Design Of Well Foundation 56654k

Well Foundation and its Design

Well Foundation  Large, thick, hollow cylindrical well sunk into the ground to transmit loads from super and substructure of bridge to the founding soil  Suitable for very heavy vertical and lateral loadings as well as large scour depths  Commonly used and popular type of foundation in alluvial plains in South Asian region  Generally costly and ineffective in utilization of materials in case of small loads and shallow depths.  Not suitable for clays and soils containing large boulders

Merit/Demerit of Well Foundation Merit  It has a larger bearing area and section modulus as compared with the cross sectional area of the main body of pier and abutment. This provides better load distribution and good lateral resistance.  Concrete is cast over ground and has better quality control, as well as good depth control.  Skilled man power and technology easily available Demerit  Obsolete technology in many parts of the world  Uneconomic for smaller bridges  Problems of tilt and shift  Time consuming in construction  Quality control problem in bottom plugging of well

Types of Well

Open Well

Pneumatic Well

Floating Box Well

Types of Well

Components of Well Foundation

Pier Well Cap

Top Plug Well Steining Dredge Hole with Granular Filling

Well Curb Cutting Edge

Bottom Plug

Components of Well Foundations and their Functions Well Steining Well steining is the main body of a well. It should be heavy enough to sink the whole well without excessive kentledge. It should be strong enough so that it is not to get damaged during sinking and from the earth pressure from outside of well.

Well Curb Lowest part of well steining to transfer load through the cutting edge to the ground. It is made of richer concrete and is heavily reinforced. It withstands the force from bottom plug due to arch action. It is made with tapering side inside the well and the taper angle is around 60 degrees with the horizontal plane. Sometimes the curb is lined with steel plate through out its height inside and outside to protect from damages due to boulders if any. Its top diameter(outer) is kept 5 to 15 cm higher than the outer dia. of the well steining to facilitate sinking.

Cutting Edge Sharp cutting edge is provided at the end of well curb, where boulders are not expected. Where as stud nose cutting edge is provided, where boulders are mixed with soil. Cutting edge should be strong enough to resist cutting pressure and rigidly fixed with well curb.

Bottom Plug Concrete layer at end of curb to stop the sinking of well, transfer load of well in wider area and to control the movement of underground water. It should be strong enough to withstand large pressure and preferably in the shape of a bulb to produce arch action and increase the bearing area. The concrete should be richer and with about 15% more cement content. It should be more workable with slump about 150 to 200 mm.

Top Plug Sand filling in the well is covered with top plug. It is usually made from lean concrete of 300 mm to 500 mm thickness. Its function is to make a smoother base for well cap.

Well Cap Well cap is a RC slab cast monolithically with the well steining and transfers load from superstructure / substructure to the well steining. Its diameter can be made larger by up to 1.0 m from the steining to accommodate the long abutment or pier. It should be strong enough to withstand the pressure from above by slab action. The top level of well cap is usually flushed with the lowest water level or at the river bed level.

Design of Well Foundation Design of well foundation is carried out in the following steps. 1.

Determine the depth of well foundation

2.

Determine the shape and size of well foundation

3.

Check the stability of well foundation Check stability at elastic state Check stability at ultimate state

4.

Perform structural design of well foundation Design well cap Design well steining Design well curb Design of bottom plug

Depth of Well Foundation

Shape and Size of Well Foundations 

Shape and size of well depends on the size of substructure ,load and type of soil. Elongated shapes are used for long piers and abutments.



Size of well is determined considering safe bearing capacity of soil at the founding level of well.



The size of the dredge hole shall not be less than 2 m to facilitate dredging .



Top diameter(outer) of curb should be higher than the outer diameter of the well steining to facilitate sinking. Usually curb offset is taken in the range of 50 to 150 mm



Thickness of steining should be sufficient so that well can be sunk by its self weight . Minimum thickness of steining shall be 500 mm.



Circular wells are most preferred because they are relatively strong, simple in construction , easy in sinking. Circular wells are not suitable for wide roads with wide substructures.



Double D and rectangular types are commonly used wells after circular wells.

