Use of GPR on two sites with voids to enable safer work and

Use of GPR on two sites with voids to enable safer work and targeted probing

By Christopher HughesSenior Engineering Geologist

opusinternational.com

Presenter

• Over 15 years of experience within the geotechnical / environmental industry.

• Experience includes planning and executing geotechnical and contaminated land investigations and the follow up assessment and reporting.

• Two case studies with potential for voids

• GPR used to help characterise the site, appraise safety for investigations and optimise scope and location of the investigation.

• Case study 1 - Mine shaft site – Thames Case study 2 – Electricity substation site – Bay of Plenty

Talk outline


Case study 1: Mine shaft site

Case study 1: Mine shaft site - Thames

• Thames – North Island – New Zealand

• Shaft collapse 2014

• Residential area

• Site cordoned off

• Concern over other possible shaft / void features at the site



Site Inspection

• Unlined shaft

• Two smaller circular depressions in the ground surface south of the shaft

• Ground surface to north hummocky and disturbed



Desk Study

• Andesite and dacite intrusivesof the Kuaotunu Subgroup of the Coromandel Group –Miocene age

• Bird in Hand number 2 Shaft

• Mining ceased / shaft capped ~1914

• Shaft completed 1874 – Approx70m deep, pump shaft


GPR / Site characterisation


Investigation Planning and Phasing

• Shallow features (<4m depth)

• Services locations

• Test pits

• Rope harnessing

• Positioning of excavator





Comparing GPR with Investigation• Anomalies to approximately 2m depth

generated by variation/physical contrast between backfill to former voids and surrounding materials.

• We consider that the risk of subsidence from collapse of shallow voids is lower than we initially believed.

• Next phase can focus on shaft and deeper anomalies – boreholes.

• Shaft remediation following CIRIA (UK) guidance.



Case study 2: Electricity Substation –Bay of Plenty

• Edgecumbe, Bay of Plenty

• ‘Tomos’ appeared on substation grounds

• Concern from voids / tomoscompromising the facility and substation components


Site Walkover


Desk study• Site underlain by recent swamp deposits

comprising dark brown to black peat, organic rich muds, silt and sand. The Edgecumbe Fault is located approximately 14km to the east of the site.

• The site lies on a flood plain next to the Rangitaiki River.

• The river is constrained by stop bank flood protection bunds

• History piping erosion through the pumice sands causing flooding, sand boils and tomo formation on the downstream sides of the bunds.


Desk study cont…

• In 2000 substantial improvement works comprising toe bunds and drainage measures were constructed adjacent to the banks to protect the surrounding land (Fig. 9).

• The desk study found historical evidence of tomos occurring outside the sub-station site during pre-2000 flood events but there was no correlation between the tomos observed on site and any recent flood events.


GPR survey


Summary

• No significant large cavities (of size greater than 2 metres) interpreted from the GPR data.

• The GPR survey also identified a lot of small features that have the characteristics of cavities but could also be natural features within the sediment. Many of these features were indicated to be present within the upper 1m to 2m and be of possibly 200mm to 300mm in size.

• The GPR aided development of a more complex ground model for the site including buried ‘natural’ channels and possible flow paths in service trenches both of which may promote rapid and/or concentrated groundwater flows leading to piping of the pumice sands and tomo formation.


Conclusions

The walkover/desk studies in conjunction with the GPR surveys have allowed us to:

1. Scope and optimise the positions of the proposed intrusive works to improve the likelihood of striking voids at these sensitive sites.

2. Develop further our ground models and understanding of the site.

3. Assess health and safety risks at these sites for both the site users and those carrying out future intrusive investigations.

The authors would like to acknowledge the clients Colliers International Ltd, Land Information New Zealand (LINZ) for the mineshaft site and our client for the substation site for their permission to discuss these two case studies.

Documents

Use of GPR on two sites with voids to enable safer work and