8 Execution

8.1 General

8.1.1 The micropiles shall be executed and supervised by trained and experienced personnel.

8.1.2 A method statement should be provided before starting the execution of micropiles. This method statement should contain (but is not limited to) the following information:

  • identification , objective and scope of the micropiles;
  • soil description; (possibly by reference to site investigation report);
  • environmental issues;
  • technical requirements;
  • equipment and working procedure for:
  • drilling and/or driving;
  • installation of reinforcement or bearing element;
  • filling, grouting or concreting;
  • measures to ensure the boring accuracy:
  • grouting parameters;
  • site installation and working areas;
  • spoil management;
  • quality control procedures.

8.1.3 Special care shall be taken for the execution of tangent or secant micropiles for the formation of walls (spacing, deviation, sequence of drilling, constitutive material).

8.1.4 Where possible the preliminary, trial or test micropiles should be installed close to positions of soil investigation.

8.1.5 Where the ground conditions differ from those stipulated in the design or where unexpected underground obstructions are encountered, appropriate action shall be taken. The execution should be conducted in such a way that any major variation of the ground conditions, from those on which the micropile design has been based, may be detected.

NOTE A drilling or driving – log can be established using simple practical identification data which can easily be recognised by the operator (e.g. class of ground, colour of flushing returns or loss of drilling fluid downhole). Any major deviation of the ground conditions shall be reported and considered in the design of micropiles as indicated in 7.3.4.

8.2 Site preparation

8.2.1 The working platform, necessary for the installation of the micropiles, shall be prepared and maintained in such a way that all operations can be carried out safely and efficiently.

8.2.2 Services and existing underground installations should be recognised and removed if necessary.

8.3 Sequence of installation

8.3.1 The sequence of installation of micropiles shall be planned.

8.3.2 This sequence shall take into account if relevant:

  • the detrimental effects of settlements on structures to be underpinned or on adjacent structures;
  • the detrimental effects on the bearing resistance of previously installed micropiles.

8.4 Drilling

8.4.1 General The drilling methods which can be used for micropiles are given in Annex A.1.

NOTE When constructing micropiles by drilling, continuous drilling with flushing for the removal of soil is the most common method. Micropile boreholes shall be drilled until they reach:

  • the specified embedment in the bearing stratum; or
  • the anticipated founding level; or
  • the prescribed length. Boreholes shall be checked for position and length. Boreholes shall be checked for inclination and orientation when specified. Loose and remolded drilling deposit shall be removed from the borehole for end bearing micropiles. For shaft bearing micropiles the drilling and flushing method shall not impede the required shaft bearing capacity. The drilling method to be used shall be decided by taking care of all requirements of soil and foundation conditions, and in such a way that the designed length can be reached and that a stable borehole and the intended cross section is created over the entire length. When uncontrolled inflow of water and soil into the borehole can occur or when there is a risk of collapse, special measures shall be taken to maintain the stability and thereby prevent the uncontrolled entry of soil and water.

NOTE 1 An inflow of water and/or soil could cause for instance:

  • a disturbance to or instability of the bearing stratum or the surrounding ground;
  • loss of support by the removal of soil from beneath underpinned or adjacent foundations;
  • damage to the unset grout, mortar or concrete in the micropile or micropiles recently installed nearby;
  • defaults in the shaft;
  • washing out of cement.

NOTE 2 There are increased risks in:

  • loose granular soil;
  • soft cohesive soil;
  • ground which is variable;
  • when using air as drilling fluid with direct circulation under the groundwater table.

8.4.2 Use of flushing Drilling can be performed with water, air and drilling fluids. When drilling methods with air as flushing medium are used for underpinning works, special care should be taken to avoid disturbance or fracturing of the ground. The drilling fluid shall not impede the success of any subsequent filling or grouting operation. Special care shall be taken when drilling through ground under artesian water pressure at the working platform. Techniques to counteract the water pressure and to prevent any blow–out, hole collapse and erosion during drilling, installation and filling or grouting operations shall be identified in advance and implemented as and when required.

NOTE For ground water levels which are higher than the working platform it may be appropriate to perform the drilling from a higher working level or to use heavy drilling fluids.

8.4.3 Boreholes supported by casings

Casings should be used when the borehole is unstable or there is a significant fluid loss or when filling or grouting is performed through the casing.

8.4.4 Drilling with continuous flight augers Drilling with continuous flight augers shall be done in accordance with 8.1.5 of EN 1536:1999. No special limitations exist concerning the inclination on the basis that the direction of excavation is controlled and the installation of the reinforcement can be achieved correctly.

NOTE Normally grout or mortar is used for the execution of micropiles with continuous flight augers.

EN 14199:2005 Execution of special geotechnical works — Micropiles