Underground Roads Manual: Construction and Maintenance

Provide the means to improve underground road pavement construction and maintenance.

Summary of Situation

Good roads are essential in moving men and materials to and from the coal face. With the application of good pavement construction and maintenance practices, the following is achievable:

• increased personnel time at the coal face.
• reduced longwall transfer times
• reduced delays associated with stores and materials delivery and
• reduced road and vehicle related injuries
• high personnel morale and
• ultimately high mine productivites

Substantial costs are incurred importing road aggregate into underground mines. With the application of good pavement practices, these costs can be maintained.

Project Scope

The project included:

• field investigations into pavement degradation processes and assessment techniques (road profiles, penetrometer testing etc);
• traffic studies relating vehicle speeds to road conditions;
• a cost survey to identify potential savings with improved road conditions; and
• providing a series of construction and maintenance guidelines to improve pavement performance, reduce costs and increase productivity.

Conclusions

Potential savings

Longwall transfer delays due to poor roads can be directly related to a lost opportunity to cut coal. With an average daily output of 8000 tonnes of coal, a loss in revenue of $300,000 per day, directly related to poor longwall roads, can occur.

Potential production time increases are estimated in the order of 20 minutes/crew/shift, with improved road conditions. This can be quantified as an approximate 6% increase in available production time or a potential $650,000 pa increase in revenue.

Injury and compensation costs to the industry due to vehicle and road related incidents equate to in excess of $2.4 million per year.

Yearly equipment costs due to poor roads:

• Major overhauls $ 12,000-250,000;
• Tyres and Wheels $ 20,000-100,000;
• Maintenance $130,000-850,000;
• Expenditure on road construction and maintenance is from $200,000 to $750,000 pa per mine.

Given these indicative figures, the total cost of poor roads to an individual mine may be in excess of $1 million per year.

Road Behaviour

In dry areas, travelling conditions are generally with minor surfacing and maintenance. Rib coal is successfully used as a road base in these situations.

In wet areas, both hard and soft floor degradation is associated with submerged pavements, swillies, low pavement permeability and poor drainage.

Soft clay floors exhibit rutting in response to water induced subgrade strength reduction.

Hard stone floors exhibit high longitudinal roughness, potholing and stepped behaviour in response to traffic induced hydraulic pumping in rock joints.

Assessment

• pavement assessment can utilise visual observation of deterioration;
• process (rutting vs potholes, clay vs stone floors and water sources);
• road roughness measurements using longitudinal and cross sectional profiling;
• measurement of vehicle speeds;
• dynamic cone penetrometer (DCP) on soft clay floors; and
• jointing, unconfined compressive strength (UCS) and seismic velocity from core for hard stone floors.

Recommendations

Assessment

Use visual assessment of road degradation processes and water conditions as the initial means of pavement assessment.

Supplement visual assessments with quantifications of pavement performance and properties by:

• road roughness measurements using longitudinal and cross sectional profiling;
• measurement of vehicle speeds and vibrations;
• dynamic cone penetrometer (DCP) measurements on soft clay floors and;
• jointing UCS and seismic velocity from core for hard stone floors.

Mining Operations

Consider cutting the floor to achieve a design/survey grade.

Construction

Ensure effective pavement drainage by:

• shaping with crossfall (4.6%), longwall (0.5-5%), and table drains using grader (or equivalent); and
• establishing water collection sumps and pumping facilities (eg in cut-throughs at regular intervals along the roadway

Consider positioning of transport roads on the high side of the panel, orientated with sufficient longfall to allow effective drainage.

Where jointed hard stone floors exhibit a high roughness and water cannot be adequately controlled, cut or rip the floor to increase permeability enabling drainage beneath the pavement and generate aggregate, reducing the need for imported material.

Rib coal can be used as a low cost material on dry roadways.

On wet roads, the following options should be considered as a last resort:

• use of high quality, moisture insensitive imported aggregate;
• use geotextile fabrics, geogrids or geowebs to reduce required aggregate thickness and quality where necessary; or
• use a mat of tyres tied together combined with coarse aggregate, in deeper swillies.

Road Maintenance

Conduct regular road maintenance to preserve ride quality and pavement condition. Maintenance should incorporate:

• reshaping the pavement (grading);
• maintaining drainage systems;
• dust control;
• compaction; and
• resurfacing

Equipment Considerations

• use low tyre pressures where possible;
• standardise vehicle track widths;
• select vehicles with balanced load distribution characteristics.

Continuous miners:

• ensure horizon control instrumentation is effective; and
• consider a balanced load distribution during operations.