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Controlling factor

Tony Barley assesses the influence of the installation process on the performance of soil nails.

Industry reservations about soil nailing in the 1980s meant a slow start for the technique in the UK, but it is now an accepted practice for the stabilisation of vertical or steep slopes and is commonly used to improve the stability of existing slopes and walls.

This relatively new technology often replaces the use of ground anchorages, but the Code of Practice for Strengthened/ Reinforced Soils and Other Fills (BS8006) does not provide comprehensive guidance for the installation of soil nails, nor does it stipulate the testing requirements for them with the same degree of detail that is provided for anchorages in the Code of Practice for Ground Anchorages (BS8081). Because of their passive nature, there is no direct monitored loading applied to the working soil nails to ensure that adequate bond capacity is provided.

Absence of both guidance and requirements for performance testing emphasises the need for strong control of the soil nail installation process to ensure that their use does not lead to catastrophic collapse of the face or slope.

Drilling and flushing

Soil nails usually stabilise slopes or faces by the use of tensile forces within the soil nail body. Although the shear and bending capacity of a nail may provide some support, these components require considerable soil movement and thus are rarely mobilised until the tensile capacity has been exceeded. Generally, shear and bending components are only considered in the design of nails which are unable to provide satisfactory tensile capacity; these are typically those installed directly in the ground by percussion or driving action without the use of grout as a bonding agent.

Where nails are designed to work primarily as tensile members and perform efficiently, it is essential to provide good bond between the tendon and the grout and between the grout and the ground. Where a relatively good bond is available at the ground interface, the bore diameter is typically 100mm to 150mm (weak rock and granular materials) while in cohesive strata, diameters of 150mm to 200mm may be required. Nail performance and bond capacity are also heavily influenced by the choice and method of application of the drilling and flushing system.

The simplest method appropriate for use in freestanding cohesive strata is the auger system which requires no drill flush (Figure 1). However, owing to the spiral transportation of the drill spoil, some smearing is deposited on the borehole wall. The effect of this smearing grows with the spoil's increasing water or moisture content and may lead to a reduction in bond.

Where mixed soils or weak rock stand open while drilling, rotary or rotary percussive drilling systems with drill flush ensuring spoil removal are appropriate. It must be remembered that the drill flush itself may lend support to the soil. Air flush provides the minimum support, water flush greater support and a dense flush medium, such as grout, the greatest soil support - although it is the most difficult to control in terms of cost and environmental considerations. The use of air as a flush medium is highly suited to dry soils, but becomes more problematic as water content increases.

In situations where the spoil laden flush is not returned in a controlled manner alongside the drill rod, it is important to assess the potential damage caused to the soil mass, to the adjacent soil nails and to adjacent properties or foundations. Persistent drilling without flush return is not acceptable in anchor practice and, unless the consequences are precisely understood, should not be tolerated during nailing operations. Loss of flush generally results either from soil collapse blocking the exhaust path or, particularly in the case of down the hole hammer works, from blockage of narrow exhaust channels.

In mixed soils and/or granular materials (sand, gravels, glacial tills etc) the use of duplex drilling techniques with either air or water flush ensures best control and minimum disturbance (Figure 2 and Figure 3). This system involves the advancement of drill casing a short distance behind the drill bit in order to support the bore and ensure the free exhaust of the flush and drill spoil to the surface via the drill casing. The rods and casing are advanced by rotary or rotary percussive drilling techniques with the production rate for long nails ( 10m) influenced by rig power and weight. With the more powerful rigs, the drill casing can also be advanced with an auger system, which allows avoidance of drill flush requirements.

Grouting

It is normal practice to use neat cement grout with a water:cement ratio of about 0.45. Slightly higher ratios may be adopted, but characteristic strengths of grout of 30N/mm2 should be attained.

Grouting the bore must, in any event, be carried out the same day as bore drilling, but in certain conditions (clays in particular), grouting should be carried out on completion of drilling to prevent deterioration of the clay and of the bond capacity.

Grout is normally tremied in after soil nail tendon installation but reverse order is acceptable.

Where drill casing is installed, grouting must be carried out prior to its withdrawal. This demands continuity of drilling, tendon installation, grouting and casing withdrawal operation.

Because of the shallow inclination of the soil nail bore, special consideration should be given to the grouting of the bore at the proximal end (Figure 4).

Where grout take exceeds the volume of the intended bore, the cause must be identified to ensure there has been no detriment to ground strength, adjacent nails, or to soil nail performance.

Where enhanced bond and nail performance is required, consideration should be given to pressure grouting techniques, although considerable experience is required by the drilling and grouting crew to ensure safe usage and improvement of ground strength without heave.

Testing

Testing of preliminary nails or sacrificial production nails should be carried out on nails installed with identical drilling and flushing and grouting systems to that used on production works.

There are generally three working components of the nail that contribute to slope/face stabilisation. Each should be tested separately by isolating each section1:

(a) the bond capacity within the resistant zone of soil;

(b) the bond capacity within the active zone of the soil;

(c) the plate bearing capacity at the surface.

The investigation of the capacity of (a) is frequent but where a structural facing is not provided the capacity of (b) and (c) are equally important components in design. Where width of the active zone (b) decreases near the base of the face or slope, the contribution of plate bearing capacity (c) increases.

Summary

The design of soil nails for the stabilisation of the slopes and faces demands the incorporation of assumed values of bond stress to transfer tensile load from the nail tendon, generally via the grout to the soil. In normal practice, site investigation only provides information on soil condition and soil strength. However, the overall performance of the soil nail, which is related to bond stress at the grout/ground interface, is heavily influenced by the drilling, flushing and grouting techniques adopted. It is therefore essential to ensure that the all stresses assumed in the design are substantiated by site trials and sacrificial production nail tests, and that the strict controls of the installation system are maintained.

References

1 Barley AD, Davies MCR and Jones NM. Proposed methods for field testing of soil nails. Proceedings of the third international conference on ground improvement systems, London, 3-5 June 1997.

Tony Barley is director of engineering at Keller Ground Engineering

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