hd 25/95 foundations (superseded)

Foundations

Summary:

THE HIGHWAYS AGENCY HD 25/95

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT

THE WELSH OFFICEY SWYDDFA GYMREIG

THE DEPARTMENT OFTHE ENVIRONMENT FOR NORTHERN IRELAND

Volume 7 Section 2Part 2 HD 25/94 Registration of Amendments

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REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Volume 7 Section 2Registration of Amendments Part 2 HD 25/94

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REGISTRATION OF AMENDMENTS

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amendments amendments

DESIGN MANUAL FOR ROADS AND BRIDGES

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VOLUME 7 PAVEMENT DESIGNAND MAINTENANCE

SECTION 2 PAVEMENT DESIGNANDCONSTRUCTION

PART 2

HD 25/94

FOUNDATIONS

Contents

Chapter

1. Introduction

2. Subgrade Assessment

3. Capping and Sub-base

4. In-situ Testing

5. References and Bibliography

6. Enquiries

Volume 7 Section 2 Chapter 1Part 2 HD 25/94 Introduction

ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYJanuary 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 1/1

1. INTRODUCTION

General

1.1 The main purpose of the foundation is todistribute the applied vehicle loads to the underlyingsubgrade, without causing distress in the foundationlayers or in the overlying layers. This is required bothduring construction and during the service life of thepavement.

1.2 The stresses in the foundation are relativelyhigh during construction, although the number of stressrepetitions from construction traffic is relatively lowand is not so channelised as normal traffic during theservice life of the pavement.

1.3 The standard practice, which is described inthis part, is to design the foundation for constructiontraffic loading. This approach provides a "standardfoundation" for the design of the pavement.

Implementation

1.4 This Part shall be used forthwith on all schemesfor the construction, improvement and maintenance oftrunk roads including motorways, currently beingprepared provided that, in the opinion of the OverseeingDepartment, this would not result in significantadditional expense or delay. Design organisationsshould confirm its application to particular schemeswith the Overseeing Department.

Mutual Recognition

1.5 The construction and maintenance of highwaypavements will normally be carried out under contractsincorporating the Overseeing Department'sSpecification for Highway Works (MCHW 1). In suchcases products conforming to equivalent standards andspecifications of other member states of the EuropeanCommunity and tests undertaken in other member stateswill be acceptable in accordance with the terms of the104 and 105 Series of Clauses of that Specification. Any contract not containing these Clauses must containsuitable clauses of mutual recognition having the sameeffect regarding which advice should be sought.

IN-SITU TESTS(Chapter 4)

CAPPINGAND SUB-BASE

DESIGNS(Chapter 3)

SUBGRADE ASSESSMENT

(Chapter 2)

NEWROAD

EXISTINGROAD

START(Chapter 1)

COMPLIANCETESTING

Chapter 1 Volume 7 Section 2Introduction Part 2 HD 25/94

ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED January 19941/2

Volume 7 Section 2 Chapter 2Part 2 HD 25/94 Subgrade Assessment

2. SUBGRADE ASSESSMENT

2.1 The subgrade is normally not strong enough to 2.5 The following equation has been derivedmpirically for typical UK soils:-

E = 17.6 (CBR) , MN/m0.64 2

carry the construction traffic without distress, unless it eis rock which is not subject to degradation byweathering. Therefore, unbound or bound foundationlayers of adequate stiffness modulus (see glossary) arerequired to reduce the stresses on the subgrade.

MATERIAL PROPERTIES

2.2 Unbound aggregates and soils can suffer frompermanent internal deformation when subjected to highstresses. They tend to have relatively poorer permanentdeformation characteristics and lower shear strengththan bound materials. There is no established test topredict susceptibility of these materials to permanentdeformation. It is common for the designer to inferfrom experience and index tests that materials have anacceptable level of stiffness modulus and shear strength. Both stiffness modulus and shear strength are usuallyreduced by increases in moisture content.

