Specific Guidance for Silt- and Clay-rich Layers of Rock Cut Slopes

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In assessing the stability of a cut slope in volcanic or granitic rock, attention should be given to establishing whether any of the following geological features are present, and if so, whether they may adversely affect slope stability: [Amd GG2/01/2017]

(i) laterally persistent (e.g. > 4 m) weak silt- and clay-rich layers (which predominantly comprise white to buff kaolin, but may contain other materials, most notably dark brown manganiferous and iron oxides), within the rock mass, regardless of thickness,

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(ii) completely and highly decomposed rock (Grades V and IV) forming planar layers that sit on slightly and moderately decomposed rock (Grades II and III), and which dip directly, or obliquely outwards from slope faces (At the Fei Tsui Road landslide site, the rock type was a eutaxitic fine-ash crystal tuff. The investigation of the Fei Tsui Road landslide (GEO, 1996a) highlighted various geological features in this rock type that influence the stability of cut slopes. The recognition of these geological features during site investigation at other cut slopes in similar rock types should raise awareness of the potential for a slope failure in similar circumstances to those pertaining at the Fei Tsui Road site.), and

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(iii) persistent (e.g. > 4 m), planar, steep joint sets and other geological contacts (e.g. dykes, faults etc.) that could form release surfaces. [Amd GG2/01/2017]

The following additional ground conditions are often indicative of the presence of

geological features listed in previous paragraph and should also be checked: [Amd GG2/01/2017]

(i) stratification dipping out of the slope (e.g. as indicated by eutaxitic foliation in some fine ash tuffs), [Amd GG2/01/2017]

(ii) zones of continuous seepage, and [Amd GG2/01/2017]

(iii) clusters of previous slope failures. [Amd GG2/01/2017]

In site investigation, the following items are recommended to facilitate the identification of geological features that may be adverse to slope stability: [Amd GG2/01/2017]

(i) The desk study should establish the history of any past failures (including the mechanism and type of failure) and continuous seepage, and, where site formation photographs are available, the presence of significant geological features.

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(ii) The engineering geological mapping should establish whether the orientation of bedding, bedding-parallel fabrics (e.g. eutaxitic foliation, which could provide an indication of the orientation of potential bedding plane structures in volcanic rocks.) or laterally continuous discontinuities (e.g.

sheeting joints) are adversely oriented, and identify any weak silt- and clay-rich layers, especially within adversely-oriented persistent discontinuities or along the weathering front (i.e. the boundary below which rock predominates in a partially weathered rock mass profile). Such adversely oriented weak layers may also occur in local depressions in the weathering front, caused for example by zones of faulting, discontinuities with close spacing, and subvertical eutaxitic foliation. Evidence of previous movement, especially that associated with any weak layers, should be noted, and could

include: [Amd GG2/01/2017]

(a) slickensiding, particularly within silt and clay-rich layers,

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(b) brecciation and shear deformation of silt and clay-rich layers, and

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(c) tension cracks and infilled tension cracks, possibly controlled by subvertical joints and particularly where associated with adversely oriented weak layers.

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Zones of continuous seepage, especially where associated with

silt- and clay-rich layers, should also be mapped.

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(iii) During the initial phase of ground investigation, emphasis should be directed to developing a representative geological and hydrogeological model rather than testing. The ground investigation should focus on examining and logging the saprolite profile in detail, with emphasis placed on identifying the presence of adversely-oriented, weak silt- and clay-rich layers, especially in the vicinity of the weathering front, regardless or not whether these layers daylight in the slope under investigation. The ground investigation should also identify any such features within the rock mass where they may influence slope stability.

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Suitable techniques for detailed examination of the saprolite profile should include:

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(a) full-face mapping and logging of cut slopes, after stripping of surface cover, and adjacent exposures, [Amd GG2/01/2017]

(b) excavation and logging of trial pits, and [Amd GG2/01/2017]

(c) logging of drillholes. [Amd GG2/01/2017]

Suitable techniques for detailed examination of the saprolite profile may also include: [Amd GG2/01/2017]

(d) excavation of trenches or adits, [Amd GG2/01/2017]

(e) continuous sampling in drillholes using triple tube core barrels with air-foam as the flushing medium, and

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(f) downhole geophysical logging and other downhole techniques, including borehole televiewer and impression packer. (Technical guidelines on the use of downhole geophysical investigation techniques in the identification of weak layers are given in Section 33).

