SLOPE STABILITY
Figure 1 - Different types of monitoring used in Sibelco mines and quarries.
A: Inclinometer readings.
B: Sibelco-Bertges dredging sen- sor monitoring deformations and water pressure.
C: Sibelco-Bertges in-house sen- sors to monitor changes in sand moisture (suction).
D: Low cost tell-tail to monitor crack movements “).
E: Sibelco-Bertges corner reflec- tors used to get a fixed reference point for InSAR deformation monitoring.
F: Screenshot of InSAR data for a Sibelco quarry. Red points show recent settlement, green points indicate stable areas.
- absolute values and changes in petrophysical properties: Measurements of changes in dielectric constants, electric impedance / resistivity (used to determine if changes in moisture content, thus suction occur in sand layers)
- rock mass, soil and structural elements: This type of monitoring is traditional construction monitoring. Samples are taken e.g. during backfilling or buttress- ing to check if the specified density has been attained.
- hydrogeological properties: Changes in pore water content of porous media are part of the petrophysical monitoring described above. Monitoring of hydrologi- cal properties in terms of measuring the position of “free water” consists of i.e. measuring location and changes of the water table.
Water has an enormous (negative) impact on the stability of
soils and rock. In “dry” quarry operations only slight changes in water pressure in the slopes determine if certain rock wedges are stable or not. Determining hydrogeological properties (e.g. borehole draw down tests with borehole pumps) and monitor- ing water level evolution is very important..
Monitoring of slopes is important during project realization
(thus excavation of the slopes) to verify design and to check if specifications are met. Note that, depending on the geotechni- cal risk, monitoring might be required and will continues after the project has been realized (in a way such monitoring of e.g. final slopes in reclaimed areas might be considered as part of the asset management of a valuable recultivated terrain).
Design and monitoring in sand operations
In order to design and plan the monitoring of the slopes in a sand operation we need to predict the behavior of soils and rocks in response to extraction or loading. Geotechnical engineers require knowledge of the:
1) physical properties of the rocks and soils
2) the nature of the action (i.e. the size of the tunnel dug into it or the size of a slope)
3) the constitutive law - the model - that links the reac- tion to the action
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For this geotechnical engineers require models: a ground model (including water described below) and mine model or mine plan (what actions will occur when) and a mechanical model (analytical or numerical). The summation of these models is the geotechnical model.
The mine model or mine plan The actions such as overburden stripping and extraction of
raw materials - causing the imbalance to the soils and rocks on a mine site - are described in the mine model or mine plan. The actions describe what will be done (at what scale) and when (time start - time end). Many geotechnical processes are time-dependent. A perfectly stable slope created in clay can be stable for months but not for years. In this example ero- sion (water) and a viscous behavior (creep) play a role. It is evident that mine planning and geotechnical engineering are interactive disciplines.
The constitutive law
The constitutive law is the link between the action (e.g. the mining) and the model consisting of soil and rock mechanical properties and defines thereby the resulting reaction.
An example of a very simple constitutive law is Hooke’s law of elasticity.
Perfectly elastic materials react linearly to forces (actions). A certain force will create a certain deformation. Hard rocks - without discontinuities - behave like elastic materials. In reality, rocks and soils undergo not only elastic but also plastic deformation.
Since the behavior of rock masses and soil masses is non-
linear, different constitutive laws have been tested and are used in practice. The simplest law is the Mohr-Coulomb law. In this case the rock mass is characterized by its elastic properties (E-modulus, Poisson’s ratio) and its plastic properties: friction angle and cohesion. Some of these properties can be measured in the field, some need to be determined in the laboratory.
A model that combines the geotechnical properties of soils,
rocks and discontinuity with the constitutive law is called a “Ground Model” or “Geotechnical Model-”.
With an appropriate ground model (with the properties of the rock mass, the soil and the discontinuities and an appropri-
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