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Pile Load Tests: A Special Analysis

Pile load tests are essential in ensuring the stability and safety of any construction project that involves piles. By conducting pile load tests, engineers can determine the ultimate capacity of a pile and assess its settlement under design load, as well as evaluate any variations in soil strata and installation quality.

There are several types of pile load tests: compressive pile load tests, pull out tests, lateral load tests, and torsional load tests. Of these, the compressive pile load test is the most common. In this article we dwell on the compressive pile load tests.

There are two types of compressive pile load tests:

  1. Initial pile load tests – These are conducted to determine the ultimate pile capacity
  2. Routine pile load tests – Routine pile load tests are carried out to confirm the pile settlement under the design load, to assess any variation in the soil strata and to ensure that the pile has been installed properly.

During a pile load test, loads are applied in increments and the settlements of the pile head are recorded. The results are plotted in the form of a load settlement curve. The results of the pile load test represent the integrated effect of the soil layers in the load transfer.

Initially, the load is mostly resisted by the shaft friction and practically no load reaches the pile base. Full shaft capacity is mobilized at a very small displacement of pile head less than about 2% of the pile diameter. On the other hand the full base resistance requires pile head movement of the order of 10% of pile diameter.

If the pile capacity comprises of mostly shaft friction,  the pile is termed as a friction pile or floating pile on the other hand if the large proportion of the load is transferred at the pile tip it is called a bearing pile. Generally the load is resisted by shaft friction as well as base resistance.

Due to the different levels of movement required, initially the load is resisted mostly by shaft friction and very little load reaches the base and when the shaft resistance is overcome any further gets fully transferred to the pile base.  

There are three types of load transfer in a pile:

  1. Mostly carried by shaft friction,
  2. Mostly by bearing at the pile base,
  3. A combination of both shaft friction and base resistance.

Load settlement curves are shown in Fig.1 for these three categories of load transfer.

This depends on the sub soil strata at a location. When shaft friction and base resistance both contribute to the pile capacity, under design load conditions, most of the load is carried by shaft friction, with a small amount of load transferred to the pile tip. In addition to the load capacity of the pile, pile load tests can provide additional information concerning the load transfer mechanism in the pile. After the shaft friction is fully mobilized i.e. at about 2% diameter pile head settlement any additional load imposed on the pile head is directly transmitted to the pile base.

Fig.1 Types of load settlement curves.

If the load increment is ΔP and the corresponding incremental head settlement is Δs

The pile base settlement Δsb = Δs – ΔP L / (AE)p ,

where the second term ΔP L / (AE)p represents the pile compression and  (AE)p is the pile sectional stiffness.

 The pile base stiffness kb may then be calculated as ΔP /Δsb. We can then substitute pile head stiffness K and pile base stiffness kb  in the following Mylonakis and Gazetas equation and obtain the parameter μ by iterative procedure.

From μ, the winkler constant k for modelling shaft friction in pile analysis can be obtained.   The winkler constant k thus determined represents equivalence of all the soil layers. As the constant k has been determined from field load test it represents the site conditions better. This parameter μ also plays an important role in pile group analysis. 

Pile load tests provide valuable information about the load transfer mechanism in the pile and help ensure that the pile has been installed correctly. They are a crucial part of any construction project involving piles.

In conclusion, understanding pile load tests is essential for anyone involved in construction projects that involve piles. These tests provide critical information about the pile’s capacity and settlement under design load conditions. By using the Mylonakis and Gazetas equation and the winkler constant k, engineers can analyze piles more accurately and design more robust structures.

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