Earthquake-Induced Ground Fissuring in Foot-Slope Positions of the Port Hills, Christchurch A thesis submitted in partial fulfilment of Master of Science in Engineering Geology in the University of Canterbury by C J Stephen-Brownie University of Canterbury 2012 Acknowledgements Over the time spent working on this thesis, I have been incredibly fortunate to receive the kind support of many people. I would like to express my sincere gratitude to my supervisors David Bell and Dr Marlène Villeneuve for their time, expertise and constructive suggestions relating to this research work. I would also like to extend my thanks to the University of Canterbury Geology Department technicians: Matt Cockcroft, Chris Grimshaw, Cathy Higgins, and Vanessa Tappenden, for their assistance with laboratory analysis and the resistivity survey. Also from the University of Canterbury, Dr Cédric Lambert who offered valuable advice on the use of FLAC, and Janet Warburton, whose administrative assistance was greatly appreciated. Many Christchurch residents allowed me to access to their properties, in particular, Alan Hawkins from number 3 Glenview Terrace, and Dr Kari Basset, of the University of Canterbury Geology Department, from number 40 Rapaki Road. Thank-you for your support, and for allowing me to dig holes in your back yards. I am particularly grateful for the insights into the behaviour of springs and groundwater in the Hillsborough area offered by Helen Rutter from Aqualinc Research Ltd., Christchurch. Thank-you to Tobi, my field assistant and champion. Finally, thank-you to my parents, whose support and encouragement has been tremendous throughout all my years of university study. I could not have done it without you. ~ 1 ~ Abstract Following the 22 February 2011, MW 6.2 earthquake located on a fault beneath the Port Hills of Christchurch, fissuring of up to several hundred metres in length was observed in the loess and loess-colluvium of foot-slope positions in north-facing valleys of the Port Hills. The fissuring was observed in all major valleys, occurred at similar low altitudes, showing a contour-parallel orientation and often accompanied by both lateral compression/extension features and spring formation in the valley floor below. Fissuring locations studied in depth included Bowenvale Valley, Hillsborough Valley, Huntlywood Terrace–Lucas Lane, Bridle Path Road, and Maffeys Road–La Costa Lane. Investigations into loess soil, its properties and mannerisms, as well as international examples of its failure were undertaken, including study of the Loess Plateau of China, the Teton Dam, and palaeo-fissuring on Banks Peninsula. These investigations lead to the conclusion that loess has the propensity to fail, often due to the infiltration of water, the presence of which can lead to its instantaneous disaggregation. Literature study and laboratory analysis of Port Hills loess concluded that is has the ability to be stable in steep, sub-vertical escarpments, and often has a sub-vertically jointed internal structure and has a peak shear strength when dry. Values for cohesion, c (kPa) and the internal friction angle, ϕ (degrees) of Port Hills loess were established. The c values for the 40 Rapaki Road, 3 Glenview Terrace loess samples were 13.4 kPa and 19.7 kPa, respectively. The corresponding ϕ values were thought unusually high, at 42.0° and 43.4°.The analysed loess behaved very plastically, with little or no peak strength visible in the plots as the test went almost directly to residual strength. A geophysics resistivity survey showed an area of low resistivity which likely corresponds to a zone of saturated clayey loess/loess colluvium, indicating a high water table in the area. This is consistent with the appearances of local springs which are located towards the northern end of each distinct section of fissure trace and chemical analysis shows that they are sourced from the Port Hills volcanics. Port Hills fissuring may be sub-divided into three categories, Category A, Category B, and Category C, each characterised by distinctive features of the fissures. Category A includes fissures which display evidence of, spring formation, tunnel-gullying, and lateral spreading-like behaviour or quasi-toppling. These fissures are several metres down-slope of the loess-bedrock interface, and are in valleys containing a loess-colluvium fill. Category B fissures are in wider valleys than those in Category A, and the valleys contain estuarine silty sediments which ~ 2 ~ liquefied during the earthquake. Category C fissures occurred at higher elevations than the fissures in the preceding categories, being almost coincident with bedrock outcropping. It is believed that the mechanism responsible for causing the fissuring is a complex combination of three mechanisms: the trampoline effect, bedrock fracturing, and lateral spreading. These three mechanisms can be applied in varying degrees to each of the fissuring sites in categories A, B, and C, in order to provide explanation for the observations made at each. Toppling failure can describe the soil movement as a consequence of the a three causative mechanisms, and provides insight into the movement of the loess. Intra-loess water coursing and tunnel gullying is thought to have encouraged and exacerbated the fissuring, while not being the driving force per se. Incipient landsliding is considered to be the least likely of the possible fissuring interpretations. ~ 3 ~ Table of Contents Acknowledgements ................................................................................................................................................. 1 Abstract ......................................................................................................................................................................... 2 Table of Contents ...................................................................................................................................................... 4 1. Introduction ................................................................................................................................................. 14 1.1 Project background .............................................................................................................................. 14 1.2 Thesis objectives .................................................................................................................................... 16 1.3 Geological setting .................................................................................................................................. 17 1.3.1 Geological evolution of Canterbury ...................................................................................... 17 1.3.2 Seismicity of the Christchurch and Port Hills area ........................................................ 19 1.4 Hydrogeology of the Christchurch Area ....................................................................................... 22 1.5 Geographical setting ............................................................................................................................. 22 1.6 Methodology and format .................................................................................................................... 23 2 Loess Soils: Review......................................................................................................................................... 24 2.1 Loess ........................................................................................................................................................... 24 2.2 Occurrence, origin and geological history of Banks Peninsula loess ............................... 27 2.3 General properties of Loess .............................................................................................................. 29 2.4 Properties of Port Hills loess ............................................................................................................ 30 2.5 Slope failures in Loess ......................................................................................................................... 33 2.5.1 Failure mechanisms observed in loess terrains .............................................................. 33 2.5.2 Fissuring behaviour in Loess .................................................................................................. 35 2.6 Loess failure case studies in natural slopes................................................................................ 37 2.6.1 The Loess Plateau of China ...................................................................................................... 37 2.6.2 Infilled Fissures in Banks Peninsula Loess ....................................................................... 41 2.7 Failure case studies in man-made structures ............................................................................ 42 2.7.1 The Teton Dam ............................................................................................................................. 43 2.8 Relevance of case studies to Port Hills fissuring ...................................................................... 45 2.9 Loess stabilisation methods .............................................................................................................. 46 ~ 4 ~ 2.10 Synthesis .............................................................................................................................................. 48 3 2010 and 2011 Christchurch