CALCULATION OF PILE CAPACITY - EXAMPLE

CALCULATION OF PILE CAPACITY - EXAMPLE

Data:

i) A Bore log furnishing the properties of the soil is given in Table-4

ii)        The pile founding level is considered at 20.0m depth from the ground level.

iii)      The pile cut off level is at 3.5m below the ground level.

iv)      The diameter of the pile is 550 mm.

v)        Concrete grade used : M20

11.1     PILE CAPACITY BASED ON SOIL SUPPORT

            The properties of the soil at depths are as under :

Depth in 'm'	'C' in kg/cm²	'f' in degree
3.5 to 5.0	0.50	28
5.0 to 10.0	0.50	31
10.0 to 16.0	0.0	34
16.0 to 19.0	0.5	29
19.0 to 25.0	0.0	36

            Since the pile founding level is at 20.0m depth, f value of 36° is considered to calculate the point resistance at the pile tip.

            From the formula 4.1

                        Ppu      =          Ap (P_D N_q + 0.5 g D N_g)

             For  f = 36°,      Nq   = 60 from fig.1 and Ng   = 56.3 from Table-1

D    = 0.55 m

L     = 16.5 m

g                 = 1.0 t/m3

For calculation of maximum effective overburden pressure P_D, fifteen times diameter is considered from pile cut off level as all the soil layers are silty soil with angle of internal friction (f) more than 28^0.

            P_D        =       15 x D x g   = 15 x 0.55 x 1.0 = 8.25 t/m²

            Ppu      =       p x 0.55²/4 [8.25 x 60 + ½ x 0.55 x 1.0 x 56.3]  = 120.0 t

            The frictional resistance of the soil is calculated using the formula 4.10

K  = 0.70,        d = 3/4 f

            Between depths 3.5 and 5.0 m

            Psu1   = [0.70 x (4.25 -3.5) x tan 21.0 + 0.5 x 5.0] p x 0.55 x 1.5    = 7.0 t

            Between depths 5.0 and 10.0 m

            Psu2   = [0.70 x (7.5 -3.5) x tan 23.25 + 0.5 x 5.0] p x 0.55 x 5.0    = 32.0 t

Between depths 10.0 and 16.0 m

Psu3   = [0.70 x 8.25 x tan 25.5 + 0] p x 0.55 x 6.0                            = 28.6 t

Between depths 16.0 and 19.0 m

Psu4   = [0.70 x 8.25 x tan 21.75 + 0.5 x 5.0] p x 0.55 x 3.0              = 24.9 t

Between depths 19.0 and 20.0 m

Psu5   = [0.70 x 8.25 x tan 27] p x 0.55 x 1.0                                     = 5.1 t

Psu     = Psu1 + Psu2 + Psu3 + Psu4 + Psu5

           =   7.0 + 32.0 + 28.6 + 24.9 + 5.1        = 97.6 t

            Wp     =   p/4 x 0.55² x 16.5 x 2.5       = 9.8 t

Ultimate Pile capacity Pu  =     Ppu + Psu - Wp  =  120.0 + 97.6 - 9.8  = 207.8 t

            Safe load on pile         P_all  =     207.8/2.5                   =          83.1 t

11.2     STRUCTURAL CAPACITY OF PILE

Considering concreting is done using tremie,

Allowable compressive stress in concrete s_c= 0.33 x 20  = 6.6 N/mm²

As the pile is wholely embedded in good soil, pile is designed as short column.

Allowable load on pile = A_p s_c -Wp = p/4 x 550² x 6.6 x10^-4 - 9.8   =  147.0 t

11.3     The safe load on the pile is least of the values obtained in 11.1 and 11.2 which is 83.1 t.