8.4 Grout placement
8.4.1 The method of grout placement will be determined by the ground condition, the works requirement and the type of grout used. The basic approaches are the following:
- a) injection in unsupported boreholes in stable ground;
- b) injection via sleeve pipes previously placed in a temporarily cased borehole, in unstable ground;
- c) injection through the drill string in unstable ground, generally considered as a pre-grouting phase and to be followed by approaches a) or b);
- d) compaction grouting is usually performed through a casing retrieved during upstage grouting.
The general implementation in soil and rock is summarized in Table 4 where a stage is defined as a predetermined length of injection bounded either by a double packer, or by a single packer and the base of the hole.
|Open hole||Sleeve pipe||Drill rod||Sleeve pipe||Lance or casing|
8.4.2 Soil grouting can be achieved with casing, grout sheath, pierced casing, and sleeve pipes.
8.4.3 Sleeve pipes which are permanently sealed into the ground by use of a support mix (sleeve grout) allow a repeated use of the injection points.
8.4.4 When grouting in soils, adjacent sleeve pipes should be flushed as a precaution against grout leakage.
8.4.5 Large openings (voids, cavities, etc.) are generally filled under gravity, either directly, or via a tremie pipe extending to the base of the opening.
8.4.6 Packers are used to isolate a grouting stage. Packers are either passive, mechanical or pneumatic, and have to be long enough to minimize the risk of grout bypass through the medium being grouted.
8.4.7 Packers shall ensure tight sealing between the grout hole wall and the injection pipe at maximum grouting pressure.
8.4.8 In instances where the grout has a tendency to bypass the packers, especially in ascending stage grouting, a flushing system should be installed to wash out bypassing grout.
8.4.9 The maximum stage length in rock should normally not exceed 10 m. In intensely fissured or disturbed rock this length should be adjusted accordingly. Injection stages in soil should not exceed 1 m in length.
8.4.10 When grouting in suspected or known conditions of flowing groundwater, excessive dilution or complete loss of grout should be prevented. Depending on the ground conditions, the purpose to be achieved and the rate of groundwater flow, the following precautions should be taken:
- the use of a grout with a short setting time, even a flash setting time (such as aquareactive resins, cement based grout with sodium silicate);
- the use of a viscous grout or/and grout with a high dry matter content;
- the use of additives to limit grout dilution.
8.4.11 If using suspensions when grouting in absorbent rock, a water retaining grout should be chosen.
8.4.12 Injection parameters (pressure, volume and flow rate) shall be adjusted to avoid unacceptable deformation and displacement of the ground, unless this is specifically intended. Particular caution shall be taken in the vicinity of sensitive structures.
8.4.13 The flow rate for permeation grouting depends on the size of the interstices and the viscosity of the grout.
8.4.14 During the placement and handling of grouts and grout components, precautions shall be taken to avoid spillage and, in particular to prevent the escape of any liquid or grout off-site.
8.5 Grouting sequences
8.5.1 The planning of grouting works is an interactive, on going process requiring on-site management. Decisions concerning changes in treatment or in parameters set out in the design and defined in the method statement, as well as continuation or termination of the grouting operation, shall be taken by all concerned parties.
8.5.2 Descending or downstage grouting is commonly reserved for the treatment of unstable rock. If several holes are grouted using downstage grouting, the uppermost stage in all holes is drilled and grouted before drilling and grouting the next stage in all neighbouring holes.
Upstage grouting is only used in open holes in stable rock or if the aim is compaction grouting. Multistage grouting, using sleeve pipes is generally used in soils and sometimes in unstable rock. Combinations of these techniques are possible.
8.5.3 The split-spacing method of first drilling and grouting primary holes, followed by intermediate secondary and finally tertiary or even quaternary holes may be used to provide a minimal uniform coverage and to allow for closer spaced holes to be restricted to those areas where progressive experience shows them to be necessary.
8.5.4 The spacing of the primary injection holes shall be decided upon on the basis of experience or grouting tests.
8.5.5 When using the split-spacing method, selected primary hole should be grouted first as exploratory grout holes except in areas of previous test grouting to:
- allow for better description of the geological and hydrogeological situation;
- permit the final selection of the grout hole depth for the remaining primary holes.
8.5.6 In order to limit the lateral migration of grout beyond the designated treatment area, injections should commence on the perimeter of the zone to be treated and move progressively inward. This procedure should not be used if there is a danger of groundwater entrapment or if the aim is to drive water out of the treatment zone.