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Measure of success

Long term instrumentation is helping to improve design of New Hampshire soil nail walls.

New Hampshire Depart- ment of Transportation (NHDOT) constructed three separate soil nail wall projects along the Kancamagus Highway in Albany, New Hampshire from 1996 to 1998.

Extensive use of geotechnical instrumentation not only provided a check on construction but has also given a means to evaluate the design and long term performance of the structures and enable more efficient design on future soil nail wall projects.

The walls support steep hillsides within the US White Mountain National Forest.

Work included wall heights up to 6.7m and lengths from 130m to 185m. Each wall was constructed in a top to bottom sequence, with the excavation and wall construction occurring in 1.5m to 2.1m lifts.

Subsurface materials within the excavations included backfill from the original walls which was replaced, and a natural glacial ice contact deposit consisting of gravelly sands with cobbles and boulders. Relatively shallow groundwater levels required the use of a temporary dewatering system throughout the wall construction period.

Geotechnical instrumen- tation used for monitoring the walls was supplied by Geokon of Lebanon, New Hampshire and included slope inclinometer casing, vibrating wire (vw) piezometers, vw temperature sensors, vw load cells and vw strain gauges.

'The advantage of vibrating wire sensors over more conventional types lies mainly in the sensor output which is a frequency rather than a voltage, ' explains Geokon's Tony Simmonds. 'Frequencies can be transmitted along cables up to 2,000m long without appreciable degradation of the signal caused by variations in cable resistance from water penetration, temperature fluctuations, or leakage to ground. Coupled with rugged design, the sensors exhibit excellent long term stability and are ideally suited for long term measurements in adverse environments.'

The general contractor for all three walls was Busby Construction Co of Atkinson, New Hampshire and the soil nail subcontractor for walls two and three was Spencer, White and Prentiss, of Swansea, Massachusetts. Geotechnical design services were provided by Haley and Aldrich, of Manchester, New Hampshire for the first and second walls and by the NHDOT geotechnical section for the third wall.

Tom Cleary, New Hampshire Department of Transportation

Wall profile Key features of the walls summarised in the general order of construction for each lift:

Shotcrete facing with a geocomposite drain system placed behind the shotcrete was used for all three walls. The shotcrete facing ranged in thickness from 170mm to 290mm for the various walls and was reinforced with either 12mm diameter reinforcing bars or steel wire mesh. The full height geocomposite drain was spaced on a 1.5m centre to center spacing behind the wall.

Permanent horizontal drains were installed for the second wall to permanently lower the groundwater level and reduce long term groundwater pressures acting on the wall. The horizontal drains consisted of 50mm diameter slotted pvc pipes, 15m in length with a five degree upward installation angle.

Soil nails consisted of epoxy coated 31mm diameter threaded reinforcing bars, with a minimum of 38mm of grout cover for additional corrosion protection. The soil nail lengths ranged from 7.5m to 15m and were generally placed on a 1.5m vertical and horizontal spacing.

The soil nails were installed before placement of the shotcrete on the first wall and after placement of the shotcrete through blockouts on the second and third walls.

The installation of nails after shotcrete placement was generally more effective in the conditions encountered.

Rigid insulation comprising two layers of rigid extruded polystyrene, total thickness of 155mm, placed over the shotcrete facing. The insulation was designed to minimize long term freeze-thaw effects behind the shotcrete.

Timber facing consisting of pressure treated 50mm by 250mm horizontal boards and 100mm by 150mm vertical posts were attached to the shotcrete facing. This provided an architectural wood face finish which was requested by the US Forest Service.

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