Innovative Assessment and Remediation of a Mine Shaft

Details:

Investigation, Assessment, Risk Appraisal, Survey, Remedial Design and Implementation of a historical 122m deep coal mine shaft in the Morriston area of Swansea.

Overview of Services Provided

• Review of historical, geological, mining, hydrogeological and investigation data;
• Detailed site reconnaissance;
• Coal Mining Risk Assessment;
• Ground Investigation – trial pitting, trenching, cable percussion and rotary boreholes;
• Field Monitoring;
• Implementing up to date risk assessments with the appropriate guidance (e.g. CIRIA SP32);
• Interpretive Assessment;
• Assessment of liability and cost risks;
• Options assessment considering the emerging condition for risk management;
• Innovative 3D Laser and 3D Sonar Surveys of the unsaturated and saturated portions of the shaft.
• Sourcing of suitable bulk infill material;
• Design, supervision and validation of remedial works.

Introduction

Earth Science Partnership were engaged to undertake a geotechnical stability and feasibility investigation for a proposed residential development in Morriston, Swansea. The development comprises 10no. properties on a 0.5Ha site known to have a history of coal mining with a 122m deep winding shaft present on the site.

History and Conditions

Historical coal mining activity was identified and a thin cap constructed at the top of a shaft in the central area was exposed during a trial pitting exercise.

General ground conditions comprised a significant horizon of Made Ground overlying Glacial Till and weathered Upper Coal Measures deposits.  The shaft was initially probed to a depth of 57m with only void space being encountered. No shallow coal mine workings are present (<30m).

Filling of the shaft occurred in the 1940’s, as detailed by the Coal Authority and it was inferred that the fill had settled to depths in excess of 57m (or the record of the filling operations was inaccurate). The shaft was capped in the 1950’s.

Commercial Barriers to Development

Since we began our involvement with this project, at least three different parties have owned the land, each in turn realising the significant commercial impacts associated with formal assessment and appropriate treatment of the mine shaft. The current site owners took a pragmatic view on the site issues and engaged us to initially scope a treatment approach, followed by a design strategy report. The client has tackled the mine shaft issue head-on and this has enabled a robust and suitable treatment approach to be implemented.

Investigation, Assessment and Risk Appraisal

Investigation methods employed include trial pits, cable percussion and rotary boreholes, followed by detailed site assessment, slope stability analysis and geoenvironmental assessment.

As part of the design strategy, further information on the shaft condition and geometry was required and this was obtained through the use of 3D laser and sonar surveys. This enabled predictions of fill volumes and confidence in treatment process. The shaft map is visualised below.

3D Laser & Sonar Survey: unsaturated upper (laser to the left) & saturated lower (sonar to the right):

 

The survey data enabled us to produce the following summary of shaft geometry and treatment requirements:

• Shaft diameter of ~2m, with local widening up to 5m;
• Shaft void of ~67m;
• Potential void volume of ~200 to ~500m³;
• Groundwater ~35m below ground level.

Remedial Design

Key factors to consider in the remedial design were:

• The presence of any near surface mine workings.
• Potential superficial soil stability;
• The nature of and mitigation measures for the hazardous conditions around proven shaft;
• Treatment measures required to prevent catastrophic failure; this encompassed an engineered cap, granular fill and grout plug;
• Grouting up to 10% void space within a defined horizon of the bulk fill.

Sourcing bulk infill, framework for use & materials management

Bulk infill was sourced from a local fixed Soil Treatment Facility that enables the Recovery and Reuse of materials utilising the Cl:AIRE CoP, as illustrated below:

• The Code of Practice facilitates recovery and appropriate re-use of soils without the need for onerous and expensive waste controls;
• This can be via an on-site, or cluster / hub scenario;
• Undertaken using a carefully planned, documented (MMP) and verified (CoP Qualified Person) operation.

 

The four main factors to consider in enabling this approach are:

• No unacceptable risk to health or environment;
• Suitable for use without further treatment;
• There is certainty of use;
• The quantities are necessary/needed.

The materials comprised engineering fill, mainly a sandy gravel:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The works were undertaken using a carefully planned, documented (MMP) and verified (CoP Qualified Person) operation. The approach was approved and signed-off by a CL:AIRE Qualified Person.

Implementation

Implementation of the shaft treatment commenced with the main steps comprising:

1. Shaft Bulk Fill with Engineering Fill.
2. Grouting of a specified zone of the Engineering Fill to limit settlements.
3. Capping design and implementation is to occur as part of the construction stage.

Validation of the works will be implemented following the reinforced concrete cap construction. The filling operations whilst in progress can be seen below:

The site is planned to be lower than current levels, so it is intended that the piled cap will be constructed at a lower elevation.

Benefits and Outcomes

The blight of historical coal mining has been overcome through:

– De-risking the project through data collection and predictive modelling;
– Low cost engineering fill availability;
– Material Management Plans and Qualified Person input;
– Minimally intrusive 3D laser and sonar survey;
– Bespoke settlement and material characteristic/performance assessment to enable efficient and targeted use of grouting.

Significant social, economic and environmental benefits have been realised, e.g. reduced use of virgin aggregates, carbon footprint and creating affordable homes. The residual land value has increased significantly as a result of our input – if you have a project that is affected bu historical coal mining hazards, please get in touch with either Matthew Eynon or Giles Sommerwill.