04/04/2018

Science Blog: Bedrock – Shattered building material

Text:
Mira Markovaara-Koivisto, Geologist

The building of underground facilities in bedrock is increasing in the modern world. In large cities, underground alternatives are being considered due to the lack of space above ground, to ease traffic and parking in city centres, and to guarantee a stable environment, for instance, for super computers and electrical lines. Bedrock can also be utilized for the safe isolation of toxic wastes. However, bedrock is a shattered construction material. It might need support when underground facilities are built, it conducts groundwater and may host shearing in the event of an earthquake. These phenomena are driven by the fracturing and brokenness of the bedrock, and they must be known before the phenomena can be controlled.

The value of fracture studies increases with the number of observations and studied properties. Drill cores provide information from deep underground, but they lack information on the fracture dimensions, larger scale surface properties and aperture. Geological mapping provides a description of fracturing but easily misses the effects of shorter fractures. Window and scanline mapping provides the most detailed fracture observations. In this study, a scanline mapping form and computer scripts were developed to analyse and visualize the observed fracturing in bedrock.

In addition to direct observations from rock surfaces, information on rock fracturing in the rock mass and the continuity of the fractures seen at the rock surface was gathered using several parallel ground penetrating radar (GPR) profiles. Reflections from adjacent profiles were carefully connected to form 3D fracture surfaces. The straightness of the reflections and reflection polarity provided information on the surface roughness and water seepage for rock quality assessment.

The results of several study methods used in this quarry-scale study underline the importance of supplementing the disadvantages of one study method with the advantages of another. Part of the information was directly collected from the quarry surfaces with geological and scanline mapping, and part indirectly with GPR and stereophotogrammetry. The fracture network model was based on indirect observations and the simulated discrete fracture network (DFN) was based on direct observations. Rock quality was calculated within the DFN using fracture properties from direct observations, as illustrated in Figure 1.

The resulting 3D fracture surfaces and the rock quality model grid can easily be transferred to any other CAD software to be utilized in the planning of underground facilities as if it was information about the building material. Building information modelling (BIM) has broken through in the infra world, and is already coming in the planning stage of underground construction.

Figure 1. Modelling of the fracture network and rock quality was based on direct, indirect and simulated input data.

This PhD thesis was published in 2017 in the Aalto University publication series DOCTORAL DISSERTATIONS, “Visualization and modelling of rock fractures and rock quality parameters in 1-3 dimensions in crystalline bedrock” by Markovaara-Koivisto (http://urn.fi/URN:ISBN:978-952-60-7754-3).

Mira Markovaara-Koivisto

Text: Mira Markovaara-Koivisto

Mira Markovaara-Koivisto, D.Sc. (Tech.), works at the Bedrock Construction and Site Assessment unit of the Geological Survey of Finland. She is specialised in rock quality and rock fracture analysis and modelling.