7.5 Grout placement

7.5.1 General Grout placement methods shall be selected specifically for each job and the choice shall be dictated by the design concept and intent. The injection process is governed by:

  • the grout volume V per pass;
  • the injection pressure P;
  • the flow or placement rate Q;
  • the grout rheology. The design should indicate how to adapt Q, V and P for a given mix design, or the rheology of the mix design, to the anticipated ground response during grout placement. For permeation, the flow (injection) rate Q should be controlled to ensure that the effective pressure remains lower than the ground fracturing pressure. The effects of the modification of the pore water pressures induced by the treatment, and related changes in the in situ stresses, should be considered.

7.5.2 Drilling pattern and borehole design The relative positioning of injection points within the mass to be grouted depends on:

  • the shape of the mass;
  • physical constraints influencing the positioning of boreholes;
  • the expected directional tolerances of the boreholes;
  • an assessment of the expected distance of grout travel (radius of efficiency) within the host material. The number, position, spacing, depth, diameter, inclination and orientation of boreholes and injection points shall be based on geological conditions, the type of structure to be grouted, the results to be obtained, the grouting method and purpose, the type of grout to be used, injection pressure and rate of grout take. The design shall make adequate provision for any variation in the above parameters. Whenever possible, the design should be refined by conducting site grouting trials. The positions of all foreseen injection boreholes shall be indicated on a plan and they shall be numbered. When impregnating a rock the spatial relationship between bedding planes and joints or fractures shall be taken into account. The boreholes shall be adjusted to the orientation and separation of the principal open features. The grout hole diameter should be based upon the type and condition of the rock to be grouted and the depth and inclination of the hole. It should be chosen to allow any foreseen in-hole tests. Care should be taken to minimize borehole deviations and the design should adjust the spacing between holes to compensate for the deviations expected. In general, the borehole axis should not deviate from the planned orientation by more than 3 % for lengths of less than 20 m. In the case of longer holes the spacing between neighbouring holes should be adjusted to compensate for deviations.

7.5.3 Grouting sequence In its simplest form, a sequence constitutes a single grout type introduced through a single hole. The grout placement sequence may progress to multiple stages, over many holes, with each stage requiring a sequence of injection passes of differing grout types. The design shall therefore specify the following variables:

  • the manner in which the treatment will progress through the mass being grouted (inwards or outwards, top-down or bottom-up, etc.);
  • the number of grouting phases;
  • the number of passes per stage;
  • the type of grout introduced for each pass.

7.5.4 Grouting pressure In general practice, the grouting pressure is measured at the grout delivery pump and/or at the hole collar. However, variations in hydraulic head, and friction losses in the delivery system, will result in this 'working pressure' being different from the 'effective pressure' acting in the ground. In rock, the value of the 'effective grouting pressure' can be assessed in relation to the pressure at which:

  • the rock breaks under tension;
  • predominantly horizontal planes in the rock are parted, and displacement occurs; or
  • widening of the treated fissures takes place. During non-displacement grouting of soils, the effective (or limit) grouting pressure is dependent on the confining pressure at the point of injection. For non-displacement grouting, the permissible injection pressure is the maximum pressure at which a grout is allowed to be introduced into the ground in order to avoid any undesirable deformation of the ground. In non-displacement grouting, a value for the permissible injection pressure shall be given in the design.

7.6 Monitoring and control criteria

7.6.1 The design shall specify the monitoring and control criteria before, during and after the grouting works and shall indicate which of the following parameters to monitor and control and how this is to be done:

  • the grout properties during processing and placement;
  • the tolerances in direction and inclination of boreholes (drilling accuracy);
  • the criteria for ending injection after each pass;
  • the results achieved after each phase of grouting and/or at the end of the project;
  • ground movement or deformations;
  • chemistry of water;
  • water levels in existing wells or observation boreholes.

7.6.2 The criteria for ending an injection pass should be based on the following:

  • in soil:
  • limiting pressure and/or volume;
  • ground movement due to grouting exceeding a limit value;
  • escape of grout to surface, into buildings or neighbouring holes;
  • bypassing of packers;
  • in rock:
  • limit pressure (refusal) and/or volume;
  • ground movement;
  • escape of grout;
  • unacceptable loss of grout into adjoining areas.

7.6.3 The design shall specify the control criteria and the tests necessary to verify that the objectives of the grouting works are achieved.

EN 12715:2000 Execution of special geotechnical work – Grouting