Well Foundation

Chapter 7. Well Foundations

CE- 471: Foundation Engineering

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. CE- 471: Foundation Engineering

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 Rectangular Wells

6.

Multiple Wells

Dredged

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Double

Holed

Components of Well Foundation The various component of a well foundations are 1. 2. 3. 4. 5. 6.

Cutting Edge Well Curb Bottom Plug Steining Top Plug Well Cap CE- 471: Foundation Engineering

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.

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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. q 13 Ds  0.473 sf where Ds  Scour depth (m) q  Design discharge(m3 /s) sf  Silt factor  1.76 Dm Dm  Mean diameter of soil particle in river bed (mm) CE- 471: Foundation Engineering

Type of material

Values of Silt Factor

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

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Grip Length for Wells 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.

1 D  .D s 3 D  Grip length of well

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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. CE- 471: Foundation Engineering

Bearing Capacity of Wells IS 3955 recommendsthe following formula for allowable bearing pressurefor sands based on its N value for safety against shear failure [5.4N2 B  16(100  N 2 )D] qa  100 where qa  Safe bearing capacity (kN/m2 ) N  CorrectedSPT value B  Smaller dimension of well D  Depth of well foundation below scour level CE- 471: Foundation Engineering

Steining 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. CE- 471: Foundation Engineering

Cement concretesteining 1. For circular and dumbbell - shaped w ells T  k (0.01DH  0.1De ) w here k  1.1 for sandy,silty and soft clay 1.25 for hard strata including hard clay, boulders, kankar, shale etc. DH  Height of w ell De  External diameter of the w ell 2. For rectangular and double - D w ells T  k (0.01DH  0.12) w here k  1.0 for sandy strata 1.1 for soft clay 1.15 for clay 1.20 for boulders, kankar, shale etc. CE- 471: Foundation Engineering

Brick Steining  De DH  T  k   8 40   where k  1.0 for sand 1.1 for soft clay 1.25 for hard clay CE- 471: Foundation Engineering

Curb 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

Wcot

De  Di 2

where Di  Internaldiameter of well W  Weight of well and curb per unit length along the centreline of steining

  Internalangle of the well

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Cutting Edge 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

qu .t fc.tan where qu  Crushing strength of rock t  Thicknessof cutting edge he 

fc  Safe compressive stressof concrete 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.

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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. CE- 471: Foundation Engineering

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 CE- 471: Foundation Engineering

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. CE- 471: Foundation Engineering

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. CE- 471: Foundation Engineering

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 CE- 471: Foundation Engineering

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 CE- 471: Foundation Engineering

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  z ( Kp  Ka )  zK ' z  DP D  DK ' CE- 471: Foundation Engineering

The well is assumed to fail as soon as the soil reaction at the bottom is equal to PD. For equilibrium at that instant ( PB ) m ax  resultant of total pressureper unit length  area of AEF - area of BCF 1 1  D 2 K ' 2DK ' D1 2 2 or 1 ( PB ) m ax  DK ' ( D  2 D1) (1) 2 Taking moment about E 1 D 1 D1 ( PB ) m ax H 1  D 2 K '  2DK ' D1 (2) 2 3 2 3 Solving for D1 2D1  3H 1  9 H 2 1  2 D(3H 1  D) CE- 471: Foundation Engineering

Putting D1 in equation (1) and solving for D. This D is the grip length required to sustain the maximum horizontal force. A safe depth can be obtained by reducing PD by a factor of safety F. This theory is based on following assumptions 1. The well is treated as a light bulk head 2. KP and Ka are Rankine' s earth pressurecoefficients 3. There is no friction at the base and wall Omision of these frictional forces yields a conservative (PB)m ax. If  1 and  2 are the horizontal displacements, then the angular deflection of the centre line of the well,  is given as 1 tan  (  1   2) D CE- 471: Foundation Engineering

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 CE- 471: Foundation Engineering

1 D3 ( PB ) m ax   ' ( KP  Ka ) 6 H D Normally around the well, scouring takes place. Beyond the well surroundings, the uncovered soil acts as a surcharge.The surchargedepth D2 is very difficult to assessand may be assumed to be equal to half the normal depth of scour.The pressuredistribution is shownin Fig(b).The equivlant maximum resistanceforceis then given as 1 D 2 ( D  D 2) ( PB ) m ax   ' K ' 6 H D CE- 471: Foundation Engineering

If d is the diameter or length of the well, the total resisting forceafter allowing a factor of safety,F is given as ( PB ) m ax d Pa  F The factor of safety should not be less than 2. The maximum pressuref at the base of the well for the no overturning moment condition is W f A where W  is the net direct load on the well base after making allowance for buoyancy and skin friction A  Area of well base z  section modulus of the well base The maximum foundation pressure should be kept within the safe bearing capacity of the soil assuming no tension occursat the base. CE- 471: Foundation Engineering

The maximum moment on the steining occurs where the resultant forceis zero. If the shear forceis zero at a depth y below the maximum level, then Pa 

 ' K' y 2d 2F

or 2FPa y  ' K' d CE- 471: Foundation Engineering

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 CE- 471: Foundation Engineering

Causes of Tilts and Shifts 1. 2. 3. 4. 5. 6.

Nonuniform bearing capacity Obstruction on one side of the well Sand blowing in wells during sinking. It will cause sudden sinking of well Method of sinking: Material should be removed from all sides equally otherwise the well may experience tilt Sudden sinking due to blasting may also cause tilting of well Irregular casting of steining will cause less friction on one side leads to chances of tilting of well.

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Rectification of Tilt 1. 2. 3. 4. 5. 6. 7.

Eccentric grabbing Eccentric loading Water jetting Arresting the cutting edge Pulling the well Strutting the well Pushing the well by jacks

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