Case Study – Lancashire CC Primary Care Trust, Colne

Client: Lancashire C.C

Value: £6.7m

Location: Colne, Lancashire

 

Brief Description

 

This former land fill site had difficult ground conditions, and had remained unused since the 1960’s. Through our tender winning design the principal contractor was able to achieve £660k of project savings, resulting in a successful bid. The development covered the whole 20,000 sqft of the site.

 

Project Challenges

 

  • Former domestic landfill site, unused since 1960’s

  • Drainage attenuation presented a problem

  • Major settlement with 6m of soft contaminated materials

  • Construction works adjacent to major river, presenting further contamination problems

 

Solution

 

Our design saved over £660k on build costs through:

 

  • Sustainable drainage system, with high BREAM rating

  • Dynamic Compaction of the large car park, avoiding piling.

  • Limited piling to building

  • Efficient and cost effective solution to accommodate excess rainfall in sub base of car park area

 

The Challenge

 

The site had difficult ground conditions. It was a former domestic landfill site that was last used in the 1960’s. As such, the site was contaminated and had over 6m of deep soft fill materialand a surface water pipe that was too small to accommodate our site surface water. In addition, the site was bounded by a stream and Pendle Water and the EA were concerned that leachates would pollute the water course. Therefore, the major issues that we would need to resolve were:

 

  • The new development covered the entire site area with a total impermeable area of almost 20,000 square meters, so drainage attenuation was a problem.

  • The depth of soft contaminated material presented problems for major settlement of the building and car park.

  • The construction works adjacent to a major river presented problems of contamination leaching into the watercourse.

 

Our Geo-Environmental Tender Winning Design

 

Our tender design resulted in a tender saving of £660K and enabled our client to win the job. This is how we did it.

 

  • Dynamic Compaction (DC) of the large car parking area, which avoided the piling solution that others had suggested. This would give a working platform, and limit future settlement.

  • Limit piling to the building only.

  • Changing the site levels to avoid any excavation into the tips capping layer and avoiding the need to cart away expensive material. We saved even more cost by specifying and approving imported material using crushed demolition material from a site just 4 Km away.

  • SUDs - To avoid large surface water storage tanks, we proposed to store the excess rainfall within the sub-base of the car park area. (15,630m2). We achieved this by draining the car park through the permeable block paving parking areas into 350mm layer of clean stone sat on top an impermeable barrier over the DC sub-base. This drainage proposal had almost no buried pipe work. Our calculations showed that little of this water will actually reach Pendle water. This design gave a major saving in cost and more importantly, the rain water is filtered through the stone and does not impact on flooding Pendle water.

  • The initial design called for a 6m deep leachate barrier around the site, which was logistically, almost impossible to construct around a 1.7ha site. However, by carrying out further site investigation into how the water moved through the site, we eliminated the need for a barrier.

 

The Benefits

 

  • A instant saving of £660K from the tender price.

  • No expensive underground piped or tank attenuation system to construct.

  • Minimal underground piped drainage that could be subject to long term displacement.

  • Sustainable drainage system for higher BREAM assessment.

  • Environmentally friendly as it provides pre-treatment of surface water as it flows through the clean stone.

  • No contaminated material or any excavated material was taken off site.

  • No risk of damaging in ground system by construction traffic

Welcome to PWA Geo-Environmental:

 

Leading Geotechnical, Geoenvironmental and Regeneration Specialists

 

What makes us different...

 

  • Addressing and understanding site abnormals

  • Significant savings on final build costs

  • Delivering time savings

  • Sustainable value engineering

  • Viability and realising full site potential

  • Strong technical background

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