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Kent County Council: Road Pavement Design GuideChart 2 Design Flow ChartSection 3Subgrade CBRCarry out Site Investigation to evaluate CBRa) during construction andb) during pavement lifeAppendix F gives appropriate methods. UseSoil Type description and moisture (with lessaccuracy) in Table 3.2Section 4Foundation designCapping and /or Sub-base optionsUse Table 4.4.1a or 4.4.1b for thicknessduring constructionUse Table 4.4.2a or 4.4.2b forthickness during pavement lifeSection 5TrafficDetailed evaluation from:Commercial vehicle (c.v.) flow count. (Para 5.2)Vehicle wear factor: Use Table 5.4.1 tochange c.v. to std. axlesDesign Life: Normally 20 years (Para 5.4.2)Traffic Growth: Para 5.4.3. and Appendix ALeft hand lane : Table 5.4.4Calculate msa (Section 5.4.6)ORObtain OGV1 and OGV2 flowUse Chart/Table in Appendix A to obtain msaORSelect traffic from standard road descriptionsTables 5.5, 5.6 or 5.7 (with less accuracy)Select worst caseUse Table 4.5.1 to provide alternativethickness for other categories of subbase;Para 4.5.2 for asphalt substitution/ soilstabilisation:Section 4.6 for recycled materialsCheck for frost protection (at end ofdesign process) (Para 4.4.3)Section 7Surfacing DesignThin surfacing thickness isselected by the contractor anddeducted from total asphaltthickness in Section 6Using commercial vehicles(Section 5) and Appendix C todetermine required PSV of coarseaggregateChoose texture depth to suit trafficspeedConsider colourSection 6Structural layer designFlexible (Para 6.1)Use Appendix B1 for total asphalt thicknessUse Appendix B2 or B3 to providealternative thickness for other classes ofasphalt stiffnessComposite Hydraulic bound (Para 6.3)Use Appendix B4 for thickness of roadbaseand total asphalt based on knowledge ofmaterial properties and costComposite Cement Bound (Para 6.2)(These include in situ recycled materials)As for hydraulic boundSection 8 Block Paving DesignUse Appendix B5 or B6 to determinestructural layerConsider skid resistance, abrasionresistance, colourRoad Pavement Design Guide.doc\HMRB CD Version 2\October 2001\Page 5 of 54 Copyright KCC

Kent County Council: Road Pavement Design Guide3 Measurement of SubgradeThe design philosophy introduced by LR1132 and described in M85/2 and M88/1 is retained here. Thepavement has to be built on subgrade of known strength. For heavily trafficked roads the principle of thedesign is that the subgrade is improved as necessary and with the sub-base form a stable platform forthe construction of the bound structural layer and surfacing above. For the longer term the cappingand/or sub-base prevent water reaching the bound layers and provide a platform on which to compactthe bound layers.The philosophy for both composite and blacktop roads is that the sub-base/ capping layer does not varyin thickness with traffic, but only with the strength of the subgrade. The same thickness is used for bothroad types.The California Bearing Ratio (CBR) is still the best indicator of soil strength despite difficulties inmeasurement particularly on mixed fine and coarse graded materials and taking account of the effects ofmoisture.There is a method currently being piloted by HA based upon the Portable Dynamic Plate, whichmeasures the stiffness of the sub-grade under dynamic loading. This is particularly advantageous wherea road is being widened so that the foundation will be at or close to equilibrium although for new roadsalternative methods will be necessary to determine equilibrium CBR values.3.1 Site InvestigationA pre-construction geotechnical site investigation shall be carried out for all sites in order to assess anumber of design issues; in particular the stiffness (CBR) of the material, its moisture sensitivity and ifnecessary its suitability for earthworks and stabilisation to form capping layer, sub-base or roadbasematerial.The site investigation should be carried out in accordance with the Association of GeotechnicalSpecialists (AGS) Guidelines for Good Practice in Site Investigation and in accordance with BS5930(Ref 4).The scale of the investigations will be dependent upon the scale of the project, but should typicallycomprise a Desk Study followed by a ground investigation. A separate chemical contamination riskassessment may also need to be undertaken.A Desk Study is beneficial for even small schemes with valuable data readily available from WellRecords, published records, geological maps and memoirs, aerial photographs, local libraries, localauthority landfill databases, speleological society records and aquifer protection maps. This literaturesearch will help to optimise and accelerate the planning of the ground investigation and the subsequentdesign and construction process.Notwithstanding, any site investigation can, by its very nature, only sample the soils at discrete locationswithin the site. Variability is inevitable and this should be included for in the design of the works. Ifnecessary advice on the likely range of CBR values may be obtained from a competent GeotechnicalEngineer.In addition to the determination of design CBR values for both short term and long term characterisationof the subgrade performance, many other factors can affect the performance of the subgrade whichmust be considered during the design stage. Typical issues to be addressed include the following:a) Depth to the water table/perched water tables.b) Chemical contamination risk assessment.c) Control of piping of fine grained soils.Road Pavement Design Guide.doc\HMRB CD Version 2\October 2001\Page 6 of 54 Copyright KCC

