Groundwater data collection

Approach to groundwater data collection

Groundwater can have a detrimental effect on slope stability. Fluid pressure acting within discontinuities and pore spaces in the rock mass reduces the effective stress, with a consequent reduction in shear strength. As noted in Chapter 1, these aspects are usually the only element of a slope design that can readily be modified by artificial intervention, usually dewatering and depressurisation.

However, dewatering and depressurisation measures are usually capital-intensive, require operator commitment to be implemented effectively and require significant lead times in design and implementation. The identification and characterisation of the hydrogeological regime in the early stages of any project is therefore of paramount importance, requiring a well-organised approach to collecting the data and building the conceptual hydrogeological model.

The development of a conceptual hydrogeological model to support a slope design analysis and depressurisation program is addressed in Chapter 6. This section focuses on the field tests and procedures used to collect the raw hydrogeological data. It describes the basic approaches to data collection then outlines the nature of each appropriate test method.

Phased approach

A typical phased approach for collecting the required hydrogeological data to support a pit slope analysis involves the following steps.

Step 1: Initial data collection

The initial step in the hydrogeological data collection program may involve the following.

■ A literature search to determine any pertinent hydrogeological information in the region surrounding the project area.

■ Identification of existing observation or groundwater production wells within or surrounding the project area, with their completion and stratigraphical details; measurement of groundwater levels in any accessible observation wells; and collection of any pumping data from available production wells. In all cases, it is important to know which formations are being measured or abstracted from.

■ Collection of hydrogeological data ‘piggy-backed’ on mineral exploration and resource drilling programs. Data collection may include:

→ measurement of groundwater levels in the open drill holes at regular intervals during and after drilling;

→ collection of water flow rate and other parameters at regular intervals during air drilling;

→ noting of loss of drilling fluids during water or mud drilling;

→ injection testing or airlift recovery testing to provide initial permeability information for individual holes. Note that the use of drilling additives to help stabilize the walls of the hole or increase recovery can alter the permeability around the hole and compromise the quality of the data.

■ Installation of groundwater observation wells using holes drilled for the geology resource evaluation or geotechnical programs.

■ Implementation of a routine water level monitoring program in all available groundwater observation wells or piezometers. For a new project, measurement of water levels on a weekly basis would be the standard procedure, possibly reducing to monthly with time as more data are collected.

■ Collection of climatological information. Depending on the climatic setting, precipitation and evaporation may exert a major influence on the groundwater flow system. They are important factors for determining pit-scale hydrological conditions. The required data may already be available from existing weather stations. For remote mine sites, installation of a dedicated weather station is often required.

■ Implementation of a hydrological database, which can be expanded as the project proceeds. Many operators use one database for all pit slope hydrogeological data, general dewatering and environmental management systems. In some cases the database is integrated with the geological modelling, geotechnical database and mine planning programs.

Step 2: Characterisation of the overall groundwater flow system

A good knowledge of the regional and mine-scale stratigraphy and geological structure is important for determining the hydrogeology around the mine site area. The geological information can typically be obtained from government agency reports, the regional exploration, resource drilling and condemnation drilling programs.

This information is used to define the hydrostratigraphical units that may influence the pit slopes. The focus for characterising the overall groundwater flow system is often the geological units that are more permeable, because most of the regional or mine-scale flow occurs within the permeable units.

The most appropriate test methods are as follows.

■ Drilling of dedicated hydrogeological test holes, which may be completed as observation wells with broader screened intervals. These are used to characterise the components of lateral groundwater flow. Several observation wells may be required for each identified hydrostratigraphical unit, to determine lateral hydraulic gradients and groundwater flow directions. Because condemnation (or sterilisation) drill holes are often located away from the immediate pit area, they may provide an opportunity to install ‘piggy-backed’ observation wells.

■ Injection testing or airlift recovery testing during drilling, to provide more detailed permeability information.

■ Installation of test pumping wells, and carrying out pumping tests to characterise permeability and storage characteristics and to assess the groundwater flow characteristics over a wider area.

■ Hydrochemical sampling and analysis of observation wells and pumping wells to characterise variations in groundwater quality which may help with the interpretation of the groundwater flow system.

Step 3: Characterisation of the local pit slopes

Once the more general data have been collected to determine the overall site setting, it will be necessary to collect data to characterise the specific conditions relevant to the immediate area of the existing or planned pit slopes.

More focused testing can then be carried out to support the detailed design of the slope depressurisation program. Information on lithology, alteration, mineralisation and geological structure will be required. It will be necessary to integrate the geology and hydrogeology information to determine structural boundaries, the influence of fault zones and groundwater flow between identified hydrostratigraphical units.

The focus for the localised area of the pit slope is often weighted towards the less permeable units in the groundwater system, because these units are typically most difficult to depressurise. However, pore pressure information will be required for each key unit in the pit slopes.

The most appropriate test methods are as follows.

■ Monitoring of airlift flow rates and pressures, and static water levels during reverse circulation (RC) drilling programs.

■ Injection testing and/or falling or rising head testing (slug testing) to provide local permeability information around individual piezometers.

■ Monitoring of fluid loss zones and static water levels during diamond/core drilling programs.

■ Installation of piezometers (vertical, horizontal, angled) with short measuring intervals, in carefully chosen locations, to characterise both the lateral and vertical variations in pore pressure and hydraulic gradients. Installation of vibrating-wire piezometers allows multi-point monitoring within a single drill hole. Depending on the nature of the slope materials and their setting, a combination of standpipe and vibrating-wire piezometers is appropriate.

■ In situ permeability testing in completed standpipe piezometers.

■ Measurement of water levels and pressures in piezometers at least weekly for a defined monitoring period.

■ Performing an integrated geotechnical and hydrogeological drilling program, as appropriate. This may include:

→ collection of groundwater levels at regular intervals during drilling;

→ in situ permeability testing and/or packer testing to characterise permeability differences within discrete hydrostratigraphical units or subunits. Packer testing is appropriate for characterising the poorly permeable units in the slope;

→ physical rock mass testing and lab testing, for porosity and density.

■ Pilot testing of depressurisation methods.

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