Loads and Loads Combinations

(IRC:78-2000, Cl. 706.1.1)

Loads and loads combination to be considered in the design of well are

Dead loads from superstructure (G), self weight of substructure including weight of sand filling (G), live load (Q), longitudinal force by braking (Fb), buoyancy (Gb), force due to water current (Fwc), frictional force due to expansion/contraction of superstructure (Ff), wind load (W), forces due to tilt and shift of the well (G), seismic load from superstructure and substructure (Feq), load due to back fill , Load due to snow (Gs), erection load (Fer), force due to water wave (Fwp), impact due to floating bodies (Fim) and centrifugal force (Fcf) Impact factor is ignored in the design of foundations. Buoyancy is separately considered for HFL and LWL. Only 15% of the total buoyant force is taken for the depth below max. scour level. The loads and forces may be evaluated as per IRC: 6 and their combinations for the purpose of the design of well will be as follows:

Combination (I):

G + Q or Gs + Fwc + Ff ± Fb + Gb + Fcf + Fep

Combination (II):

(I) + W + Fwp or (I) + Feq + Fwp or (I ) + Fim + Fwp

Combination (III):

G + Fwc + Gb + Fep + Fer + Ff + W or Feq

Loads on Well Foundation W

W, H, M - Resultant vertical force, Horizontal force and Moment due to externally applied load

H

M Maximum scoured level

µ‘P

P-

Force due to net lateral earth pressure

µ’ P -

Frictional force along the embedded height of well

R-

Vertical reaction from base

M’-

Moment at base due to unequal distribution of base pressure

F-

Frictional force at base

P

M’ F R

Base of Well

Tilt and shift of well Soil stratum through which the wells are sunk are very rarely uniform and therefore, the resistance offered by these layers to the sinking is different in different parts of the wells due to which tilt and shift of well my occur. The effect of tilt and shift is to cause extra foundation pressure and this pressure shall be considered in design. IRC 78-2000 Provision

The well shall be shank vertically without any tilt and shifts. However a tilt of 1 in 80 and shift of 150 mm due to translation in a direction which will cause most severe effect shall be considered in design of well.

Translational shift Total shift

Original C/L of Well

Shifted C/L of Well

Tilted C/L of Well

Base of Well

Stability of Well Stability of well under the action of lateral loads depends on the resistance of soil on its sides and base. For a given vertical load the deformation of load increases with the increase in lateral loads, therefore resistances offered by the sides and the base of well also change. The behaviour of the well at ultimate failure is different than at the elastic state. Therefore, in the design of well foundation, stability foundation should be checked at elastic state and at ultimate state . (Ref. IRC 45)

I.

Check the stability of well at elastic state under working load Assumptions 1. Soil surrounding the well and below the base is perfectly elastic. 2. Under design working load unity soil reaction increases linearly with increasing lateral deflection 3. Coefficient of horizontal subgrade reaction increases linearly with the depth in cohesionless soil. 4. The well is assumed to act as a rigid body subjected to unidirectional lateral load and moment at scour level of well.

Steps for checking stability of well at elastic state Step 1 Having determined the grip length of well, calculate -Total downward load consisting of DL, LL acting on the base of well (W) -Total lateral load applied above the scour level (H) -Total external moment applied at the base of well due to eccentricity of LL, tilt, shift etc (M).

Step2 Using the dimensions of well calculate the following geometrical properties IB – MI of base section in the plane of bending about the axis perpendicular to the direction of lateral force

Iv – MI of vertical projected rectangle of well below scour level

B – Dia. of well L – Projected width of well in with soil offering ive resistance L=0.9 × B m = Ratio of horizontal and vertical subgrade modulus at base level ,

μ - Coefficient of friction between well sides and the soil

φ – Angle of internal friction of soil

Df – Depth of grip of well

Step 3

Check the point of rotation of well lies at the base by ensuring that the frictional force at the base is sufficient to restrain the movement of well forward or backward

µ - coefficient of friction at base of well

Step 4 Check that the soil on sides remain elastic by ensuring the earth pressure below the pressure line γ – Unit wt. of soil (dry or submerged) KA , KP – Coefficient of active and ive earth pressure

Step 5

Check the pressure at the base of well

σ1 ,σ2 – maximum and minimum base pressure P – Total horizontal reaction from the side

A – Area of base section of well qallow - Allowable bearing capacity of soil

If the above conditions are not satisfied, the grip length of well shall be increased.

II. Check the stability of well at ultimate state under ultimate load

Steps for checking stability of well at ultimate state Step 1

Compute ultimate vertical load at base (Wu), Ultimate moment about the point of rotation of well which is taken at 0.2 Df from base (Mu) and ultimate horizontal load at the scour level (Hu ) for various ultimate load combination

Step 2

Check maximum pressure at base with allowable bearing pressure OR

Step 3

Check ultimate moment with total ultimate moment of resistance of well OR Mb – M.R. of base section Q – Shape factor Ms – M.R. Due to the well sides earth pressure Mf - M.R. due to side friction

Structural Design of Well I. Design of Well Cap

Critical section for BM and SF Pier/Abutment Well

Plan of Well Cap

D

Dia. of well cap (D) –

Diameter of well cap depends on the sizes of abutment/pier and diameter of well. Diameter well cap is kept at least 150 mm larger than well and pier/abutment in all sides to maintain offset.

