Use of temporary anchors in reinforced soil wall construction for the M4 Motorway widening

Tristan McWilliam and Loges Paramaguru

The M4 Motorway has in excess of 100,000 vehicle movements per day with the projection exceeding 150,000 vehicle movements per day by 2030. To facilitate the projected traffic flow increase, the M4 Motorway has been upgraded by adding an additional lane in each direction (eastbound and westbound) to the existing three lanes. The upgrade comprises widening a 7.5 kilometre section of motorway between Parramatta and Homebush including the construction of 2 kilometres of viaduct with 49 spans, 3 bridges and 24 retaining walls (including RSWs, soldier pile walls, soil nail walls and reinforced concrete walls).

This paper focuses on the sections of motorway widened using Reinforced Soil Walls (RSW) and presents a case study of the use of temporary Platipus anchors, to retain up to 9 metres of existing engineered fill embankment of the motorway. The construction of the RSWs required excavation of the existing motorway embankment at between 45° to 75° for excavated heights typically between 6 metres and 9 metres. Platipus anchors were selected in preference to soil nail or sheet pile solution to increase the stability of the steep temporary batter, to expedite the excavation process, reduce the amount of excavation of the embankment and reduce the total construction cost. The design considered the stage excavation process and ensured adequate factor of safety at each stage of the excavation process. In addition, the design confirmed that the settlement of the existing M4 pavement immediately adjacent to the excavation was within tolerable limits.

The paper provides discussion on the feasibility assessment of the use of the ‘Platipus’ anchors and the design undertaken for the temporary retention system. It also provides a discussion on the installation process for the anchors, anchor testing undertaken, instrumentation and monitoring; and construction challenges for integration of the temporary support into the permanent works.