Abstract

Bluff Road is a low use, single lane, coastal road linking two beach resorts on the north east coast of the Coromandel Peninsula. It is a popular pedestrian short cut between the two beaches, is the access way to attractive fishing marks and provides important options for access to a number of properties.

At one location, close to the western end of the road, the road is overhung by a steep rock bluff at a point where it is partly formed on reclaimed land and supported by retaining structures. Following significant rockfalls and erosion damage in the years since Cyclone Wilma in 2011 a detailed assessment of possible remedial works to improve the safety and resilience was instigated.

Remedial works comprising blasting, scaling, bolting and a passive netting scheme commenced in late 2015. During this work further significant rockfalls occurred, promoted by previously unseen fractures. Following further risk and cost benefit assessment the decision was made to close the affected section of the road. This meant the popular recreational route for pedestrians and cyclists was no longer available and that the Matarangi fire service had a potential additional 11km route to reach the east end of Rings Beach.

Throughout 2016 the public repeatedly removed fences, barriers and warning signs erected to warn of the high risk of rockfalls and prevent access.

In this paper we describe the history of the site, the works carried out and discuss issues around rockfall risks, road closure and public perception of risk.

1 INTRODUCTION

Bluff Road is a low use, single lane, coastal road linking two beach resorts on the northeast coast of the Coromandel Peninsula, Figure 1. It is a popular pedestrian short cut between the two beaches, is the access way to attractive fishing marks and provides important options for private and emergency access to several properties.

At one location close to the western end of the road, the road is overhung by a near vertical 20m high bluff of variably weathered and hydrothermally altered andesite, that is also locally sheared and closely fractured, Figure 2. Three major discontinuity sets are present. Two of them, dipping out of the face at angles of between 30 and 50° and a second dipping out of the face at 70 to 80° promoted planar and toppling failure respectively. The third, near vertical set, has a strike at almost 90° to the face and act as release planes for block failures. Weathering and hydrothermal alteration has particularly picked out the vertical discontinuities trending into the face, which are locally concentrated and may be localised fault or shear zones. The published geological information indicates these to be the Matarangi Andesite of the Coromandel Group of Miocene age (New Zealand Geological Survey 1976).

Downloads

NZGS_Symposium_20_Read1-1.pdf

Collection

NZGS Symposium

Authors(s)

Christopher Hughes, Ken Read

Compilation

NZGS Symposium>20th NZGS Symposium

Type

Paper

ISSN

0111-9532