PRESTON WESTERN DISTRIBUTOR ROAD (PHASE 1-4)

The scheme also includes two new roads connecting to new and existing housing areas of North West Preston and Cottam. The new East-West Link Road and Cottam Link Road will provide convenient access to the Preston Western Distributor, avoiding already congested local routes.

Due to the key properties of LECA LWA including reduced compaction rates, cost effectiveness, low density and phi values, LECA LWA was selected as a key engineering solution for this ambitious redeveloping project. 
 

There were many proposed materials available for the structural backfill requirements for this project and this included traditional rock aggregates and alternative solutions, but as with many projects where LECA has been specified as a design solution.
 

Leca® LWA can be placed in the same way as other conventional aggregates. This can be either by tipping and spreading using a tracked dozer, or tracked excavator. However, a value adding property (unique to LECA LWA) is the ability for vehicles to continue operations over the placed material without the risk of potential surface crushing of the placed material. Compaction of Leca® LWA material requires substantially less compaction effort when compared to conventional fills - layer depths can be greater, typically up to 1000mm. Compaction should be conducted using a vibrating plate compactor with a maximum ground bearing pressure of 50kN/m². Typically, 3-4 passes of this vehicle are required to limit potential surface crushing of the Leca® LWA compaction layer. Compaction is expected to provide a reduction in volume of 8-12% with the average being approximately 10%. 

This ease of compaction, following the placement of the material through the traditional placement method of using a combination of Excavator/ Dozer, provided further value for the engineering team at Costain, Rosey Thurling goes onto explain, “Following the guidance from the manufactures instructions the material was easy to place, it can be tripped straight from the delivery wagon into position, and compacted with just a few passes of a standard tracked machine. The ordering and delivery system was also easy to arrange and any questions were answered quickly from the knowledgeable team at LECA.” 

Low Density Design Solution Properties:

The specification of Leca® LWA on large scale cut and fill operations and construction on soft soils or bad ground can overcome stability problems, reducing the risk of landslide and deformation. When used in road embankments, Leca® LWA exerts much lower horizontal earth pressures compared with other backfill materials, helps improve stability and reduce the need for counterfill.

The developers recognised that Leca® LWA has an installed bulk density of approximately 20% of that of general fill materials and will considerably reduce settlement of the road or rail carriageway, both immediately and in the long term. Widening and replacing existing carriageways is also simplified by using Leca® LWA with no specialist techniques necessary. Because the compaction of Leca® LWA material requires substantially less compaction effort when compared to conventional fills - layer depths can be greater, typically up to 1000mm. “The main property for the use of LECA on PWDR was its low density enabling the structural design to be as efficient as possible. The phi value (angle of repose) of the material was also an important factor when choosing this material for use at Becconsall Bridge in particular.”  

Confidence was established when similar case studies were provided which presented the effectiveness of LECA LWA in similar scale bridge projects, when installed behind abutments, including the FARRR’s project where 27,000 m3 was installed in a MSE system. And a similar bridge refurbishment project, where the old back fil had overloaded the abutments, causing the bridge deck to hogg – creating a bridge failure - the solution was to remove and replace the abutment back fill with the Leca®LWA 10-20mmR.