GROUND IMPROVEMENT BY PRELOADING AND DRAINAGE.

1. GROUND IMPROVEMENT BY
PRELOADING AND DRAINAGE
Prepared By:-
Sagar Halwawala (11BCL014)
Himanshu Manolkar (11BCL015)Himanshu Manolkar (11BCL015)
Satish Kambaliya (11BCL016)
Abhinav Karmilla (11BCL017)
Civil Engineering Department
Institute Of Technology Nirma University,
Ahmedabad

2. Introduction
• In times of urbanization, growth of population
and associated developments, construction
activities are more and more focused on soils
which were considered unsuitable in the past
decades. These soft soil deposits have a lowdecades. These soft soil deposits have a low
bearing capacity and exhibit large settlements
when subjected to loading. It is therefore
inevitable to treat soft soil deposits prior to
construction activities in order to prevent
differential settlements and subsequently
potential damages to structures.

3. Introduction cont.
• According to Bergado et al. (1996) ground
improvement techniques can be divided broadly
into two categories.
1. Technique which require foreign materials and
utilisation of reinforcements. They are based on
stiffening columns either by the use of astiffening columns either by the use of a
granular fill (stone columns), by piling elements
which are not reaching a still soil stratum (creep
piles) or by in situ mixing of the soil with
chemical agents (deep stabilisation).
2. Technique which includes strengthening of the
soil by dewatering, i.e. preloading techniques
often combined with vertical drains.

4. Preloading OR Precompression
• Preloading is surcharging the ground with a
uniformly distributed surface load prior to the
construction of the structure such as Buildings,
Embankments, Motorways, and Runways Tanks
etc.)etc.)
• The purpose is to take up the settlements under
the Civil Engineering structures before they are
built.
• Soils suitable for preloading: Compressible soft to
medium soft saturated clays and silts, organic
clays, peats.

5. Preloading cont.
Primary consolidation using Surcharge loading
Primary consolidation by using surcharge loading

6. • Types of preloads:
Earth fills (Most common), Water in tanks or
ponds, vaccum application under a membrane,
special anchor and jack systems, groundwater
lowering, electro osmosis.
• The surcharge results in;
Preloading cont.
• The surcharge results in;
1. Primary Consolidation Settlement
2. Secondary Consolidation Settlement
3. Increase in the undrained shear strength of the
soil.

7. Preloading Techniques
A. Conventional Preloading
• The simplest solution of preloading is a preload,
e. g. by means of an embankment.
• When the load is placed on the soft soil, it is
initially carried by the pore water. When the soil
is not very permeable, which is normally theis not very permeable, which is normally the
case; the water pressure will decrease gradually
because the pore water is only able to flow away
very slowly in vertical direction.
• In order not to create any stability problems, the
load must mostly be placed in two or more
stages.

8. Conventional Preloading cont.
The principle of
Conventional
Preloading
If the temporary
load exceeds theload exceeds the
final construction
load, the excess
refers to as
surcharge load.

9. Conventional Preloading cont.
• The temporary surcharge can be removed when the
settlements exceeds the predicted final settlement.
• This should preferably not happen before the
remaining excess pore pressure is below the stress
increase caused by the temporary surcharge.
• By increasing the time of temporary overloading, or
the size of the overload, secondary settlement can
be reduced or even eliminated.be reduced or even eliminated.
• This is because by using a surcharge higher than the
work load, the soil will always be in an over
consolidated state and the secondary compression
for over consolidated soil are much smaller than that
of normally consolidated soil. This will benefit greatly
the subsequent geotechnical design.

10. B. Vacuum Preloading
• Sometimes soft soil may be so weak that even a
common 1.5 m embankment might cause
stability problems. Then it can be suitable to use
the method of vacuum preloading.
• In 1952 Kjellman was the first who introduced
vacuum preloading to accelerate consolidation. Invacuum preloading to accelerate consolidation. In
vacuum consolidation the surcharge load is
replace by atmospheric pressure.
• In its simplest form the method of vacuum
consolidation consists of a system of vertical
drains and a drainage layer (sand) on top.

11. Vacuum Preloading Conti….
• The common advantages of vacuum preloading
are that there is no extra fill material needed, the
construction times are generally shorter and it
requires no heavy machinery. Moreover, norequires no heavy machinery. Moreover, no
chemical admixtures will penetrate into the
ground and thus it is an environmental friendly
ground improvement method.

12. Vacuum Preloading cont.

13. Vacuum Preloading cont.
Possible problems associated with vacuum
preloading are:
• To maintain an effective drainage system under
the membrane that expels water and air
throughout the whole pumping duration.
• Keeping non-water saturated medium below the• Keeping non-water saturated medium below the
membrane.
• To maintain an effective level of vacuum.
• To maintain a leak proof system in particular at
the pumps / membrane connections and over the
entire membrane area.

14. Vacuum Preloading cont.
• Anchoring and sealing of the system at the
periphery.
• Reducing lateral seepage towards the vacuum
area.area.

15. Principles of Preloading
Figure illustrates schematically a vertical stress profile
when a vacuum load (assuming 100 % efficiency) is
applied to the ground surface in comparison with initial
conditions and conventional surcharge.
a) initial in situ conditions

16. Principles of Preloading cont.
b) conventional surcharge c) vacuum induced surcharge

17. Vertical Drains
• Preloading technique may not work sometimes
alone due to a thick uniform soft clay layer or
permeability of the clay is very low.
• so time for precompression is very long and not
practical or surcharge will be very high for
reasonable waiting periods.reasonable waiting periods.
• Because of its low permeability, the consolidation
settlement of soft clays takes a long time to
complete. To shorten the consolidation time,
vertical drains are installed together with
preloading either by an embankment or by
means of vacuum pressure.

