Remediation of Galligu Contaminated soil with Powercem and cement - The Redevelopment of Hutchinson Street, Widnes -

News release: Background
The Borough of Halton lies centrally within an area known as the Mersey Belt. This area was at the forefront of the Victorian industrial revolution and consequently developed a significant chemical manufacturing base and a contaminated land legacy. In particular Halton was home to the Le Blanc chemical process, a process that converted common salt into soda ash using sulphuric acid, limestone and coal. The waste by-products from this process were highly toxic. The development of Hutchinson Street, where two Le Blanc process plants existed in the late nineteenth century, into a site with recreational facilities has required a novel approach to the remediation process.

Site Investigation
Typical Leblanc alkali waste deposits, or "Galligu" were found in the majority of locations on the site. The original Ground Investigation reported moisture contents for the Leblanc waste ranging from 31% to 138%, with a range of 40 to 60% being the typical value. Indiscriminate waste tipping over the years had added to the problem, and, as a result the land was classified as highly or very highly contaminated. Contamination included; arsenic, lead, zinc, cadmium, copper, sulphurous waste deposits (galligu), nickel and chromium. The pH of the contaminated fill material varied from 2.6 to 12.2 pH. Typically, the low pH values were recorded in the upper 1.0 to 2.0m of fill.

One of the main environmental concerns about the contaminated fill was the level of heavy metal contamination, particularly arsenic, cadmium, copper and zinc. At low pH values (typically less than 7 pH), the solubility and mobility of these substances is high. This combination of factors can result in the heavy metal being easily and readily dissolved and mobilised, resulting in a high leachability potential that would pose a risk of pollution to the River Mersey, located only 500m away from the site

Solution
To prevent heavy metal pollution of the Mersey via cross-contamination as a result of surface water infiltration, a dig and dump solution was considered, however, this was rejected due to the volume of contaminated material requiring removal (approximately 87,000m3). A more sustainable solution was therefore investigated.
Several remediation techniques were investigated, however, an in-situ stabilisation and solidification (s/s) technique was opted for finally, as this was the most cost effective. Due to the high levels of contamination an ordinary cement binder was not considered appropriate to result in adequate contaminant encapsulation. Therefore a Powercem/cement binder matrix was adopted. Powercem is a specialist secondary additive developed by Powerbetter Environmental Services to enhance the s/s process where heavily contaminated sites are encountered.

The Powercem/cement binder was utilised to treat the contaminated galligu soil to create a 350mm deep impermeable cap to prevent surface water infiltration. On top of the cap an imported reclaimed granular material, geotextile, sand and topsoil was placed in order to develop the recreational facilities.

Performance
Initial site investigation revealed the following; Strength of the galligu soil varied from 0-3% CBR, heavy metal concentrations were very high with leachate extracts from lead (1,290 µg/l), zinc (1,570 µg/l) and arsenic (580 µg/l) being classified as excessively high. Based on these initial soil properties the end criteria of the treated capping layer, developed by Halton Borough Council, were;

  • To reduce the permeability of the Galligu contaminated soil to at least 10-8
  • To increase the strength of the Galligu soil to achieve at least a soaked CBR strength of 15%
  • To reduce the leachate potential of the Galligu soil and thus the risk to water courses and the River Mersey.

Tests undertaken seven days after remediation operations to validate the process demonstrated the following;

  • Soaked CBR strengths ranged from 50-85%
  • Permeability of the treated Galligu soil varied from 0.13 x 10-9 - 0.86 x 10-9
  • Leachate values for arsenic (<1µg/l), cadmium (<1µg/l), chromium (10µg/l), lead (<10µg/l), zinc (19 - 250µg/l), Copper (20 - 200µg/l), Selenium (<8µg/l) and Nickel (<30µg/l, with one at 110µg/l) were less than the maximum concentration values stipulated in the Water Supply (Water Quality) Regulations 1989 of England and Wales.

Summary
The Powercem and cement treated Galligu soil achieved and exceeded the clients requirements.
A sustainable solution was achieved that minimised landfilling and import of inert material.
Lorry movements were reduced from an estimated 12,500 (remove-replace) to just 32 (cement deliveries).
The development costs of the site were reduced by £600,000 when compared to the remove and replace option.



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