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Pile Load Test Failure Criteria

by Lois Earles

Do you recall when your project was put on hold due to pile load test failure criteria? If yes, you must know how frustrating it can be – especially when there is no clear solution. Understanding the causes and underlying factors of this kind of problem is essential for finding an effective solution as soon as possible. 

In this article, we will look at the different types of failure criteria associated with conducting pile load tests, what they mean and why they are important so that we can better identify potential issues before they become major problems.

What is Pile load Test?

Pile load testing is essential to any construction project involving deep foundation structures, such as piles and caissons. This type of testing is used to determine soil’s bearing capacity and settlement behaviour in response to the loading conditions applied to a pile. 

From this data, engineers can evaluate the safety margin for a given structure and make sound decisions about future construction activities. 

Failure Load and Settlement

In most cases, a straightforward method is to calculate the capacity of bearing that will ultimately be used by the pile and then multiply it by a factor of safety that is proven to restrict the amount of settlement at the workload to the acceptable degree to the structural engineer. In cases where settlements are important, it is important to analyze the proportions of the applied load carried by end-bearing and shalt friction. 

Then analyze the interaction between the elastic compression force of the shaft, the elastoplastic deformation of the soil surrounding the shaft, and the compression of the soil under the pile’s base to calculate the settlement of the pile head.

In all instances when piles are completely supported through soils and set into groups, steps for the calculation of the allowable pile load are as follows:

• Determine the base level of the piles, which must be maintained to avoid over-expansion of the piles. The feasibility of reaching this level using the current methods of constructing the piles is to be considered.

• Calculate the needed size or width of the piles so that the chance of settlement of the particular piles at the set working load won’t cause an excessive settlement for the pile group.

• Study the economics of changing the size and number of the piles within the group to help support the entire group load.

The ultimate goal is to put the maximum workload for each pile while keeping the number of piles within each group as low as feasible. This means that the pile cap will become less and more affordable, and group settlement will be kept at the lowest. 

However, an excessive settlement that results in unacceptably high settlements between neighbouring piles or groups could happen if the safety level on each pile needs to be increased.

The size and width of piles can be determined exclusively by analyzing the degree of settlement of the pile isolated at the load it is working on in the event of piles isolated from each other or arranged in very small groups. Installation techniques significantly affect the capacity of the piles. 

The relationship between the soil and soil is determined by several factors, including whether a pile is cast or driven in situ within the form of a bore, tapered or straight-sided, and whether it is constructed from concrete, steel, or wood.

Engineers should set low expectations of the formulas used in determining the capacity of piles. Stay calm if the calculations show that failure loads are off by more than or less than 60 per cent of the load determined by testing. Keep in mind that the complete foundation is being evaluated whenever the pile is put under load tests.

It’s not surprising that there might be significant differences in failure loads on any particular site due to the usual variation in the conditions of the ground and the impact of installation methods on resistance. If the full-scale strip or pad foundations were loaded to the point of failure, engineers wouldn’t be surprised to find huge variances.

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Another option is to determine the allowable load and design-based bearing capacity using dynamic formulae. They will provide even more variation than the soil mechanics method. In all likelihood, dynamic formulas are generally discredited by foundation engineering experts without using them as part of tests and analyses that are dynamic with the standard equipment.

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