5 Site investigation

5.1 General

5.1.1 The general requirements for site investigation are contained in ENV 1997-1:1994, chapters 3 and 7 and the relevant national documents.

5.1.2 For design purposes, the site investigations should:

  • provide a comprehensive geotechnical report;
  • establish the ground susceptibility to grout;
  • furnish a basis for selecting the grout types.

5.1.3 For the execution, the ground investigation report should contain the following specific information:

  • the relevant physical and chemical characteristics of the ground;
  • the ground level at investigation and testing locations relative to a recognized datum;
  • the location, founding level and condition of existing or envisaged structures;
  • the presence of any anisotropies or permeable horizons which could influence the grouting works;
  • the orientation, frequency, and width of rock joints and the composition and nature of any infill material;
  • the location and nature of filled or open cavities;
  • the presence of obstructions that require special drilling and grouting methods or equipment;
  • the presence and characteristics of ground that is likely to loosen, soften or become unstable, dissolve, collapse or swell as a result of drilling or grouting;
  • the groundwater levels and gradients and their variation with time;
  • strata with high groundwater velocities and permeabilities;
  • the temperature, chemical composition, organic and bacteriological content of the groundwater or ground, if problems are expected.

5.1.4 The geological and geometrical model, and the void structure and its anticipated evolution should be described in the investigation report. The precision and limits of the model should be highlighted.

5.1.5 Where relevant, the following specific drilling information should be recorded:

  • location and cause of core losses;
  • unstable zones and stabilization measures taken;
  • water level at the beginning and end of a run, zones of water loss and gain, measurements of return water, water colour and changes in colour;
  • action of the drill rig (jerky, rough, smooth, steady motion);
  • recording of drill parameters in the case of destructive boreholes;
  • rate of advance.

5.1.6 Special care shall be taken when investigating for injections in high stress regions and in polluted ground:

  • for injections in high stress regions, the in situ state of stress shall be investigated before designing a grouting project;
  • for the injection of organic gels into polluted soils, a bacteriological investigation of the groundwater and ground shall be carried out.

5.1.7 Site investigation boreholes shall be suitably backfilled when no longer required.

5.2 Permeability testing

5.2.1 The hydraulic conductivity of the ground may be:

  • determined from in situ permeability tests or large scale pumping tests;
  • estimated from laboratory tests on samples of undisturbed soil or recompacted soil;
  • estimated from the particle size distribution of the soil, and/or density of the ground.

5.2.2 Suitable correlations with actual field permeabilities should be established.

5.2.3 Each site investigation hole in rock should be tested to determine the water take and to identify the water bearing or open zones in the hole. The hole should be either tested as it is drilled or tested by the use of packers after drilling is completed.

5.2.4 Special care shall be taken to isolate artesian zones before testing.

5.2.5 Lugeon tests are used in rock to obtain a general impression of the transmissivity of the ground. They do not necessarily provide a reliable correlation with the absorption of a specific grout.

5.3 Field grouting trials and grouting tests

5.3.1 Field grouting trials are executed in order to define or validate a grouting method. Field grouting trials should be considered as part of the initial site investigation. They should be conducted during the final design phase, or the first part of the construction phase if they did not form part of the site investigation. They should be executed where initial investigations and local or comparable experience is insufficient to support or justify the effectiveness of the grouting project. The trials should provide information on borehole spacing, grouting pressure and grout take and type.

5.3.2 Grouting trials shall be planned and executed in close collaboration with the designer of the final grouting program.

5.3.3 The limit criteria for the properties of the proposed grout should be established on the basis of experience gained during the trial grouting.

5.3.4 Detailed records shall be kept of each operation performed during the grouting trials.

5.3.5 An indication of in situ conditions may be obtained by permeating reconstituted soil samples with trial grouts, under laboratory conditions. Permeability measurements made before and after grouting the sample may provide indicative information which will facilitate decisions on the frequency of injection points, the desired properties of the grout mix, and the required grout volume.

EN 12715:2000 Execution of special geotechnical work – Grouting