How to prepare for a static plate load test at your site:

A seven-step guide for practicing geotechnical engineer

The static plate load test (PLT) is essentially a rigid plate placed at the ground surface that is stressed under a specified loading protocol, while the plate settlement is measured throughout. The results represent a direct test of the performance of the ground under representative loading, not reliant on correlations, correction factors, or inferences. It is conceptually a straightforward test; however, many geotechnical engineers consider the test to be costly, or that it requires an excessive amount of mobilisation and preparation at the site. Some may be unsure what they will achieve with the PLT or find the interpretation of the results challenging. All are valid concerns, but with the right level of preparation and understanding, the testing procedure will go smoothly at any site, no matter the complexity of the project.

In terms of its uses in practice, static plate load testing is used extensively for assessing the bearing capacity and stiffness of gravel platforms for safely carrying loads from cranes, piling rigs, or other types of heavy plant during construction. The test is often used in the construction of railways and roading infrastructure, or subdivision projects involving earthworks. The PLT can even be used in remote locations where, for example, wind turbines are under construction. In any construction project, there is no time to be wasted on the day and the mobilisation costs can be substantial, so, the key to success is excellent preparation and scoping.

The static plate load equipment itself is heavy: it consists of a stack of plates, a jack, a load cell, and an electronic settlement gauge, all connected to a controller. Even if the modern test uses digital technology, the mobilisation of the equipment remains an important and costly logistics exercise. In addition, the static plate load test requires the presence of heavy plant at the site, for using its mass as a reaction load. This raises some health and safety issues during the test that need to be addressed. All the above may add significant cost; however, this additional investment easily pays for itself in terms of guaranteeing the infrastructure has been constructed to perform as intended.

Undertaking static plate load testing at any site requires excellent preparation and scoping. But what are the questions that need to be answered during the preparation and scoping phase in the process of requesting the PLT? Seven questions need to be answered by the geotechnical engineer at the scoping and preparation stage, and these are the following.

 

Seven-step guide for static plate load test:

Which testing standard will be applied?

There are many international standards available for conducting the plate load test. It is recommended to use the DIN 18134:2012 standard, as this is the only standard that provides formulae to estimate the desired modulus Ev (Ev1 and Ev2), and the subgrade modulus (K), which leads to the estimation of CBR.  The formulae provide an objective basis for interpreting the test results. The Ev2 characterizes the state of compaction of the soil and whether the material is sufficiently stiff for the given application, and the ratio Ev2/Ev1 is a measure of the relative stiffness measured during the loading and reloading phases of the test. When combined this information aids with accepting or rejecting the compaction effort. Additionally, the DIN 18134:2012 standard provides a formula to estimate the subgrade modulus and as a result, the CBR value can be estimated.

There are other standards available; however, most do not provide any formula for the estimation of the above parameters which leads to challenges with consistency in the interpretation of the results.

What is the plate diameter to be used during the test?

What should the mass of the plant be at the site?

How much should be the maximum stress applied to the ground?

How many loading steps should be used in the load protocol?

What is the purpose of the test?

Who should I engage for establishing QA/QC?

Since 2017, our team at Cook Costello has conducted thousands of plate load tests.

We are here to help with the scoping and preparation for your plate load tests at any site, please do not hesitate to get in touch with our team of experts. We can offer professional advice that will lead to the smooth preparation and scoping for your plate load test requirements.

To add more value to your project, contact our Geotechnical Team at Cook Costello today. 

The Cook Costello Geotechnical Team