Highway Slope Manual

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Notes: (1) The responsibility for the evaluation of rockfall and design of mitigation measures should be clearly
specified. The levels of supervision, qualifications and experience of supervisory staff at different
stages of construction should also be specified. All available geotechnical information, initial traffic
impact assessment reports, risk assessment reports and information on contractual constraints which
may affect the execution or progress of the works (e.g. traffic restrictions and access constraints)
should be provided to the tenderers. Where existing slopes are to be modified, information on the
extent and details of any existing engineering measures such as coverage and details of rock bolts and
dowels installed on the slopes should also be provided.
(2) For Design and Build contracts, only preliminary design reports are prepared at the pre 叫 tender stage
and detailed design reports are prepared by the contractor at the contract award stage.
(3) Evaluation of hazard/risk and traffic impact assessment findings should be included.
Figure 7.2 Processes to Address Safety Concerns Arising from Rock Breaking Above and Adiacent to
Roads at Various Stages of a Proiect
Except in negligible risk situations, the evaluation of potential rockfall risk should
include a comprehensive geotechnical investigation to assess the potential failure modes and
the likely scale of rockfalls (sizes of rock blocks and kinetic energy). Rockfall trajectory
analyses using computer simulations may be carried out to support decisions on appropriate
risk mitigation options. Where such analyses are used, the uncertainties associated with the
model and parameters should be borne in mind in interpreting the results of the simulations.
The type and capacity of any protective barrier to be provided and the requirements for any
slope treatment prior to excavation would depend on the severity of the rockfall hazard posed
to the carriageway (Figure 7.3).
If rockfall protective barriers or fences are required along the road, their location,
height and capacity, in terms of the maximum kinetic energy that they can absorb, should be
specified. The typical temporary protective fences used in Hong Kong (see Plate 7.1) often
have energy-absorbing capacities of only about 10 kJ (equivalent to an approximately 1 m 3
rock block impacting at a velocity of about 3 m/s). Commercially available specialised
rockfall fences (see Plate 7.2) have energy-absorbing capacities of up to 2 500 kJ, but even