Vicki G. Moon

MICROSTRUCTURAL CONTROLS ON THE GEOMECHANICAL BEHAVIOUR OF IGNIMBRITE

Abstract “Ignimbrite” is a genetic name relating to the pyroclastic origin of the material. Ignimbrite materials have a wide range of geomechanical characteristics; the materials range from soft, non-jointed soils with dry compressive strenghts −2 , to hard rocks with extensive systems of cooling joints and dry compressive strengths 50MN m −2 . Tensile strengths of oven-dry rocks range from −2 , cohesion from 0.14 to 13 MN m −2 , and friction angles from 27° to 35°. Porosities of 17 to 51% lead to a considerable loss of strength on saturation, and second-cycle slake durabilities range from 30 to 99%. The groundmass microstructure exerts the primary control over the geomechanical behaviour of ignimbrite. Compressive strength and slake durability are controlled by the closeness of packing of the groundmass shards, and the nature of the welding between individual shards at their points of contact. These factors control the ease with which microfractures can propagate through the groundmass. Crystal and clast size exerts a secondary control on compressive strength by influencing the initiation of microfractures through stress concentration around inhomogeneities; the proportion of such inclusions is insignificant, so long as a small number are present to initiate microfractures. Tensile strength is primarily influenced by the degree of shard alignment, with only a small anisotropy existing in terms of the direction of shard alignment with respect to the principal stress direction. Shear strength is not clearly related to microstructure.

TEXTURAL AND MICROSTRUCTURAL INFLUENCES ON THE DURABILITY OF WAIKATO COAL MEASURES MUDROCKS

Abstract Data on durability, petrography and microstructure are presented for 27 specimens of Waikato Coal Measures mudrocks: fine-grained, carbonaceous rocks, which are dominated by kaolinite clays, and have low durabilities. The poor durability is not caused by swelling clays. Rather, slaking is believed to result from the release of residual stresses within the rock following weakening by water adsorption on to clay surfaces and a consequent loss in cohesiveness. This mechanism favours fragmentation into small, water-stable fragments, rather than dispersion of clays into the water. The amount of day-sized material in the rocks provides the most direct control on the durability, but mierostructural features are also recognized as important: specimens with a discontinuous matrix have a slightly lower durability than those with a continuous matrix.

GEOTECHNICAL CHARACTERISTICS OF IGNIMBRITE: A SOFT PYROCLASTIC ROCK TYPE

Abstract The ranges of a variety of geotechnical properties encompassed by the ignimbrite lithology are established. Ignimbrites are of low density (1212–1928 kg m −3 ) and high porosity (18–51%), are very weak to weak in compression (0.23–54 MN m −2 ), have low tensile strength (0.12–7.1 MN m −2 ) and cohesion (0.06–9.0 MN m −2 ), yet the angle of internal friction is comparatively high (27°–38°). The second-cycle slake durability index ( I d2 ) ranges from very low (30%) to very high (99.3%). Considerable softening (softening factors of 1.3–10.8) occurs on saturation, and all ignimbrites undergo significant plastic deformation prior to failure. Extensive systems of open, continuous, vertical joints occur in many ignimbrites, typically forming a very widely spaced (3–5 m), irregular columnar pattern, while other ignimbrites are effectively non-jointed, though occasional closed, continuous vertical joints occur at extremely wide spacings (10–15 m). Large changes in strength and jointing may occur within a single profile. Two broad categories of ignimbrite are recognised: durable ignimbrites with I d2 ⩾90% , and non-durable ignimbrites with I d2 ⩽30% . Durable ignimbrites behave as weak rocks for which the rock mass characteristics exert the primary influence upon slope stability and engineering behaviour. Steep slopes and precipitous gorges result from the predominance of vertical jointing, and high cleft water pressures have contributed to historical engineering failures. Non-durable ignimbrites are typically non-jointed and are the weakest materials; they are classified as soft rocks, and the intact rock characteristics are the dominant control on their behaviour. High friction angles allow relatively steep slopes, but low durability makes them prone to gully erosion. They are typically very sensitive materials and susceptible to piping. Second-cycle slake durability index and effective porosity together allow classification of ignimbrites and prediction of likely material characteristics. These tests require small specimens and simple laboratory equipment.

