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ACTIVITY CENTRE

SLOPE ENGINEERING

A new actively stressed soil nail system has been trialled in New Zealand. Tony Barley reports on developments.

Queenstown, a popular resort on the edge of Lake Wakatipu in New Zealand's Southern Lakes region, is surrounded by impressive mountain peaks and ridges.

Development of its level shoreline reached capacity several years ago. Queenstown's modern expansion has encroached into hillside areas. The demand for lake and mountain views from multilevel developments requires excavation and retention of steep or vertical cut faces.

Soils typically comprise dense silty sands with gravel but, in areas where they contain weak alluvial silt bands, the interbedding poses a problem for slope designers. These mixed overburden materials are typically underlain by weathered schist of varying degrees of degradation and thickness and are not necessarily appropriate for high bond capacity in ground anchors.

The area also experiences seismic events, an additional difficulty that has to be accommodated in the safe design of the steepened slopes.

The rigid and flexible facings of these slopes are supported by the latest soil nail and ground anchor technology, including actively stressed soil nails. In April this year the licensee Austress Menard completed the first trials of the latter system on a slope in Queenstown.

This nail system complements the multiple unit anchor concept of the Single Bore Multiple Anchor system, which was also recently used on a job in Queenstown town centre by foundation contractor March Construction.

Early development of UK soil nailing technology used fully bonded, passive tensile members for retention of active zones of soil.

Surface soil movement was initially controlled by placement of geogrids or similar materials over the exposed slope surface, and the grid was often restrained by driven pins.

As confidence in soil nail technology increased, cut faces became steeper and surface soil retention extended to semi-flexible and stiff facing systems, most commonly provided by shotcreting.

The concept of building the facing and leaving both the face retention and the soil nail in a passive state gave engineers reason for concern.

Face movement would initially be required to develop the nail retention forces, and subsequently active movement within the soil would take place to generate tension in the deeper component of the passive nails.

The simplest and most practical solution appeared to be to consider the head of the nail as an anchor which justified the application of prestress. However, load cannot be applied to a bonded elastic tensile member without debonding progressively along the nail. This results in the applied face retention force being resisted by load transfer into the active zone of the soil mass.

The actively stressed soil nail system ensures the face retention force is transferred entirely into the resistant zone of the soil mass which is consistent with the original principles of wall retention.

This is effected by the installation of two tensile members in each nail bore; one being actively stressed against the face and bonded in the resistant zone. The second member passively ties the active zone soil to the resistant zone consistent with soil nail technology.

The two tensile members in the borehole are fully isolated from the surrounding material, encapsulated in a plastic corrugated duct. This complies with the latest code recommendations and eliminates tendon corrosion, increasing life expectancy of the nail. And it has potential benefits in seismic areas.

In Queenstown, the active soil nail system has been proposed for both stabilisation of the soil mass and for the prestressed retention of a 14m high, 80infinity shotcrete face.

Seven rows of actively stressed nails will be used with a single row of toe level SBMA anchors. The nails will be founded in the silty sand and gravels and the multiple anchors in the weak silts.

Trials, supervised by Single Bore Multiple Anchor, were carried out on the new nail system, achieving load capacities as high as 864kN without failure in the distal 6m length of a 12m long nail.

Tests on the short fixed length post-grouted multiple anchors investigated ultimate bond capacity in the weak silts. They attained between 544kN and 864kN.

Keller, the UK patent holder, is now preparing for trials on the actively stressed nail system.

Tony Barley is director of Single Bore Multiple Anchor.

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