Sheeting Joints: Characterisation, Shear Strength and Engineering
- First Online:
- Cite this article as:
- Hencher, S.R., Lee, S.G., Carter, T.G. et al. Rock Mech Rock Eng (2011) 44: 1. doi:10.1007/s00603-010-0100-y
- 3.4k Downloads
Sheeting joints are extensive fractures that typically develop parallel to natural slopes. Embryonic sheeting joints initially constitute channels for water flow and then become the focus for weathering and sediment infill accompanied by progressive deterioration and dilation. Slabs of rock fail along them periodically because of their adverse orientation and long persistence. They are however rough and wavy and these characteristics contribute highly to their shear strength and improve their stability. This paper reviews several landslide case histories and on the basis of these provides guidelines for characterising sheeting joints and determining their shear strength. Engineering options for stabilising sheeting joints in natural and cut slope configurations are then examined with reference to case examples.
KeywordsSheeting joint originsShear strengthLandslidesEngineering measuresRoughness characterisation
2 Development of Sheeting Joints
Sheeting joints are common in granite and other massive igneous rocks but also develop more rarely in other rock types including sandstone and conglomerate. Ollier (1975) provides an excellent review of early research and observations on their occurrence and development and Twidale and Vidal Romani (2005) discuss their occurrence specifically in granitic terrain.
Additional evidence for the great age of some sheeting joints is the fact that they can sometimes be observed cutting through otherwise highly fractured rock. Most sheeting joints occur in massive, strong rock and it is argued that if the rock mass had been already highly fractured or weathered then the topographic stresses would be accommodated by movements within the weak mass rather than by initiating a new tensile fracture (Vidal Romani and Twidale 1999). Therefore where sheeting joints are found in highly fractured rock masses, it is likely that they predate the gradual development of the other joints as mechanical fractures during unloading and weathering (Hencher 2006; Hencher and Knipe 2007).
Some extensive, hillside-parallel joints have many of the characteristics of “true” sheeting joints but owe their geometry instead to the opening up of pre-existing weakness directions such as doming joints in plutonic igneous rock or bedding in sedimentary rock. In this case the pre-existing fracture network defines the hillside shape rather than the other way around (Twidale 1973). The opening up of these pre-existing joint systems is probably largely in response to the same topographic stress conditions that encourage the formation of virginal sheeting joints in massive rock but development may be more gradual. That being so, such joint sets are more likely to retain intact rock bridges between sections of fully developed mechanical fractures than will true sheeting joints and these rock bridges will provide real cohesion, improving overall hillside stability.
3 Geometry and Occurrence
3.1 Sheeting Joints Within the Weathering Profile
3.2 General Shape, Occurrence and Relationship to Micro Fractures
3.3 Surface Characteristics
4 Engineering Considerations
It is a paradox that sometimes the entire stability of steeply cut slopes in otherwise excellent quality rock can be compromised by the presence of discrete sheeting joints. They are also a major source of landsliding in natural terrain.
4.2 Shear Strength of Sheeting Joints
Based on observations of many natural slopes it appears that failure of slopes along sheeting joints occurs predominantly by translational sliding of slabs of rock, often initiated by water pressure in the joint network. The problem is therefore relatively tractable to evaluate as it essentially only involves planar failure calculations rather than more complex wedge intersection displacements involving two or more joints. The common persistent nature of such joints means that the difficult judgmental issue of the contribution from true cohesion from rock bridges is of minor importance although cohesion might be a real factor for infilled and weathered zones within sheeting joints and for stepped situations where different sections of joint terminate against a pre-existing cross joint. Key factors that always need consideration are geometry (orientation and roughness at all scales), shear strength and the potential for ingress and development of adverse water pressure.
4.3 Basic Friction, ϕb
Basic friction of natural joints can be measured by direct shear testing but tests need very careful setup, instrumentation and analysis if they are to make sense. A series of tests on different samples of a joint will often yield very wide scatter which is simply not interpretable without correcting for sample-specific dilation as described by Hencher and Richards (1989) and Hencher (1995). Dilation reflects work being done in overriding asperities. The dilation angle, measured during a shear test will vary especially according to the original roughness of the sample and the stress level. It is test-specific, will vary throughout a test and with direction of testing. It is not the same as the dilation angle, i°, which needs to be assessed at field scale, although the mechanics are the same. To avoid confusion the laboratory-scale dilation angle measured during a test is here designated, ψ°, whereas the field-scale dilation angle to be judged and allowed for in design is, i°, as defined by Patton 1966).
