CMW Geosciences was engaged to monitor construction and to provide geotechnical engineering support during the construction of Horsham Downs Link Road, a one-kilometre stretch of road connecting Horsham Downs Road to the recently opened Waikato Expressway.
The design included cuts of up to four metres and fill embankments up to six metres in height, over both soft organic peat and stiff Hamilton Ash Clay soils.
The design also included a shared path and underpass to futureproof the area, promoting cycling and walking in the rapidly developing Horsham Downs area.
The team encountered a few challenges along the way.
Prior to construction starting, the team located stockpiles of uncontrolled fill on site that would be very difficult to re-condition and compact to meet the specification for embankment fill construction. This led to additional fill being used to replace the unsuitable soil.
Due to this large amount of uncontrolled fill, the shared path was redesigned so that some uncontrolled fill could be left in place for its construction, where a lower specification was required.
The team also went back to the drawing board to carry out a detailed displacement analysis of the road embankment under the design seismic loads, and was able to meet the Waka Kotahi serviceability requirements without the use of high strength geofabrics. This resulted in significant cost savings.
Once the unsuitable soil was removed, an unexpected soft, organic layer was found approximately one metre below the cut, requiring further excavation. Our team responded quickly to supervise the removal of the organics, and test and approve the road embankment subgrade, minimising disruptions to the construction programme.
During construction, fill material was continuously monitored to ensure settlement was within the acceptable range. However the soil onsite produced low quality fill, and the subgrade fill continually failed to meet the specified CBR (California bearing ratio).
Large settlements were anticipated as a consequence of the construction of the large fill embankments. It was critical that the construction team knew when these settlements had been completed, in order to minimise the risk of civil works becoming damaged after construction. To this end, settlement plates were installed during construction, and monitored frequently to better understand the settlement response of the underlying ground, and to estimate when majority of the settlement had occurred.
Due to the low CBR achieved on the subgrade fill, lime stabilisation was carried out on the subgrade of the road to meet the specified CBR. The CMW Geosciences team worked with the specialist contractor and provided guidance on appropriate lime application rates.
What we achieved
With our adaptive approach over the course of this project, our team built a solid relationship with the Waikato District Council and the contractor, Dempsey Wood, to ultimately produce a high-quality stretch of road. The Link was successfully completed, and its opening ceremony was carried out on 21 October 2022.