2.3 Ideally, a knowledge of the stiffness modulusand shear strength of the subgrade would be required todetermine the thickness of the overlying pavementlayers in order to avoid under- or over- design.However, these two parameters are dependent on soiltype (particularly plasticity), degree of remoulding,density and effective stress. Effective stress isdependent on the stress due to the overlying layers, thestress history and the pore water pressure or suction. Inturn, suction is dependent on the moisture contenthistory, the soil type and the depth of the water table. The number of factors involved makes it necessary toadopt simplifications and to use index tests.

Index Tests

2.4 Since direct determination of stiffness modulusand shear strength is not always practical, the CaliforniaBearing Ratio (see CBR - paragraphs 4.6, 4.7) isfrequently used as an index test: CBR is quoted inpercent to two significant figures. The CBR is not adirect measure of stiffness modulus or of shear strengthbut it is widely used and considerable experience with ithas been developed. It thus provides a common meansof comparison.

IEbad

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23pccTw

pp(ELECTRONIC COPY - NOTJanuary 1994 PAPER COPIES OF THIS ELECTRO2.6 If it is not possible to determine a CBRvalue using the tests described in Chapter 4 thenTable 2.1 provides a simple means of assessing theequilibrium in- service (ie. long term) CBR of thesubgrade. The table shall be used to derive adesign in-service CBR unless site or laboratory testdata clearly indicate otherwise. Considerable careis required in assessing the lower values of CBR. Note that Table 2.1 is based on calculations ratherthan measurement. Even though CBRs are quotedto the nearest %, this degree of accuracy shouldnot be implied as achievable. As subgrades getsofter so the CBR values become less consistent. Values should be rounded down unless positiveand consistent CBR determinations have beencarried out.

t provides a means of assessing the stiffness modulus,, which is approximately valid for values of CBRetween 2 and 12 %. This may be used with care innalytical design HD 26 (DRMB 7.2.3.6). For moreetailed information refer to CR72 (1987).

ETERMINATION OF SUBGRADE CBR

.7 In Table 2.1, a `high' water table is one within00mm of formation (or sub-formation if a capping isresent). A `low' water table is 1 metre down. `Thick'onstruction represents a 1200mm pavement (includingapping); a `thin' pavement is 300mm of construction. he construction condition referred to relates tohether the subgrade is allowed to become wet, ie.rotection from rain, and the quality of drainagerovided. More detailed advice is given in LR11321984). FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 2/1

Chapter 2 Volume 7 Section 2Subgrade Assessment Part 2 HD 25/94

TYPE OF SOIL PI HIGH WATER TABL

CONSTRUCTION CONDITPOOR AVERAGE

Thin Thick Thin Thick Th

HEAVY CLAY 70 1 2 2 2

SILTY CLAY 30 2 3 3 4 3SANDY CLAY 20 2 4 4 5 4

60 1 2 2 250 1 2 2 240 2 2 2 3 2

10 1 3 3 6 3

SILT* 1 1 1 1

SAND ---------------------------------------------------------(POORLY GRADED)

SAND ---------------------------------------------------------(WELL GRADED)

SANDY GRAVEL ---------------------------------------------------------(WELL GRADED)

* estimated assuming some probability of material saturating

TABLE 2.1 Equilibrium SELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO2/2E LOW WATER TABLE

IONS: CONSTRUCTION CONDITIONS:

GOOD POOR AVERAGE GOOD

in Thick Thin Thick Thin Thick Thin Thick

2 2 1 2 2 2 2 2

5 3 3 4 4 4 6 7 3 4 5 6 6 8

2 2 1 2 2 2 2 22 2 2 2 2 2 2 2 3 2 2 3 3 3 3

7 2 4 4 7 6 >8

2 2 1 1 2 2 2 2

---------------- 20 -------------------------------------------------------------------------

---------------- 40 -------------------------------------------------------------------------

