EFFECT OF WATER TABLE ON SOIL

DURING C0NSTRUCTION

BHARAT INTITUTE OFTECHNOLOGY,MEERUT

SUMITTED TO:Mr.Ehtesham anwarAsst.prof.Civil.engg.Bit meerut

SUBMITTED BY:Roop kishorCE.4th yearRoll.No.1012800090

What Will Be Covered:1.Water table2.Selection of foundation3.Effect of water table on bearing capacity 4.Mechanism of failure5.Procedure of under water construction

WATER TABLE: water table is the surface where the water pressure head is equal to the atmospheric pressure (where gauge pressure = 0)

(from Keller, 2000, Figure 10.9)

The water table is actually a sloping surface.Slope (gradient) is determined by the difference in water table elevation (h) over a specified distance (L).Direction of flow is downslope.Flow rate depends on the gradient and the properties of the aquifer.

Groundwater Movement -

Overpumping will have two effects:1. Changes the groundwater flow direction.2. Lowers the water table, making it necessary to dig a deeper well.• This is a leading cause for desertification in some areas.• Original land users and land owners often spend lots of money to drill new, deeper wells.• Streams become permanently dry.

Groundwater Overdraft

Groundwater -- Artesian Conditions

Ephemeral Stream(influent)

• Semiarid or arid climate• Flows only during wet periods (flashy runoff)• Recharges groundwater

Shallow foundations: › Where the ratio of embedment depth to min plan

dimension is less or equal to 2.5 › Embedment depth is the depth below the ground surface

where the base of foundation rests.

a. plain concrete foundation,b. stepped reinforced concrete foundation,c. reinforced concrete rectangular foundation,d. reinforced concrete wall foundation.

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1 Obtain the required information concerning the nature of the superstructure and the loads to be transmitted to the foundation.

2. Obtain the subsurface soil conditions.3. Explore the possibility of constructing any one of the types of

foundation under the existing conditions by taking into account (i) the bearing capacity of the soil to carry the required load, and (ii) the adverse effects on the structure due to differential settlements. Eliminate in this way, the unsuitable types.

4. Once one or two types of foundation are selected on the basis of preliminary studies, make more detailed studies. These studies may require more accurate determination of loads, subsurface conditions and footing sizes. It may also be necessary to make more refined estimates of settlement in order to predict the behavior of the structure.

5. Estimate the cost of each of the promising types of foundation, and choose the type that represents the most acceptable compromise between performance and cost.

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Total Overburden Pressure q0

Effective Overburden Pressure q'0

The Ultimate Bearing Capacity of Soil, qu

The Net Ultimate Bearing Capacity, qnu

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Gross Allowable Bearing Pressure, qa is expressed as:

where Fs = factor of safety.

Net Allowable Bearing Pressure, qna

Safe Bearing Pressure, qs

qs is defined as the net safe bearing pressure which produces a settlement of the foundation which does not exceed a permissible limit.

Note: In the design of foundations, one has to use the least of the two values of qna and qs.

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Ultimate Bearing Capacity of Soil Strip Footings:› Terzaghi developed his bearing capacity equation for strip

footings by analyzing the forces acting on the wedge abc in Fig.

where Qult = ultimate load per unit length of footing, c = unit cohesion, /the effective unit weight of soil, B = width of footing, D,= depth of foundation, Nc, Nq and Nɣ are the bearing capacity factors. They are functions of the angle of friction ɸ.

where Kp = passive earth pressure coefficient

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The determination of bearing capacity of soil based on the classical earth pressure theory of Rankine (1857) began with Pauker, a Russian military engineer (1889).

It was modified by Bell (1915). Pauker's theory was applicable only for sandy soils but the theory of Bell took into account cohesion also.

The methods of calculating the ultimate bearing capacity of shallow strip footings by plastic theory developed considerably over the years since Terzaghi (1943). Terzaghi extended the theory of Prandtl (1921).

Taylor (1948) extended the equation of Prandtl by taking into account the surcharge e Terzaghi (1943) first proposed a semi-empirical equation for computing the ultimate bearing capacity of strip footings by taking into account cohesion, friction and weight of soil, and replacing the overburden pressure with an equivalent surcharge load at the base level of the foundation effect of the overburden soil at the foundation level. 14

1. Terzaghi's bearing capacity theory2. The general bearing capacity equation3. Field tests TERZAGHI'S BEARING CAPACITY THEORY

› Terzaghi made the following assumptions for developing an equation for determining qu for a c-ɸ soil.

