Clean up of an old gasworks in Scarborough by Sirius Group is being fast-tracked through the first UK use of new equipment from Komatsu.
Remediation of a site with limited working room to a tight timescale while improving the ground conditions is the tough challenge facing Sirius at an old gasworks site in the seaside Yorkshire town of Scarborough.
But despite the potential difficulties, Sirius technologies manager Elliott Haywood remains undaunted and is confident the project will be completed on time.
The key to this confidence is the UK debut of a Komatsu Reterra mobile materials reprocessor, which Haywood says is helping to stabilise and improve the contaminated material without dust and minimal noise.
The gasworks opened in 1893 and was closed in 1971, since when the site has had several owners and was initially used by Transco before the Scarborough Building Society (now part of Skipton Building Society) took over the offices. It is soon to be taken over by Scarborough Borough Council.
“Our client Skipton wanted to sell the site,” explains Haywood. “Some remediation was carried out in the 1990s but this only involved excavating and disposal of the material off site.
This targeted the process areas and worked around the existing structures to meet the guidelines at the time.
“Our client Skipton wanted to sell the site”
But to complete the sale of the site, work needed to be carried out to ensure the contamination was below the levels required by modern legislation.”
Sirius carried out a rigorous site investigation involving cable percussive techniques and trial pitting, after completing a review of existing information.
“This helped us to identify the remaining areas of residual contamination,” says Haywood. “There were pollutants that are typical for those found around gasworks that had the potential to leach into the groundwater and to reach receptors downstream from the site.”
The pollutants that caused most concern were total petroleum hydrocarbons (TPHs) poly-aromatic hydrocarbons (PAHs), benzene, toluene, ethylbenzene and xylene (BTEX) and phenol.
The geology at the site is formed by between 1.8m and 8m of made ground over glacial till or alluvial clay.
The till is cohesive in nature and generally consists of sandy clay, overlying clayey sands and gravels.
The alluvial clay features gravel inclusions becoming a fibrous peat with bands of laminated clay.
The investigation was completed at the end of last year and, with a buyer waiting to take possession of the site, Skipton was keen to fast-track the clean up.
Sirius presented a number of options, but the best in terms of timescale and remediation was using a solidification/stabilisation technique.
“It is a rapid process capable of achieving the required remedial targets in a very short time-frame,” explains Haywood. “The speed enabled the material to be excavated, treated and redeposited quickly so avoided the need to stockpile untreated and treated materials for a significant length of time.
“The alternative of disposal off-site would have been difficult, prohibitively expensive and unsustainable.
Bioremediation was not a viable option due to recalcitrant contaminants, space limitations and required programme.”
Sirius further improved the potential of the stabilisation approach by using the Komatsu Reterra soil improver to screen and add a binder to the extracted material before it is placed back into the excavation.
Once excavated, the material is inspected on site and further testing for contaminants is carried out as necessary.
The material is then separated into the three types expected at the site - stony cohesive, chalk-based and cement stabilised silty cohesive.
The stony and silty materials are placed back after processing in 275mm and 250mm layers respectively, and compacted with a Rammax RW3005 vibratory pad foot compactor with nine and 12 passes respectively at 7,083kg/m.
“The Reterra’s mixing chamber uses a soil cutter, three rotary hammers and an after-cutter that can instantly loosen and mix soil,” explains Haywood.
Although he would not reveal the actual binder used, he describes it as carbon additive with primary and secondary binders that “locks in” the contaminants and improves the geotechnical properties when placed back in compacted layers.
“We are very pleased with it and we’re already looking to use it on other projects”
According to Haywood, the main benefit of the Comets machine is its compact dimensions.
“Normally you’d have to use several different crushers and screeners to achieve the same result, but the Reterra is mounted on one chassis,” he says.
In addition to it being easier to use on site, the machine also generates less noise and dust compared to the multiple machine approach.
“We are currently halfway through the work and it has all gone well so far,” says Haywood. “The technique is tried and tested, but using the Reterra was a new factor. We are very pleased with it and we’re already looking to use it on other projects.”