Analysis Of Well Foundation 1b386d

Analysis of Well foundation Correct evaluation of ive relief is a important factor in the design of a well foundation.Views differ regarding the assumptions and methods.to be adopted for working out its values. The main points to be addressed while deciding for the method are. I)Whether the rotation of the foundation takes place only at the base or at points above or below the base. ii)Whether the skin friction on the sides can be considered in the calculation of well resistance,and iii)What fraction of applied moment is resisted by soil resistance on the sides and base resistance. Method of Analysis Several methods are available for computing the stability of a well foundation. 1)Terzaghis Analysis based on Bulkhead concept(1943). 2)Banerjee and Gangopadhyay method(1960) . 3)IRC Method(IRC:45-1972). i)Elastic Theory Method. ii)Ultimate soil resistance method. Theoretical study of Terzaghis Analysis based on Bulkhead concept. The simplest approach of analysing the stability of well foundation.The following are the assumption considered in analysis. i)Bulkhead is weightless. ii)There is no contribution of base reaction. iii) There is no wall friction. Methods for Light Wells: Let qmax be the horizontal force per unit length. Resultant total pressure per unit length = qmax= Area of pressure diagram =(P1,- P2,)at a point of ultimate failure =Area of BGE-CGF (

) (

( )(

) )………………………….1

(

Moment of Force about the base ,

)

From equation 1 (

)(

)

(

) [

(

)

(

√

(

)

)]

From D1, get qmax Qmax= qmax x L Assuming FS=2 Applied force

Method for heavy Well A heavy well under a lateral load will rotate above its base The value of qmax becomputed by taking moment about the base, (

) (

)

For unscoured soil of thikness above maximum scour level effect of earth pressure considered ( Total resisitance horizontal force i.e

)

(

)

= qmax x B

With FS against ive resistance of soil, Allowable horizontal force . IF

Qa=

Qa

Moment at the base Mb due unbalance horizontal force (Q- Qa ) . MB= (Q- Qa ) (H+D)

The maximum and minimum foundation pressure at the base. ⁄

Theoretical study of Banerjee and Gangopadhyay method . As per Banerjee and Gangopadhyay the well foundation can be analysed based on the following assumption. 1)The well is acted upon by a unidirectional force P in a direction across the pier.

2)The well is founded in the sandy stratum. 3)The resultant unit pressure on soil at any depth is in simple proportion displacement. 4)The ratio between pressure and corresponding displacement is independent of the pressure. 5)The coefficient of vertical subgrade reaction has the same value for every point of the surface acted upon by pressure. Horizontal soil reaction. Let a rigid well embedded in sand move parallel to its original position under the action of horizontal force P.assume the wall movement e1moblised fully the active and ive earth pressure on the well. The unit pressure(ive) at depth Z below the surface= The unit pressure(active) at depth Z below the surface= Therefore, resultant unit pressure (

Here

)

=unit weight of soil

From assumption 3

(

)

…………………………………….

(

)

m is know as the coefficient of horizontal soil reaction. In fig let the well rotate at a point “A”,situated at a depth D1 below the scour line.the induced reaction are shown in fig. Let e1=horizontal displacement of the centre line of the well at the scour line. e2=horizontal displacement of the line of the well at the base level of the well. e3=vertical downward displacement of one edge of the well at its base. =vertical upward displacement of the well on the lower part of right face . P1=Resultant ive reaction of the well on the left face. P2=Resultant ive reaction of the well on the lower part of the right face. R= Resultant vertical reaction at the base of the well. c=Skin friction on the left face of the well. µ P2=Skin friction on the right face of the well. µ R=Frictional resistance of the soil at its base. From statics, the conditions of equilibrium are as follows. P=P1-P2- µ R PH=M3+M2-M1+ µ RD+ µ(P1-P2)

And W= µ(P1+P2) M1=Moment at the scour line produced by P1. M2=Moment at the scour line produced by P2. M3=Moment of the vertical soil reaction at the base. The reaction between the various displacement is as follows. ⁄

Determination of P1 from equation zz

(

) (

)

∫

And

(

∫ [

) ]