SRUDENT

1. EFFECT OF WATER
TABLE ON SOIL
DURING
C0NSTRUCTION
BHARAT INTITUTE OF
TECHNOLOGY,MEERUT
SUMITTED TO:
Mr.Ehtesham anwar
Asst.prof.
Civil.engg.
Bit meerut
SUBMITTED BY:
Roop kishor
CE.4th
year
Roll.No.1012800090

2. What Will Be Covered:
1.Water table
2.Selection of foundation
3.Effect of water table on bearing capacity
4.Mechanism of failure
5.Procedure of under water construction

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

4. (from Keller, 2000, Figure 10.9)

5. The watertable 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 -

6. Overpumping will have two effects:
1. Changes the groundwater flow
direction.
2. Lowers the watertable, 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

7. Groundwater -- Artesian Conditions

8. Ephemeral
Stream
(influent)
• Semiarid or arid climate
• Flows only during wet periods (flashy runoff)
• Recharges groundwater

9.  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.
9

10. 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.
10

11.  Total Overburden Pressure q0
 Effective Overburden Pressure q'0
 The Ultimate Bearing Capacity of Soil, qu
The Net Ultimate Bearing Capacity, qnu
11

12.  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.
12

13.  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
13

14.  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

15. 1. Terzaghi's bearing capacity theory
2. The general bearing capacity equation
3. Field tests
 TERZAGHI'S BEARING CAPACITY THEORY
› Terzaghi made the following assumptions for developing
an equation for determining qu for a c- soil.ɸ
› 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,
15

16. 16

17. 17

18.  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 18

19.  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.
19

20. 20

21. 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.
21

23. 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.

25.  (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.

26.  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.

27.  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

28.  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.

29.  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.

30.  Completely sealed
system.
 Must cater for
upthrust.
 Only direct rainfall
needs to be
pumped out.
 Horizontal barrier
can be concrete,
clay, ground
freezing etc.

31.  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.

32.  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

33.  For processes that can be carried out
underwater.
› Welding
› Concreting
› Assembly work
› Inspections
 Divers
 Remote controlled equipment
 Remote handling

34.  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.

35.  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

36.  For processes that can be carried out
underwater.
› Welding
› Concreting
› Assembly work
› Inspections
 Divers
 Remote controlled equipment
 Remote handling

37. 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 depression
flowflow