Measuring rock stress in the construction of new mines, tunnels, caverns and underground stations for hydroelectric plants

Vattenfall Power Consultant conducts reliable measurements for determining rock stress using two different methods: overcoring and hydraulic methods.

The overcoring method uses the Borre probe that we have developed, which is the only probe that is able to measure stress in three dimensions in boreholes filled with water. The hydraulic methods are also partly based on our own unique equipment which has a pressurization capacity far greater than conventional equipment.

Knowing the prevailing state of stress (pressure) in a rock mass is vital for all rock mechanical work, such as mine design, construction of railway tunnels and caverns for the deposition of nuclear fuel, etc. This knowledge enables the behaviour of the rock mass to be analysed, provides boundary conditions for rock mechanical modelling, increases understanding of groundwater flow in the rock mass and, on a larger scale, provides an insight into the mechanisms that cause tectonic plate movements.

Stress measured from the expansion of rock

The procedure for the overcoring method consists of a number of steps. The first is to drill a hole (φ76 mm) down to the pre-determined measurement depth. A smaller hole (φ36 mm) is then drilled from the bottom of this hole in the centre. A set of strain gauges are then fixed to the rock wall on the inside of this smaller hole using glue. Drilling of the larger hole then continues using a hollow drill bit, which results in a hollow cylinder of rock.

The stresses are calculated from the expansion that occurs when the rock cylinder is overcored and is cut off from the surrounding state of stress. The overcoring method requires 10-12 hours for the glue to bond and usually only one measurement is taken per day.

Existing fractures are re-opened or new ones induced

In contrast to overcoring, measurement using hydraulic methods takes place in pre-existing boreholes. The basic principle of this method is to use water pressure to determine the stress acting perpendicular to the fracture plane (normal stress) at different depths along the borehole under test. The fractures under test may be either existing or induced fractures and are isolated in the borehole using steel-armoured rubber packers.

The Vattenfall Quadruple Packer Tool (VQPT) is a unique packer tool that we have developed. It has been specially adapted for the mining industry and areas with high rock stress. This tool has a pressurization capacity of up to 700 bar, which is far greater than conventional equipment. Using hydraulic methods, stresses are calculated by measuring the normal stresses and the orientation of the fracture under test. With hydraulic methods, 3-6 measurements can usually be taken per day.

Integrated data analysis

Vattenfall Power Consultant has a unique range of inversion software which enables the state of stress to be determined using many different types of stress data, such as data from hydraulic methods, data from four types of overcoring probes, stress-induced collapse in boreholes and fractures induced by drilling. The main benefits of integrated analysis are that it facilitates comparisons between different methods and makes it easier to identify the sources of errors. In addition, the final integrated solution is based on a larger amount of data and is therefore more reliable.

Experience from assignments around the world

Overcoring and hydraulic methods are tried and tested ways of measuring rock stress and we have been using them for around 30 years. Our consultants carry out rock stress measurements for clients around the world. Below you can find some of the clients that Vattenfall Power Consultant has helped: