Geotechnical site investigation

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The successful design of underground excavations requires a thorough understanding of the geologic structure, the rock mass strength, and the stress and groundwater regime of the area affected by the excavations. Failure of the rock mass around an underground excavation is usually a result of unfavourable geologic structure, high induced stresses exceeding the strength of the rock mass, or a combination of both. Therefore, it is important that structure and stress are addressed in the geotechnical site investigation so that they can be properly incorporated into the mine design.

Structural discontinuity features will vary widely on the mine scale. The discontinuity type, spacing, persistence, orientation, and shear strength of the discontinuities must be quantified for the rock mass. A geotechnical field program should also facilitate an estimation of the intact rock strength parameters, so that the competency of the rock between the structural discontinuities can be determined. This will allow the rock mass to be classified as a bulk material, with consideration of both the intact strength of rock and the strength of the discontinuities. By superimposing the strength of the rock mass on the in-situ stress regime and defining the sources and magnitudes of the stresses prior to underground mining, the response of the rock mass to an excavation can be predicted, allowing adequate support measures to be developed or modifications to the excavation to be designed. The stress distribution around the proposed excavation will vary depending on such factors as: rock type, depth, support installed, proximity to other excavations and structures, and pore pressure distribution. Thus, it is important to design the geotechnical investigation program to obtain a good representation of the structure, strength, and stress within a rock mass.

To properly characterize the rock mass, a basic understanding of the engineering geology of the site is required. This will involve close cooperation and strong communication between the engineer and the geologist. Understanding the genesis of the rock mass is fundamental to determining its potential behaviour during development and mining. This may require review of regional geologic studies and examination of satellite images or aerial photographs to identify major geologic structures, structural trends, geologic contacts, etc. Ideally, this is carried out prior to detailed site investigations so that drilling, mapping and laboratory testing can be properly focused early in the field program.

Planning a geotechnical site investigation program

The level of effort put into the assessment will be dictated by the level of the study for which the geotechnical program is required. For example, the requirements for a scoping level study are significantly less than the efforts required for detailed design. A suggested level of effort (modified after Read and Stacey, 2009) is outlined in Table 1 below.

Table 1: Suggested level of geotechnical effort by project stage.
Project level status Conceptual Pre-feasibility Feasibility Design and construction Operations
Geotechnical level status Level 1 Level 2 Level 3 Level 4 Level 5
Geological model Regional literature; advanced exploration mapping and core logging; database established; initial country rock model Mine scale outcrop mapping and core logging, enhancement of geological database; initial 3D geological model Infill drilling and mapping, further enhancement of geological database and 3D model Targeted drilling and mapping; refinement of geological database and 3D model Ongoing underground mapping and drilling, further refinement of geological database and 3D model
Structural model (major features) Aerial photos and initial ground proofing Mine scale outcrop mapping; targeted oriented drilling; initial structural model Trench or exploration drift mapping; infill oriented drilling 3D structural model Refined interpretation of 3D structural model Structural mapping on all working levels; further refinement of 3D model
Structural model (fabric) Regional outcrop mapping Mine scale outcrop mapping; targeted oriented drilling; database established; initial stereographic assessment of fabric data; initial structural domains established Infill trench mapping and oriented drilling; enhancement of database; advanced stereographic assessment of fabric data; confirmation of structural domains Refined interpretation of fabric data and structural domains Structural mapping of all drifts and cross-cuts; further refinement of fabric data and structural domains
Hydrogeological model Regional groundwater survey Mine scale airlift, pumping and packer testing to establish initial hydrogeological parameters; initial hydrogeological database and model established Targeted pumping and airlift testing; piezometer installation; enhancement of hydrogeological data base and 3D model; initial assessment of depressurization and dewatering requirements Installation of piezometers and dewatering wells; refinement of hydrogeological database, 3D model, depressurization and dewatering requirements Ongoing management of piezometer and dewatering well network; continued refinement of hydrogeological database and 3D model
Intact rock strength Literature values supplemented by index tests on core from geological drilling Index and laboratory testing on samples selected from targeted mine scale drilling; database established; initial assessment of lithological domains Targeted drilling and detailed sampling and laboratory testing; enhancement of database; detailed assessment and establishment of geotechnical units for 3D geotechnical model Infill drilling, sampling and laboratory testing; refinement of database and 3D geotechnical model Ongoing maintenance of database and 3D geotechnical model
Strength of structural defect Literature values supplemented by index tests on core from geotechnical drilling Laboratory direct shear tests of saw cut and defect samples selected from targeted mine scale drill holes and outcrops; database established; assessment of defect strength within initial structural domains Targeted sampling and laboratory testing; enhancement of database; detailed assessment and establishment of defect strengths within structural domains Selected sampling and laboratory testing and refinement of database Ongoing maintenance of database
Geotechnical characterization Pertinent regional information; geotechnical assessment of advanced exploration data Assessment and compilation of initial mine scale geotechnical data; preparation of initial geotechnical database and 3D model Ongoing assessment and compilation of all new mine scale geotechnical data; enhancement of geotechnical database and 3D model Refinement of geotechnical database and 3D model Ongoing maintenance of geotechnical database and 3D model

Table 1 summarizes geotechnical investigation techniques that apply to different levels of design, with the main difference being the level of effort expended to achieve a required confidence level in the answers being obtained. The primary methods of data collection involve core logging, mapping, and various types of downhole surveys. These methods are discussed in greater detail in the following sections. Where applicable, additional references on more specialized techniques not commonly used have also been provided for further clarification, and for brevity of presentation.

The primary objective of the geotechnical investigation is to collect enough information to support the desired level of study. Prior to undertaking the investigation the following questions must be addressed:

1. What are the targets?

There must be sufficient geotechnical information from the rocks comprising both the ore body and the host rock(s). The amount of drilling, mapping and sampling depends on the number of different rock types that will be encountered in the proposed mine and/or left remaining in the areas around the stopes. The variability of the different rock types will also determine how many samples of each are required to adequately characterize the rock mass.

2.What is appropriate for drillhole placement and direction?

Access restrictions or topographic constraints may limit the direction in which holes can be drilled, resulting in blind zones, data bias, or shortage of data for key targets.

3. What is the most appropriate method to collect information in those targets?

The rock quality will dictate which core orientation techniques can be utilized. For example, more competent rock will allow the core to be pieced back together (e.g. imprint methods), whereas poorer quality rock will require some sort of downhole survey to allow the geologic structures to be inspected in-situ.

4. What types of downhole testing are required?

Hydrogeologic testing, in-situ stress testing, and geophysical logging requires specialized equipment. The type and size of the equipment may dictate the size of the drillhole, or may impose limitations on the drilling equipment that can be used, or require that additional drilling equipment not normally stocked by diamond drillers be brought to the site.

5. Which rock mass classification method should be used?

Numerous classification systems exist. Depending on the proposed mining method, different rock mass classification systems may be required to properly characterize the rock so that the mining method can be optimized.

6. What is a representative sample size?

Drilling of small diameter core may not provide sufficient information if an understanding of large scale roughness or continuity of the specific geologic structures is critical to the design of the excavation. Mapping of adits and cross-cuts may be necessary to achieve greater certainty in design.

7. Who collects the data (e.g. geologist, engineer, or technician) and what level of detail is required for the mine design?

For example, detailed assessments and characterization of alteration type, if considered critical to the design, may require specialized geologic expertise or training, whereas simple measurements, such as rock quality designation or point load index, may only require limited guidance and can be carried out by technicians.

See also

Discontinuity characterization
Geotechnical logging techniques
Geotechnical model
Geotechnical drilling

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