8.3 Grouting

8.3.1 General

Grouting meets one or more of the following functions:

  • a) to form the fixed anchor length in order that the applied load may be transferred from the tendon to the surrounding ground;
  • b) to protect the tendon against corrosion;
  • c) to strengthen the ground immediately adjacent to the fixed anchor in order to enhance ground anchor capacity;
  • d) to seal the ground immediately adjacent to the fixed anchor length in order to limit the loss of grout.

NOTE If a grout volume injected is in excess of three times the borehole volume at pressures not exceeding total overburden pressure, then general void filling is indicated which is beyond routine anchor construction. In such cases general void filling may be necessary before grouting the anchor. For functions c) and d) above only nominal grout consumptions should be expected.

In order to form the fixed anchor length without an uncontrolled loss of grout over this length, the following operations may be considered:

  • borehole testing;
  • pregrouting;
  • anchor grouting.

8.3.2 Borehole testing

On completion of the borehole or during the grouting of the anchor measures shall be taken to ensure that the fixed length is fully grouted after the grout has set. This may be done for example, by water testing, falling head grout tests or by pressure grouting.

NOTE 1 Water Testing. - The likelihood of cement grout loss can be assessed in rock from an analysis of a water injection test. Routinely a falling head test is applied to the borehole or the fixed anchor length via a packer. Pregrouting is not usually required if leakage or water loss in the hole or fixed anchor length is less than 5 l/min at an excess head of 0,1 MPa measured over a period of 10 min.

NOTE 2 Falling Head Grout Test. - When pressure grouting of the fixed anchor length is not carried out as part of the routine anchor construction, the borehole may be pre-filled with grout and the grout level observed until it becomes steady. If the level continues to fall it should be topped up and after sufficient stiffening of the grout, the borehole should be redrilled and retested. The test may be applied to the entire borehole or restricted to the fixed anchor length by packer or casing over the free anchor length.

NOTE 3 Pressure Grouting. - For anchor types where grouting of the fixed anchor length is done under pressure, this activity is generally isolated during controlled withdrawal of the lining tube or by the use of a packer or tube-a-manchette system. During injection a controlled flow rate at a measured pressure indicates a satisfactory grouting operation. On completion of grouting of the fixed anchor length the efficiency of this phase can be checked by monitoring the response of the ground to further grout injection when the back pressure should be quickly restored.

8.3.3 Pregrouting

Pregrouting should be carried out by filling the borehole with cement based grout. Sand/cement grout is commonly employed in rock and very stiff to hard cohesive deposits with partially filled or open fissures and in permeable cohesionless soils to reduce grout consumption.

On completion of pregrouting the borehole should be retested and if necessary, the grouting process should be repeated after redrilling.

8.3.4 Pregrouting of rock

In soft rocks, the time of redrilling in relation to grout strength gain is critical to avoid problems of drilling alignment.

Chemical grouting should not be necessary in normal practice but, if employed, it should be established that the chemical will have no deleterious effect on the anchor or on the environment (i.e. contamination of soil and groundwater).

Where a water test indicates an hydraulic connection to an adjacent unstressed anchor, then stressing of that anchor should not be carried out before the grout has set.

8.3.5 Pregrouting of soil

Where borehole testing has identified that the soil is highly permeable or that the grout may be injected at a high flow rate without generating back pressure, pregrouting may be required. This may not be a routine procedure but a prudent precautionary measure if the above soil conditions are suspected to prevail.

In exceptional circumstances it may be necessary to carry out general void filling for overall ground strengthening. In this case such work should not be considered part of routine ground anchor construction.

8.3.6 Anchor grouting

Placement of grout should be carried out as soon as possible after completion of drilling.

When grouting by the tremie method, the end of the tremie pipe shall remain submerged in grout within the fixed anchor length and grouting shall continue until the consistency of the grout emerging is the same as that of the injected grout.

The grouting process should always start at the lower end of the section to be grouted. For horizontal and upward inclined holes, a seal or packer is required to prevent loss of grout from either the fixed anchor length or the entire hole.

Air and water shall be able to escape to permit complete grout filling.

When installing nearly horizontal anchors, special measures, such as multi-stage pressure grouting should be used to prevent any voids being left in the section to be grouted. When multi-sequence grouting along the fixed anchor length or regrouting is envisaged, a sleeve pipe (tube-a-manchette) system should be incorporated in the anchor assembly.

In certain ground conditions where the grout column is adequately confined in the free length some load can be transferred from the fixed length into the free length and onto the back of the structure. Where appropriate, one or more of the following actions may be taken:

  • flush the grout out from behind the structure;
  • replace the free length grout with a non-load transferring material;
  • locate a packer at the proximal end of the fixed length.

High pressure multi-stage grouting may be used to increase anchor 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 tendon installation.

Artesian water gain within a borehole should be counteracted by an excess head of grout or by pregrouting irrespective of the rate of the water gain.

8.4 Stressing

8.4.1 General

Stressing is required to fulfil the following two functions:

  • to ascertain and record the load carrying behaviour of the anchor;
  • to tension the tendon and to anchor it at its lock-off load.

Stressing and recording shall be carried out by experienced personnel under the control of a suitably qualified supervisor, provided preferably by a specialist anchor contractor or stressing equipment supplier.

8.4.2 Equipment

Stressing equipment and load cells in regular use shall be calibrated at intervals not exceeding six months and the calibration certificate shall be made available for inspection on site at all times.

Stressing equipment for bar and strand tendon should tension the complete tendon as a single unit. Stressing - equipment which tensions individual strands not simultaneously should be provided with or be supplemented by measuring devices which establish the total load in the multiple strands at any time during testing. Alternatively accurate lift-off checks should be undertaken.

NOTE The equipment should be capable of safely tensioning the tendon to the specified proof load within the rated pressure capacity of the pumping unit.

8.4.3 Stressing procedure

If the loading on the structure is required to control the sequence or the phase loading of the anchors then this shall be specified at the design stage.

The anchored structure should be designed to provide reaction to allow load testing of the ground anchors in accordance with clause 9.

The methods of stressing and load recording to be used in each testing or stressing operation should be detailed prior to any tensioning work.

The equipment should be used strictly in accordance with the manufacturer's operating instructions.

Stressing or testing should not be carried out until sufficient hardening of the grout in the fixed length has been achieved, which normally requires seven days.

In sensitive cohesive soils it may be appropriate to stipulate a minimum time period for soil recovery after completion of the ground anchor installation and prior to stressing.

During the testing or stressing of production anchors no indents resulting from tendon gripping should be formed in the tendon below the anchor head and no damage should be allowed to the corrosion protection.

EN 1537:1999 Execution of special geotechnical works – Ground anchors