8.1 Drilling of holes
Holes for anchors shall be drilled to the tolerances specified.
NOTE 1 In the case of unforeseen conditions on site, design modifications or execution alternatives may be introduced. During execution the borehole diameter may be increased due to the necessary use of casing etc. Compliance with the positioning tolerance is important if the design of the anchored structure is to remain valid. In general horizontal boreholes are avoided because of problems in filling the borehole completely with grout.
The hole diameter shall provide for the specified grout cover to the tendons along the fixed anchor length.
Allowance for extra drilling depth should be added to the specified depth when detritus cannot be removed from the bottom of the hole.
Unless otherwise specified the choice and the set-up of the drilling equipment should satisfy the following conditions:
- the borehole collar axis at the anchor head should be positioned within a radial tolerance of 75 mm;
- the initial alignment when setting up the drilling rig should not deviate by more than 2° from the specified axis of the borehole.
The deviation should be checked after the borehole has been advanced 2 m.
During drilling the overall borehole deviation tolerance should be limited to 1/30 of the anchor length. On occasion ground conditions may dictate the need for a relaxation of this tolerance.
NOTE 2 The drilling rig assembly and any working platform must be rigid if the desired borehole alignment is to be achieved. In case of doubt the positioning should be rechecked during drilling. Compliance with the angular tolerance is important in relation to the interaction between the fixed anchor lengths. Longer anchors may require smaller angular tolerances, if fixed anchor interference is to be avoided.
NOTE 3 Compliance with the deviation tolerance is important if difficult homing, undesired friction during stressing and interaction between fixed anchors are to be avoided. Measurement of borehole deviation is not common practice but in special cases inclinometer devices can be applied. Borehole deviation can be minimized by using rigid and large diameter drill rods and associated casing. Deviation of rigid systems usually results from obstructions or inclined bedding planes. Other means and procedures, not detailed here, are available to check and control misalignment.
8.1.2 Drilling methods
The drilling method shall be chosen with due regard to the ground conditions so as to cause either minimum ground modification or the modification most beneficial to the anchor capacity and to allow the design anchor resistance (Rd) to be mobilized.
NOTE 1 The reasons for minimum ground modification are:
- to prevent collapse of the borehole wall during drilling and tendon installation (where necessary a casing should be utilized);
- to minimize loosening of the surrounding ground in cohesionless soils;
- to minimize change of ground water levels;
- to minimize softening of the surface of the borehole wall in cohesive soils and degradable rocks.
The modification to the ground shall be limited in such a way as to reduce the negative effects e.g. splitting, preconsolidation, postconsolidation, associated with each operation. The drilling fluid and possible additives shall have no adverse effect on the tendon, tendon protection, the grout or on the borehole walls especially in the tendon bond length.
NOTE 2 The relationship between area of inlet of the drilling fluid, the annular area of flush return, the particle size and density of the drill spoil and the density of the drilling fluid are critical to the efficiency of the drilling system. The use of air flush with wet cohesive spoil may cause blockage and result in unnecessary disturbance of adjacent ground. Clays, marls and marly rock may be liable to swelling or softening if exposed to water flush for unnecessarily lengthy periods.
Special care should be taken when drilling through ground under artesian water pressure.
NOTE 3 Sands may be loosened and destabilized by adverse hydraulic gradients in the soil surrounding the borehole.
Techniques to counteract the water pressure and to prevent any blow-out, hole collapse and erosion during drilling, installation and grouting operations shall be identified in advance and implemented as and when required. In high water table situations it may be appropriate to use heavy drilling fluids.
NOTE 4 Possible preventative measures include:
- the use of special auxiliary drilling equipment such as seals or packers;
- the lowering of the water table, after the risks of general settlement of the ground have been assessed;
- pre-grouting of the ground.
Drilling operations should be conducted in such a way that any major variation in ground characteristics, from those on which the ground anchor design has been based, may be detected immediately.
An indicative drilling log should be established using simple practical identification data (e.g. class of ground, colour of flushing returns or loss of drilling fluid downhole), which can be easily recognized by the operator.
Any major deviation from the indicative log shall be reported immediately to the designer.
8.2 Manufacturing, transport, handling and installation of tendons
During manufacture and storage, the tendons and their components shall be kept clean and free from corrosion, mechanical damage and weld splash.
The tendons shall not be coiled to radii less than any minimum specified by the manufacturer.
Where tendons comprise pre-coated greased strand or wire, exposed elements in the tendon bond length shall be cleaned and degreased thoroughly using steam or solvents.
When solvents are used care to degrease the tendons shall be taken to ensure that they are not aggressive to any of the anchor components and that after application the tendon/grout bond is able to transfer the design tensile loads without creeping.
Centralizers capable of ensuring the required cover to the tendon should be firmly attached to the tendon.
NOTE The spacing of centralizers will primarily depend on the stiffness and weight per unit length of the tendon.
8.2.2 Transport, handling and installation
During loading, transporting and installation of the tendon, care shall be taken not to kink the tendon or cause damage to its components and corrosion protection elements.
Prior to tendon installation, the borehole should be checked for obstructions and cleanliness in addition to length. Tendon installation should be carried out in a controlled manner with care being taken to avoid relative displacement of the components. In upward inclined ground anchors the installed tendon should be securely fixed to prevent movement during grouting.
The time intervals between the different operations required for the construction of an anchor should be related to the properties of the ground. They should, however, be kept as short as possible.
NOTE Where there is a risk of ground swelling or softening, installation and grouting of the tendon should follow immediately after the drilling of the borehole. As a general rule, tendon installation and grouting should be carried out on the same day as drilling of the fixed anchor length. If a delay cannot be avoided, each hole should be plugged to prevent the entry of deleterious material.