› The soil is semi-infinite, homogeneous and isotropic,› The problem is two-dimensional,› The base of the footing is rough,› The failure is by general shear,› the load is vertical and symmetrical,› The ground surface is horizontal,› the overburden pressure at foundation level is equivalent to a

surcharge load › Coulomb's law is strictly valid, that is,

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Terzaghi's bearing capacity Eq. has been modified for other types of foundations by introducing the shape factors. The equations are: › Square Foundations: › Circular Foundations:› › Rectangular Foundations:

Ultimate Bearing Capacity qu in Purely Cohesion-less and Cohesive Soils Under General Shear Failure:

For cohesion-less soil (for c = 0) and cohesive soils (for ɸ = 0) as follows.

Strip Footing Square footing .circular footing

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In case the water table lies at any intermediate depth less than the depth (Df+ B), the bearing capacity equations are affected due to the presence of the water table.

Case 1. When the water table lies above the base of the foundation.

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The zones of plastic equilibrium represented in this figure by the area gedcf may be subdivided into three zones:

› 1 . Zone I of elastic equilibrium› 2. Zones II of radial shear state› 3. Zones III of Rankine passive state

› When load qu per unit area acting on the base of the footing of width B with a rough base is transmitted into the soil, the tendency of the soil located within zone I is to spread but this is counteracted by friction and adhesion between the soil and the base of the footing.

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Caissons – usually refers to structures which are constructed offsite and then brought to site in one piece or in a series of independent modules.

Cofferdams – usually refers to structures in water that are constructed on site, often from standard parts. Identical structures on land are not usually called cofferdams and the name seems to be falling out of use.

(a) Box caisson floated into place with ballast as required. (b) Caisson filled with appropriate material – water may be

pumped out first. Hollow caissons can be used to house equipment – filled

they can be used as foundations.

Open caissons permit excavation or other work to be carried out inside the caisson.

The caisson will sink down into the soil as excavation proceeds.

Sections can be added on top to increase height.

Water can be pumped out to permit dry work.

Pneumatic Caissons can be sunk with the aid of compressed air.

Provides a dry working chamber.

Regulations apply › Volume air supply› Caisson sickness› The bends› Structural integrity› Man management

Cut off walls sunk into low permeability material› Sheet piles

Usually steel interlocking

› Contiguous bored piles Problems with seals at

joints› Vibrated beam wall

Vibrate “H” pile into ground and inject grout as pile removed – usually permanent.

Pump water from sump. System can be used for construction below water table on land or in rivers etc.

Can lower water table by sinking wells and pumping water (at a rate faster than the re-entry rate) to a suitable location. Must consider silt content etc. of pumped water and effect on ground water flow.

Completely sealed system.

Must cater for upthrust.

Only direct rainfall needs to be pumped out.

Horizontal barrier can be concrete, clay, ground freezing etc.

Effectively confined to land sites › with low permeability soils› to lower water table slightly over large

area Sink a series of wells

› generally on a grid pattern. Pump water from wells

› Ground water will flow towards excavation› Consider environmental effect of pumped

water.

Freeze the water.› Requires a lot of energy.› Soil mass expands

can cause damage changes properties of soil mass

Cement grouting› Cement reacts with water› Permanently changes properties of soil mass› Generally used as ground strengthening

Other chemical reactants

For processes that can be carried out underwater.› Welding› Concreting› Assembly work› Inspections

Divers Remote controlled equipment Remote handling

Effectively confined to land sites › with low permeability soils› to lower water table slightly over large

area Sink a series of wells

› generally on a grid pattern. Pump water from wells

› Ground water will flow towards excavation› Consider environmental effect of pumped

water.

Freeze the water.› Requires a lot of energy.› Soil mass expands

Cement grouting› Cement reacts with water› Permanently changes properties of soil mass

Other chemical reactants

For processes that can be carried out underwater.› Welding› Concreting› Assembly work› Inspections

Divers Remote controlled equipment Remote handling

Pumping water from a well causes a cone of depression to form in the water table at the well site.

Groundwater Movement -- Cone of Depression

Water table

Cone of depressionflowflow