Kent County Council: Road Pavement Design Guided) Risk of encountering loose Made Ground.e) Need for ground improvement of foundation soils (e.g. soft Alluvium, loose Made Ground etc.)f) Risk of collapse settlement of dry engineered fill.g) Risk of landslips.h) Risk of underground caves, deneholes etc.i) Impact of adjacent developments for sites on soft Alluvium.j) Frequency and treatment of subgrade solution features in Upper Chalk and Hythe Beds.k) Treatment of solution features below drainage runs.l) Frequency and treatment of other subgrade soft spots.m) Frost susceptibility of subgrade.n) Differential settlement risks/need for ground improvement.o) Chemistry of subgrade soils if in-situ lime/cement stabilisation is considered.p) Shrinkage/swelling potential of over-consolidated clays. (particularly where trees removed)q) Risk of open fissures in underlying rock.r) Risk of soft clay layers in a granular soil.The above list illustrates that laboratory and/or field CBR testing may not be exclusively used todetermine the performance of the road pavement foundation. Foundation failure can occur for manyother reasons including settlements induced in underlying soft Alluvium/loose Made ground, shrinkageand swelling of cohesive soils, softening/washouts produced by groundwater/poor drainage and frostheave. Many of these are borne out by case histories in Kent which have often been costly to rectify.Designers should asses the relative impact of the above on the pavement design and incorporate anynecessary works to allow for them. Additional specialist advice may need to be sought3.2Selection of method for determination of CBR.There are two cases to be considered for design, the likely CBR at time of construction and the longterm equilibrium value. For the purposes of design both of these are required. Should the ‘as found’ CBRat time of construction be lower than that at site investigation, a change in foundation layer thicknessmay be required.The strength of many Kent soils, e.g. Folkestone Sand, Gault Clay, Weald Clay and Chalk are verydependent upon moisture content, condition and density with, for example, rapid loss of strength as themoisture content increases. On such soils, protection during construction as described in theSpecification for Highway Works is particularly important together with a conservative view whenconsidering the effect of subgrade drainage.Road Pavement Design Guide.doc\HMRB CD Version 2\October 2001\Page 7 of 54 Copyright KCC

Kent County Council: Road Pavement Design GuideThe method to be selected for determination of CBR should be based upon the size of the scheme, theaccuracy required and the likely soils to be encountered, as detailed in Appendix F.For cohesive soils the plasticity index should be determined, or for other soils the description of the soiltype from a grading analysis of a bulk sample and some knowledge of the possibility of saturation in thefuture evaluated. From this information the CBR can be approximately determined using Table 3.2below.Table 3.2 Equilibrium CBR values and Plasticity IndexSoil TypePlasticity IndexDesign CBR %Plastic ClayGreater than 50Less than 2Silty Clay403Sandy Clay303Sandy Clay20Less than 2Silt10Less than 2Poorly graded Sand-7*(20)Well Graded sand-10*(40)Well Graded sandy gravel-15*(60)Made Ground 2%**Engineered FillMinimum 2% ***Notes* This assumes some probability of the material saturating in service. If the drainage and watertable position make this very unlikely the figures bracketed may be used.**Specialist investigations will be required to determine the extent of ground improvementrequired.***NoteDesign CBR dependant upon choice of fill material. Minimum 2% CBR assumed limit oftrafficability of fill.For CBR less than 2% special measures are necessary to provide adequate foundationsupport. These are described in Section 4.3.Road Pavement Design Guide.doc\HMRB CD Version 2\October 2001\Page 8 of 54 Copyright KCC