18. Vertical Drains cont.
• Therefore, the vertical drain installation reduces
the length of the drainage path and,
consequently, accelerates the consolidation
process and allows the clay to gain rapid strengthprocess and allows the clay to gain rapid strength
increase to carry the new load by its own.

19. Vertical Drains cont.
Preloading with vertical drains

20. Maximum Length [m]

21. Types of Vertical Drains
• Sand drains are basically boreholes filled with
sand. As for the displacement type of sand drains,
a closed mandrel is driven or pushed into the
ground with resulting displacement in both
vertical and horizontal directions.
• The installation causes therefore disturbances,
especially in soft and sensitive clays, whichespecially in soft and sensitive clays, which
reduces the shear strength and horizontal
permeability.
• The low- or non-displacement installations are
considered to have less disturbing effects on the
soil. Drilling of the hole is done by means of an
auger or water jets. In terms of jetting, however,
installation is very complex.

22. Types of Vertical Drains conti.
• The installation of prefabricated vertical drains is also
done by a mandrel and it is a displacement
installation. The dimensions of the prefabricated
drains are much smaller compared to sand drains
and subsequently are the dimensions of the
mandrel. Thus, the degree of soil disturbance caused
by the size of the mandrel during installations is
lower.
by the size of the mandrel during installations is
lower.
• At the tip of the mandrel is detachable shoe or
anchor made of a small piece of metal (given in
following figure). Sometimes it might also be a piece
of drain itself. The purpose of the anchor is to
prevent soil from entering the mandrel and plugging
it during penetration. It also keeps the drain at the
desired depth as the mandrel is withdrawn.

23. Some disadvantages of sand drains
• To receive adequate drainage properties, sand has to be
carefully chosen which might seldom be found close to
the construction site.
• Drains might become discontinuous because of careless
installation or horizontal soil displacement during the
consolidation process.
• During filling bulking of the sand might appear which
could lead to cavities and subsequently to collapse due tocould lead to cavities and subsequently to collapse due to
flooding.
• Construction problems and/or budgetary burdens might
arise due to the large diameter of sand drains.
• The disturbance of the soil surrounding each drain
caused by installation may reduce the permeability, the
flow of water of water to the drain and thus the
efficiency of the system.
• The reinforcing effect of sand drains may reduce the
effectiveness of preloading the subsoil.

24. Drain Properties
• Equivalent diameter for band-shaped drains
The theory of consolidation with vertical drains assumes that the
vertical drains are circular in their cross-section. Since most of the
prefabricated drains are rectangular in cross-section, the
rectangular drain has to be converted into an equivalent cylindrical
shape. That implies that the equivalent diameter has the same
theoretical radial drainage capacity as the band-shaped drain. The
equivalent diameter (dw) of a band-shaped drain with width (a) and
thickness (b) as given below.

25. Discharge Capacity
The discharge capacity depends on the following factors:
• Lateral earth pressure: By increasing lateral pressure, the filter
passes into the core and subsequently decreases the discharge
capacity due to a reduction of the cross-sectional area available for
flow.
• Large settlements: During consolidation, the ground will be
subjected to large settlements. Thus, the drains tend to settle
together with the ground which will result in bending of folding of
the drain.
• Clogging of drain: In the initial filtering process of flow from the soil• Clogging of drain: In the initial filtering process of flow from the soil
through the drain filter, the displaced water will contain a small
portion of fine particles. These may be deposited with the core
channels and may lead to clogging of the drain.
• Time: The discharge capacity may be reduced due to aging in the
soil after installation, possibly due to biological and chemical
activities.
• Hydraulic gradient: The measured discharge capacity varies with
different hydraulic gradients and is smaller when a higher hydraulic
gradient is used. This might be due to the loss of flow energy as a
result of turbulent flow at a high hydraulic gradient.

26. Properties of the filter
The filter has to meet the following requirements:
• The permeability of the filter should be high enough
not to influence the discharge capacity of the drain
system.
• On the contrary, the permeability of the filter should
be low enough to retain fine soil particles. The soil
particles might penetrate through the filter into the
core, which eventually might be filled with soil and getcore, which eventually might be filled with soil and get
clogged.
• The filter needs to be strong enough to withstand high
lateral pressure in order not to be squeezed into
channel system of the core.
• The filter should be strong enough not to break during
installation, and
• The filter should not deteriorate with time because this
would reduce the discharge capacity of the drain.

27. CONCLUSION
• A system of prefabricated vertical drains(PVDs) combined with
vacuum preloading is an effective method for accelerating soil
consolidation.
• It is clear that the application of PVDs combined with vacuum and
surcharge preloading has become common practice, and is now
considered to be one of the most effective ground improvement
techniques.
• Analytical and numerical modelling of vacuum preloading is still a• Analytical and numerical modelling of vacuum preloading is still a
developing research area. There has always been a discrepancy
between the predictions and observed performance of
embankments stabilised with vertical drains and vacuum pressure.
• Vacuum assisted consolidation is an innovative method which has
recently, and successfully, been used for large scale projects on very
soft soils in reclamation areas. The extent of surcharge fill can be
decreased to achieve the same amount of settlement and the
lateral yield of the soft soil can be controlled by PVDs used in
conjunction with vacuum pressure.

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