Peer Reviewed Article
Apparent Cohesion and the Design of Slopes in Sand Quarries
Dr. ir. Robrecht Schmitz CPG-11917, CEng.,
M.ASCE, Global Leader Mining (pictured left) and Geotechnics Sibelco Hon. Prof. Dr. ir. Christian Schroeder
Abstract
This article discusses the background for the design, design-based monitoring (and the standards that allow this) of slopes in sand deposits in order to reach optimum extraction at sand extraction sites. We explain the benefits of taking the apparent cohesion into account when designing slopes. We provide a step by step overview detailing how the design should take place if the apparent cohesion is taken into account in the design of slopes in sand quarries.
Key words: Sand, slopes, monitoring, stability, apparent cohesion Introduction
Sand is extracted in many operations world-wide. It can be mined above (dry operations) or below the water table (dredg- ing operations). In deep “dry” quarries sand is extracted from a sequence of progressively deeper benches. This entails the creation of ‘temporary’ and ‘final’ slopes: the final slopes define the mine’s final shape, and need to be stable in perpetuity or until any backfilling is completed. The temporary slopes are the working faces at which daily extraction occurs. They are not as high and are not required to be stable for as long as the final slopes.
All of these slopes require a design, whether simple (based
on experience) or more complex (using limit equilibrium methods and or numerical methods). The required detail of design depends on the local setting, local regulations and the geotechnical risk class.
The design is the basis for extraction and extraction is then followed by monitoring. Monitoring can also be simple (visual inspection) or complex (using inclinometers, piezometers, slope deformation monitoring).
The design aim for these slopes is part of the overall goal to determine the optimal extraction of the sand deposit. That is, the profitable extraction of as much sand as possible without compromising safety through an unacceptable risk of slope failure, and without the need for costly slope remediation (such as buttressing). The slope design, general layout of the different benches, final slope design and monitoring require- ments should be described in the mine (or extraction) plan. For Sibelco’s quarries and mines the requirements for design and monitoring are based on Eurocode 7, as is explained in the two sections below.
28 TPG •
Oct.Nov.Dec 2020
Geotechnical design of mines and quarries and dredging operations
Is a detailed geotechnical assessment based on samples, sample analysis in the laboratory and numerical simulations required for every geotechnical problem? The answer is no. What needs to be done depends on the Geotechnical Category (Table 1) of the problem, as defined below, and as determined by a preliminary investigation.
For the geotechnical design of, for example, slopes, tun-
nels and caverns, Sibelco follows the principles of Eurocode 7 (implemented as e.g. British Standard EN 1997-1:2004 Eurocode 7: Geotechnical design) and uses the concept of the Geotechnical Category. Although not directly aimed at sand extraction, the design philosophy of Eurocode 7 is typical for processes used historically in mining, such as relying on moni- toring using the observational method. Therefore, it is wise to cross-check the geotechnical aspects of mine or quarry design with the principles laid down in the Eurocode.
A preliminary classification of a structure according to Geotechnical Category should normally be performed prior to the geotechnical investigations.
The category should be checked and changed, if necessary,
at each stage of the design and extraction process. For each category Eurocode 7 will provide information on the extent of site investigation required, the design type and degree of monitoring.
Depending on the nature of the design requirements, analytical, kinematic and numerical methods are used. We cooperate with universities to validate and introduce new con-
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