Published on March 21, 2014
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 isflexible loads, like a beam the well under horizontal 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 • 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 • Cutting Edge • Well Curb • Bottom Plug • Steining • Top Plug • 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. w here Ds = Scourdepth (m) q = Design discharge(m3 /s) sf = Silt factor= 1.76 Dm Dm = Mean diameter of soil particle in river bed (mm) Ds = 0.473 Q f
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. D = 1 .Ds 3 D = Grip length of well 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 3 m. The final size is should be decided after satisfyingthe lateral stability condition of the wells.
Bearing Capacity of Wells + + 100 w here qa = Safe bearing capacity(kN/m2 ) N = CorrectedSPT value B = Smaller dimension of w ell D = Depth of w ellfoundation below scourlevel IS3955 recommendsthe follow ing formula for allow able bearing pressurefor sands based on its N value for safety against shear failure 2 2
This is subjected to of the different stresses. sinking it to water types At is subjected and earth pressure. At dredging stage, inside surface issubjected to water pressure while outside surface to the pressure. recommends earth IRC some rules of thumb for fixing the thickness of steining which are given below. Steining
Cement c onc retesteining 1.For c irc ular and dumbbell - shaped w ells T = k (0.01DH + 0.1De) w here k = 1.1for sandy,silty and soft c lay 1.25 for hard strata inc luding hard c lay, boulders, kankar,shale etc . DH = Height of w ell De = External diameter of the w ell 1.For rec tangular and double - D w ells T = k (0.01DH + 0.12) w here k = 1.0 for sandy strata 1.1for soft c lay 1.15 for c lay 1.20 for boulders , kankar, s hale etc .
Brick Steining T = k De + DH 8 40 w here k = 1.0 for sand 1.1for soft clay
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 w here Di = Internaldiameter of w ell W = Weight of w elland curb per unit length along the centreline of steining = Internalangle of the w ell 2 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 w here qu = Crushing strengthof rock t = Thicknessof cutting edge fc = Safe compressive stressof concrete The value of θ is usually taken as 300. The choice of this angle has been proved to be suitable for easy access to the cutting edge. fc.tan he = qu.t 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 base 3. Cancels steining the tensile forces at the hoop stresses induced in
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 • Self weight of the well and its superstructure • Live loads • Water currents and buoyancy • Temperature, wind and earth quake • Breaking and tracking forces • Resistance of the well walls • 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 = z(Kp − Ka) = zK ' z = DPD = DK '
The well is assumed to fail as soon as the soil reaction atthe 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 D2 K '− 1 2DK ' D1 Solving for D1 2D1 = 3H 1 +9H 2 1 − 2D(3H 1 − D) (2) 2 3 2 3 1 1 (PB) m ax = 1 DK '(D − 2D1) 2 Taking moment about E (1) 2 2 or
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 factorof safety F.This theoryis based on follow ing assumptions 1.The w ellis treated as a light bulk head 2.KP and Ka are Rankine' s earth pressurecoefficients 3.Thereis no friction at the base and w all Omision of thesefrictional forcesyields a conservative (PB)m ax. If 1 and 2 are the horizontal displacements, then theangular deflection of the centreline of the w ell, is given as tan = 1 (1 − 2) D
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
1 D3 (PB) m ax = '(KP − Ka) 6 H + D Normally around the w ell, scouring takes place. Beyond the w ellsurroundings, the uncovered soil acts as a surcharge.The surchargedepth D2 is verydifficult to assessand may be assumed to be equal to half the normaldepth of scour.The pressuredistribution is shownin Fig(b).The equivlant maximum resistanceforceis then given as 1 D2 (D + D2)
If d is the diameter or length of the w ell, the total resisting forceafter allow ing a factorof safety,F is given as Pa = ( PB ) m a x d F The factorof safetyshould not be less than 2. The maximum pressuref at the base of the w ell for theno overturning moment condition is f = W A w here W = is the net direct load on the w ell base after making allow ancefor buoyancy and skin friction A = Area of w ell base z = section modulus of the w ell base The maximum foundation pressure should be kept w ithin thesafe bearing capacity of the soil assuming no tension occursat the base.
2FPa ' K' d Pa = ' K' y d 2F or The maximum moment on thesteining occurs w here the resultant forceis zero.If the shear forceis zero at a depth y below the maximum level, then 2 y =
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
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