Treatments Of Existing Abutment Rsw Using Permeation Grouting And Horizontal Soil Nailing

The construction of Bangalow Interchange on the Pacific Highway Upgrade Project was undertaken using a staged approach. All traffic was initially diverted onto the northbound carriageway of the existing Pacific Highway whilst the southbound carriageway was decommissioned and excavated down to a maximum of 6.7m below the existing fill embankment surface. A temporary soil nail retaining wall was required to support the vertical cut face at the median. The temporary wall and the northbound carriageway would be decommissioned after the interchange construction work.

One challenge to the temporary design was the presence of a Reinforced Soil Wall (RSW) at the abutments of the existing twin bridges over Bangalow Road. The proposed temporary wall alignment was to cut through the middle of the RSW block in order to allow the eastern half of the RSW block, along with the southbound bridge, to be removed. The diverted traffic was to travel over the existing northbound bridge, which would be supported by the remaining western half of the RSW block. One further challenge was that the existing RSW abutments were designed to support the bridge sill beams without any pile foundations.

This paper focuses on the design treatment of the existing bridge abutment subjected to partial demolition of the abutment RSW. Initially, the adopted strategy was to inject low pressure permeation grout into the existing RSW block prior to excavation. A grouting trial was conducted, which indicated that because of the presence of the relatively high fines content within the existing RSW, the injected grout was unable to permeate through the RSW fill to form a coalesced grouted mass. An alternative support system using horizontal soil nailing was subsequently adopted through the vertical cut of the abutment RSW. The design of the horizontal soil nails would not only need to satisfy the stability requirements but also require controlling the movements of the RSW to within a tolerable limit. A comparison of the survey monitoring results with the design predictions of the wall movements is outlined.