86877098 well foundation

CE- 471: Foundation Engineering

Chapter 7. Well Foundations


Introduction

Well foundations are being used in India from very early days. Taj Mahal was built on such foundations. Wells are classified as deep foundations. The main difference between a well and a pile foundation is that, while a pile is flexible like a beam under horizontal loads, the well undergoes rigid body movement under such loads.


Types of Wells

Wells have different shapes and accordingly they are named as

1. Circular Wells

2. Dumb bell

3. Double-D Wells

4. Double Octagonal Wells

5. Single and Double Rectangular Wells

6. Multiple Dredged Holed Wells


Components of Well Foundation

The various component of a well foundations are

1. Cutting Edge

2. Well Curb

3. Bottom Plug

4. Steining

5. Top Plug

6. Well Cap


Design of Wells

Design of wells basically involves finding

1. Depth of the well

2. Size of the well and

3. Design of the other components.


Depth of Scour

Well foundations are constructed in river beds, they should be taken to a safe depth well below the anticipated scour level. Scour around piers depends on several factors like flood discharge, the angle of attack of the flow, flow obstruction etc. The scour depth is calculated as follows.

(mm) bedriver in particle soil ofdiameter Mean D

D1.76 factor Silt s

/s)(m dischargeDesign q

(m)depth Scour D

where

473.0

m

mf

3

s

31

f

s

s

qD


Values of Silt Factor

Type of material Mean diameter (mm) sf

Coarse silt 0.04 0.35

Fine sand 0.08 0.5

Fine sand 0.15 0.68

Medium sand 0.3 0.96

Medium sand

0.5 1.24

Coarse sand

0.7 1.47

Coarse sand

1 1.76

Coarse sand

2 2.49


The foundation should be taken well below the scour level to protect it from any movement due to the force of the stream and other external forces.

welloflength GripD

.3

1

sDD

Grip Length for Wells


Size of Wells

The size of dredge hole of a well varies. In small and shallow wells, the minimum diameter of the dredge hole should be 1.8 m. In larger wells, the minimum size of the dredge hole should be 3 m. The final size is decided after satisfying the lateral stability condition of the wells.


Bearing Capacity of Wells

levelscour below foundation wellofDepth

wellofdimension maller

valueSPT Corrected

)(kN/mcapacity bearing afe

where

100

)D]N16(100B[5.4Nq

failureshear against

safety for valueN itson based sandsfor pressure bearing

allowablefor formula following therecommends 3955 IS

2a

22

a

D

SB

N

Sq


This is subjected to different types of stresses. At the sinking it is subjected to water and earth pressure. At dredging stage, inside surface is subjected to water pressure while outside surface to the earth pressure. IRC recommends some rules of thumb for fixing the thickness of steining which are given below.

Steining


etc. shale kankar, boulders,for 1.20

clayfor 1.15

claysoft for 1.1

stratasandy for 1.0

where

)12.001.0(T

wellsD-double andr rectangulaFor 2.

well theofdiameter External

wellofHeight

etc. shale kankar, boulders, clay, hard

including strata hardfor 1.25

claysoft andsilty sandy,for 1.1

where

)1.001.0(T

wellsshaped-dumbbell andcircular For 1.

steining concreteCement

H

k

Dk

D

D

k

DDk

H

e

eH


clay hardfor 1.25

claysoft for 1.1

sandfor 1.0 k

where

40

D

8

DkT

SteiningBrick

He


The curb of a well transfers all the superimposed loads to the soil through the cutting edge while sinking. The material used for curbs may be timber or RCC. The forces acting on well curb are shown in Fig(b). The total horizontal force on the well curb on both sides is

well theof angle Internal

steining of line centre thealonglength unit per

curb and wellof Weight W

wellofdiameter Internal D

where

2

DDWcot

ie

i

Curb


The cutting edge is provided at the bottom of the well below the curb to cut through the soil during sinking. It is generally made of steel and welded to an angle iron to fit the outer dimensions of the well steining. The height of the cutting edge is given by

concrete of stress ecompressiv Safe f

edge cutting of Thickness t

rock ofstrength Crushing

where

tanfh

c

c.

.e

u

u

q

tq

Cutting Edge

The value of is usually taken as 300. The choice of this angle has been proved to be suitable for easy access to the cutting edge.


Bottom Plug

After final grounding of the well to the required foundation level, a concrete plug is provided. The bottom plug transfer the entire load to the ground. The bottom plug functions as an inverted dome supported along the periphery of the steining. As it is not feasible to provide reinforcement at the bottom, it is generally made thick and a rich concrete mix (M20) is used.


Sand Filling

The bottom plug concrete is cured and after curing, the well is filled with sand in saturated condition. Sand filling provides

1. Stability to the bottom of the well.

2. Eliminate the tensile forces at the base

3. Cancels hoop stresses induced in steining


Top Plug

The top plug is provided after the filling is completed. Top plug helps in transferring the load of the pier and superstructure to the steining. The thickness of the top plug is generally kept greater than 50 % of the smaller dimension of the dredge hole. If sand filling is used, the top plug is simply constructed using PCC of 1:2:4 otherwise it is reinforced with steel bars and lean concrete of 1:3:6 is used.


