameter of disturbed zone, ds, can be relatedcross-sectional dimension of the mandrel as follows:

where dm is the diameter of a circle with an area equal to the sectional area of the mandrel. At this diameter, the theoreticastrain is approximately 5 % as shown in Fig. below

2

)65(m

s

dd



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Hansbo (1987) recommended the following expression basedresults of Holtz and Holms (1973) and Akagi (1979):

This relationship has been verified in the reconstituted soft Ba

clay by Bergado et al. (1991) using a specially designed laboratesting apparatus as plotted in Fig. below.

ms dd 2


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The influence of smear increases with increasing drain diamet

sand drain or mandrel diameter for prefabricated drains (Han1981). The time-settlement relationships obtained from full stest embankment (Bergado et al. 1991) is shown in Fig. belowmandrel area together with the settlement prediction. Theperformance of PVD is well predicted with smear effect taken

consideration using kh/kv = 10 and ds =2dm. (Bergado et al. 19



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prefabricated vertical drains (pvd)

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