Estimating the Load Capacity of Pile | Foundation | Soil Engineering

In this article we will discuss about:- 1. Estimation of Load Capacity of Pile from SCPT 2. Estimation of Load Capacity from SPT 3. Estimating the Load Capacity from Load Test on Piles.

Estimation of Load Capacity of Pile from SCPT:

The load capacity of piles can be indirectly estimated using the results of in-situ test SCPT described as follows:

Point-Bearing Resistance:

For driven piles in sand, unit point-bearing resistance is given by –

q_P = q_c …(20.33)

where q_c is the average static cone resistance over the depth of 4 d, as shown in Fig. 20.18, and d is the diameter of the pile.

Equation (20.33) is valid on the basis that the pile is driven to a minimum depth of 5 d into the bearing stratum.

Skin Friction Resistance:

As per Meyerhof, unit skin friction resistance for displacement piles is –

f_s = q_c/2 …(20.34)

where f_s is the unit skin friction resistance of the pile in kN/m² and q_c is the average cone resistance in kgf/cm² over the length of the pile.

For low-displacement piles, such as H piles, unit skin friction resistance is given by –

f_s = q_c/4 …(20.35)

Estimation of Load Capacity from SPT:

The load capacity of the pile can be estimated indirectly based on the in-situ test, SPT.

Point-Bearing Resistance:

From SPT, unit point-bearing resistance is given by –

q_p = 40NL/d …(20.36)

where q_p is the unit point-bearing resistance in kN/m², N is the standard penetration resistance without correction for overburden pressure, L is the length of the pile, and d is the diameter or width of the pile. When L/d ratio in Eq. (20.36) exceeds 10, it should be restricted to 10 so that the maximum value of q_p in Eq. (20.36) becomes 400 N.

Skin Friction Resistance:

From SPT for displacement piles, unit skin friction resistance is given by –

f_s = 2N ≤ 100 …(20.37)

where f_s is the unit skin friction resistance in kN/m² and N is the average value of N over the length of the pile. For low-displacement piles, such as H piles, unit skin friction resistance is given by –

f_S = N ≤ 50 …(20.38)

Estimating the Load Capacity from Load Test on Piles:

Load test is the direct method of determining the allowable load on piles. Being an in-situ test, load test is consid­ered to be the most reliable of all the methods of estimation of pile load capacity, especially in cohesionless soils, where it is difficult to collect undisturbed samples.

Pile test may be carried out on a single pile or a group of piles as required. In the case of pile groups, caps will be provided such that the required conditions of actual use are fulfilled. Generally, the load application and deflection observation will be made at the pile top.

The test should be carried out at the cutoff level wherever practicable; otherwise, suitable allowance shall be made in the interpretation of the test results/test load if the test is not carried out at the cutoff level. The cutoff level is the level where the installed pile is cutoff to support the pile caps or beams or any other structural components at that level.

Types of Pile Load Tests:

Depending on the type of load that is determined, pile load test is of four types, which are as follows:

1. Vertical load test – It is used to determine the safe load in compression.

2. Cyclic pile load test – It is a special case of vertical load test and is used to separate skin friction and point bearing resistance with in the vertical load capacity.

3. Lateral load test – It is used to determine the safe lateral load.

4. Pull-out test– It is used to determine the safe uplift capacity of the pile.

1. Vertical Load Test on Piles:

This is the most commonly used test to determine the ultimate or safe load capacity of the pile.

i. Principle:

The test consists of applying a vertical load in increments on the pile top and measuring the settle­ments at the end of each load increment. A load-settlement curve is drawn and the ultimate load is determined from it.

ii. Preparation of Pile Head:

The pile head is chipped off till the sound concrete is met. The projecting reinforce­ment is cut off or bent suitably and the top surface is finished smooth and levelled with plaster of Paris or similar synthetic material where required. A bearing plate with a hole at the center is placed on the head of the pile for the jacks to rest.

iii. Application of Load:

The test is carried out by applying a vertical downward load by means of a hydraulic jack in increments of 20% of safe load on the pile.

