Subbase and hydraulically bound materials (HBM)

Transcription

1 Subbase and hydraulically bound materials (HBM) Subbase and hydraulically bound materials (HBM) This section includes summary sheets for the following technical solutions: Treat existing soil to make HBM subbase/ballast; Recycled/secondary aggregates to make HBM subbase; Recycled/secondary aggregates as unbound subbase; and Geogrids/geotextiles to reduce thickness. 66 Designing out Waste - Part 2: Technical Solutions

2 Technical solution: Subbase and hydraulically bound materials (HBM) Treat existing soil to make HBM subbase/ballast Application: Construction and maintenance of railway ballast and base and subbase for pavements for highways, airports, utilities, harbours, docks and waterways, power generation and in the development of brownand greenfield sites. Design for Reuse and Recovery. Treating existing soil with binders/activators is known as stabilisation of the soil to form a bound layer. It may be a one stage or two stage process. The two stage process uses lime to break down cohesive soils and then they are treated as in the single stage process, for non-cohesive soils, by adding a binder. The binder may be cement, ground granulated blastfurnace slag (ggbs) or pulverised fuel ash (pfa) or a combination of cement with either ggbs or pfa or lime. This process is usually undertaken in situ but may be done ex-situ. The production of HBM requires higher levels of binders/activators than the production of capping or the drying out of soils to be suitable as general fill. Waste reduction: the use of the existing soil reduces the quantity of material sent to tip. Cost reduction: is cheaper to mix in-situ than off site, does not require primary aggregate. Recycled content: increases the recycled content of the scheme. Programme: no significant impact on programme. Carbon footprint: the use of the existing soil gives a saving on lorry movements and fuel. Maximising the use of pfa and ggbs compared to cement and lime reduces the carbon footprint of the solution. Other environmental benefits: reduction in congestion, noise, vibration and fumes by reduction in lorry movements, and valuable nonrenewable aggregate sources are not wasted. Reduced resource depletion. This process may be used as a subbase in any pavement construction or as a base in lightly trafficked pavements. Sulfates and sulfides in the soil can lead to expansive reactions with the binders and activators, so the soil should be checked thoroughly for these constituents. Lime and cement were used to stabilise insitu glacial till deposits to form a soil cement equivalent to Type 1 unbound subbase for one carriageway of a 24km scheme to upgrade the A120 in Essex between the M11 and the existing Braintree Bypass of_ra_and.html The solution is covered by BS EN Parts The methodology for stabilisation is given in the Specification for Highway Works, Clause 840, co.uk/mchw/vol1/pdfs/series_0800.pdf and the associated Notes for Guidance, Clause 840 discusses the testing procedures, vol2/pdfs/series_ng_0800.pdf Guidance on the use of stabilised materials in earthworks is given in the Appendix to Network Rail Model Clause 52 for Specifying Civil Engineering Works and generally takes the same approach to that in the Specification for Highway Works. There are two useful Britpave documents; Britpave BP/08 Technical data sheet Stabilised soils as subbase or base for roads and other pavements and Britpave BP/15 Soil stabilisation- Guidelines for best practice. Subbase and hydraulically bound materials (HBM) Designing out Waste - Part 2: Technical Solutions 67

3 Subbase and hydraulically bound materials (HBM) Technical solution: Subbase and hydraulically bound materials (HBM) Recycled/secondary aggregate to make HBM subbase Application: Construction and maintenance of subbase for pavements for highways, airports, utilities, railways, harbours, docks and waterways, power generation and in the development of brown- and greenfield sites. Design for Reuse and Recovery. Aggregate resulting from the processing of inorganic material previously used in construction, aggregate recovered from demolished concrete and aggregate from residues of industrial processes including mining. Typical materials include blastfurnace slag, burnt colliery spoil, china clay sand/stent, coal fly ash and pulverised fuel ash, foundry sand, furnace bottom ash, incinerator bottom ash aggregate, phosphoric slag, recycled aggregate, asphalt and concrete, recycled glass, slate aggregate, spent oil shale/blaise, steel slag and unburnt colliery spoil. Waste reduction: the use of recycled or secondary aggregates in HBM reduces waste and can lead to thinner layers being required compared to unbound subbase. Cost reduction: is cheaper than using primary aggregates. Recycled content: increases the recycled content of the scheme. Programme: no significant impact on programme. Carbon footprint: the use of waste materials are usually available locally, therefore there is a saving on transport in lorry movements and fuel and use of primary aggregates. Maximising the use of pfa and ggbs compared to cement and lime reduces the carbon footprint of the solution. Other environmental benefits: reduction in congestion, noise, vibration and fumes by reduction in lorry movements. Reduced resource depletion. Recycled/secondary aggregates may be used for hydraulic bound subbase beneath any pavements. A higher grade use of HBM is as base course for lightly trafficked pavements. Some of the secondary materials suitable for use in subbase and base are indicated in the Design Manual for Roads and Bridges, HD 35/04, co.uk/dmrb/vol7/section1/hd3504. pdf Guidance on the use of HBM in road foundations is given in Interim Advice Note 73/06 Revision 1 (2009) Design Guidance for Road Pavement Foundations (Draft HD25), available at co.uk/ians/pdfs/ian73rev1.pdf Use of HBM enables a foundation with higher stiffness to be produced, which can lead to reductions in the thickness of the subbase and overlying pavement layers if the design process in IAN73/06 is followed. For the subbase in pavements construction the aggregate requirements are indicated in the Specification for Highway Works, Series 800, mchw/vol1/pdfs/series_0800.pdf and the associated Notes for Guidance, Series 800, vol2/pdfs/series_ng_0800.pdf There are two useful WRAP documents for HBMs; a WRAP Guidance document: Hydraulically bound mixtures incorporating recycled and secondary aggregates www. aggregain.org.uk/opportunities/materials/ hydraulically_bound/index.html and a WRAP Quality manual for hydraulically bound mixtures construction/how_do_i_reduce_waste/ sectors/utilities/utilities_guidance.html There is also a Britpave document which indicates the requirements in the European standards for HBMs, Britpave BP/13 Technical data sheet Cement and other hydraulic bound mixtures (The new European Standard BS EN 14227, Parts 1-5). 68 Designing out Waste - Part 2: Technical Solutions

