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Mixed results

REMEDIATION

Geotechnical contractor Bachy Soletanche is using its Colmix process to contain heavy hydrocarbons at a site at Pumpherston, west of Edinburgh. The process involves mixing cement grout into soil, usually to strengthen ground, but in this case to immobilise the highly mobile contaminants.

Established in 1864 as home to Young's Paraffin and Light Mineral Oil Company, the site was developed because it is underlain by oil shale, which was mined and then heated to drive off the hydrocarbons it contained. These consisted of paraffin and wax, along with heavier hydrocarbons, mostly tar. The paraffin and wax were processed and sold, with the tar and burnt shale (locally known as 'blaes') as waste products. The tar was stored on site and the blaes piled up in massive red bunds.

When production of paraffin ceased in 1963 a detergent factory was set up, which itself closed in 1993. It caused further contamination, including groundwater pollution with detergent.

Edinburgh-based environmental engineer The Robertson Partnership has been working on the site since the mid-1980s and has been overseeing the £7.7M remediation programme since it began in 1993.

Detergent in the groundwater is being managed using a drainage and reed bed system. Two main drains, between 2.5m and 5.5m below ground level, run along the eastern and southern edges of the main site area, intercept groundwater flow and feed it into reed beds, where the detergent is removed.

Dealing with the tar is more involved. It is generally located in contamination 'hot spots' - including buried pipelines and storage tanks - but it is highly mobile and can soon migrate to the surface if not contained.

Close examination of the problem involved the research and development department of client BP, who acquired the site when it took over Young's. Robertson senior engineer George Kerr explains that a number of possible clean-up solutions were examined, including conventional dig and dump, insitu treatment and burning the tar at a local cement works.

It was decided to deal with the tar on site, as the other solutions were more costly. As ever, the choice was a balance between environmental considerations and cost, which largely depends on final use, in this case an extension to the neighbouring public golf course.

A number of trials were carried out on site, including bioremediation and immobilisation. For the lighter tars, bioremediation was found to be the most appropriate. These works were carried out by geoenvironmental contractor Celtic Technologies. Bachy Soletanche's Colmix process was chosen as the most suitable method for the heavier, more stable tars.

First developed in the late 1980s for strengthening a SNCF (French Railways) embankment, the Colmix method uses overlapping and contra-rotating hollow stem augers which break up and mix the soil with a cement binder to form homogenous soil/cement columns up to 25m long. Its primary use is to strengthen ground but it has also proved suitable for treating contaminated ground.

However, in an earlier contract carried out by another contractor, treating the tar insitu proved difficult, with the auger struggling to cope with underground obstructions. For this contract, the pure tar has been excavated, and along with tar from other sites, placed in two purpose-built repositories, which are now being treated and fixed using Colmix.

Both 20m by 75m repositories are 3.5m deep and and lie side by side in one corner of the site. They are lined with a 1m thick layer of clay and filled with alternating 150mm layers of blaes and 100mm layers of tar. These layers are being mixed with grout consisting of 50% PFA (supplied by Scottish Power) and 50% cement.

The system used at Pumpherston employs four augers in a square arrangement. While four augers have been used before, says Bachy Soletanche environmental manager Peter Barker, this is the first use of 750mm diameter augers (the largest used previously were 500mm diameter). The larger augers can be used because the nature of the ground is fully known and the columns are all short at only 3.5m.

The system is mounted on a modified Casagrande C50 B18 piling rig, although Barker says that the lighter Colmix arrangements can be attached to the arm of a backhoe excavator. The rig works in rows across both repositories at a time (ie 40m passes). Each successive row is slightly staggered to ensure all the material is mixed thoroughly.

It takes about 3.5 minutes to drill to the base of each column, with grout injected through the hollow stems of the augers as they travel down. Grout injection is computer controlled from four pumps linked to a batching plant and is continually adjusted to match the rate of penetration, ensuring the specified amount is mixed with the ground. Optimum injection rate is 425 litres/m, equivalent to about 300 litres/m3, which takes into account the overlap of the columns and the volume of the augers.

Once at the base, the grout is switched off and the auger direction reversed. As the augers are withdrawn, this reversal causes the soil/grout mix to be compacted. This process is slightly slower, taking about 4.5 minutes. Barker says that the mixing method keeps the amount of neat tar coming to the surface to a minimum. Compaction also minimises spoil at the ground surface, which reduces the health and safety risk to site operatives.

In all, 2142 columns will be formed to stabilise some 10,500m3 of material, 40% of which is pure tar. Each is roughly 1.2m square and overlaps neighbouring columns by 160mm. Typically 40 columns are formed every day, although Barker says that 54 have been installed in a single shift.

As the columns are completed, a mini excavator is used to level the area. After all the columns have been finished, the repositories will be capped with another 1m thick clay layer and the area landscaped to form a driving range for the new golf course. Work began in October last year and was due to finish as Ground Engineering went to press.

George Kerr explains that the ideal mix - the minimum amount of blaes needed to keep treated volumes as low as possible, together with the amount of cement needed to fix the tar - was obtained during the trials of the method on site. Samples of this mix were tested for density and strength and these results are used to empirically check Bachy's stabilisation work.

About 15 cores will be taken at the end of the job. These will allow a visual check of the mixing and then triaxial testing will be carried out to confirm that the columns meet the specified unconfined compressive strength of 75kPa. Density testing will also be carried out for comparison.

Bachy Soletanche is also checking columns throughout the contract. French subcontractor Sol Solution is carrying out strength testing of columns seven and ten days after they are formed using the Panda insitu dynamic penetrometer system. This lightweight, portable system uses steel rods tipped with an oversize sacrificial steel cone driven into the ground using a club hammer. For each blow, it calculates and displays the cone resistance and records the data to generate depth/resistance plots.

Barker says that the results so far have been good. 'The strength is higher than specified,' he says. The company also produces detailed drilling records of all the columns, showing penetration and grout injection rates, which gives an indication of the quality of mixing.

Kerr adds that this, and all other aspects of the remediation project, are also checked independently by the Scottish Environmental Protection Agency which is 'heavily involved'. 'SEPA's contaminated land team carry out monthly site checks and other testing is carried out as part of the site's waste management licence,' he says.

Barker says there is potential for using Colmix on other remediation jobs, especially as there is now a move away from traditional dig and dump. 'And with increased use, the cost should drop further as it becomes more competitive,' he adds.

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