Ground improvement for a big new retail development in Glasgow has been assured by a combination of high energy impact compaction and continual monitoring.
Auchinlea Retail Park is a huge, £44M shopping centre taking shape in Glasgow. Its 60 units will be spread over 27ha at Easterhouse, off Junction 10 of the M8 motorway to the east of the city.
Landpac Ground Engineering recently completed a five-month ground improvement contract at Auchinlea, working for main contractor Laing O'Rourke (Scotland).
Work involved insitu improvement of 20ha of ground to a depth of 3m and compaction of 250,000m 3of cut to fill material using Landpac's High Energy Impact Compaction (HEIC) system.
Geotechnical contractor Van Elle had carried out deeper treatment work on the numerous abandoned shallow coal mines underlying part of the site last summer (GE August 2003).
Landpac's treatment area was previously a golf course and is underlain by a backfilled dolerite quarry. Site investigations revealed the backfill to be up to 17m thick and highly variable - a mixture of brick, ash, clay, gravel, metal and concrete.
Treated ground had to be able to have a bearing pressure of 35kPa beneath buildings, 20kPa beneath roads and 10kPa beneath car parking areas.
HEIC is a dynamic compaction technique that uses a pair of linked three- or five-sided drums pulled by a tracked machine. As the drums rotate, their flat sides repeatedly strike the ground to create a compacted layer.The hollow drums weigh between 10t and 12t and have drop heights of 150- 230mm, with a depth of influence of 2-4m.
Compaction was controlled using Landpac's continuous impact response (CIR) monitoring system, surface level surveys carried out after each batch of 10 surface 'coverages' and postcompaction verification testing.
Between 30 and 60 coverages were used across the treatment area. Level surveys allowed calculation of average induced settlements after each set of 10 coverages.
Landpac Ground Engineering managing director Dermot Kelly says: 'As a general rule the amount of settlement induced decreases with increasing numbers of surface coverages until a point is reached when the average induced settlements no longer decreases.'
HEIC is carried out until induced settlements between each set of 10 surface coverages are less than a figure set at the start of the contract.
On the Glasgow job, this figure was 20mm, although this was revised to 30mm during the project - Landpac concluded that less stringent induced settlement could be used to control work.
Induced settlement for the final set of 10 surface coverages was generally between 10mm and 30mm, Kelly says.
The CIR monitoring system records the rate of deceleration of the drum as it impacts the ground, and the location of each impact is recorded by an on-board global positioning system.
These measurements are used to produce a digital CIR plot that shows the deceleration of the impact drum at any point on the site. The ranges of the plot are set and correlated to site materials and expressed in terms of gravitational acceleration, g.
Kelly explains that CIR is used to highlight areas of concern: 'For example, if the impact drum records a deceleration of 2g in area A and 6g in area B, this means the drum takes more time to stop moving downwards in area A than in area B. This provides a broad indication that the materials in area A are not as hard (or stiff ) as those in area B.'
Areas that are relatively less stiff than other parts of the site can be identified for further investigation.
At Auchinlea, Landpac provided CIR digital plots to identify the soft spots needing more treatment. Figure 1 shows the condition of the site as recorded by the CIR system after the initial HEIC surface coverages. This shows decelerations were generally lower (less than 4g) in the northern half of the site (the red areas) than in the southern half, indicated by yellow (4-6g) and green (6-50g).
Ground conditions recorded by the CIR monitoring system after HEIC work was finished are shown on Figure 2. This shows a general increase in the rate of deceleration of the impact drum (ie improvement). This improvement was most significant in the northern half.
As well as undertaking CIR, Landpac commissioned site investigation firm CL Associates to carry out a range of other verification tests, particularly in those areas remaining red after treatment.
CL Associates carried out 150 post-HEIC dynamic probes, 132 plate bearing tests and 13 zone tests across the site. Kelly says the results confirmed the required 3m depth of treatment and overall compliance with the specification.
A tracked-mounted Competitor rig was set up to carry out 'Super heavy' dynamic probing.
This uses a 63.5kg hammer falling 750mm, with a 50.5mm diameter cone.
No post-compaction probing was carried out in areas not treated by Landpac; where there had been significant amounts of 'soft spot' remediation (coarse stone fill used to remediate these areas would distort the post treatment blow counts, Kelly explains); or where areas had become inaccessible because of ongoing site work.
The tested red areas were all found to have met settlement requirements, Kelly says. The ground had to be able to support 350kPa under a 600mm plate, with settlement when loaded to 35kPa of not more than 2.5mm.
Without CIR monitoring, Kelly says the number of tests would have doubled. 'The small number of tests carried out were placed using the CIR system, which concentrated on the potential problem areas below ground level.
'The CIR monitoring reinforced the confidence and physical test data obtained and provided further confidence that the HEIC works satisfied ground improvement requirements, ' he adds.