Design of Pile Foundation by Indian Standards

DESIGN CONSIDERATION FOR PILE FOUNDATION

Pile foundations shall be designed in such a way that the load from the structure it supports can be transmitted to the soil without causing any soil failure and without causing such settlement, differential or total, under permanent or transient loading as may result in structural damage and/or functional distress. The pile shaft should have adequate structural capacity to withstand all loads (vertical, axial or otherwise) and moments which are to be transmitted to the subsoil.

The ultimate load capacity of a pile may be estimated using a suitable static formula. However, it should preferably be determined by an initial load test on a trial pile.

The settlement of a pile obtained at safe load/working load from load test results on a single pile shall not be directly used in forecasting the settlement of a structure unless experience from similar foundations on its settlement behaviour is available.

LOAD CARRYING CAPACITY OF PILES (Q_f)

(A) Ultimate Load Carrying Capacity

The ultimate load carrying capacity or ultimate bearing capacity, or the ultimate bearing resistance Q_f of a pile is defined as the maximum load which can be carried by a pile, and at which the pile continues to sink without further increase of load.

The load carrying capacity of a pile can be determined by the following methods:

• Dynamic Formulae
• Static Formulae
• Pile Load Test
• As per IRC-78 & IS 14593

1. DYNAMIC FORMULAE

When a pile hammer hits the pile, the total driving energy is equal to the weight of hammer times the height of drop or stroke. In addition to this, in the case of double acting hammer, some energy is also imparted by the steam pressure during the return stroke. The total downward energy is consumed by the work done in penetrating the pile and by certain losses.

The following are some of the commonly used dynamic formulae:

i) ENGINEERING NEWS FORMULA

\[ Q_a = \frac{W \cdot H}{F(S + C)} \]

Q_a = allowable Load

W = Weight of hammer, Kg

H = Height of fall, cm

F = Factor of safety = 6

S = penetration per blow in cm

C = empirical constant, generally = 2.5 cm

ii) HILEY’S FORMULA

\[ Q_f = \frac{\eta_h W H \eta_b}{S + \frac{C}{2}} \]

Q_f = Ultimate bearing capacity

C = C₁ + C₂ + C₃ = total elastic compression

η_h = Efficiency of hammer (Vary from 65% to 100%)

η_b = Efficiency of hammer blow

2. STATIC FORMULAE

The static formulae are based on assumption that the Ultimate load carrying capacity (Q_f) of a pile is the sum of the total ultimate Skin friction (R_f) and total ultimate point or end bearing resistance (R_p).

\[ Q_f = R_f + R_p \]

\[ Q_f = A_s \times r_f + A_p \times r_p \]

Where:

• A_s = Surface area of pile upon which the skin friction acts
• A_p = Area of C/s of pile on which bearing resistance acts
• r_f = Average skin friction
• r_p = unit point or toe resistance

A factor of safety of 3 may be adopted for finding the allowable load.

i) FOR COHESIVE SOIL

\[ Q_f = A_p \times N_c \times C_p + \alpha \times C \times A_s \]

Where:

• N_c = Bearing capacity factor usually taken as 9
• C_p = average cohesion at pile tip in kg/cm²
• α = Reduction factor
• C = avg. cohesion throughout the length of pile in kg/cm²

ii) FOR NON-COHESIVE SOIL

(Formula for non-cohesive soil was incomplete in original document)

3. LOAD CARRYING CAPACITY FROM PILE LOAD TEST

The pile load test can be performed either on a working pile or on a test pile (trial pile).

The ultimate load carrying capacity is preferably determined by an initial load test on trial pile.

An initial load test should be conducted on a trial pile, particularly in any locality where experience of piling is not available. (For detailed methodology see chapter 5 testing of pile)

4. LOAD CARRYING CAPACITY OF PILE IN ROCK AS PER IRC-78 & IS 14593

i) As per IRC-78

\[ Q_f = A_s \times q_s + A_p \times \frac{K_p \times q \times d_f}{\text{factor of safety}} \]

q_s = ultimate shear along the socket

• for hard rock = 50 Kg/cm²
• For weathered rock = 20 kg/cm²

K_p = An empirical coefficient = (0.1 to 0.4)

q = Average uniaxial compressive strength at the tip level

d_f = depth factor = 1 + 0.4 × diameter of socket

ii) As per IS 14593

\[ Q_f = (q_c \times \pi \times D \times L_s \times \alpha \times \beta) + (q_c \times N_j \times N_d \times A_p) \]

Negative Skin Friction or Dragdown Force

When a soil stratum through which a pile shaft has penetrated into an underlying hard stratum, compresses as a result of either it being unconsolidated or it being under a newly placed fill or as a result of remoulding during driving of the pile, a drag down force is generated along the pile shaft up to a point in depth where the surrounding soil does not move downward relative to the pile shaft.