Barton (1990) suggested that the dilation-corrected basic friction angle might be partly scale-dependent as assumed for the asperity damage component in the Barton–Bandis model (Bandis et al. 1981) but further research using the same shear apparatus and modelling setup as Bandis (1980), but with better instrumentation, indicates that this is unlikely (Hencher et al. 1993; Papaliangas et al. 1994). Rather it appears that the dilation-corrected basic friction, once the effects of small-scale roughness have been corrected for, as described above, remains fairly constant and seemingly independent of the length of the sample. Scale effects do however need to be considered as a geometrical effect when deciding on the appropriate field-scale i° value to add to the dilation-corrected ϕb as discussed below.
This suggested procedure of first testing joints to determine a dilation-corrected basic friction angle and then adding the field-scale roughness angle component is best illustrated by some case examples.
Roughness at the field scale will be the controlling factor for the stability of most sheeting joints and for engineering design must be added to the basic friction ϕb of the effectively planar yet naturally surfaced and textured rock joint. Roughness is expressed as an anticipated dilation angle, i°, which accounts for the likely geometrical path for the sliding slab during failure (deviation from mean dip). There are two main tasks for the geotechnical engineer in analysing the roughness component for a typical sheeting joint slab failure: firstly, to determine the actual geometry of the surface along the direction of likely sliding at all scales, and secondly, to judge which of those roughness features along the failure path will survive during shear and force the slab to deviate from the mean dip angle. This is the most difficult part of the shear strength assessment, not least because it is impossible to establish the detailed roughness of surfaces that are hidden in the rock mass. Considerable judgement is required and has to be balanced against the risk involved. Hack (1998) gives a good review of the options and the difficulties in exercising engineering judgement are discussed in an insightful way by Baecher and Christian (2003).
Defining the scale at which roughness will force dilation during sliding rather than being sheared through requires considerable judgement. Some assistance is provided by Schneider (1976) and by Goodman (1980) who indicate that for typical rough sheeting joint surfaces, where slabs are free to rotate during shear, as the length of the slab increases (at field scale) the dilation angle controlling lifting of the centre of gravity of the upper block will reduce. As noted earlier, sheeting joints are often wavy and major waves, where opposing the shearing direction, can almost always be relied upon to cause dilation at field scale from the mean dip of the overall sheeting joint plane, especially at the low stress levels appropriate for most sheeting joints despite the obvious stress concentrations at overriding contacts. Simple geometry shows that for wave amplitude of 1 m over a wavelength of 20 m the minimum dilation angle would be about 6°; over a wavelengths of 10 m, 11°; and over 6 m, 18°. This is an example of where geometrical scale effects operate and must be taken into account.
4.5 Infilled Joints
Richards and Cowland (1982) suggest that where there is a thick band of weathered rock along a joint (say grades IV and V) as shown in Fig. 5, zero dilation should be allowed when assessing stability. Where the joint is infilled with a mixture of weathered rock and rock fragments however the Hoek–Brown strength criterion might be used to provide some estimate of strength without laboratory testing (Carter et al. 2002) although Brown (2008) cautions against applying the original criterion outside the original data set and expresses specific concern for application for rocks with uniaxial compressive strength (UCS) below about 30 or 40 MPa. Carter et al. (2008) and Carvalho et al. (2007) discuss a modified Hoek–Brown criterion for low strength rocks that may be more applicable for such application.
4.6 Estimating Shear Strength Using Empirical Methods
Because of the inherent difficulties, need for quality equipment and expertise required for measuring shear strength of rock joints, various empirical criteria have been proposed for estimating shear strength based on index tests and idealised joint shapes. The most widely used strength criterion is that proposed by Barton (1973). This takes the “basic friction” measured for saw-cut or other artificially prepared planar surfaces then adding in a component to account for roughness adjusted for the strength of the rock asperities and for scale. Details are given in many text books including Brady and Brown (1985) and Wyllie and Mah (2004). The advantages of this criterion are its apparent ease of use and application in numerical modelling but there are difficulties in determining each of the various parameters. Basic friction is taken to be a lower bound component with a “limiting value” of 28.5–31.5° (Barton and Bandis 1990) but the friction measured for a saw-cut surface is not necessarily a lower bound either for natural or artificial joints (see Fig. 17). Hencher (1976) for example reports the sliding angle reducing from about 32° to only 12° for saw-cut surfaces of Darleydale Sandstone after about 4 m in tilt tests with continual removal of rock flour between test runs. Furthermore, considerable variability is sometimes reported from tests carried out on artificially prepared surfaces. Stimpson (1981) measured values ranging from 24° to 38° using limestone core pieces in sliding tests. Tests reported by the Norwegian Geotechnical Institute (NGI) for the Åknes landslide investigation gave values ranging from 21° to 36.4° for tilt tests on saw-cut joints with about 73% of data between 25° and 30.2° (Kveldsvik et al. 2008). Nicholson (1994) reports a variation in 12.5° for tests carried out on carefully prepared saw-cut, lapped surfaces of Berea sandstone, all suggesting that the recommendation of some lower “limiting value” of 28.5 to 31.5° may not be universally applicable. There is also some confusion in the literature regarding application of some of the Barton early equations as to whether ϕb (which is stipulated as sawn surface value determinations) or ϕr (residual values from multi-reversal shear box testing) is the appropriate parameter for application in the equation. In the authors’ opinion is also extremely unwise to rely on the widely publicised Schmidt Hammer relationships proposed between residual strength and base friction angle as a means for sorting out the correct value for shear strength determination.
