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Hit the ground running

An industrial site sounds like the ideal place to relocate a warehouse. But one such site required a not so straightforward approach to improving the ground. Robert Hutchison reports.

Industrial estates on the fringes of major cities typically comprise prime locations with respect to transport links and regional positioning to major conurbations.

As an established commercial and industrial area, Holbrook Industrial Estate, to the south of Sheffield, in the north of England and within the M1 motorway corridor, is one such example.

Evolution Power Tools chose a site on the estate to relocate its European distribution warehouse and act as new headquarters and offices for its international tools business.

The proposed development, designed by consultant Opus Joynes Pike, consists of three warehouse units – up to four storeys – with high bay racking and onerous infrastructure loading.

The consultant and development team faced the challenge of redeveloping the site, both economically and sustainably, within severe time constraints; international consignments were planned to arrive at the new warehouse facility before construction works had even begun.

The challenges
The site presented a number of key challenges:

- Although only 1.4ha, the site sloped at an angle of 1 in 10, resulting in a 10m drop in level from the north to the south boundary. The nature of the warehousing and offices required that a level development platform had to be created

- The south part of the site was historically part of the Cow Lane opencast site, which was worked between 1949 and 1951. Abandonment plans showed that a buried high wall was present within the site boundary

- The site had been used as a disposal point for surplus soil created by the development of the remainder of the industrial site. The nature of these soils were generally unknown and almost impossible to delineate

- Taking the site levels into account, there was a surplus of soil. This meant either abnormally high retaining walls would have to be constructed along the south boundary, which would conflict with maximum gradients on the access roads, or that along the north boundary retention would be necessary, which would clash with the office and warehouse units.

An extension of a philosophy
The overall philosophy of ground improvement is well known and given the current climate of brownfield development, vibro stone columns are the normal solution to improving ground. However, sometimes single, or even multiple, traditional ground improvement techniques may not be viable.

Looking at the development issues, the design team concluded that a range of ground improvement and ground enhancement techniques would be required, along with re-masterplanning the chronology (or programme) of the development.

Ground conditions
Overall, the natural ground conditions comprised of a brown, gravelly clay overlying rockhead. The depth of natural overburden varied between 1m and 4m. Rockhead level also dipped from the west to the east at 1 in 20.

The rock consisted of coal measures strata (rock with multiple coal seams). Within the site area, two coal seams sub-cropped – the main Dunsil seam and a lower thin floor coal, entitled Cannal for the purposes of the development.

The Dunsil seam typically consists of a minor leaf or band and a thin floor coal about 100mm below it. These had a thickness of 1m and 300mm respectively, and were separated by about 900mm of grey clay.

On top of the natural strata lay various made-ground deposits from the previous development of the industrial estate.

The design solution and programme
Opus Joynes Pike proposed the following ground and site improvement techniques to overcome the various design challenges:

- The first step involved stripping the brown, gravelly clay for use in building a reinforced soil (RS) retaining wall along the south boundary of the site

- Site workers used the brown, gravelly clay as engineering fill behind the RS retaining wall to raise the site levels to support the structures

- Once the overburden had been stripped from the north of the site, the team extracted and sold the two coal seams, creating a reduction in levels across the north of the site

- During coal extraction and the bulk cut and fill, the team sorted and screened earthwork materials for use in further ground improvement processes

- After completing the main bulk of earthworks, site workers carried out a programme of lime/cement modification and lime/cement stabilisation to make use of remaining materials (the majority of which would have been traditionally classed as unsuitable for earthworks)
- The programme was completed in three main phases. The first being the bulk earthworks and construction of the RS retaining wall. This allowed the first unit to be constructed. The second phase involved the stabilisation works, which allowed the main warehouse and last unit to be built.The third phase comprised the earthworks and retention works for the last unit in the south west of the site.

Specific design issues and benefits

Detailed investigation and testing for the earthworks proved that the brown, gravelly clay was a Class 2B (dry cohesive fill) material in accordance with The Manual of Contract Documents for Highway Works Volume 1 – Specification for Highway Works, Series 600 – Earthworks.

The investigation proved that the typical undrained shear strength for the material varied between 45kPa and 65kPa. The consultant decided that re-engineering this material provided an opportunity to improve the clay in two ways.

The first increased the soil's moisture content as it was excavated and re-compacted. This allowed a lower air voids content to be achieved and had the added benefit of enabling the use of plant that imparted a lower compactive effort.

Second, computation of a traditional bearing capacity for the clay material in situ would have been based on limited in situ testing, lab testing and incorporating a factor of safety on the ultimate limit state. Re-engineering the material allowed more in situ testing to be carried out on the final product, including load-settlement analyses of the material.

