Mining companies that are planning shaft construction to access salt and potash deposits typically require grouting work to control water-bearing formations that will be encountered during shaft sinking operations. The following commentary provides relevant guidelines for consideration when planning for grouting activities.
It is anticipated that multiple holes will be drilled from the surface to pre-condition permeable formations and adjacent rock formations in proximity to any mine shafts.
The objective of surface drilling and grouting for pre-conditioning of major aquifer formations is to intercept and seal existing permeable formations, thereby achieving permeability reduction in advance of shaft excavation.
While grouting from the surface is not expected to dissipate in-situ formation pressures to be encountered while shaft sinking, potential inflow volumes are expected to be significantly reduced, with a corresponding time/cost reduction for in-shaft grouting activities.
Approximately 8-12 in-shaft grout curtains will typically be constructed at various depths within each shaft as shaft sinking proceeds, with a typical duration of 3 weeks at each shaft grout curtain location.
Depending upon specific conditions to be encountered at various depths, one or more rings of cover grout holes will be drilled and grouted to depths of 40 to 60 m below shaft bottom.
Due to anticipated formation pressures and ground conditions, standpipes will be installed to depths of 8-9 m, drilled out and pressure tested up to 14,000 kPa prior to drilling to ultimate depth.
All primary, secondary and tertiary standpipes for each ring will be installed and must satisfy pressure test criterion prior to advancing any cover holes.
Standpipes will be equipped with suitable BOP devices prior to advancing cover holes.
After all standpipes have been installed and successfully pressure tested, a selected set of primary cover holes will be advanced for testing and grouting purposes. Subsequent sets of primary, secondary and tertiary cover holes will only be drilled after completion of all initial primary cover hole drilling/grouting operations.
Depending upon conditions encountered, it is anticipated that each cover hole will be drilled, flushed, tested and grouted more than once, possibly several times.
Upon completion of grouting at each cover hole, such holes will be drilled out for a final time and then abandoned using a specific grout mix intended for this purpose.
All grouting plant and equipment will be situated on the surface adjacent to the shaft collar.
Grouting slurries and fluids will be transferred to shaft bottom using dedicated high-pressure product lines.
Shaft bottom grouting activities will include connection of product lines through high-pressure manifolds, monitoring of grouting parameters and valve operations as directed by the grouting supervisor.
Grouting plant will incorporate electronic flow and pressure measurement devices and transmitters, video/audio communications with shaft bottom crew, real-time computer monitoring, data acquisition, software for assessment of flow/pressure trends and remote equipment operating controls at a dedicated surface location.
All grouting operational decisions will be taken by the grouting supervisor in consultation with engineers and technicians situated at the dedicated surface location.
It is anticipated that pre-conditioning through surface drill holes of permeable ground in advance of shaft sinking will utilize precipitation grouting materials consisting of super-saturated solutions with precipitation inhibitors that will provide for gradual crystallization of calcium carbonate or other precipitate materials within the grouted formation as the inhibitors dissipate over time.
Precipitation grouting ingredients are expected to be chemically similar to dissolved minerals that presently exist within the formation.
In-shaft grouting operations are expected to utilize the following types of grouting materials:
Slag-based ultrafine cement with maximum particle size of 10 microns and median particle size of 3 microns for grouting of permeable rock fissures
Acrylamide chemical grout with reaction inhibitor to facilitate penetration within low permeable zones prior to material solidification
Water-activated, hydrophobic urethane grouts for special grouting applications involving cut-off of residual water inflows
Cement-silicate blends for special grouting applications where rapid set times are required
Conventional cements and additives for routine grouting applications such as abandonment of cover grout holes
Grouting for shaft construction purposes incorporates various grouting technologies that have been well established over the past 40 years.
Ultrafine cements have been produced by several manufacturers over the past 30 years and are now widely used in heavy civil construction and mining applications.
Safe application of acrylamide chemical grout has become well established over the past 40 years with a proven record of successful and permanent applications in high-pressure, saline formation conditions.
Water-activated, hydrophobic urethane grouts have been manufactured by several manufacturers over the past 40 years and are now widely used in heavy civil construction and mining applications.
Significant progress has been made with cement-based grouting technology over the past 40 years, incorporating various admixtures and ingredients for enhanced standards of performance and durability.
All major grouting projects that involve groundwater control in complex site conditions utilize robust real-time monitoring and data acquisition equipment, with a central grouting control station staffed with qualified personnel to provide ongoing assessment of dynamic performance – and corresponding response – while drilling and grouting operations are underway.
Safe work protocols for all aspects of shaft sinking, including drilling and grouting operations, have been significantly enhanced over recent decades.
In addition to shaft miners and equipment operating personnel, shaft construction involves a significant team of multi-disciplined engineers, technicians and specialists to design, plan and oversee all aspects of drilling and grouting operations.
Drilling and grouting operations require substantial research and development to identify, test and prove candidate grouting materials and techniques that may have relevance for shaft construction for each shaft sinking project.
Critical technical skills and site specific knowledge required for execution and optimization of grouting operations will not be found externally but must be developed in-house by the shaft sinking contractor and other members of the project team.
The specific grouting methodologies that are required to support shaft construction will be unique to each site. It can be expected that site conditions and corresponding grouting methodologies will not be identical in any adjacent shafts, so grouting methodologies must be adapted accordingly.
Significant opportunity exists for time/cost savings through optimization of drilling and grouting operations based upon specific site conditions that will be encountered at each shaft bottom drilling/grouting location.
While certain aspects of grouting operations, such as standpipe installation, can be undertaken in accordance with required performance standards within a predictable period of time, drilling and grouting operations will encounter numerous and complex site conditions that will require prompt assessment and comprehension to develop an appropriate, time efficient and cost effective response.
For each unique set of site conditions that are encountered, it will be required to select an optimum drilling pattern – neither too many holes nor too few – as well as an optimum grouting cycle – neither too much grout volume nor too little.
Site specific shaft grouting operations cannot be arbitrarily prescribed in advance but must be continuously adapted in accordance with encountered site conditions.
Shaft grouting methodology for complex site conditions does not rely upon subjective assessments but upon rigorous analysis and technical decisions based on the available relevant information.
Peter White, P.E., is a senior engineer and grouting specialist with over 25 years of experience working with mine grouting projects around the world.