Results / Swelling and weathering rock

The effect of swelling and weathered rock in hydropower tunnels

A better method for testing rocks can save the power industry a lot of money and contribute to safer hydropower tunnels, both in Norwegian and foreign hydropower projects. This is shown by research from HydroCen.

Sufficient securing of hydropower tunnels is important to avoid expensive and time-consuming repairs from rocks collapsing inside the tunnel. The trick is to secure and reinforce enough of the tunnel wall where it is needed, while at the same time not spending time, effort and building materials on securing parts where it is not necessary.

Testing the properties of the rock material is therefore important in order to say something about where and how much of the tunnel needs to get reinforced.

Today's measurements are too bad

In order to know where in the tunnel one should reinforce, samples of the rock material are taken, they are processed and analyzed to test for the properties of the rock material. Unfortunately, there are several cases where these assessments are not sufficient enough as they are too general to capture all that conditions the rock material is exposed to in the water tunnels. In addition, the rock masses and their properties can change over time when they are exposed to major physical changes such as differences in pressure and temperature. It is therefore important to also take these potential changes into account to avoid the tunnel from collapsing.

Have tested rocks in the laboratory and in the field

In collaboration with SINTEF Energi and NTNU, Lena Selen has through her work looked at the effect swelling mechanisms and the weathering of rock material have on already weathered or weak rock.

This has been done by looking at the different methods that are often used to assess different rock materials' composition, strength, swelling properties (how they react with water), as well as how the properties change when the rock material gets wet and dried in several cycles, as they often do in the hydropower tunnels.

In addition, she has worked to improve laboratory assessments of swelling or weak rock in order to predict potential challenges with stability and the need for protection in water tunnels. She has also tested measurements of in-situ swelling potential and weathering during full-scale operation of the hydropower plant.

Through the research carried out in the project, several critical limitations of current test methods for swelling and weak rock material have been highlighted, these can in turn lead to too little securing of weak areas or an unnecessary reinforcement in parts of the tunnel that do not need it.

FACTS ABOUT THE PROJECTS MAIN DISCOVERIES

Rock samples taken from the same area may look the same even though they are worn or weathered to different degrees, and therefore potentially weak. It is therefore very important to take into account the mechanical properties of the rock material, and not just look at whether they are visually similar.
By only looking for specific rock and clay minerals known to have a high swelling potential, other swelling rock components may be neglected. Misinterpretation can occur when several mechanisms interact on the swelling behavior of intact rocks.
It is very important to be able to identify weathered structures in the rock material, since changes in structure due to weathering can lead to swelling in the rock material even if it contains little or no swelling clay minerals.
The test method for measuring the "worst case scenario" of the swelling potential of the rock material used today can be misleading when used to estimate the swelling potential of intact rock material. The method can lead to an increased amount of swelling from swelling clay minerals in the rock material, without other important structural factors that would normally limit this swelling behavior being included.
XRD analysis, which looks at the composition of minerals in the rock mass, is not always able to measure the content of minerals in weathered rock material well enough. Weak and bulging parts in rock material can either be camouflaged, destroyed or neglected in the analysis process.
Different laboratories operate with different methods for carrying out measurements on the rock material. This makes it difficult to create a common database with comparable results for classification purposes.
The distribution of swelling clay materials in intact rock material can greatly affect the swelling potential of the intact rock material. When the distribution of swelling clay minerals is not suitable for the water to penetrate the material and swelling, it can lead to an overestimation of the need for support in the tunnel. On the other hand, it can also lead to an underestimation of the need for reinforcement of the tunnel if weathered rock material favors other swelling mechanisms.

More research and updated tests are needed

In order to obtain more accurate analyzes and measurements of both the content of swelling minerals in partially intact and intact rock, and the effect of swelling minerals in partially intact and intact rock, there is a need for improvement of methodology and analysis techniques and more research.

The results from the project point to the need for an update of the current guidelines when it comes to the choice of examination methodology and the choice of laboratory examinations.

There is also a great need for hydropower-specific laboratory investigations of swelling rocks. In addition, established survey methods should be specified so that results can be compared across projects.