Steve Hencher

Assessing the Shear Strength of Rock Discontinuities at Laboratory and Field Scales

This paper sets out an approach to assessing shear strength of rock joints at project scale based on measurement and analysis rather than empiricism. The role of direct shear testing in this process is discussed in detail and the need for dilation measurement and correction emphasised. Dilation-corrected basic friction angles are presented for various rock types. The characterisation of first and second order roughness features and their contribution to shear strength at project scale are discussed with reference to possible scale effects. The paper is illustrated by a case example of a spillway slope for a dam in the Himalayas.

Engineering in weathered rock

Abstract The potential problems to be faced when engineering in zones of weathered rock are reviewed. Difficulties in formulating realistic geotechnical models by standard methods of site investigation and routine testing are emphasized and the consequences of being wrong are illustrated by examples. Alternative approaches for delineating weathered zones for design are presented. The constraining influence of particular characteristics of weathered zones on methods of construction are discussed with reference to brief case histories.

Characterizing discontinuities in naturally fractured outcrop analogues and rock core: the need to consider fracture development over geological time

Abstract This paper reviews aspects of the procedures for characterizing rock masses from outcrop mapping and core logging. It is argued that current definitions of discontinuities and joints are too simple and too coarse to deal adequately with the range of geological features that are found in the field and that range from open fractures through to incipient joint traces. A generic approach is proposed that differentiates between discontinuities on the basis of relative tensile strength compared to the intact parent rock. Examples are provided of how fracture frequency and extent vary with degree of weathering and erosion, and it is suggested that the concept of dynamic development of geological discontinuities needs to be appreciated by geotechnical engineers and structural geologists when analysing fracture networks. This concept has major implications for the use of rock mass classifications to zone the rock mass into engineering units.

Prediction of rainfall-induced transient water pressure head behind a retaining wall using a high-resolution finite element model

Abstract Retaining walls are used to provide additional support to slopes thought to be at risk of failure. The design of such structures requires the inclusion of pore water head in the appropriate calculations, whilst subsequent construction can have major implications for the overall slope pore water head regime. Despite the obvious importance of pore water head in this context there has been no rigorous model development facilitating the temporal as well as the spatial high-resolution computation of pore water head. This paper reports on the development of a soil water finite element model (ESTEL-2D), which is capable of achieving this. The model is applied to a typical Hong Kong retaining wall scenario where the model is shown to be capable not only of establishing ‘dynamic steady state initial conditions’, but also of illustrating the complexities of the pore water head changes occurring immediately adjacent to a retaining wall. The results suggest that it is entirely appropriate to consider the utilisation of such modelling capability to assess drainage requirements (including optimal drainage locations) and appropriate piezometric monitoring strategies.

Discussion of Alejano, Gonzalez and Muralha (2012)

This is a useful contribution serving to illustrate the difficulties in defining a ‘‘basic friction angle’’ for rock joints. The fact that the authors find that the sliding angle of planar surfaces of rock in tilt tests can vary between 10 and 40 for a single granite block (their Fig. 12b) may come as a surprise to some engineers and researchers. Many textbooks and papers lead one to believe that there is a unique friction angle, øb, for a planar joint in ‘‘fresh’’ rock that can be taken as a lower bound for estimating the shear strength of natural joints empirically. Similar variability has perplexed many authors such as Nicholson (1994) who found that friction angles for sawcut Berea sandstone in direct shear tests varied by 12.5 despite great attention to sample preparation and reproducibility. Kveldsvik et al. (2008), in their investigations of the Åknes rock slope, found that the ‘‘basic friction angle’’ derived from tilt testing of core varied between 21 and 36.4 . Coulson (1971) demonstrated that the friction angle of planar surfaces of rock varies with surface finish. Krahn and Morgenstern (1979) reported similar variation for surfaces prepared in different ways and with different surface finishes. Hencher (1976, 1977) showed how repeated tilt testing of saw-cut and lapped rock sliders could reduce the sliding angle from over 30 to almost 10 after metres of displacement where rock flour was removed between runs. Continuing tests and allowing sliding debris to accumulate between runs, the sliding angle increased again. The results from two such tests using sliders of Darleydale sandstone weighted with steel blocks are presented in Figs. 1 and 2. Similar results were obtained using slate and limestone. All these data are valid strengths for planar rock surfaces; the sliding angle at each stage simply reflects different conditions of surface finish, wear and the presence and nature of any debris. As Harrison (2008) noted in his review of 60 years of papers in Géotechnique: ‘‘Unfortunately, these valuable contributions seem to have been ignored by the rock mechanics community in its subsequent development of tilt tests. Furthermore, the principle that friction angle may reduce as the shear displacement continues to increase up to very large values is probably—and erroneously—not accounted for in the majority of analyses undertaken by geotechnical engineers.’’ The test data presented above simply illustrate that there is no single and simple ‘‘basic friction’’ angle for planar rock joints. Most planar rock surfaces can be roughened to the point where the friction angle approaches 40 ; natural rock joints often have such strength even without dilation (Papaliangas et al. 1995). Much of the frictional strength is derived from ploughing and deformation of surface textural components (Engelder and Scholtz 1976). The same surfaces could be polished so that the strength reduces towards the purely adhesional contribution to friction, which, for many rocks seems to be about 10 . Slopes sometimes fail at sliding angles lower than that of a saw-cut surface, which belies the concept of a lowerbound basic friction angle measurable by simple tilt tests on saw-cut or cored samples. One example investigated in detail was reported in Hencher (1982) and is summarised in Hencher (2012). Similarly the extensive, naturally polished surfaces in the Coal Measures of South Wales have been associated with large landslides. The friction angle of these natural discontinuities can be as low as 10 whereas a sawcut sample through the parent rock gives more than twice S. R. Hencher (&) Halcrow China Ltd., Professor of Engineering Geology, University of Leeds, West Yorkshire, UK e-mail: stevehencher@btinternet.com