This enabled a much more realistic approach for the design of the allowable bearing capacity of the material. The team also back-analysed load-settlement results to provide equivalent undrained shear strength of the material following re-engineering. The strengths were calculated at between 70kPa and 100kPa – a theoretical average improvement of over 50% (theoretical as improvements were also made on the design as well as the materials side).

At first the coal presented the design team with another challenge. Where coal was near the surface there exists a risk of spontaneous combustion through oxygenation in contact with the atmosphere. The team found a simple solution to this problem, which involved extracting the two coals from the ground.

This gave rise to a number of benefits – enabling overall site levels to be reduced by 1.3m, thereby dealing with the problem of surplus soils. It also allowed the coal to be sold to a local power station, which not only gave rise to power production but also provided income to help fund the earthworks. In all, the works resulted in over 8000t of site-won coal.

But even with this reduction in levels, the cut and fill exercise totalled 40,000m3.Lime/cement modification/improvement
By the beginning of 2007, approximately 70% of the cut and fill exercise had been completed. The remaining 30% consisted of materials that traditionally would not be incorporated into earthworks.

The breakdown of this 30% was as follows:

- 4000m3 of highly organic clay – this material had a mean organic content of 10% with an extreme of 20%

- 500m3 of topsoil – workers discovered this during the earthworks as a buried mound that had been deposited from a site strip elsewhere on the estate

- 5000m3 of a material that could be technically graded as a 6F1 material. However, the more granular materials were held in a very soft clay matrix that made the workability of the soil almost impossible

- 2200m3 of very soft, sloppy brown and grey clay – typically generated by using a certain amount of site-won materials for sacrificial activities such as trafficking and sealing during the winter months.

Opus Joynes Pike worked together with The Independent Stabilising Company and Mid Sussex Testing to design the programme of lime/cement modification and lime/cement stabilisation. Between January and March last year, they carried out two rounds of testing on various stockpiled soils to account for the organic content of the materials and design appropriate binders.

Test results led to four different design binder mixes for the various materials. As a result, site workers carried out lime modification using 2% to 3% lime (the higher percentage in wetter material) and stabilisation using 2%/4% and 3%/7% lime/cement mix (with the higher lime/cement content in the worst soils).

This allowed all materials, apart from the topsoil, which was segregated for landscaping, to be used on site. It also created a Type 1 replacement material – to be used under the main floor slabs, car parks and hardstandings – that further negated the need to import materials.

Site workers set up an area of overfilling so all materials could be incorporated into the site works at the appropriate time and be modified and/or stabilised on site. This ensured that when the third-phase earthworks concluded the works on the third unit, materials would be suitable for inclusion. The stabilisation and modification works were undertaken between March and July 2007 with a contract value of £265,000.

Retaining wall
Along the south boundary the RS retaining wall is 135m long and retains to a maximum height of 7.5m. Its ability to be constructed over the buried high wall and the changing ground conditions on site is another reason the team chose this option. Strain was imparted to the geogrids during construction to minimise post-construction strains. In addition, the wall was designed as if it was a bridge abutment and post-construction strains on the geogrids were limited to less than 0.5%.

The consultant faced a particularly difficult challenge with the retaining wall as the footprint of the building ended less than 2.5m from the top of the wall. This change in levels, not only caused a problem with the pressures imparted by the building to the geogrids forming the wall, but also with differential settlement.

The team overcame these difficulties by building the structural foundations into the reinforced earth retaining wall. This in itself presented some issues with regard to the durability of the geogrids for the high pHs experienced when in contact with the concrete. But the use of Huesker Fortrac M grids, affording a higher chemical protection than normal, overcame the issue.

Site workers built the foundations at various levels throughout the building, generally rising in elevation from south to north. The settlements were calculated for each gridline of the structure with the next gridline being founded at an increased level above rockhead, within the maximum tolerable differential settlement limits of the structure.

Project conclusion
The consultant finished the third phase of the works at Holbrook in March. By this time, the first two units, including the main warehouse, were operational and accepting deliveries from national and international distribution points.

The site proved challenging and stretched the design and construction teams to the maximum. It has proven traditional ground improvement techniques can be applied to challenging sites and infrequently encountered engineering situations.

But these need to be carried out in conjunction with variations in the construction programme and the general logistics of the overall site, and not just be applied to individual areas, as may be the case on some more straightforward sites. The cost savings – because of the nature of the approach taken and its flexibility – is estimated at over £180,000.

Robert Hutchison is associate director at Opus Joynes Pike.

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