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

Introduction

• In times of urbanization, growth of populationand associated developments, constructionactivities are more and more focused on soilswhich were considered unsuitable in the pastdecades. These soft soil deposits have a lowdecades. These soft soil deposits have a lowbearing capacity and exhibit large settlementswhen subjected to loading. It is thereforeinevitable to treat soft soil deposits prior toconstruction activities in order to preventdifferential settlements and subsequentlypotential damages to structures.

Introduction cont.• According to Bergado et al. (1996) ground

improvement techniques can be divided broadlyinto two categories.

1. Technique which require foreign materials andutilisation of reinforcements. They are based onstiffening columns either by the use of astiffening columns either by the use of agranular fill (stone columns), by piling elementswhich are not reaching a still soil stratum (creeppiles) or by in situ mixing of the soil withchemical agents (deep stabilisation).

2. Technique which includes strengthening of thesoil by dewatering, i.e. preloading techniquesoften combined with vertical drains.

Preloading OR Precompression

• Preloading is surcharging the ground with auniformly distributed surface load prior to theconstruction of the structure such as Buildings,Embankments, Motorways, and Runways Tanksetc.)etc.)

• The purpose is to take up the settlements underthe Civil Engineering structures before they arebuilt.

• Soils suitable for preloading: Compressible soft tomedium soft saturated clays and silts, organicclays, peats.

Preloading cont.

Primary consolidation using Surcharge loading

Primary consolidation by using surcharge loading

• Types of preloads:

Earth fills (Most common), Water in tanks orponds, vaccum application under a membrane,special anchor and jack systems, groundwaterlowering, 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 thesoil.

Preloading TechniquesA. 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 isinitially carried by the pore water. When the soilis not very permeable, which is normally theis not very permeable, which is normally thecase; the water pressure will decrease graduallybecause the pore water is only able to flow awayvery slowly in vertical direction.

• In order not to create any stability problems, theload must mostly be placed in two or morestages.

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.

Conventional Preloading cont.• The temporary surcharge can be removed when the

settlements exceeds the predicted final settlement.

• This should preferably not happen before theremaining excess pore pressure is below the stressincrease caused by the temporary surcharge.

• By increasing the time of temporary overloading, orthe size of the overload, secondary settlement canbe reduced or even eliminated.be reduced or even eliminated.

• This is because by using a surcharge higher than thework load, the soil will always be in an overconsolidated state and the secondary compressionfor over consolidated soil are much smaller than thatof normally consolidated soil. This will benefit greatlythe subsequent geotechnical design.

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.

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.

Vacuum Preloading cont.

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.

Vacuum Preloading cont.

• Anchoring and sealing of the system at the

periphery.

• Reducing lateral seepage towards the vacuum

area.area.

Principles of PreloadingFigure illustrates schematically a vertical stress profilewhen a vacuum load (assuming 100 % efficiency) isapplied to the ground surface in comparison with initialconditions and conventional surcharge.

a) initial in situ conditions

Principles of Preloading cont.

b) conventional surcharge c) vacuum induced surcharge

Vertical Drains

• Preloading technique may not work sometimesalone due to a thick uniform soft clay layer orpermeability of the clay is very low.

• so time for precompression is very long and notpractical or surcharge will be very high forreasonable waiting periods.reasonable waiting periods.

• Because of its low permeability, the consolidationsettlement of soft clays takes a long time tocomplete. To shorten the consolidation time,vertical drains are installed together withpreloading either by an embankment or bymeans of vacuum pressure.

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.

Vertical Drains cont.

Preloading with vertical drains

Maximum Length [m]

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 theground with resulting displacement in bothvertical and horizontal directions.

• The installation causes therefore disturbances,especially in soft and sensitive clays, whichespecially in soft and sensitive clays, whichreduces the shear strength and horizontalpermeability.

• The low- or non-displacement installations areconsidered to have less disturbing effects on thesoil. Drilling of the hole is done by means of anauger or water jets. In terms of jetting, however,installation is very complex.

Types of Vertical Drains conti.

• The installation of prefabricated vertical drains is alsodone by a mandrel and it is a displacementinstallation. The dimensions of the prefabricateddrains are much smaller compared to sand drainsand subsequently are the dimensions of themandrel. Thus, the degree of soil disturbance causedby the size of the mandrel during installations islower.by the size of the mandrel during installations islower.

• At the tip of the mandrel is detachable shoe oranchor made of a small piece of metal (given infollowing figure). Sometimes it might also be a pieceof drain itself. The purpose of the anchor is toprevent soil from entering the mandrel and pluggingit during penetration. It also keeps the drain at thedesired depth as the mandrel is withdrawn.

Some disadvantages of sand drains• To receive adequate drainage properties, sand has to be

carefully chosen which might seldom be found close tothe construction site.

• Drains might become discontinuous because of carelessinstallation or horizontal soil displacement during theconsolidation process.

• During filling bulking of the sand might appear whichcould lead to cavities and subsequently to collapse due tocould lead to cavities and subsequently to collapse due toflooding.

• Construction problems and/or budgetary burdens mightarise due to the large diameter of sand drains.

• The disturbance of the soil surrounding each draincaused by installation may reduce the permeability, theflow of water of water to the drain and thus theefficiency of the system.

• The reinforcing effect of sand drains may reduce theeffectiveness of preloading the subsoil.

Drain Properties• Equivalent diameter for band-shaped drains

The theory of consolidation with vertical drains assumes that thevertical drains are circular in their cross-section. Since most of theprefabricated drains are rectangular in cross-section, therectangular drain has to be converted into an equivalent cylindricalshape. That implies that the equivalent diameter has the sametheoretical radial drainage capacity as the band-shaped drain. Theequivalent diameter (dw) of a band-shaped drain with width (a) andthickness (b) as given below.

Discharge Capacity

The discharge capacity depends on the following factors:

• Lateral earth pressure: By increasing lateral pressure, the filterpasses into the core and subsequently decreases the dischargecapacity due to a reduction of the cross-sectional area available forflow.

• Large settlements: During consolidation, the ground will besubjected to large settlements. Thus, the drains tend to settletogether with the ground which will result in bending of folding ofthe 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 soilthrough the drain filter, the displaced water will contain a smallportion of fine particles. These may be deposited with the corechannels and may lead to clogging of the drain.

• Time: The discharge capacity may be reduced due to aging in thesoil after installation, possibly due to biological and chemicalactivities.

• Hydraulic gradient: The measured discharge capacity varies withdifferent hydraulic gradients and is smaller when a higher hydraulicgradient is used. This might be due to the loss of flow energy as aresult of turbulent flow at a high hydraulic gradient.

Properties of the filterThe filter has to meet the following requirements:

• The permeability of the filter should be high enoughnot to influence the discharge capacity of the drainsystem.

• On the contrary, the permeability of the filter shouldbe low enough to retain fine soil particles. The soilparticles might penetrate through the filter into thecore, which eventually might be filled with soil and getcore, which eventually might be filled with soil and getclogged.

• The filter needs to be strong enough to withstand highlateral pressure in order not to be squeezed intochannel system of the core.

• The filter should be strong enough not to break duringinstallation, and

• The filter should not deteriorate with time because thiswould reduce the discharge capacity of the drain.

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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