Monday, March 24, 2008

Aggregate Supply

Gravel sources within a radius of about 160 km were sampled and tested. Within any general area, only one or two sources were sampled because the nature of gravel within each of these areas was generally very similar. These sources are contained within glaciofluvial outwash, deltaic, and ice-contact deposits laid down ~ 10 to 12 thousand years ago during deglaciation. Clast types within these deposits are derived mainly from Precambrian igneous (e.g. granite, gneiss) and minor sedimentary (e.g. argillite, sandstone) rocks. Materials were tested for grading, shape, resistance to abrasion using the Los Angeles Petro graphic Number, unit weight, and bulk relative density.

The samples from North Bay and Huntsville had the poorest shape because they contained the highest proportion of strongly foliated gneisses. The foliation gave the particles an elongated rather than cubical shape. The sample from Thessalon also had relatively poor shape due to the presence of the sedimentary rocks that often had a tabular shape.

The two MTO sources were of most interest. They were located within about 10 km of each other and had very similar geological composition; however, they were found in deposits of different glaciofluvial origins. The Deux Rivieres source was topographically lower than the Randon source by about 100 m and represented material from a relatively lower energy regime. The Randon source represented material that had been transported in a very high-energy environment. This was a coarse outwash deposit formed during a time of enormous water flow down the Ottawa River during the melting of glacial ice and discharge of water from glacial lakes.

The Randon pit contained large rounded boulders (up to 0.5 m diameter) that could only be transported in a high energy environment. The Random material had significantly better sphericity and rounding than any other source that was investigated. This was the material that was ultimately selected by the contractor for supply of the truck arrester bed gravel.

The supply of the arrester bed aggregate was governed by a special provision in the contract that specified the materials and defined the quality assurance process. After placement of material in the bed, it was to be covered to prevent the ingress of foreign materials during any subsequent construction work. Following production of material for the truck arrester bed from the Randon source, two series of tests were conducted to investigate the frozen strength of the aggregate.

Following installation of the truck arrester bed, full scale testing was made with a 4 axle, 20 tonne tractor-trailer combination owned by the Ministry of Transportation. The driver was highly experienced and wore a helmet. The trailer/truck combination was also fitted with anti-jackknife chains. The truck entered the bed at a variety of speeds and the bed performed as expected, stopping the truck in a short distance that increased as entry speed increased. Distance to full stop was measured. The rolling resistance was calculated as an average of about 0.25.

No comments: