Test: Deep Foundation - Civil Engineering (CE) MCQ

Cyclic pile load test method is used in case of initial test to find out skin frictional resistance and point bearing load separately for a single piles of uniform diameter.
Alternate loading and unloading shall be carried out at each stages and each loading and unloading stage shall be maintained for at least 15 min and the subsequent elastic rebound in the pile should be measured accurately by dial gauges or LVDTs.
The details procedure for pile load test is given in IS 2911-Part IV.
There are two types of tests, namely, initial and routine test for each type (mode) of loading (that is, vertical, lateral and pull-out).
Initial Test: This test is required for one or more of the following purposes:
(a) Determination of ultimate load capacity and arrival at safe load by application of factor of safety.
(b) To provide guidelines for setting up the limits of acceptance for routine tests.
(c) To get an idea of suitability of piling system.
(d) To have a check on calculated load by dynamic or static approaches.
Routine Test:
This test is required for one or more of the following purposes:
(a) Checking the safe load as determined from static analysis [As per IS 2911 (Part 1/Sec 1 to 4), IS 2911 (Part 2) and IS 2911 (Part 3)].
(b) Detection of any unusual performance contrary to the findings of the initial test, if already done.
Other Important Points:
In vertical, lateral and pull out mode of loading compressive, shear and tensile strength of pile is found out respectively.

Group Efficiency is defined as the ratio ultimate bearing capacity of a pile group to the sum of the ultimate load capacity of individual piles

For sandy soil: η > 1.0
This is because the soils get densified due to the driving of piles in sandy soil and hence soil strength properties increase.
For clayey soil: η < 1.0
This is because the pressure bulbs of individual piles overlap the pressure bulb of each other resulting in a reduction of pile group carrying capacity.
∴ Pile group efficiency will be more than 100% for pile group in cohesionless soil and less than 100% for those in cohesive soils.

Black cotton soil:

• Black cotton soils show swelling and shrinkage behavior on wetting and drying.
• Black Cotton Soil also consists of the excess of Montmorillonite mineral.
• These show volumetric changes with the change of atmospheric conditions.
• Black cotton soil shows swelling when moisture content increases and shrinkage behavior when moisture content decreases. 
• Under reamed piles are used for these types of soils.

Important Points

• As black cotton soils are expansive soils i.e having swelling and shrinkage behavior and due to this Lightly loaded structures are more susceptible to damage as a result of volume changes in the soil. Therefore these soils need to be stabilized first before loading.
• Investigation says that lime is a good stabilizing agent for the construction of a road in black cotton soil.
• Lime forms the bonding between the soil particles which is missing in black cotton soil and hence stabilizes.

Plate load Test

• Plate load test is done at the site to determine the ultimate bearing capacity of soil and settlement of foundation under the loads for clayey and sandy soils. 

Cyclic load Test

• It is used for separating the load carried by the pile into the skin friction and point bearing resistance.

Dynamic load Test

• Dynamic load testing with the aid of Pile Dynamic Analyzer (PDA) is a fast and reliable way to evaluate the bearing capacity of the pile.

Pile Integrity Test

• It is a non-distracted test (NDT) method that can evaluate the continuity and consistency of pile material, length, and cross-sectional area. 

Negative skin friction: It is usually a downward shear drag acting on a pile or pile group because of the downward movement of surrounding soil relative to the piles. Such a force can exist under one of the following conditions:

• If a fill of clay soil is placed over a granular soil layer into which a pile is driven, the fill will exert a downward drag force on the pile due to consolidation.
• If a fill of granular soil is placed over a layer of soft clay, it will induce the process of consolidation in the clay layer and thus exert a downward drag on the pile.
• Lowering of the water table will increase the vertical effective stress on the soil at any depth, so it can induce negative skin friction on piles.

Now the axial capacity of a pile is the summation of the upward reaction due to bearing at the base and net upward skin frictional resistance. As the negative skin friction (acting downward) lowers the net skin resistance, it in turn reduces the axial capacity of piles.
A schematic diagram of negative skin friction on piles is shown below:

∴ Negative skin friction on a pile under vertical compressive load acts downwards and reduces the load-carrying capacity.