Rock Bolts

Horizontal toe support for steel sheet piles on bedrock

Steel sheet piles retaining walls need to be embedded a minimum depth in the ground in order to generate sufficient passive earth resistance at the bottom of the wall. 

However, in Nordic countries the soil consists quite often of soft clay deposits overlying bedrock, and similar ground conditions exist in other countries. 

Although it is possible to install sheet piles into the bedrock with installation aids such as drilling or blasting of a small trench in the upper layer of the bedrock, it is easier and faster to apply less destructive installation methods.

For excavations in such soft soils with limited thicknesses of soft soil layers above the bedrock, or where the excavation level lies below the bedrock level, it is common practice in Nordic countries to support the toe of the sheet piles with additional rock bolts (dowels).

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The design and installation of rock bolts must be based on a comprehensive geotechnical investigation with detailed information on the location and inclination of the bedrock, both longitudinally and perpendicularly along the future axis of the wall. Borelogs must be drilled along the wall axis.

Due to the significant influence of the gap between the bedrock and the toe of the sheet piling on the design of the rock bolt solution, it is of major importance to collect reliable data about the bedrock horizon, especially if this horizon is quite irregular with large and / or fluctuating slopes. The estimated bedrock horizon is also required to order the minimum length of each sheet pile. 

The distance between boreholes should not exceed 5 m along the sheet pile wall axis, and should be complemented with boreholes to determine the bedrock inclination perpendicular to the wall axis. Some of the boreholes should penetrate at least 3 m into the bedrock, to rule out the possibility of misinterpreting large boulders as bedrock, and to collect sufficient information on the quality of the bedrock. 

In most cases, the sheet piles should be chamfered to fit the actual inclination of the bedrock (see Figure 5), so as to minimize the gap between sheet piles and bedrock.

In Norway, rock bolts have been used with success since the 1990’s, in rock with different strength conditions. Typical rock strength (uni-axial compressive strength) varies from 50-100 MPa (e.g. shales and phyllites) and up to around 300 MPa (e.g. granite and basalt). 

As the uni-axial compressive strength of the rock is seldom tested during the project execution, the bedrock quality is often estimated based on the drilling rate (seconds per meter) when drilling into the bedrock. At drilling rates above 100 – 200 seconds per meter, somewhat dependent of the hammer drill, the rock quality is normally acceptable. 

Jointing and fissuring, together with the inclination of the bedrock, can influence significantly the resistance capacity of the rock bolts. Use of rock bolts therefore should be based on previous experience with similar bedrock characteristics.