The contribution to shear strength from roughness for small-scale roughness can be measured or estimated from standard shape profiles, but this can be difficult in practice and varies according to shearing direction and with scale, requiring appropriate judgement for its effective application. Beer et al. (2002) carried out an online survey of people’s estimates of joint roughness coefficient (JRC) for three randomly selected joints. Considerable scatter was reported and for one of the three joints a possibly bi-modal distribution of estimates was determined with the two centres of population at 8.9 and 17.9, perhaps reflecting different individual’s perception of controlling roughness scale. Like any other stochastic parameter, considerable difficulties can occur when representing joint roughness with a single value JRC estimate, as clearly demonstrated by determinations for the Åknes landslide by workers from NGI and MIT (Kveldsvik et al. 2008) where JRC measured for foliation joints at a 0.25-m scale ranged from 2.5 to 20 with a mean of 10.6. At a 1-m scale, JRC estimates covered the full possible range (from 0 to 20) with a mean of about 8 and standard deviation of ~4. The range of calculated factor of safety for this range of JRC was from about 0.8–2.0 taking all other parameters at their mean values. As is obvious, considerable judgement is still needed in application of such empirical procedures so that overall estimates for joint surface strength can be considered realistic. Furthermore, once the second order roughness contribution has been decided upon, then an additional roughness angle, i°, still needs to be determined and added, to account for larger scale roughness not sampled in the JRC assessment (Barton 1990).
An important point that arises from this review of empirical strength criteria for estimating field strength of rock joints is that it needs to be emphasised that the correct base-line parameters must be utilised within the equations whatever approach is adopted. It is a prevalent misconception in the literature (e.g. Simons et al. 2001) that dilation-corrected data from direct shear tests on natural joints can be used interchangeably in empirical equations. This is incorrect because the dilation-corrected strength already includes a frictional component contributed from textural and roughness damage (part equivalent of JRC) and its substitution for the saw-cut or residual ϕb of Barton could lead to overestimations of field-scale strength by maybe 10° in many cases.
5 Case Examples of Landslides Involving Sheeting Joints
A number of landslides involving sliding on sheeting joints have been studied in some detail in Hong Kong and provide some insight into operative shear strength and mechanisms of failure.
5.1 Sau Mau Ping Road, Hong Kong, Early 1970s
5.2 Hui Ming Street 2000 and 1993
5.3 Above Leung King Estate: 2000
5.4 Lessons from Landslide Case Studies
These case examples of landslides involving sliding on sheeting joints have provided some useful insights into the nature and characteristics of such failures. In particular, the failure above Leung King Estate gave considerable evidence of long-term deterioration involving intermittent movements by sliding along the joint along which detachment eventually occurred. Such deterioration with sediment infill and natural pipe systems may be taken as indications that the slope may be failing. The importance of the development of cleft water pressure is evident in triggering most sheeting joint failures investigated in Hong Kong. The ground investigation reported by Richards and Cowland (1986) which demonstrated the complex reaction of water pressures in joints to rainstorm events indicates the difficulties in designing drainage measures to prevent failure.
The case studies demonstrate the difficulties in extrapolating the geometry of sheeting joints into the rock mass from measurements in exposures. In particular local increases in dip hidden in the rock mass can allow significant and unpredicted rock falls.
Back analysis of landslides adopting reasonable estimates for active water pressures confirms that the current approaches to assessing shear strength based on dilation-corrected basic friction plus an i° value judged from roughness measurements on a grid basis at field scale can provide realistic parameters for design use.