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10.9 Specific Guidance for Tunnel Works [Amd GG2/01/2017]

10.9.1 Preliminary Design Stage [Amd GG2/01/2017]

In the preliminary design stage, recommendations are made on a preferred tunnel

alignment and the scope of works, including risk mitigation measures based on assumed methods of excavation. Ground investigation with a geophysical survey could be undertaken to refine the geological model, to gain additional information on significant geological features, identify sensitive receivers and to establish baseline conditions.

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Boreholes should extend well below the anticipated depth of the tunnel and shafts to allow for any subsequent changes in the vertical alignment of the tunnel in the detailed design stage, and because the zone of influence of the tunnel may be extended by the nature of the

ground at a greater depth. [Amd GG2/01/2017]

Long horizontal boreholes parallel to the proposed tunnel alignment are extremely useful, particularly where the location of the proposed tunnel is overlain by thick layers of deeply

weathered rock (McFeat-Smith, 1987). [Amd GG2/01/2017]

Soil permeability tests and Lugeon tests at close spacings should be undertaken in the boreholes to assess the soil mass and rock mass permeability, respectively. It is important not to overlook areas of apparently strong bedrock, as the mass permeability of these areas may be high and may affect significantly construction of the tunnel works. The test results should be used for development of the hydrogeological model, defining the hydraulic boundary conditions for the design and for assessing the need to control groundwater inflow/drawdown

during construction. [Amd GG2/01/2017]

10.9.2 Detailed Design Stage [Amd GG2/01/2017]

In the detailed design stage, when the tunnel alignment has been fixed, the main aim of the ground investigation should be to obtain information for the reference design or detailed design of the tunnel works and the associated temporary works. The ground investigation should also identify conditions at likely problematic areas along the chosen alignment. The ground investigation data should be adequate for preparing the design of the ground support, ground treatment, groundwater control works and the risk mitigation measures. It should also be adequate for planning the inspection, testing and monitoring works during construction.

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Where existing boreholes are found close to or intercepting the tunnel, the risk of these boreholes not properly grouted should be assessed and mitigation measures, where necessary, should be carried out to ensure that these boreholes would not form preferential flow paths that

could jeopardize the tunnel construction. [Amd GG2/01/2017]

New boreholes along the chosen tunnel alignment and shaft locations should generally extend to a sufficient depth below the invert of the tunnel/shafts to obtain information for the assessment of possible failure mechanisms/limit states, and/or construction of the tunnel/shafts.

For tunnels in rock, this should be a least 2.5 times the tunnel diameter (or the crown to invert

dimension) below the invert. [Amd GG2/01/2017]

Directional coring along the tunnel alignment should be considered. If this is to be carried out, it should preferably be done immediately on commencement of the detailed design

stage, in order to yield early data to maximise its benefit for the design. Despite the cost, the directional coring together with pumped down packer tests could provide useful information on the geology and hydrogeology along the tunnel alignment which could not be obtained from vertical or inclined boreholes. The information along the tunnel alignment would help to enhance the management of ground risks in tunnel excavation. [Amd GG2/01/2017]

For ground investigation to support the design of shafts, particular attention should be given to identifying poor ground conditions, which could lead to collapse, excessive ground deformation/vibration or excessive groundwater inflow/drawdown. For deep shafts with a significant length in rock, the ground investigation should assess the hydrogeology and inflow into the unlined sections of the rock mass, and the need for ground treatment and groundwater control works to prevent excessive drawdown of piezometric pressures in the rock and the soil

overburden. [Amd GG2/01/2017]

For significant temporary works to be designed by the contractor, e.g. major ground treatment, groundwater control and ground support works, the pre-tender site investigation should provide sufficient geological and hydrogeological data for the pre-tender reference designs of such works, which should be carried out to adequately define the scope of the works required to meet the safety standards and the performance criteria specified.

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