A Methodology for Assessing Landslide Hazard Using Deterministic Stability Models

Deterministic stability models are used to assess the potential for mass movement within Hamilton City, New Zealand using sensitivity analysis for critical environmental variables. Discrete geomorphic zones are recognised on the basis of material properties and slope characteristics; generic slope profiles are derived for each of these zones by averaging slope profiles determined from a Digital Elevation Model. Stability analysis models are used to derive critical failure surfaces for these profiles using measured or estimated material properties, and sensitivity analysis allows the conditions of water table level and seismic acceleration under which the slopes become unstable to be determined. This method is applied to Hamilton City to assess the citywide hazard associated with mass movement. For the slopes studied, conditions of elevated water table alone may initiate failure, but this is seen as unlikely as the materials are well drained. Combinations of water tables above 10% of the slope elevation together with seismic accelerations of approximately 0.2 g (150 year return period) represent likely failure conditions for many slopes. This information provides emergency management planners with estimates of the likely extent of failure in different regions of the city, and hence facilitates identification of lifelines and infrastructure at risk. The method cannot provide site-specific information, but in combination with knowledge of cultural features gives indications of critical locations where detailed engineering assessments are required.

The value of rock mass classification systems for weak rock masses: a case example from Huntly, New Zealand

Three published rock mass classification systems (RMR, SMR, RMS) were applied to weak Waikato Coal Measure mudrocks in order to assess their value as indicators of rock mass conditions and stable slope angles. The SMR classification gives the most sensitive measure of rock mass conditions for the slopes studied, but none of the systems adequately predicts observed slope angles. Regression analysis indicates that where conditions for failure by sliding along discontinuities exist the slopes are most sensitive to the discontinuity parameters of parallelism, dip angle and spacing. Further, when the discontinuities are oriented favourably with respect to the slope, slope angles are most sensitive to intact rock strength and groundwater. This is supported by geomorphic evidence. Low angle natural slopes developed on unfavourable discontinuity sets are well predicted by published equations associated with both the RMR and RMS classification systems. Steeper slopes developed on favourable discontinuities are at a much lower angle than the equations predict and show evidence of extensive creep. Application of the present rock mass classification systems to these weak rocks is thus only appropriate when conditions exist under which the rocks fail by sliding on unfavourably oriented discontinuities. Where this does not occur, the contribution of intact strength to the rock mass strength is greatly overestimated by all of the rock mass classification systems studied. Development of a separate rock mass classification for these conditions is not seen as appropriate. Hence, recognition of the favourability or otherwise of the discontinuity sets is crucial to dealing with these rocks. A stereonet overlay that facilitates this division is presented.

Large-scale mass wasting in ancient volcanic materials

Abstract Giant landslides are significant hazards associated with many active volcanic edifices. We describe a similar feature on ancient (>4 Ma) volcanic deposits subject to active tectonism. The landslide is approximately 3 km long by 1 km wide, with an estimated depth of 400 m. Side margins are straight and parallel, mimicking regional structure; narrow valleys incised down these margins provide low-strength side-release surfaces. Between these is a giant slump consisting of at least four, largely intact, discrete blocks that have moved down-dip a distance of >500 m. A series of flows with areal extents ranging from 0.01 to 0.5 km 2 extends from the front of the failure. The materials represent an eroded sequence of andesite flows on the flanks of a stratovolcano. These have undergone two phases of hydrothermal alteration, and are deeply weathered to low-density (1040±80 kg m −3 ) silt (59%) and clay (35%) materials with strength properties typical of weathered silts ( c =26±3 kN m −2 ; φ =42±8°). The size and location of this landslide preclude detailed geotechnical investigation of the failure. The worth of numerical stability analysis as an alternative technique in assessing the nature of the failure and hence the risk it poses to nearby communities is investigated. Sensitivity analysis identified likely conditions under which initial failure may have occurred: analyses for sensitivity to strength and earthquake acceleration needed conversion to critical combinations ( F =1.0) of water table and strength/acceleration to remove the overriding influence of water table fluctuations. Failure was likely initiated either by a high water table level (83–84%), or some combination of intensity VII–IX earthquake waves together with water table heights of 40–80%. A general hazard assessment indicates that the risk associated with creep and catastrophic failure of the main mass is small, whereas the risk from flow failures near the toe of the landslide may be high. Important parameters (hydrological regime, flow failure morphology, age of initiation, and rates of movement) requiring closer investigation are identified. Development of a model is crucial to assessing the hazard associated with a feature such as that described here. With limited resources, a detailed stability analysis is a powerful tool as an initial stage in hazard analysis.

