Engineering geological investigations of two North Canterbury landslide complexes.

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Geology
Degree name
Master of Science
Publisher
University of Canterbury
Journal Title
Journal ISSN
Volume Title
Language
English
Date
1994
Authors
Justice, Thomas Richard
Abstract

Engineering geological investigations of the Mt. Vulcan and Coringa Landslide Complexes, situated near Motunau Beach, North Canterbury, New Zealand, have been carried out with the intention of determining the nature and causes of the complexes. Descriptions of the landslide complexes and field investigations undertaken for each site are discussed. In particular, the forms of slope movement termed 'earthslides' in this thesis are examined in detail. The tem1 'earthslide' is defined in this study .as a slow moving, lobate or elongate mass of accumulated debris which advances primarily by sliding on discrete bounding shear surfaces. The average grain size distribution of the earthslide mass contains more than 50% sand, silt and clay combined.

Mt. Vulcan Landslide Complex comprises an area of about 85 ha, and is developed within a · lower Tertiary marine succession, specifically failing within smectitic silty clays (Ashley Mudstone) and massive glauconitic sands (Waipara Greensand), underlying strong limestone (Amuri Limestone). Rotational slumping occurs at the head of the complex, involving limestone and/or clayey silt/sand blocks, which subsequently develop into translational slide/earthslide movements downslope of the head zone. An age of formation for the complex proved difficult to ascertain, however, the probable minimum and maximum ages of the complex are postulated at about 5000 b.p. and 100 000 b.p. respectively.

Coringa Landslide Complex comprises an area of about SO ha and is developed within a structurally disturbed sequence of Ashley Mudstone and Waipara Greensand. The silty clay and sandy units, and the blocks of overlying limestone are interpreted to have initially failed by rotational sliding in smectitic silty clays and to have subsequently developed a southwesterly slide motion on a stiff mudstone (Loburn Mudstone), stratigraphically underlying the complex. The landslide is interpreted to have developed at about 100 000 years b.p. in response to uplift on thrust faults which underlie the complex.

Earthslide A and Earthslide 3 occur within the Mt. Vulcan and Coringa Landslide Complexes respectively. Detailed examination was made of both of these features, in terms of surface and boundary morphology, displacement rates and characteristics, the effect of rainfall, and geotechnical properties. In general, it has .been found that the earthslides examined in this thesis move primarily by sliding or plug-flow on discrete bounding shears. The surface morphology typically has an undulating appearance which may show transverse tension cracking or bulging, depending on the longitudinal strain rate. Regions under extension typically display tensional cracking, while compressive regions display transverse bulges and/or multiple basal thrust development. Lateral shear zones display typical features analogous to those developed along strike-slip faults, and lateral bulges formed immediately adjacent to the lateral shear zone are often (but not universally) associated with zones of constricted earthslide movement.

Geotechnical properties determined for the earthslides examined in this study indicate plastic limits of 27-38%, liquid limits of 15-43%, plasticity indices of 14-45%, clay fractions of 14- 47% and bulk densities of 1633-2004 kg/m³. This wide range in values is attributed to both the variable sand fraction encountered in different areas of both earthslides, and the various sources for debris supply. Typical residual friction angles were found to be just over .13°, while effective residual cohesion was found to be close to 0.

Both Earthslide A (part of the Mt. Vulcan Landslide Complex) and Earthslide 3 (part of the Coringa Landslide Complex) are found to move by sliding and/or plug-flow on basal and· lateral shear zones, and the average particle size distribution contains more than 50 % sand, silt and clay. Furthermore, Earthslide 3 was found to move at slow (0.5-2.3 m/yr) rates. Because of these characteristics, the previous term widely used in New Zealand for these types of failure (creeping earthflow) has been modified to 'slow earthslide'.

Movement rates and characteristics determined for Earthslide 3 on the Coringa Landslide Complex indicated regions of compressive, extensional and plug-flow within the earthslide. Likewise, regions of steady and unsteady state behaviour were recognised. The accumulation zone of Earthslide 3 was found to be a region of unsteady- state compressive flow, while the majority of the track zone was found to be a region of steady-state plug flow.

A correspondence was noted between earthslide movement rate and precipitation, and a lag time of 2-3 weeks was indicated. The mechanism of momentum transfer of head zone disturbances for the earthslide is found to be a mixture of plastic (advective) and viscous (diffusive) styles.

Remedial options for both Earthslide A (Mt. Vulcan Landslide Complex) and Earthslide 3 (Coringa Landslide Complex) involve either reforestation or drainage and/or stream control of key areas of both earthslides.

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