Thick. of well cap (d) -

Thickness of well cap is determined to resist BM and SF at critical section.

Area of steel (Ast) -

Area of steel bars (Ast) required for well cap are designed for the BM found at the critical section of well cap.

II. Design of Well Steining 1. Determine the thickness of well steining Thickness of well steining should be such so that well is sunk by its self weight without excessive kentledge. Thickness of steining is fixed based on the following considerations.

d - External diameter of well (m) f – Skin friction of well k – Constant, which depends on the type of soil D – Depth of well below GL or LWL (m) γc – Unit wt. of concrete t – Thickness of well steining

t

Well steining

2. Check the pressure on well steining during sinking Bed level Water level

h’

Outside of well

KAγs h’ + KA γsub h

h

On the outside of well , the soil as well as water exert the pressure. On the inside of well, water exerts the pressure, which partly cancelled the outside pressure. The net pressure (p1 = KAγs h’ + KA γsub h - γw h) causes hoop stresses in the well steining . Hoop compressive stress along the inner face (f1) and outer face of steining (f2) should not exceed the allowable compressive stress of concrete

γw h

≤

Allowable compressive stress of concrete

p1 -

Net pressure on outside of well

r1 , r2 -

Internal and external diameter of well

KA –

Coefficient of active earth pressure

γs , γsub – Unit wt. of soil above water level and submerged unit wt. of soil

γw –

Unit wt. of water

3. Check the stresses in well steining due to all possible loads Calculate all the vertical forces, horizontal forces and moments at the level of maximum scour. Find the maximum bending moment due to all possible loads at the section of zero shear and check stresses in well steining. Section of zero shear lies at ‘x’ distance from the MSL and found as follows.

Scour level (MSL)

x

Section of zero shear force

Df B

Bottom of well

F – Factor of safety (F = 2) H – Resultant horizontal force at scour level γb – Submerged unit wt. of soil (γb = γsoil - γw) Mmax – Maximum value of BM at x level Mo – BM at scour level ka , kp – Coefficient of active and ive earth pressure B – Diameter of well Df – Grip of well

Stresses in steining of well are found at the section of maximum moment and checked as follows.

f1 shall be less or equal to the allowable bending compressive stress of concrete f2 shall be less or equal to the allowable bending tensile stress of concrete Where, VVertical loads at the level of X, i.e. at the section of zero SF Mmax - Maximum moment at section of zero SF ( Mmax = M0 + 2/3Hx) INet moment of inertia of the well section (I = I outer – I inner) yDistance from the centroid to the outer face of well (y = outer radius of well) 2 ANet cross sectional area of the well steining ( A = π r outer - π r2 inner ) r outer, r inner - Outer and inner radius of well

4. Determine vertical and transverse reinforcement of well steining To determine the vertical reinforcement, well is considered as column section subjected to axial load, shear and bending moment . However the amount of vertical reinforcement provided in steining should not be less than 0.2% of actual cross sectional area of the steining. The transverse reinforcement in the steining should be provided in accordance with provision for a column but in no case should be less than 0.04% of the volume per unit length of the steining.

III. Design of Well Curb Well curb shall be designed for the loads subjected A. While sinking the well B. While the curb rest on the bottom plug of well

A. Design of curb while sinking the well

Well Curb

N

N d

H θ

Q P

Q P

H

Where, d – Mean diameter of curb N – Weight or steining in KN/m θ – Angle of inclination of bevelling face of curb θ ≈ 600 μ - Coefficient of friction between soil and concrete of curb P – Force in KN/m acting normal to bevelling face of curb Q – Force in KN/m acting parallel to bevelling face of curb

B. While the curb rest on the bottom plug of well River bed

Under the conditions when the cutting edge is not able to move downwards , reaction can be resolved into horizontal and vertical components. For the condition hoop tension developed in curb is given by

Df

d

p2 b

r

p1 q

In granular soil, the hoop tension ‘H’ is relieved by the active pressure around the curb.

At junction of the curb and steining , a moment ‘M0‘ is developed due to the horizontal force H caused by bevelled action i.e.

IV. Design of Bottom Plug For circular well, thickness of the seal ‘t’ is given by the following relation

Reinforcement of Well Well Cap Well Steining

Sectional Elevation Well Curb

Sectional Plan

Reinforcement of Well Cap Pier

Well Cap

Bottom Reinforcement

Top Reinforcement

Reinforcement Detailing Well Curb