Halloysite behaving badly: geomechanics and slope behaviour of halloysite-rich soils

Abstract Halloysite-rich soils derived from in situ weathering of volcanic materials support steep stable slopes, but commonly fail under triggers of earthquakes or rainfall. Resulting landslides are slide-flow processes, ranging from small translational slides to larger rotational failures with scarps characteristic of sensitive soils. Remoulding of failed materials results in high-mobility flows with apparent friction angles of 10-16°. The materials characteristically have high peak-friction angles (∼25- 37°), low cohesion (∼12-60 kN m-2) and plasticity ( plasticity index ∼10-48%), and low dry bulk density (∼480-1,080 kg m-3) with small pores due to the small size of the halloysite minerals. They remain saturated under most field conditions, with liquidity indexes frequently >1. Remoulded materials have limited cohesion (<5 kN m-2) and variable residual friction angles (15°-35°). Halloysite mineral morphology affects the rheology of remoulded suspensions: tubular minerals have greater viscosity and undrained shear strength than spherical morphologies.

A new attraction-detachment model for explaining flow sliding in clay-rich tephras

Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap–shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.

Discovery of halloysite books in altered silicic Quaternary tephras, northern New Zealand

Abstract Hydrated halloysite was discovered in books, a morphology previously associated exclusively with kaolinite. From ∼1.5 to ∼1500 μm in length, the books showed significantly greater mean Fe contents (Fe2O3 = 5.2 wt.%) than tubes (Fe2O3 = 3.2 wt.%), and expanded rapidly with formamide. They occurred, along with halloysite tubes, spheroids and plates, in highly porous yet poorly permeable, silt-dominated, Si-rich, pumiceous rhyolitic tephra deposits aged ∼0.93 Ma (Te Puna tephra) and ∼0.27 Ma (Te Ranga tephra) at three sites ∼10-20 m stratigraphically below the modern landsurface in the Tauranga area, eastern North Island, New Zealand. The book-bearing tephras were at or near saturation, but have experienced intermittent partial drying, favouring the proposed changes: solubilized volcanic glass + plagioclase→halloysite spheroids→halloysite tubes→halloysite plates→ halloysite books. Unlike parallel studies elsewhere involving both halloysite and kaolinite, kaolinite has not formed in Tauranga presumably because the low permeability ensures that the sites largely remain locally wet so that the halloysite books are metastable. An implication of the discovery is that some halloysite books in similar settings may have been misidentified previously as kaolinite.

Utilizing cone penetration tests for landslide evaluation

Pore pressure and shear strength are two important parameters that control the stability of slopes. These parameters can be derived in-situ by cone penetration testing (CPT) with pore pressure measurements. This paper presents the results from three static, vibratory and dissipation CPT profiles deployed into a landslide headwall at Pyes Pa, Bay of Plenty, New Zealand. The landslide strata consist of volcanic ashes and ignimbrites. Studying the stability of slopes in this area using in-situ geotechnical testing is of societal-economic importance since several other landslides within comparable strata caused considerable property damage. Three CPT profiles were collected across the headwall of the slide scar with 2 m spacing in undisturbed sediments using static, vibratory and dissipation test modes. Static CPT results are used to evaluate soil grain size variations, geotechnical parameters of sediments such as shear resistance, probable slip surface and sensitivity of sediments. Liquefaction potential of sediments is assessed using vibratory CPT results. For dissipation tests, the cone remained stationary in the sediment for ∼60 min to monitor pore pressure dissipation at the depths of 6, 9 and 11 m. With the use of pore pressure dissipation data, values of soil horizontal permeability are calculated. The liquefaction probability from static CPT results is compared to liquefaction potential evaluation from vibratory CPT. Last but not least, an unstable soil layer is defined based on static CPT, vibratory CPT and dissipation results.