The EDC Geotechnical team does a deep dive into Geotechnical Assessments and Remediation Solutions.
A landslip on the slope above the carpark at the Tauwhare Pa Scenic Reserve in Ohope occurred in 2021, during a period of heavy rainfall. The Department of Conservation (DoC) approached EDC in early 2024 to offer remediation solutions for the specific instability, as well as to provide a geotechnical assessment of all the slopes on the reserve, to support decisions related to the protection of archaeological sites.
View of Top of Slip from Walking Track, 2024 (3 years after slip)
Tauwhare Pa Scenic Reserve is one of the oldest pa sites in the Bay of Plenty and has close ancestral ties to Ngati Awa, kaitiaki of the land. The steep topography of the site allows for panoramic views from the pa site out to the north (Whakaari/White Island) and to the east, so it was important to ensure the safety of visitors and continued use of such a valuable taonga.
View to the North near the centre of the Site
Minimising our intrusive investigation footprint was a high priority for EDC in order to preserve the integrity and beauty of the site, while collecting enough data to give an overview of the subsurface conditions across the approximately 20.6Ha property. EDC undertook a subsurface investigation using minimally invasive technologies including Cone Penetrometer Testing (CPT) to more than 20m depth, as well as shallow soils investigation (approximately 3m deep).
Using the data collected, our geotechnical team was able to model the conditions which lead to the existing slip. The ‘back analysis’ was critical to:
- Understand specific causes of the instability (i.e. soil type/properties, slope angle, degree of soil saturation, etc) and their interactions.
- Apply parameters of failure across the Pa site to understand where future instabilities are likely to occur.
- Identify remediation solutions specific to the existing slip.
Our analyses showed that slopes angled 40° or more were at elevated risk of failure when saturated.
Further minimising site disturbance, EDC engaged a drone survey subcontractor to obtain detailed LiDAR data for the purposes of developing a Digital Terrain Model (DTM) for the property. The data was then used to correlate areas of overland flow paths (where water was most likely to seep into the ground) with slope angles greater than 40°. Based on the results of our slope stability modelling, these locations were identified as areas of heightened instability hazard, and EDC provided remediation solution recommendations for the consideration of DoC.
Overland Flow Paths and Slope Angles within the Southern Portion of the Site
Our investigation and analyses allowed EDC to identify potential risk of further instability even though there may not have been instability noted at the time, as well as provide parameters retaining at the top and base of the existing slip. We are pleased to have been able to assist DoC with their continued efforts to preserve this taonga for future generations.