Kent County Council: Road Pavement Design Guide4Design of Foundation Layer4.1 Role of Foundation LayerThe strength of the foundation layer is dependent upon the three factors applicable to all pavementengineering design.i)the support provided by the underlying material, in this case the subgrade. This is measured asdescribed in Section 3.ii)the strength of the foundation material itself.iii)the thickness of the layer.During construction, it is important that an adequate base is provided to support the traffic which will usethe road foundation. In addition the foundation layer enables subsequent structural layers to beadequately compacted during construction. This will vary with the season, the moisture susceptibility ofthe soil, any subsurface drainage that is provided and whether the surface water can drain readily. Noteson drainage are included in Section 4.2.Unless the works are such that the subgrade may be exposed for periods long enough for the climate toaffect it, the equilibrium CBR (that pertaining at a moisture content 0.5m. below the surface) should beselected as the construction CBR. The sub-grade in winter may have a lower CBR, this will need to bechecked at the time, and if necessary the design thickness increased.For the long term condition, the equilibrium subgrade stiffness needs to be used. This is the conditionthe soil will reach under a pavement in the long term. It is affected by the condition at the time ofconstruction, provision of sub-soil drainage and to an extent the kind of pavement construction used;stabilised and hydraulic bound sub-base materials are generally less permeable than unboundaggregates, cement bound materials are prone to cracking which can permit ingress of water.The foundation layer has a role in providing a uniform layer with known characteristics and solvingpotential problems from pore water pressure, the water table and the drainage of water which maypenetrate the overlying bound layers.Foundation layer strength is dependent upon the material being fully compacted as described in theSpecifications. If possible and in addition the actual stiffness of the material prior to overlaying should bemonitored for stiffness using the Portable Dynamic Plate. This may for example permit materials otherthan Type 1 sub-base to be used.Materials must also be laid and compacted within the specified level tolerances for such layer, as givenin the Specification. The thickness of materials in the design tables are nominal thicknesses, applicationof tolerances means that actual thickness may be more or less.4.2 DrainageIn times past, axle loads and speeds were not large enough to create a problem in the sub-base orsubgrade if these were saturated. Ancient roads are therefore unlikely to have any sub-soil drainage.However the foundation will have been consolidated and compacted. The existing road may thereforeperform much better than a new road of the same construction. When reconstruction, statutoryundertakers works or maintenance is carried out, it is unlikely that the new materials can be installed tothe same level of compaction.Road Pavement Design Guide.doc\HMRB CD Version 2\October 2001\Page 9 of 54 Copyright KCC

Kent County Council: Road Pavement Design GuideSurface water can enter the pavement construction though the porous surface, through cracking as thepavement ages and at the edge of the carriageway if it cannot enter gullies grips or edge drainageeasily. Wherever possible drainage via grips to ditches, kerbs and channels or continuous channeldrainage should be provided. Over the edge drainage to combined surface water/sub-soil drainage is notrecommended as it softens the carriageway edge and clogs with time.Ground water will rise beneath any pavement in cuttings or where the water table is near the surface.Sidelong ground can lead to saturation from surface and sub-soil water flow. Cut-off sub-soil drainageshould be provided in these circumstances. The design of such sub-soil drainage is a specialist skilloutside the scope of this document.Modern traffic loads can create serious problems in the road foundation if it is saturated, as follows:-reduction in strength of the subgrade, capping layer and unbound sub-base as pore water pressuresare generated and particle interlock is lost.-movement of fines within the capping layer and unbound sub-base leading to further loss ofaggregate interlock, loss of strength and possible risk of frost damage.-degradation of unbound aggregate generating even more fine material.-friction between the sub-base and structural layers is reduced, lowering the strength of the totalconstruction.-the base of the asphalt layer may be subjected to scouring by water stripping the bitumen, creatingvoids and reducing strength. Water can also be forced into micro-cracks leading to rapid failure.If the water table is well below the formation level and both the capping layer material and sub-basehave adequate permeability to carry away surface water, it is possible that sub-surface drainage is notnecessary.For unbound sub-base, it is necessary to ensure that the sub-base cannot become saturated, byinstalling sub-base drainage.For ‘sub-base only’ options on sub-grade CBR less than 3%, as an additional safeguard, it isrecommended that a non-woven geotextile separation layer is inserted to prevent fines migration into thesub-base should drainage become ineffective with time. This should be designed based upon thegrading of the sub-gradeFor bound sub-base, it is necessary to ensure that the interface between the sub-base and asphalt roadbase is drained so that friction is maintained at the interface.Designs are based upon the sub-grade and sub-base being wet but not saturated, drainage should beprovided as necessary to ensure this is the case. This is particularly pertinent where the saturated statecould lead to a CBR of 2% as this leads to a dramatic increase in the total pavement thickness.4.3 Foundation Material SelectionThe foundation can be constructed of capping layer plus sub-base or sub-base alone. The choice ofwhether to use capping layer plus sub-base or sub-base alone can be made on economic grounds orother construction programme considerations (unless the CBR is less than 2% in which case a cappinglayer is obligatory). It may be advantageous to provide the contractor with both options as he is in thebest position to make the selection.Where the CBR is 2%, a significant increase in capping layer is necessary. It may be more economicalto dig out the soft area separately and refill with material similar to the surrounding materials. If the softarea is as a result of localised water ingress this may be solved by local drainage. As a furtheralternative, and taking specialist advice, it may be economic and require less imported materials toreinforce the subgrade with a geogrid, together with a geotextile membrane if necessary.Road Pavement Design Guide.doc\HMRB CD Version 2\October 2001\Page 10 of 54 Copyright KCC

