Stability (Pre-operational Phase)
Stability in Quarries
Ensuring stability in quarries is primarily a safety issue, but there can also be economic and environmental issues relating to stability, which are relevant before, during and Post-operation.
Pre-operation (planning and design phase)
Mineral operators have a statutory duty to maintain a safe working place and not to jeopardise the safety of those either inside or outside a quarry. There is also a common law obligation to maintain the support of third party land and a specific planning requirement, outlined in PPG 14 9 and further developed in MPG 5 L0232, to maintain the stability of all external slopes to avoid failures that extend outside a planning permission boundary. There is, in addition, a requirement to maintain stability of all slopes to avoid failures affecting subsequent uses of the quarry.
A safe working place in a quarry has two elements:
- A designed geometry for all excavations (including haul roads) and tips that is capable of being worked in a safe manner and gives rise to a completed excavation that is stable in perpetuity and suitable for its intended after-use. Key elements of such a design geometry will be:
stable slopes and retaining structures;
safe haul roads with edge protection, adequate width and appropriate drainage, maximum gradients and crossfalls;
working face alignments and working directions that avoid, where possible, adverse geotechnical settings (such as undercut bedding or other discontinuities);
and bench and tip heights appropriate to the equipment that will be forming them (Quarry Design Handbook L0060 ); and
- A management system that incorporates working rules requiring safe methods of working for all aspects of the quarry operation.
The first of these elements is paramount; without it, it may not be possible to achieve a safe working place, however good the management systems and working rules at the quarry. Re-design of a quarry because of unforeseen stability problems once quarrying has started (e.g. reduction of overall slope angles or changes to the orientation of working faces), can be very costly and may not be possible within the terms of the extant planning permission and associated Environmental Statement.
There are important environmental considerations relating to the appearance of quarry excavations and the suitability and sustainability of the landforms they create to support acceptable landscaping and after-uses (Secure and sustainable final slopes for SME aggregate quarries L0056). All of this underlines the fundamental importance of taking account of stability when planning and designing a quarry, alongside environmental impact and commercial considerations (Quarry Design Handbook, An overview of design and management approaches to reducing the environmental footprint of the supply chain for land-won aggregates L0075 )
A successful quarry design that gives rise to a working quarry incorporating stable slopes and structures during operation and on completion of restoration must be based on reliable geological and geotechnical information. The information relevant to stability that is needed for quarry design and planning includes:
- A geological model providing, in three dimensions, all relevant geological surfaces (including original ground surface, base of soil, rockhead, top of mineral (if different from rockhead), base of mineral, any faults or other major discontinuities).
- For rock, discontinuity models from which modes of failure and geotechnical settings may be determined.
- Intact and rock mass geotechnical parameters for all materials to be excavated, and disturbed geotechnical parameters for all materials that will be stored in tips (including restoration slopes).
- A hydrogeological model providing all necessary information for stability analysis (and prediction of inflows and/or drawdown effects around the workings).
- Using this information, a quarry design can be developed that incorporates stable final excavated and tip slope angles and working faces that are aligned to avoid adverse geotechnical settings.
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