Soil nailing is a common type of construction for retaining walls. For retaining wall structures with “passive” nails, mobilisation of the nails is required in order to retain the ground. An approximately 400 m long vertical soil nail retaining wall of up to 7.5 m height has recently been completed at the NZ Transport Agency’s Waterview Connection Project in Auckland, New Zealand. The soil nails comprise inclined glass-fibre reinforced plastic (GRP) bars, drilled and grouted in predominantly firm to very stiff Tauranga Group alluvium and East Coast Bays Formation residual soil. The nails are connected to a vertical reinforced shotcrete facing, constructed in a top-down sequence. The wall and retained ground surface have been monitored for movements during and after construction using survey markers and in-ground inclinometers. This paper presents the results of the monitoring, including observation of movement associated with soil shrink and swell. The paper also discusses how the results are compared with the design prediction, as well as movements observed from bored pile retaining walls of similar retained height and ground condition in other parts of the project.
A new retaining wall has recently been completed along the southern edge of the recently re-aligned Great North Road westbound off-ramp at the NZ Transport Agency’s Waterview Connection Project in Auckland, New Zealand. The Great North Road westbound off-ramp was required to be re-aligned to allow for a geometrically acceptable connection to be established between the existing State Highway 16 (SH 16) and the new Great North Road Interchange. The re-alignment works involved removal of the existing cantilevered post and panel retaining wall, and the excavation and construction of a new retaining wall to support the new alignment.
The new retaining wall consists of three types of wall structure:
- Cantilevered reinforced concrete bored pile wall structure, which forms about twenty percent of the retaining wall, mainly within the eastern section.
- Carrington Road overbridge retention, which is a retention system provided by the existing Carrington Road bridge south abutment columns.
- Soil nail wall, which makes up the majority of the retaining wall and the focus of this paper.
Figure 1 below shows the aerial view of the completed retaining wall.
Figure 1: Aerial view of the Great North Road westbound off-ramp new retaining wall
The soil nail wall structure was monitored for movement, more extensively than the other wall structure types, during and after construction. This was because, besides the soil nail wall forming the majority of the retaining wall, the “passive” nails within the wall structure are required to mobilise to achieve the resistance required to retain the ground. Therefore, it was essential to monitor the soil nail wall movements to confirm that the wall has achieved the intended performance as predicted in the design.
This paper focuses on the results of the soil nail retaining wall monitoring results, in particular with respect to the wall horizontal movements, and provides discussion of the observations made from the monitoring outcome.
2 SOIL NAIL RETAINING WALL DESCRIPTION
The soil nail retaining wall section is approximately 400 m long in total; approximately 260 m long to the west of the Carrington overbridge and 140 m long to the east of the Carrington overbridge. The western soil nail wall has a maximum retained height of approximately 7.5m, and the eastern soil nail wall has a maximum retained height of 3.8 m. The nails comprise BluGeo Powerthread K60 glass-fibre reinforced plastic (GRP) 25 mm diameter solid bar, with associated glass-fibre nail plates, nuts and stainless steel couplers. The nail lengths vary from 5 m to 15 m long and the spacings vary from 0.8 m to 1.3 m depending on the ground condition and the retained height. The nails are placed and grouted into 150 mm diameter holes on a triangular grid.
The construction of the soil nail retaining wall was undertaken in a top-down sequence. The excavation and nail installation were carried out row by row, with the excavated face and nail heads shotcreted immediately upon completing the nail installation on each row. Following completion of the shotcrete and excavation works for the entire retaining wall, precast concrete panels were placed in front of the completed retaining wall. Bored drains, strip drains and weep holes were also installed within the retaining wall to maintain a drained retaining wall condition.
One of the typical sections of the soil nail retaining wall is illustrated in Figure 2 below.