5. TYPE 3 PRESCRIPTIVE MEASURES
5.7 PRESCRIPTIVE ROCK DOWELS FOR ROCK CUT SLOPES .1 Collection and Assessment of Discontinuity Data
No. 13/99 : Geotechnical Manual for Slopes - Guidance on Interpretation and Updating (Works Bureau, 1999), provided that the conditions for ‘existing’ walls are satisfied, and no upgrading works would be required in this case.
(b) Otherwise, determine the required prescriptive skin wall thickness, ts, using the following equation:
ts = 0.056He + 0.22 ... (5.2)
where He = maximum effective height of slope feature (m)
Typical skin wall details are given in the latest version of CEDD Standard Drawing No. C2521 : Typical Details of Skin Wall without Soil Nails.
5.6 PRESCRIPTIVE CONCRETE BUTTRESSES FOR ROCK CUT SLOPES
Where a rockfall has occurred leading to the formation of a cavity on the slope face, it may be necessary to construct a concrete buttress in the cavity to prevent further rockfalls.
A buttress serves two functions, viz. to retain and protect areas of weak rock and to support the overhang. It may also be used to prevent local toppling failure of the rock face. Rock dowels are commonly used in conjunction with concrete buttresses to stabilise and tie the rocks together. Typical details of a concrete buttress are shown in the latest version of CEDD Standard Drawing No. C2203 : Typical Details of Concrete Buttress Type A.
The size of a concrete buttress is generally governed by geometrical considerations such that it is large enough to provide physical support to a rock block or overhang. The stability of its foundation should be considered. It should be founded on a level, clean and sound rock surface. If this surface is not at right angles to the direction of resultant force acting on the buttress, the buttress should be anchored to a solid base using dowels to prevent sliding failure. In addition, the top of the buttress should be set at a higher elevation than the top of the overhang to ensure good contact.
5.7 PRESCRIPTIVE ROCK DOWELS FOR ROCK CUT SLOPES 5.7.1 Collection and Assessment of Discontinuity Data
Stability in rock is controlled principally by discontinuities in the rock mass. The role of discontinuity data collection is primarily to aid the identification of the possible modes of failure. Rock outcrop mapping is the best field way to obtain discontinuity data.
Geoguide 2 : Guide to Site Investigation (GCO, 1987) and Geoguide 3 : Guide to Rock and Soil Descriptions (GCO, 1988) describe the requirements for rock discontinuity mapping for rocks in Hong Kong.
Rock joint discontinuity data should be recorded in a proforma similar to Figure 1 of Geoguide 3 (GCO, 1988).
accuracy of the collected data. In the event that the rock slope face is fully exposed, there should be sufficient good quality data for rock mass discontinuity assessment. If little or no exposure is available on the slope, knowledge of the local geology may permit extrapolation from areas outside the slope. The key to this lies in the recognition of discontinuity patterns.
Where extrapolation is necessary, designers should determine whether the rock mass and discontinuity pattern in the area of data collection are akin to those of the rock slope by consideration of the local geological conditions.
Where there are doubts on this, the discontinuity data should be collected from the covered rock slope direct. Techniques for investigating partially and fully covered rock faces include surface cover stripping, window opening, coring and drillhole inspection.
Where stripping is used, a scanline survey may be undertaken as opposed to stripping the whole slope. Where prescriptive measures are specified without a close inspection of all the rock blocks to be treated, the actual rock block size and the measures needed should be reviewed once the conditions and dimensions of the block can be examined on site more accurately during the construction stage, particularly when the slope surface cover is removed and/or safe access for close inspection is provided.
A qualitative assessment is required to determine the potential instability problem and the likely scale of failure. If local zones of instability are observed, the prescriptive measures items given in this document can be applied. However, if there are potential global instability or large zones of potentially unstable rock blocks with a volume greater than 5 m3 , the use of these prescriptive measures items alone is not considered adequate, and suitable stabilisation measures based on analytical design should be implemented.
During the assessment, kinematic analysis could be used to facilitate judgement to be made on the stability of the slope. Where stereoplots are used, their limitations should be recognised (Hoek & Bray, 1981; Hencher, 1985). It is important that designers exercise due care when interpreting stereoplots and that correct judgement is applied. It should also be noted that assessment of discontinuity data only provides a reference for designers. The stability of an existing rock slope, particularly local stability of individual rock blocks, should always be assessed based on field inspections. Indeed, the step of discontinuity data collection may be omitted if the rock face to be treated is fully exposed, such that detailed examination of the rock face can be carried out to identify all potential instability problems.
Designers should review the overall stability of a rock slope before concentrating on stabilising small unstable rock blocks by means of prescriptive measures. Desk study, collection of relevant rock joint data and assessment of the stability of an existing rock slope through detailed visual inspection in the field should be carried out as needed.
5.7.2 Prescriptive Rock Dowels
Loosening and detachment of small rock blocks on the slope face can be prevented by the installation of passive rock dowels, which are composed of reinforcing steel bars grouted into holes drilled in the underlying stable rock.
Prescriptive rock dowels can be applied as upgrading works to rock cut slopes which
criteria are satisfied, the rock dowels may be used as preventive maintenance works.
Standard rock dowel design has been developed as shown in Figure 5.11 for prescriptive application. The rock block sliding angle and rock block volume should be estimated prior to using the design table to determine the number of dowels required. By reading off from the design table with the appropriate volume of potentially unstable rock block, the required number of dowels can be estimated. Typical details of a rock dowel are shown in the latest version of CEDD Standard Drawing No. C2202 : Typical Arrangement of Rock Dowel.
Table 5.12 Qualifying Criteria for Application of Prescriptive Rock Dowels to Rock Cut Slopes as Upgrading Works
Subjects Qualifying Criteria for Application
Geometry
1. The volume of the rock block should not be greater than 5 m3 and the rock block is not supporting any foundations of structures or surcharge.
2. The angle between the slope at the rock face and the potential sliding surface should not be smaller than 10°.
3. The angle of the rock block basal sliding surface should be smaller than 60°.
Engineering and geology
1. The rock type is granitic or volcanic and of decomposition grades I to III.
2. No daylighting clay-infilled or silt-infilled discontinuities.
Volume of Potentially
Dowel bars shall be of 32 mm in diameter (hot dip galvanised type 2 high yield steel bars to be used).
Angle of dowels to be approximately perpendicular to potential sliding surface of the rock block.
Dowel length = 3 x thickness of potentially unstable rock block, subject to a minimum length of 3 m and a maximum length of 6 m.
The layout of the rock dowels as applied to a sliding rock block/wedge shall be at least 0.3 m from the identified periphery of the rock block/wedge in order to provide effective stabilisation.
The vertical and horizontal spacing of the rock dowels shall be from a minimum of 0.3 m to an effective spacing evenly distributed to cover the sliding area of the block.
Figure 5.11 Prescriptive Rock Dowels on Rock Cut Slopes