---------------- 60 -------------------------------------------------------------------------

ubgrade CBR Estimation2.8 If full information is not available forTable 2.1 to be used, then certain assumptions canbe made. The worst condition of a high watertable can be taken together with construction beingcarried out to the Specification (MCHW1) andthus at least `average' construction conditionspertain. The pavements discussed in this Sectionvary between "thick" and "thin" constructions; byinterpolating between the values in Table 2.1, atable of acceptable Equilibrium Values can bederived. This is shown in Table 2.2. Backgroundinformation on this table is available in HA 44/91(DMRB 4.1.1). Table 2.2 should be used wherefull information is not available. The followingmethods may be used as a check for the CBRvalue, but shall only supersede the use of Tables2.1 and 2.2 with the prior approval of theOverseeing Department.

Laboratory Testing

2.9 CBR values can be measured in the laboratoryon recompacted specimens, in accordance with BS1377(1990), during the site investigation stage and when theequipment and experience are available. Tests shouldbe carried out over a range of conditions to reproduce,as far as possible, the conditions of moisture contentand density which are likely to be experienced duringconstruction and in the completed pavement. Cohesivesoils should be compacted to not less than 5% air voids,to reproduce the likely conditions on site. Equilibriummoisture content can be deduced from measurements ona suction plate (LR889, 1979).

Site Testing

2.10 For design, the CBR must be estimated beforeconstruction commences. For fine grained soils in-situCBR values can however be measured for checkingpurposes (not to allow design changes) in pits or on trialstrips during construction. Equilibrium CBR valuesrequire the testing of existing pavements and HA 44/91(DMRB 4.1.1) suggests a suitable procedure. Platebearing tests are necessary for coarse materials(BS5930, 1981). FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED January 1994

Volume 7 Section 2 Chapter 2Part 2 HD 25/94 Subgrade Assessment

Suction Method

2.11 For remoulded cohesive specimens the suctionmethod of Blood and Lord (1987) may be used. However, this method effectively considers a worst caseconstruction condition with very high water table, poordrainage and full wetting. Its application is also limitedto soils of plasticity index 13% to 35%.

Type of Soil PI PredictedCBR %

Heavy Clay 70 2January 199460 2

50 2

40 2 to 3

Silty Clay 30 3 to 4

20 4 to 5

Sandy Clay 10 4 to 5

Sand (Poorly graded) 20Sand (Well graded) 40Sandy gravel (Well graded) 60

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Volume 7 Section 2 Chapter 3Part 2 HD 25/94 Capping and Sub-base

3

3.suththw

laadco

3.foce300

200

100

01 2 3

FIGURE 3.1 Capping a

Example 1 : CBR 3.5% ExampELECTRONIC COPY - NOJanuary 1994 PAPER COPIES OF THIS ELECT

Alternative Designsa. Sub-base 150mm on Capping 330mmb. Sub-base 280mm No CappingSubgrade CBR (%)4 5 8 10 15 20 30

nd Sub-base Thickness Design

le 2 : CBR 8%SUB - BASETHICKNESS

CAPPINGTHICKNESS

(mm)

(mm)

400

300

200

100

0

600

500

400

Key:Capping/Sub-base Design

Sub-base only Design for flexible and flexible composite pavements, capping not required

SUB - BASE

For low CBR valuessee Paragraphs

3.7 - 3.10

CAPPING

. CAPPING AND SUB-BASE

1 Capping is used to improve and protect weak composite pavements.bgrades by using a relatively cheap material betweene subgrade and the sub-base. The aim is to increase 3.3 The grading for unbound granular sub-base ise stiffness modulus and strength of the formation, on intended to provide a dense layer of relatively highhich the sub-base will be placed. Capping with a stiffness modulus, which is reasonably impermeableboratory CBR value of at least 15% should provide an and will thus shed rain water during construction, givenequate platform for construction of the sub-base when adequate fall. It is not necessarily free draining andmpacted to the appropriate thickness. may exhibit suction, and thus increase in moisture

