8.8 Grouting

8.8.1 General The following methods may be employed for filling and grouting the borehole(see Figure 6):

  • filling the borehole with grout;
  • grouting:
  • single step grouting through a temporary casing;
  • single step grouting through a bearing element;
  • grouting during driving and/or drilling;
  • single or multiple step grouting through tubes-à-manchettes, special valves or post-grouting tubes (= multi–stage grouting).

NOTE Filling or grouting meets one or more of the following functions:

  • to create or improve the bond between the micropile shaft and the surrounding ground to allow the design shaft bearing capacity to be mobilised;
  • to protect the reinforcement against corrosion;
  • to improve the bearing capacity of the micropile;
  • to strengthen and seal the ground immediately adjacent to the micropile in order to enhance the micropile bearing capacity. The method of grout placement shall be determined by the ground conditions, the required shaft and end bearing capacity, the type of grout and equipment used and shall be specified in the project specifications. For shaft bearing micropiles, high pressure second stage grouting may be used to increase the friction resistance by introducing further grout into the ground and raising the normal stresses at the ground/grout interface. This may be carried out before or after installation of the reinforcement.

8.8.2 Grout preparation The grout preparation and the filling or grouting procedure shall be carried out in such a way that the required design strength of the material is assured. Contamination of the grout and its constituents shall be avoided during storage, handling and delivery. The proportioning of grout components shall be carried out with certified measuring devices and respecting the tolerances given by the suppliers. Batching and/or mixing processes shall be controlled in accordance with Mixers shall be selected to ensure the homogeneity of the grout. An intermediate holding tank should be located between the mixing tank and the pump(s). The mix in the holding tank should be agitated to prevent segregation and/or premature setting. Pumps and grouting systems shall be compatible with the selected filling or grouting method. The grouting pressure should be measured as close to the point of placement as it is practicable.

8.8.3 Borehole testing and pregrouting

For micropiles installed into weathered or strongly fissured rock, borehole testing and pregrouting may be necessary to avoid uncontrolled loss of grout into the surrounding rock mass and to guarantee the required grout cover of the reinforcement or load bearing element.

NOTE General information on borehole testing and pregrouting is given in informative annex E.

8.8.4 Filling the borehole with grout The interval between the completion of the borehole drilling and the filling up of the borehole with grout shall be kept as short as possible. Measures shall be taken to ensure that the micropile length is completely filled with grout. When filling the borehole with the tremie pipe or through the drill rods or tubular bearing elements, the end of the tremie pipe or drill rods shall remain submerged in the grout and grouting shall continue until the consistency of the grout emerging on top is almost the same as that of the injected grout (Figure 6 a). When filling the borehole, air and drilling fluid shall be able to escape to permit complete grout filling. For drilled boreholes the remaining cuttings shall be able to escape when filling the borehole.

8.8.5 Single step grouting through a temporary casing The reinforcement shall be placed before the temporary casing is extracted. During extraction of the temporary casing the grout level within the casing shall be brought back up to ground level before the next length of casing is removed (Figure 6 b). The grouting pressure should be applied at least every 2 m during the extraction of the casing.

8.8.6 Single step grouting through a load bearing element When tubes are used as bearing element, single step grouting can be applied at the bottom of the bearing element (Figure 6 c). When the specified grouting pressure cannot be applied, re–grouting shall be performed after a certain waiting period until the specified grouting pressure can be applied.

8.8.7 Grouting and filling during driving

For driven micropiles grouting and filling shall be done in accordance with EN 12699:2000.

8.8.8 Grouting during drilling When grouting is applied during drilling, the bearing elements are fitted with a drill bit and they are drilled into the ground. For grouting during drilling, the grout pressure and flow rate should be adjusted depending on the grout susceptibility to penetrate the ground loosened by the drilling process and contained within the annulus around the reinforcing element. When grouting during drilling, grout flushing should be carried out at a constant rate and the flush should be re–established each time a new section of bearing element are added prior to advancing the drill bit.

8.8.9 Multi–stage grouting Multi–stage grouting may be executed by single step grouting through tube-à-manchettes (Figure 6 d) or by multiple step grouting through tube-à-manchettes or special valves (Figure 6 e) or by single step grouting through several post-grouting tubes staggered in length (Figure 6 f). The multi–stage grouting phase(s) shall take place only after the grout placed into the borehole according to 8.8.4, 8.8.5 or 8.8.6 has set. The grouting shall be carried out either in single or multiple step(s) and in single or multiple stage(s), according to the project specifications. When the specified grouting pressure cannot be applied additional step(s) of grouting shall be performed after a certain waiting period until the specified grouting pressure can be applied. The grouting tubes shall be flushed with water after each grouting step and filled with grout at the end of the whole grouting process.

8.9 Concreting

8.9.1 Concreting in submerged conditions shall be in accordance with EN 1536:1999, subclause 8.3.3.

8.9.2 Concreting through a continuous flight auger shall be in accordance with EN 1536:1999 subclause 8.3.6.

8.9.3 Concreting in dry conditions shall be in accordance with EN 12063:2000 subclause

8.10 Trimming of micropiles

8.10.1 The trimming operation:

  • shall be carried out only when the grout or concrete has obtained sufficient strength;
  • shall remove all grout or concrete which is contaminated or of lower quality than required.

8.10.2 Cutting off and stripping of the micropile head shall be done carefully to avoid shattering and damage of the rest of the micropile.

EN 14199:2005 Execution of special geotechnical works — Micropiles