Forensic Excavation of Rock Masses: A Technique to Investigate Discontinuity Persistence

True persistence of rock discontinuities (areas with insignificant tensile strength) is an important factor controlling the engineering behaviour of fractured rock masses, but is extremely difficult to quantify using current geological survey methodologies, even where there is good rock exposure. Trace length as measured in the field or using remote measurement devices is actually only broadly indicative of persistence for rock engineering practice and numerical modelling. Visible traces of discontinuities are treated as if they were open fractures within rock mass classifications, despite many such traces being non-persistent and actually retaining considerable strength. The common assumption of 100% persistence, based on trace length, is generally extremely conservative in terms of strength and stiffness, but not always so and may lead to a wrong prediction of failure mechanism or of excavatability. Assuming full persistence would give hopelessly incorrect predictions of hydraulic conductivity. A new technique termed forensic excavation of rock masses is introduced, as a procedure for directly investigating discontinuity persistence. This technique involves non-explosive excavation of rock masses by injecting an expansive chemical splitter along incipient discontinuities. On expansion, the splitter causes the incipient traces to open as true joints. Experiments are described in which near-planar rock discontinuities, through siltstone and sandstone, were opened up by injecting the splitter into holes drilled along the lines of visible traces of the discontinuities in the laboratory and in the field. Once exposed the surfaces were examined to investigate the pre-existing persistence characteristics of the incipient discontinuities. One conclusion from this study is that visible trace length of a discontinuity can be a poor indicator of true persistence (defined for a fracture area with negligible tensile strength). An observation from this series of experiments was that freshly failed surfaces through pre-existing rock bridges were relatively rough compared to sections of pre-existing weaker areas of geologically developed (though still incipient) discontinuities. Fractographic features such as hackle and rib marks were typical of the freshly broken rock bridges, whereas opened-up areas of incipient discontinuity were smoother. Schmidt hammer rebound values were generally higher for the rock bridge areas, probably reflecting their lower degree of chemical and physical weathering.

Landslide mechanisms in Hong Kong

Abstract This paper reviews the nature and mechanics of landslides in the weathered terrain of Hong Kong. The vast majority of landslides are very shallow (a few metres depth) and occur during intense rainstorms. Deeper-seated landslides, in contrast, may occur days or weeks after intense rainstorms. The time of occurrence of landslides can be linked to hydrological and hydrogeological factors, and a hydrogeological grouping of landslide mechanisms is introduced related to timing in a storm. A relationship is presented that links intensity of landsliding to 24 h rainfall. The gradual deterioration and internal erosion of slopes prior to detachment is discussed and allows some realistic opportunity for identifying progressive major landslides. In particular, the growth of natural piping systems and infilling of dilated fracture networks are recommended as important indicators of landslide development. The conclusions are supported by case examples of slope failures, the study of some of which has been taken to a forensic level.

Assessing the Stability of a Geologically Complex Slope Where Strong Dykes Locally Act as Reinforcement

This paper presents a case study of aspects of the design of large slope cuttings in complex geological conditions in South Korea. During the original cutting of the slopes to geometries prescribed by Korean standards for rock slopes, several translational slides occurred on daylighting bedding planes. From observation it was evident that away from the areas of distress, some parts of the slopes appeared to be relatively stable, apparently because of the strengthening influence of relatively strong and massive igneous dykes and sills through the sedimentary rock. This paper describes how the geological conditions were assessed and stability analysed, section-by-section, along the road. This was achieved using a form of the method of slices to account for those sections of potential failure surfaces where sliding could occur along bedding and others where failure would necessitate shear through the intrusive igneous rock. Results were checked using UDEC models of critical sections. Following these analyses, recommendations were made for localized additional preventive measures including anchors and drainage.

Geomorphological landslide models for hazard assessment: a case study at Cloudy Hill, Hong Kong

Abstract Geomorphological landslide models developed to characterize natural hill slope landslides can assist significantly in the evaluation of natural hill slopes for hazard and risk assessment and for the selection of appropriate mitigation measures. Intense rainstorms in 2000 and 2001 resulted in about 45 natural hill slope landslides on a small area in the vicinity of Cloudy Hill, Tai Po, Hong Kong. A detailed study was carried out to examine systematically the characteristics and mechanisms of the landslides for the purposes of identifying the geomorphological and geological factors influencing susceptibility to landslide occurrence and with the aim of improved hazard assessment for similar hill slopes elsewhere in Hong Kong. Site-specific landslide models were developed from an assessment of the geomorphological setting and landslide characteristics, using a combination of morphological and morpho-chronological mapping based on aerial photograph interpretation together with field reconnaissance and inspection. Detailed field mapping was carried out for selected landslides following initial review, to confirm the relevant geomorphological factors and document relevant landslide characteristics. An outline of the study at Cloudy Hill is given and the development of the landslide models is presented, together with an overview of how the models were used to assist in the evaluation of natural terrain landslide hazards.

Damage to the Bangudae Petroglyph in South Korea and a Scheme for Its Conservation

The Bangudae petroglyph is one of the most important examples of Korean prehistoric art. The petroglyph however has been subjected to annual submergence and drying since the impounding of a reservoir in 1965, which has led to some deterioration. The deterioration and need to preserve the petroglyphs has become an important matter of public concern. This paper presents some of the options for conservation of the Bangudae petroglyph and reviews their various advantages and disadvantages.