Pile Capacity Based on SPT Results and Static Cone Penetration (CPT) Results

Pile Capacity Based on SPT Results

On the basis of SPT results Meycrhof has suggested the following relationships for the ultimate capacity of piles.

Displacement piles

Q_u  = 400NA_b + 2N’A_s   ———- (1)

H Piles

Q_u = 400NA_b + N’A_s  ———-  (2)

Bored Piles

Q_u = 133NA_b  + 0.67N’A_s  ———- (3)

Where, Q_u = Ultimate total load in kN

N = Average SPT value below pile tip

N’= Average SPT value along the pile shaft

A_s = Shaft surface area of pile in m²

A_b = Base area of pile in m²

The minimum factor of safety of 4 is recommended for driven pile. The allowable load is given by:

   ———- (4)

Pile Capacity Based on Static Cone Penetration (CPT) Results

The cone penetration test may be considered as a small scale pile load test. As such, the results of this test yields the necessary parameters for the design of piles subjected to vertical load.

Vander Veen’s Method for Piles in Cohesionless Soils

In the Vander Veen’s method (1951), the ultimate end bearing-resistance of a pile is taken, as being equal to the point resistance of the cone. In this method, the average cone resistance over a depth equal to three times the diameter of the pile above the pile point level and one pile diameter below point level is taken as shown in Fig.1.

The ultimate load capacity of pile is,

Q_u = Q_b + Q_s

Base resistance

Q_b = A_bq_p‘  ———- (5)

Where, A_b = Area of base of pile

q_p‘ = Average point resistance taken over the depth 4D as shown in the Fig.1.

Shaft resistance

Meyerhof (1956) suggested the following relationships,

Large displacement piles

  ————–(6)

Where, q_c‘ = Average point resistance along the length of the pile (kN/m²)

Small displacement piles

 ———- (7)

Where, q_c = average cone resistance in kg/cm² over the length of pile shaft under consideration

The allowable load is,

   ———- (8)