6 Engineering Works
6.1 Assessing Risk and the Need for Preventive Engineering Measures
Slopes in sheeting joint terrain often appear extremely threatening because of the persistent, daylighting and steeply dipping nature of the joints. The fact that such steeply dipping joints are associated with failures at all scales from small rock falls to major translational movements has, over the years, necessitated that engineering works be implemented to reduce the risks.
A modern approach to assessing the need for preventive measures is to use quantified risk assessment as described by Pine and Roberds (2005) for the widening of the Tuen Mun Highway in Hong Kong (Fig. 1b). This project involved remediation and stabilisation of several sections of high cut and natural slopes dominated by potential sheeting joint failures and by the potential for failure of rock blocks and boulders bouncing down exposed sheeting joints to impact the road below. Design of the slope cut backs and stabilisation measures was based on a combination of reliability criteria and conventional Hong Kong standard factor of safety design targets aimed at achieving an ALARP (as low as reasonably possible) risk target which, in actuarial terms, translated to less than 0.01 fatalities per year per 500 m section of the slopes under remediation.
6.2 General Considerations
Remediation of sheeting joint-controlled stability hazards on high rock slopes is often not trivial and implementation of the works can itself increase the risk levels albeit temporarily. Factors that will influence the decision on which measures to implement include the specific nature of the hazards, topographic and access constraints, locations of the facilities at risk, cost and timing. The risks associated with carrying out works next to active roads both to road users and to construction workers themselves and how to mitigate these are addressed in some detail in Geotechnical Engineering Office (2000a) and Halcrow China Limited (2002c). Pre-contract stabilisation works will often be needed to allow initial site access and preparation. Preventive measures such as rock bolting may be carried out at an early stage to assist in the safe working of the site and designed to form part of the permanent works. Options for the use of protective barriers and catch nets to minimise disruption to traffic during the works also need to be addressed, as do contractual controls and alternatives for supervision of the works. The use of a risk register, as piloted for tunnels (Brown 1999), with clear identification of particular risks and responsible parties, helps to ensure that all hazards and consequences are adequately dealt with during construction. Decision analysis is now widely applied at an early stage to assess whether to mitigate slope hazards (e.g. by rockfall catch nets) or to remediate/resolve the problem by excavation and/or support approaches. If construction of intrusive engineering measures to stabilise hazards might be unduly risky, then passive protection can be adopted instead. A hybrid solution is often the most pragmatic solution for extensive, difficult slopes such as at Tuen Mun Road where some sections were stabilised by anchors and buttresses and other sections were protected by nets and other measures (Carter et al. 2002; Pine and Roberds 2005).
7 Engineering Options
7.1 Surface Treatment
Rockfall trajectory analysis using widely available software allows prediction of energy requirements and likely bounce heights and run-out damage zone extent. Where energy considerations allow, toe-zone protection measures, catch benches, catch ditches, and toe fences provide the earliest viable mitigation approach without requiring access on the slope.
Surface drainage is a very important consideration for all slopes but particularly for slopes comprising part rock (with very high runoff) and soil sections which might be eroded and undermined from high surface flow concentration.
7.2 Mesh Drapes
Where slope heights are significant and ramp or bench approach is difficult, mitigating hazards can be problematic even using rope access techniques because face stability may be too unstable to even allow rock climbing personnel onto the face. Under such conditions surface mesh draping may allow some effective protection to be achieved preventing ski jump-style bouncing of rock progressively down slope (Carter et al. 2002). Application of drape mesh (varying from chain-link, triple twist, hex-mesh to ring-net in increasing order of energy capacity) can be effected by a variety of techniques ranging from climber controlled unrolling of the mesh to helicopter access placement. Typically, crest restraint is provided by dowels or tie-back anchors usually cabled back some distance from the crest zone to provide a safe anchorage.
7.3 Fences, Catch Nets and Barriers
Drainage can be very effective in preventing the development of adverse water pressures, but there is a need to target subsurface flow channels many of which will be shallow and ephemeral. The paths may be tortuous and hard to identify and drainage measures can therefore be rather hit or miss (Hencher 2010). Regular patterns of long horizontal drain holes can be very effective, but it must never be expected that all drains will yield water flows and the effectiveness of individual drains can change with time as subsurface flow paths migrate. With exposed sheeting joints forming ledges on a slope, care must be taken that the step zones are not shotcreted otherwise free drainage may be impeded and water might dam up behind the shotcrete. If the exposed joint is weathered the weak material may back-sap and possibly pipe leading to destabilisation, partially caused by lack of free drainage. This can be rectified by installing closely spaced horizontal drains with geotextile filter fabric sleeves so as to prevent blocking together with protection of the weathered material. No-fines concrete whilst appearing to be suitable to protect weathered zones often ends up with lower permeability than designed and should not be relied upon without some additional drainage measures.