The reaction of the hydraulic jack is borne by:

a. Dead Weights:

It is known as Kentledge, placed on a platform supported clear of the test pile. The platform consists of rolled steel joist or suitable load frame.

b. Anchor Piles:

With center-to-center (c/c) distance with the test pile not less than three times the test pile shaft diameter subject to minimum of 2 m. Adjacent working piles may also be used as anchor piles, in which case it should be ensured that their residual uplift is within the limits.

Settlements under each load increment are recorded by minimum two dial gauges for a single pile and four dial gauges of 0.01-mm sensitivity for groups, each positioned at equal distances around the piles. The dial gauges are held by datum bars resting on immovable supports at a distance of 3 D or 1.5 m, whichever is more, from the edge of the piles, where D is the pile stem diameter of circular piles or the diameter of the circumscribing circle in the case of square or non-circular piles.

Each load increment is kept and settlement observations are made until the rate of settlement is less than 0.1 mm in 30 min or 0.2 mm in 1 h or till 2 h whichever occurs first.

The routine test is carried for a maximum load of 1.5 times the working load for individual piles. The maximum settlement in the routine test should not exceed 12 mm. In the case of pile groups, routine test is carried for a maximum load equal to the working load. The maximum settlement for pile groups in the routine test should not exceed 25 mm.

iv. Safe Load:

The safe load on a single pile for the initial test is taken as least of the following:

a. Two-thirds of the final load at which the total settlement is 12 mm.

b. Fifty percent of the final load at which the total displacement is 10% of the pile diameter in the case of uni­form-diameter piles.

c. Fifty percent of the final load at which the total settlement is 7.5% of the bulb diameter in the case of under- reamed piles.

The safe load for pile groups for the initial test is taken as least of the following:

a. Final load at which the total settlement is 25 mm.

b. Two-third of the final load at which the total settlement is 40 mm.

In the case of pile load test in cohesive soils, the data obtained should be used with caution because of the fol­lowing limitations:

a. The soil is disturbed due to pile driving or boring and regaining of the lost strength takes at least 30 days.

b. Pore pressure is developed in pile driving or boring operation, which results in underestimation of the load capacity. The minimum number of load tests to be conducted on piles is 1% for the initial test and 0.5%-2% for the routine test, depending on the variability of the soil profile involved over the site.

2. Cyclic Pile load Test:

Cyclic pile load test is useful to separate skin friction resistance and point-bearing resistance of the pile. After the settlement observations are completed under the first load increment, the load is released and the elastic rebound of the pile is determined. The load is then gradually applied up to a value equal to twice the load increment. After the settlement observations are completed under the new load increment, the load is removed, and the elastic rebound is measured. The procedure is repeated until the load is equal to the maximum load. The load-settlement curve is shown in Fig. 20.19.

Total settlement of the pile at any load –

S = S_e1 + S_e2 + S_p …(20.39)

where S_e1 is the elastic compression of the pile, S_e2 is the elastic compression of soil at the base of the pile, and S_p is the plastic compression of soil at the base of the pile; the plastic compression of the pile is very small and is, there­fore, neglected.

Steps involved in the separation of skin friction resistance and point-bearing resistance are as follows:

i. Determine the elastic compression of the soil and the pile from the cyclic load test data for each load level.

ii. Assume that the elastic compression of the pile S_e11 = 0 and draw a graph between S_e21 and Q, as shown by curve I of Fig. 20.20.

iii. Draw a straight line through the origin parallel to the straight line portion of curve I.

iv. Determine the point-bearing resistance (Q_p) and skin friction resistance (Q_s1) in the first iteration as shown in Fig. 20.20 at different load levels.

v. Determine the elastic compression of the pile at different load levels using the value of Q_s1 obtained in the first iteration from –

where S_e12 is the elastic compression of the pile after first iteration, Q is the load on the pile, Q_s1 is the skin fric­tion component correspondent to load Q obtained from Fig. 20.20 in first iteration, I is the length of the pile, A is the cross-sectional area of the pile, and E is the modulus of elasticity of the pile material.

