Rangitaiki River and Floodway

Edgecumbe, NZ

Background

The Bay of Plenty Regional Council (BOPRC) has an ongoing programme of reviewing the stopbanks on the Rangitaiki River and floodway, located in Edgecumbe, NZ. Investigations and remedial works have been prioritised following recent flood events.

The BOPRC requested that CMW assess the ability of the stopbanks (at the identified locations) to withstand a flood event with the identified return period.  Results and recommendations were presented in a report which BOPRC require to help establish the details of a forward works programme of stopbank improvements (maintenance) and construction details of capital works (floodway).

Scope

CMW carried out a geotechnical investigation of the required areas comprising of:

• Supervision of drilling of 18 machine boreholes with SPT tests completed at 1.5m intervals to a final depth of up to 17m;
• Undertaking of 24 hand auger boreholes to up to 5m deep;
• Undertaking of 24 Dynamic Cone Penetrometer (DCP) tests to 5m depth or refusal;
• Laboratory sampling on 18 representative soil samples to determine particle size distribution;
• Seepage, stability and settlement analysis for each of the 24 cross sections;
• Preparation of a final report to present findings and recommendations to BOPRC.

Findings/Challenges

This project required close planning and communication with BOPRC in order to meet their requirements.

Detailed review of previous geotechnical analyses completed in the area was undertaken in order to assess the remaining reaches and corroborate CMW’s own data.

The subsoils encountered varied significantly across the site due to the alluvial nature of their deposition as well as past and current land use activities. This variability resulted in complex seepage analysis results and careful consideration was required when determining remedial options and recommendations.