Design of a vertical drainage using wick drain

DESIGN VERTICAL DRAINAGE USING WICK DRAIN.
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Designing Vertical Drainage Using Wick Drains. 1

Abstract.
For an improved and accelerated radial drainage and soil consolidation, it is effective to
employ the services of vertical drain system with surcharge preloading. This technical report
paper illustrates the procedure of vertical drain design using prefabricated vertical drains (PVDs),
also known as wick drains. This paper also extensively digs into covering such techniques as
lateral permeability reduction, effects of smear zones overlapping and undrained shear strength
gain after consolidation.
The illustration of the vertical drain construction procedure methodologies gives
practitioners a wider scope to solving soil consolidation problems. Soil consolidation makes soil
reduce its water content when subjected to some external loading, making its particles to be
closely compacted together. Drainage time is faster in loose soils than consolidated soils.
Title.
Design of a vertical drainage using wick drain
Introduction.
Vertical drain consolidation is preferable when looking for a shorter length of the
horizontal drainage leading to a decrease in the time taken for adequate consolidation to be
achieved (Agarwal et al., 2016).
Soft, deep clayey soils require routine consolidation process for the purposes of
improving the soil strata. Increased input of vertical pressure originally raises the soil pore water
pressure (PWP) (Fellenius., 2017). Over time, the excess pore water pressure releases when
water starts flowing from the soil leaving the particles to be packed closer to one another. For
deep and low permeable clay soils the procedure of consolidation needs time, probably years or
Designing Vertical Drainage Using Wick Drains. 2

months for sufficient soil strata improvement (Serpico., 2017). Time is money and time waits for
no man. Most projects in construction require that the processes of soil consolidation take the
shortest periods of time possible in the order of few weeks or utmost less than three months. In
this situation, prefabricated vertical drains, better known as wick drains are applied.
There are several factors that influence the timeframe necessary for complete
consolidation process for soils which are deep and low permeable, but the main factor that affects
the timeframe is the length of the flow path (Sobhanmanesh et al., 2017). For deep natural clay
soils with extensive plans and continuous plans, there is a vertical flow of water in the upward
direction. Similarly, in situations where sand or gravel, soils that are highly permeable, lies under
the clay soil, there will be a vertical flow of water I both upward and downward direction. This
kind of scenarios usually leads to flow paths of longer lengths, usually in tens to hundreds of
meters, for soils with deep strata (Zamierowski., 2017).
To effectively and significantly minimize the required time for adequate soil
consolidation, prefabricated vertical drains (PVD) or wick drains must be installed down through
the concerned soil strata in a vertical orientation (Le et al., 2018). These drains make the vertical
flow path highly permeable thus facilitating water flow to the top within the shortest time
possible, or to a drainage layer at the bottom of the soil strata if any. By using wick drains the
length of the flow path in the soils of low permeability is reduced to about half of the drain
spacing. To install the drains, they are spaced triangularly or squarely having a spacing of 2.5m
to 6.5m centers. Moreover, in some soil types, it is advantageous to use a horizontal flow path
due to higher permeability than vertical flow path. Thus, it is wise and more effective to involve
horizontal flow paths for intersecting the vertical drain than typically decreasing the flow path
length. vertical drains have come a long way in terms of evolution. The first ones were known as
Designing Vertical Drainage Using Wick Drains. 3

sand drains, developed by placing clean sand in a borehole. Subsequently, cardboard drains
followed to address challenges like difficulty in getting the most suitable sand as well as
reducing time and cost of installing drains. This was followed by the invention of the wick
drains, with a higher capacity and has significantly improved cost efficiency of the drains and the
potential installation depth as well as the performance output (Alostaz., 2014).
Smear effects and well resistance connected to vertical drains are considered to better
understand consolidation theory with regards to radial drainage. Mandrel driven Wick drains
have smear zones with radius in the range of twice or thrice the mandrel radius, with the
undisturbed soil permeability to smear zones ratio being between 1 - 8. Laboratory tests prove
that there is a substantial decrease of the horizontal permeability towards the drain located in the
smear zone in a non-linear way. It is possible to reduce horizontal permeability till it matches the
vertical stability, closer to the drain, a process known as complete remolding (Hu et al., 2017).
During a sharp decrease in this ratio, while approaching the drain, the vertical
permeability along the radial vector becomes generally constant automatically (Kelly., 2014).
The disparity in the consolidation degree acquired from the linear and non-linear variation of
horizontal permeability lack significance until the value approached by the undisturbed
horizontal permeability coefficient and the minimum horizontal permeability coefficient in the
smear zone is different (Li and Yang., 2015). Drain performance can further be affected by the
reduction of drain spacing causing smear zone to overlap (Tomlinson, 2015).
To arrive at the most suitable drain spacing during design, several iterations are
conducted. One of the New designs tends to avoid the trial and error drain spacing determination
approach. The trial and error approach suffers from limitations like being time-consuming,
tedious and cumbersome (Small, 2016). However, this new design ignores the effects of quasi-
Designing Vertical Drainage Using Wick Drains. 4