vi. Compute S_e22 at different load levels using the equation

S_e22 = S_e – S_e12 …(20.41)

vii. Now, draw a graph between S_e21 and load in the same Fig. 20.20 as curve II.

viii. Draw a straight line from the origin parallel to the straight line portion of curve II.

ix. The new values of Q_s2 and Q_p2 are obtained from this straight line of Fig. 20.20 at different load levels.

x. Substitute the value of Q_s2 at different load levels in Eq. (20.42) to obtain the elastic compression of the pile –

where S_e13 is the elastic compression of the pile after the second iteration and Q_s2 is the skin friction component of resistance corresponding to different load levels obtained from the second iteration.

xi. Now determine the elastic compression of the soil at different load levels from –

Se₂₃ = S_e – S_e13 …(20.43)

xii. Draw a graph between S_e23 and load in Fig. 20.20 as curve III. Usually, curve III will be very close to curye II and no further refinement in the computation of Q_s and Q_p is required.

3. Lateral Load Test on Piles:

The lateral load test is conducted to determine the safe lateral load capacity of a pile. The lateral load is applied on the pile by a hydraulic jack with gauge between two piles or pile groups. The loading should be applied in incre­ments of about 20% of the estimated safe load. Each load increment is kept until the rate of displacement is about 0. 1.mm per 30 min.

Displacements are measured by using at least two dial gauges of 0.01-mm sensitivity spaced at 30 cm and kept horizontally one above the other on the test pile. For testing of raker piles, it is essential that loading is along the axis.

The safe lateral load on the pile shall be taken as the least of the following:

i. Fifty percent of the final load at which the total displacement is 12 mm.

ii. Final load at which the total displacement is 5 mm.

4. Pull-Out Test on Piles:

In the pull-out test, the uplift force is applied by means of a hydraulic jack(s) with gauge using a suitable pull-out setup. The hydraulic jack is made to rest on rolled steel joists resting on two supports on the ground. The reaction from the jack is borne by a frame attached to the top of the test pile such that when the jack is operated, the pile gets pulled up.

The test pile should have adequate steel to withstand pulling. In some cases, to allow for neck tension in a pull-out test, it may be necessary to provide additional reinforcement in the piles to be tested.

The pull-out load increments and consequent displacement readings are read as in the case of vertical load test. The initial test is carried out up to twice the estimated safe load or until the load-displacement curve shows a clear break (downward trend). The routine test is carried out to 1.5 times the estimated safe load or until total displace­ment reaches 12 mm, whichever is earlier.

The safe load is taken as the least of the following:

i. Two-thirds of the load at which the total displacement is 12 mm.

ii. Half of the load at which the load-displacement curve shows a clear break (downward trend).

Depending on the purpose of conducting the test, pile load test is again of two types, which are as follows:

1. Initial test.

2. Routine test.

Let us look have a look at them in detail:

1. Initial Test:

There should be a minimum of two initial tests when specific information about strata and past guiding experience are not available. The initial test also helps to provide guidelines for setting up the limits of acceptance for routine tests.

It also fulfills the following objectives:

a. To study the effect of piling on adjacent existing structures.

b. To check the suitability of the piling system.

c. To verify the calculated load by dynamic or static approaches.

The initial test is conducted to determine the ultimate load capacity of the pile at failure. From the ultimate load, the safe load is determined by applying the factor of safety. The pile used for conducting the initial test, known as test pile, is discarded and should not be used to support the structure.

2. Routine Test:

The routine test is conducted to verify whether a constructed pile can carry the design safe load. The pile used for conducting the routine test, known as working pile, forms part of the foundation system used for supporting the structure and is not discarded as done in the initial test.

The number of routine tests may generally be 0.5% of the total number of piles in the project.

The number of routine tests may be increased upto 2% under any of the following conditions:

a. Routine test is used as one of the criteria to determine the safe load of the pile.

b. In the case of detection of any unusual performance contrary to the findings of the initial test.

The test procedure is the same both for initial and for routine tests, except for the difference that the pile is tested until failure in the initial test, whereas the maximum load applied on the pile in the case of routine test is restricted to the safe load as per design.