The Gunwharf Quays area of Portsmouth has been in constant use for many hundreds of years. In this instance, Dean and Dyball, contracting for Berkley Homes was retained to construct a Piling Mat over a 550m2 area to a depth of 900mm as part of a new development – giving a total excavated area of 4950m3.
It was originally envisaged that around 10,000 tonnes of spoil would be removed from the site and transport local landfill, being replaced with Type1 MOT stone that would achieve an engineering performance of 30% CBR as required to construct the piling mat. However, upon investigation, while 4000 tonnes of spoil could be taken to a local landfill site, it was discovered that 6000 tonnes was contaminated with mixed metals and therefore classed as ‘hazardous waste’. The problem was that the nearest hazardous landfill site is located in Swindon, approximately 90 miles away.
Upon investigation the chosen solution was to batch mix SMR onsite with the existing spoil; drying it out, bonding it together with all the particles in the spoil and subsequently compacting it to create a sub-layer with an engineering performance in excess of the required 30% CBR.
Upon testing it was calculated that one and a half 25KG bags of SMR were needed per cubic metre to treat the excavated spoil and achieve the desired result. Therefore 7425 25KG bags (a total of 186 tonnes) of SMR were needed. All spoil would remain on site and all re used.
Disposal of contaminated spoil at £100 a tonne x 6000 tonnes = £600,000
Disposal of uncontaminated spoil at £10 a tonne x 4000 tonnes = £40,000
Replacement Type1 MOT stone at £15 a tonne x 10,000 = £150,000
TOTAL = £790,000 plus associated transportation costs
186 tonnes of SMR at £284 a tonne = £52,824
Other costs (labour, plant, fuel): £20,000
The SMR was thoroughly mixed with the excavated spoil using a 30 tonne, 360 degree excavator with one bag of SMR being added per bucket load. The curling action of the bucket was all that was needed to mix the SMR into the excavated spoil. The SMR treated spoil was then returned and compacted using a Bomag 140 Roller to achieve the required 550m2 Piling Mat.
The sub-layer was also analysed following compaction to ensure it met the required 30% CBR and testing showed that an average CBR of 500% was achieved – far in excess of what was needed.
Opting for the SMR methodology over traditional method resulted in a number of significant benefits:
Improved engineering performance: While a 30% CBR was required for the Piling Mat on this project, testing showed that having used the SMR methodology an average CBR of 500% was achieved.
Reduced lorry movements: by retaining and reusing all the spoil onsite an estimated 999 lorry movements were saved.
Contaminants locked in: As all the spoil, including the 6000 tonnes deemed to be contaminated with mixed metals was re-used on the project; this negated the need for expensive disposal.
Zero spoil to landfill: By diverting all 10,000 tonnes from the envisaged landfill, significant environmental savings were achieved, helping to significantly minimise the Carbon footprint and impact for the project.