Well Cap

As the shape of the well pier and cap are different, the well cap forms an interim layer to accommodate the pier. The well cap is so designed that the base of the pier is provided with a minimum all round offset. The centre of the well cap is made to coincide with that of the pier and not with that of the well. Such positioning nullifies the effect of the minor shifts which might have occurred during well sinking.


Stability Analysis of Well Foundations

A well foundation supporting a bridge pier is subjected to vertical and horizontal forces. The various forces acting on the well are

1. Self weight of the well and its superstructure

2. Live loads

3. Water currents and buoyancy

4. Temperature, wind and earth quake

5. Breaking and tracking forces

6. Resistance of the well walls

7. Base and skin friction


Terzaghi (1943) gave an approximate solution based on the analysis of the free rigid bulk.

Resolve all forces in vertical direction and obtain the resultant PV.

Resolve the forces in two horizontal directions i.e along and across the pier and get the values of PB and PL


The resultant vertical force PV and the resultant horizontal force PB are considered for analysis.

The forces and earth pressure distribution acting on the well are shown in the figure.

Pressure at any depth z below the scour level is

'

')(p

DKDPz

zKKKz

D

ap


The well is assumed to fail as soon as the soil reaction at the bottom is equal to PD. For equilibrium at that instant

)3(2932D

Dfor Solving

(2) 3

'22

1

3'

2

1 )(

Eabout moment Taking

(1) )2('2

1 )(

'22

1'

2

1

BCF of area-AEF of area

length unit per pressure totalofresultant )(

112

11

1

11

21max

1max

12

max

DHDHH

DDDK

DKDHP

DDDKP

or

DDKKD

P

B

B

B


)(D

1 tan

asgiven is well, theof

line centre theof deflectionangular then the

nts,displaceme horizontal theare and If

)(P veconservati

a yields forces frictional theseofOmision

walland base at thefriction no is There 3.

tscoefficien pressureearth sRankine' are K and K 2.

headbulk light a as treatedis wellThe 1.

sassumption

followingon based is theory This F.safety offactor

aby P reducingby obtained becan depth safeA

force. horizontal

maximum esustain th torequiredlength grip the

is D This D.for solving and (1)equation in D Putting

21

21

max.B

aP

D

1


Stability Analysis of a Heavy Well

In the Terzaghi approximate analysis, it is assumed that the bulkhead tends to rotate about some point O above the lower edge and tends to transfer the soil from elastic to plastic equilibrium. But in case of heavy wells embedded in cohesionless soil, the well is assumed to rotate about its base and the assumed pressure distribution is given in Fig(a). Taking the moment about the base, the value of (PB)max


DH

DDDKP

DH

DKKP

B

aPB

)(''

6

1 )(

asgiven then is force resistance maximumequivlant

The Fig(b).in shown ison distributi pressure

The scour. ofdepth normal thehalf toequal be

toassumed bemay and assess todifficult very is

Ddepth surcharge The surcharge. a as acts soil

uncovered thegs,surroundin well theBeyond

place. takesscouring well, thearoundNormally

)('6

1 )(

22

max

2

3

max


base. at the occurs tension no assuming soil theof

capacity bearing safe n thekept withi be should

pressure foundation maximum The

base well theof modulussection z

base wellof Area A

frictionskin and

buoyancy for allowance makingafter

base wellon the loaddirect net theis W

where

f

iscondition moment goverturnin no for the well

theof base at the f pressure maximum The

2. than less benot shouldsafety offactor The

)(Pa

asgiven is

F safety, offactor a allowingafter force resisting

total the well, theoflength or diameter theis d If

max

A

W

F

dPB


dK''

2FP

2F

dyK''P

thenlevel,

maximum thebelowy depth aat zero is force

shear theIf zero. is forceresultant thewhere

occurs steining on themoment maximum The

a

2

a

y

or


IRC and IS Design Recommendations

The IRC and IS 3955 publications recommend the following procedure for design of well foundations in sand deposits (for clay the expressions should be suitably modified)

1 Check the stability of well under working loads, assuming elastic theory

2. Find the factor of safety of the well against ultimate failure using ultimate load theory


Causes of Tilts and Shifts

1. Nonuniform bearing capacity

2. Obstruction on one side of the well

3. Sand blowing in wells during sinking. It will cause sudden sinking of well

4. Method of sinking: Material should be removed from all sides equally otherwise the well may experience tilt

5. Sudden sinking due to blasting may also cause tilting of well

6. Irregular casting of steining will cause less friction on one side leads to chances of tilting of well.


Rectification of Tilt

1. Eccentric grabbing

2. Eccentric loading

3. Water jetting

4. Arresting the cutting edge

5. Pulling the well

6. Strutting the well

7. Pushing the well by jacks

86877098 well foundation

Documents

foundation engineering

pile foundation

shallow wells

larger wells

circular wells

foundations ce

double rectangular wells

doubled wells