STRUCTURAL CAPACITY

DESIGN OF PILE FOUNDATION - A Case Study

Design of Pile foundation for Proposed Project having following design data.

• Type = Multistoried Residential Building (Ground + 11 Storey)
• Floor to floor height = 3.0 m
• Imposed Load = As given in Appendix -B
• Exposure condition = Moderate
• Earthquake Zone = III, Zone factor = 0.16
• Basic wind speed at 10 m height = 44 m/s
• Bearing capacity of soil = 40 t/m² (as per soil investigation Report)
• Material = M-30, Steel = Fe500
• Design Philosophy: Limit state Method conforming to IS 456-2000, IS-2911 Part-I section-2, IS 14593

FIG. _______ ARCHITECTURAL PLAN OF BUILDING

STEP – 1: STRUCTURAL PLANNING

The width of the beam will be kept 230 mm to meet the fire resistance requirements. The depth of the beam kept twice the width of beam.

The width of the column is kept 300 mm for earthquake requirement. The ratio of depth of column to width of column will be limited to 3.

STEP – 2: ANALYSIS

The analysis for this project is done in STAAD and load combination for different nodes (column) is converted into Excel format which are shown in appendix-A.

Node Numbers

Building STAAD Model

STEP – 3: To Determine the Ultimate Load carrying capacity of Pile

As per the soil investigation Report, The Average Uniaxial Compressive Strength = 411 Kg/cm² = 4110 t/m²

Assume the dia of pile = 500 mm, Length of Socket in Rock = 1000 mm

\[ Q_s = q_c \times N_j \times N_d \times A_p + q_c \times \pi \times D \times L_s \times \alpha \times \beta \]

Calculation yields: Q_s = 563.36 Tonne

Structural capacity: Q = 125.6 Tonne

Hence pile capacity is restricted to structural capacity = 125 t

STEP – 3: DESIGN OF PILE

i) Design of Main Reinforcement for Pile

Provide: 10 Nos – 16 mm dia bars (A_st = 2009.60 mm² > required)

ii) Design of Lateral Ties

Provide 8 mm dia lateral ties @ 200 mm c/c

Fig. Reinforcement Details of Pile

STEP – 4: DESIGN OF PILE CAP

Example: Node No. 1 (Column size = 300 × 750 mm)

Maximum axial load = 1901.323 kN

No. of piles required ≈ 1.52 → Adopt 3 piles

Combined Node Example: Nodes 14 & 15 (300 × 600 mm)

Maximum Axial Load on Node 15 = 1920.57 kN

Adopt 2 piles per node (common pile cap)

Combined Node: Nodes 64 & 83

Maximum Axial Load on Node 64 = 2307.87 kN

Adopt 3 piles

Combined Node: Nodes 5 & 10

Maximum Axial Load on Node 5 = 2586.53 kN

Adopt 3 piles

Pile Cap Arrangement – Combined Nodes

TESTING OF PILE

INTRODUCTION

Construction of pile foundations requires a careful choice of piling system depending upon the subsoil conditions, load characteristics of the structure and limitations of total and differential settlement.

Installation of piles demands strict control of position, alignment and depth and involves specialised skill and experience. Hence testing of piles becomes necessary.

Testing of piles is generally carried out for following reasons:

• To obtain back-calculated soil parameters for design of other piles.
• To confirm pile lengths and hence contract costs.
• To counter-check results from geotechnical and pile driving formulae.
• To determine load–settlement behaviour for group settlement prediction.
• To verify structural soundness of piles.

PILE LOAD TEST

Pile load test is intended to validate computed capacity of pile foundation and also to provide information for improving design rational.

Test to pile failure should preferably be conducted instead of testing only up to specified termination load.

Pile load testing is carried out as per IS 2911 (Part 4).

Information required before pile testing:

• Pile type, material and reinforcement details.
• Method of driving with driving records.
• Pile depth and cross-section details.
• Type of test proposed.
• Layout of pile group.
• Water table depth and soil strata details with test results.
• Safe and ultimate load capacity.