4 Technical solution: Subbase and hydraulically bound materials (HBM) Recycled/secondary aggregates as unbound subbase Application: Highways, railways, airports, capital utilities, bridges and structures and development sites. Design for Reuse and Recovery. The use of high quality recycled and secondary aggregates are permitted in unbound subbase applications under the Specification for Highway Works. The recycled/secondary aggregates give the same performance as primary aggregates for the same applications. Waste reduction: allows use of materials that might otherwise be used for lower value applications or disposed of to landfill or exempt sites. Cost reduction: recycled aggregates are normally cheaper than primary aggregates, particularly in urban areas. Recycled content: increases the recycled content of the project Programme: no effect on programme. Carbon footprint: reduced compared to primary aggregates because of lower transport distances. Other environmental benefits: reduction in resource depletion from quarrying. The use of crushed slag, crushed concrete, recycled aggregates and well-burnt nonplastic shale is permitted in Type 1 and Type 2 unbound subbase. For recycled aggregates, the content of recycled asphalt must not exceed 50%, the content of glass must not exceed 25% and the content of other materials must not exceed 1%. Crushed blast furnace slag and recycled concrete aggregate are permitted in Type 3 (open graded) unbound mixtures and Category B (close graded) unbound mixtures. For recycled concrete aggregate, the content of asphalt must not exceed 5% and the content of other materials must not exceed 1%. Type 4 unbound mixture shall be made from recycled aggregates containing asphalt arisings, and may contain crushed rock, crushed slag, crushed concrete or well burnt non-plastic shale. It shall have a recycled asphalt content greater than 50% and not contain more than 25% glass and 1% other materials. Full details are available in the 800 Series of the Specification for Highway Works, available at co.uk/mchw/vol1/pdfs/series_0800.pdf Numerous case studies of the use of recycled and secondary aggregates in unbound subbase are available at Use as Type 1 and Type 4 unbound subbase is illustrated by the M25 widening between J12 to 15 and Heathrow T5 spur case_studies/m25_j12_to_15.html Spent railway ballast and blast furnace slag were used as Type 1 unbound subbase in the Newport Southern Distributor Road _performance.html The use of recycled and secondary aggregates in highway works is covered by HD35/04, which is available at www. standardsforhighways.co.uk/dmrb/vol7/ section1/hd3504.pdf Subbase and hydraulically bound materials (HBM) Designing out Waste - Part 2: Technical Solutions 69

5 Subbase and hydraulically bound materials (HBM) Technical solution: Subbase and hydraulically bound materials (HBM) Geogrids/geotextiles to reduce thickness Application: Construction and maintenance of subbase/ballast for pavements/railtrack for highways, railways, airports, utilities, harbours, docks and waterways, power generation and in the development of brownand greenfield sites. Design for Materials Optimisation. Geogrids/geotextiles work by creating mechanical interlock of the aggregates and acting as a separator above the soil foundation; this allows a reduction in the thickness of the subbase/ballast layer in pavement or railtrack construction. Mechanically stabilised foundation layers can sustain increased loadings, or provide the required performance with a reduced layer thickness. Waste reduction: improvement/ strengthening of the subbase/ballast means that it may not need to be as thick. Cost reduction: some cost savings may result because of the reduction in subbase/ballast thickness. Recycled content: the quantity of imported primary aggregates is reduced. Programme: additional time required to place the geogrids, but balanced by reduced requirement for material; overall no significant impact. Carbon footprint: Some reduction in lorry movements needed to import additional aggregate. Other environmental benefits: lorry traffic on adjoining roads is reduced in many cases. A more durable product may require less maintenance. Geogrids/geotextiles can be used when constructing new pavements, carrying out major remediation of an existing pavement, strengthening the ballast for railway track construction. Geotextile fabric acts as a good separator limiting deformation at the aggregate surface. Geogrids are particularly effective in reducing lateral movement because of granular interlock within the subbase as a result of stones being forced into the apertures of the grid. The improved mechanical properties of the subbase/ballast may lead to an increased life. Design for traffic/railway loading is based on the stabilisation factor which will vary according to geogrid/geotextile type. The manufacturer s performance data and design methodology should be taken into account. Various TRL Reports, e.g. RR140 (Deformation of road foundations with geogrid reinforcement) and RR382 (Installation damage trials on geotextiles), give trial results. WRAP document Sustainable Geosystems in Civil Engineering Applications, 70 Designing out Waste - Part 2: Technical Solutions

6 This information is an extract from Designing out Waste: a design team guide for civil engineering Designing out Waste: a design team guide for civil engineering was originally published July To downloaded the full guide visit: While we have tried to make sure this document is accurate, we cannot accept responsibility or be held legally responsible for any loss or damage arising out of or in connection with this information being inaccurate, incomplete or misleading. This material is copyrighted. You can copy it free of charge as long as the material is accurate and not used in a misleading context. You must identify the source of the material and acknowledge our copyright. You must not use material to endorse or suggest we have endorsed a commercial product or service. For more details please see our terms and conditions on our website at