Kent County Council: Road Pavement Design GuideCapping layer can be formed from a wide range of materials including lime and cement stabilisedsubgrade soils, secondary aggregates and other quarry or demolition waste.Any of these materials may be used provided they satisfy the requirements of the Specification. Furtherinformation is provided in Section 4. 6. The capping layer is not expected to have the same stiffness assub-base and so requires a greater layer thickness. This should be offset by the reduced cost,particularly of in-situ recycling. However even potentially strong materials will not achieve high stiffnesswhen they are not fully compacted, achieving this may be more difficult on a weak sub-grade.Experience suggests that the use of imported capping layer material may only be economic on asubgrade stiffness with CBR less than 3% at time of construction.Sub-base materials are formed from cement stabilised soils, hydraulic bound materials and well gradedgranular material. The former are particularly relevant below similarly bound roadbase materials. Wellgraded granular material has been put into three Categories. An example of Category A material onwhich the design charts are based is Type 1 sub-base. Further details on materials that are suitable foruse as sub-base is given in Section 4.6.4.4 Design of foundation layer thicknessTwo cases need evaluation and the worst case design selected1 Construction foundation2 Equilibrium foundationsub-base only (Table 4.4.1 a)Sub-base only (Table 4.4.2 a)capping layer and sub-base (Table 4.4.1 b)Capping layer and sub-base (Table 4.4.2 b)The tables below are based on a capping layer having a stiffness of 75MPa (Approximately equivalent toan in-situ CBR of 8%), and Category A material (cf. Section 4.6) being used as a sub base.Normally the construction phase will be the worst case and only a check required for the long termcondition.4.4.1Foundation during constructionThe standard level of construction traffic assumed within the Department of Transport Design Manual forRoads and Bridges Vol 7 HD25 is 1000 standard axles. For new works or larger areas of reconstructionthis value has been used.LR1132 gave the thickness of sub-base required for a range of construction traffic loadings andsubgrade strengths. This permits other levels of construction traffic to be accommodated.For maintenance schemes, patching, haunching, local reconstruction or the like, or where in siturecycling is used, the sub-base will not be trafficked during construction and hence can be significantlythinner. For design purposes, however, the minimum construction traffic loading has been taken to be100 standard axles. The thickness given presumes adequate compaction as described in theSpecification has been achieved. This may require special measures to control the moisture contentwithin the permitted range and use of suitable plant.Table 4.4.1a – Thickness of Sub-Base (mm) : ConstructionSub-base thickness (mm)for Type of WorkCBR of Subgrade (%)Road Pavement Design Guide.doc\HMRB CD Version 2\October 2001\Page 11 of 54 Copyright KCC

Kent County Council: Road Pavement Design GuideHaunching, In situ recyclingor Local ReconstructionUse Table4.4.1b275*225175150150New Works or Major Areasof ReconstructionUse Table4.4.1b400*300225200150Note 1 * denotes a non-woven geotextile is recommended to prevent contamination of granular subbase material (if used). In addition the contractor may choose to increase the thickness of thesub-base or use

Subgrade CBR Carry out Site Investigation to evaluate CBR b) during pavement life Appendix F gives appropriate methods. Use Soil Type description and moisture (with less accuracy) in Table 3.2 Use Table 4.4.1a or 4.4.1b for thickness during construction Use Table 4.4.2a or 4.4.2b for thickness during pavement life Select worst case