The factor of safety against slab sliding can be improved by a variety of options. For sheeting joints specifically, provided there has not been previous movement, the rough interlocking nature of these tension fractures provides considerable shear strength (where not severely weathered) and this needs to be accounted for in design in order to avoid over-conservatism. If the joint can be prevented from sliding by reinforcing at strategic locations then full advantage can be taken of the considerable natural frictional resistance. Active stabilisation of blocks is possible if they are of relatively small size and access is feasible either by rope access techniques down the slope, using “spyder” drills or even better if tracks can be constructed, using more conventional drilling equipment. Depending on configuration, rock blocks may be stabilised by dowelled concrete buttressing (to provide direct support to a well-defined potential release block), through various forms of tie-down and/or overturning control tie-back reinforcement, comprising deep sub-vertical dowelling. Sub-horizontal cable anchors can be used if capacities larger than about 20 tonnes per reinforcement member are required. Often the most significant reinforcement is needed where extensive sheeting joint zones define slabs of large proportions. In such cases, the preferred method in Hong Kong is to use passive dowel designs rather than tensioned bolting for necessary shear constraint. This is because it is considered that active reinforcement members are more subject to corrosion damage and that passive dowels allow both mobilisation of a normal force (due to the restraint provided by the full column bond against asperity ride during shear), plus active shear restraint provided by the steel of the dowels resisting block slide mobilisation (Spang and Egger 1990).
The Geotechnical Engineering Office in Hong Kong has published some guidelines on prescriptive measures for rock slopes and in particular gives guidance on rock dowelling for rock blocks with volume less than 5 m3 (Yu et al. 2005). In essence, it is advised to use pattern dowels with one dowel per m3 of rock to be supported with minimum and maximum lengths of 3 and 6 m, respectively and where the potential sliding plane dips at less than 60°. The dowels are to be installed at right angles to the potential sliding plane, with the key intention to allow the dowels to act in shear, whilst also enhancing the normal restraint due to asperity ride during sliding. In practice dowels frequently need to be used in more variable orientations.
Similarly on the slopes above Tuen Mun Highway many combination buttresses were employed with parts of blocks dowelled and part buttressed as access and local geometry dictated.
Sheeting joints develop due to topographical or residual tectonic stresses close to the Earth’s surface. Those that develop in natural slopes are often of adverse geometry with respect to natural hillside slopes and as such, may pre-dispose the slope to repeated failure. Sheeting joint terrain often comprises a series of simple slabs resting on one another and often these are geologically young. Many other sheeting joints are very old however, as evidenced by their association with deep weathering profiles, by the propagation of other fracture systems through the rock mass after formation of the sheeting joints and from geomorphological interpretation.
Sheeting joints appear to always have originated by tensile opening and as such often occur as persistent, mechanical fractures extending laterally over many tens to even hundreds of metres. Detailed assessment of the configuration of sheeting slabs on various slopes in Hong Kong, Korea and other well-defined sheeting geologies around the world, suggests that in general, remnant slabs sitting on persistent sheet structures owe their stability more to the roughness and undulating/wavy character of the sheet structure (and associated dilation) rather than to rock bridges (and true cohesion) which is commonly the case for other types of joints. Definition of controlling shear strength is thus amenable to evaluation either through a testing programme combined with field measurement and assessment of roughness and analysis of the way that roughness will cause dilation or by employing empirical methods. Both approaches require considerable judgement.
Rock slope failure mechanisms based mainly on pseudo-statistical analysis of defect data should not be the sole basis of defining ground models. More intelligent analysis of the data is required and in the case of sheeting joints, recognition of lateral variation in orientation, roughness and degree of weathering and openness. The possibility for composite landslides, partly involving rough joints and partly through more weathered sections should be recognised. It is clear that sheeting joint failures are often associated with the development of cleft water pressures and that failure may be incremental over long periods and many storm events.
Landslide preventive works often necessitate reinforcement, drainage and rockfall protection (such as fences, catch nets and ditches). On steep hill slopes where detached or partially detached sheeting structures exist, buttresses and/or anchor blocks have application for preventing initial movement that would otherwise lead to progressive deterioration. Preventing initial movement will optimise the contribution from peak shear strength. Seepage points on faces can help to identify the likely routes for channel flow which should be targeted with raking drains.
This research was supported in part by a grant (NEMA-06-NH-05) from the Natural Hazard Mitigation Research Group, National Emergency Management Agency (NEMA), Ministry of Public Administration and Security, Korea.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.