5 Site investigation

The ground is a vital element of the ground anchor system, therefore a good quality geotechnical investigation is essential. A common cause of individual anchor failure at the acceptance testing stage is the lack of accurate information on the ground conditions local to the anchor.

Since inclined ground anchors are installed as commonly as vertical anchors, lateral variations in ground properties should be investigated as thoroughly as the vertical variations.

All geotechnical investigation shall be undertaken in accordance with the requirements and recommendations of ENV 1997-1Eurocode 7, Part 1.

Geotechnical investigation should be extended to site extremities so that the strata profile may be interpolated between the investigation locations rather than extrapolated outside the area investigated. Where possible it should be extended to include ground formations outside the actual site if stresses induced by anchors are extended there.

Depths of geotechnical investigation should be adequate to ensure that:

• a) a known geological formation is proved; or
• b) no underlying stratum will affect design; and
• c) groundwater conditions are well defined.

In addition to the lithology and structure of the ground in accordance with ENV 1997-1 Eurocode 7, Part 1, the following shall also be known, where applicable:

a) for soils:

• soil description and classification (grading, moisture content, unit weight, relative density, Atterberg limits);
• shear strength, compressibility and radial stiffness;
• permeability;
• ground water conditions;
• corrosion potential of soil and ground water;
• existence of stray electric currents;

b) for rocks:

• classification (geometry of discontinuities, unit weight, degree of weathering, index tests);
• rock stratification;
• unconfined compression strength of intact rock;
• shear strength and deformability of rock mass;
• permeability;
• ground water conditions;
• corrosion potential of rock and ground water;
• existence of stray electric currents.

From this information, it should be possible to determine the likelihood of difficulties relating to:

• potential obstructions to drilling;
• the process of borehole drilling (drillability);
• borehole stability;
• flow of ground water into the borehole;
• loss of grout from the borehole.