2 Granular and cemented sub-bases are permitted at least 30% should provide an adequate platform forr flexible and flexible composite pavements but only construction of the pavement when compacted to themented sub-bases are permitted for rigid and rigid appropriate thickness.

content. Granular sub-base with a laboratory CBR ofT FOR USE OUTSIDE THE AGENCYRONIC DOCUMENT ARE UNCONTROLLED 3/1

Alternative Designsa. Sub-base 150mm on Capping 210mmb. Sub-base 190mm

No Capping

Chapter 3 Volume 7 Section 2Capping and Sub-base Part 2 HD 25/94

3.4 The thickness of capping and sub-baseshall be obtained from Figure 3.1. The sub-basemay be omitted on hard rock subgrades thatare intact or, if granular would have alaboratory CBR of at least 30%, and which donot have a high water table. For a subgradehaving a CBR greater than 15 %, the thickness

3.6 The final design thickness shall bespecified to the nearest 10 mm greater than thevalue obtained from Figure 3.1. On subgradeswith a CBR of less than 15 %, the minimumthickness of a layer of aggregate (either cappingor sub-base) placed directly on the subgrade

THICKNESS DESIGNof sub- base is 150 mm, this being controlled bythe minimum practicable thickness forspreading and compaction. When the subgradeCBR is between 2.5 and 15% for flexible andflexible composite construction, there are twooptions available:

1. 150mm of sub-base can be used on avarying thickness of capping dependingon the CBR value or,

2. An increasing thickness of sub-base canbe used with the decreasing CBR, withno requirement for capping.

For all pavements on subgrades with CBRvalues below 2.5%, and for rigid and rigidcomposite construction on CBRs below 15%,150mm of sub-base on the varying thickness ofcapping must be used. See Figure 3.1. Whenthe subgrade CBR is sufficiently below 2% suchthat capping with sub-base is insufficient tosupport the pavement, then refer to Paragraphs3.7 to 3.10.

3.5 It is not intended that the foundationdesign should vary frequently along the roadbut that an appropriate value shall be selectedfor each significant change in the subgradeproperties. For this reason changes infoundation design should not be made forlengths less than 100 m and rarely less than 500m.ELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO3/2shall be 150 mm. At and below 3 % CBR, thefirst layer of aggregate shall be at least 200 mmthick. The thickness of all foundation layersshall be constant over the full width of thepavement.

Soft Subgrades

3.7 When a subgrade has a CBR suficiently below2% such that it becomes unsuitable as a pavementfoundation, (a subgrade would tend to deform and`wave' under construction traffic), then a number ofoptions are available.

3.8 The material can be removed and replaced bymore suitable material; if the depth is small, all can bereplaced but it may only be necessary to replace the toplayer. The thickness removed will typically be between0.5 and 1.0 m. Although the new material may be ofgood quality, the subgrade should be assumed to beequivalent to one of a CBR value just under 2% (ie.600mm capping), in order to allow for movements inthe soft underlying material. A total constructionthickness about 1.5 m thick will often result. Ageosynthetic may also be useful.

3.9 If the soil is cohesive, a lime treatment may bean economic option, subject to soil suitability beingshown. Details of various soil treatments are given inHA44/91 (DMRB 4.1.1). The overlying capping isagain designed on the basis of a subgrade with a CBRjust under 2% (ie. 600mm capping).

3.10 If the soil is reasonably permeable, a deeperthan normal drainage system may be considered,together with a system of monitoring the improvementexpected. Design of the main foundation may then bebased on whatever conditions are achievable in the timeavailable. FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED January 1994


standard unbound material for use with flexibleand flexible composite pavements. Granularsub- base, Type 2, may be used in pavementswhich have a design traffic loading of less than5 msa at opening, provided that, when tested, alaboratory CBR of 30 % or more is obtained(see Specification (MCHW1), Series 800);particular care is required to ensure that dryingout does not occur before covering.

CAPPING MATERIALS

3.11 The Specification (MCHW1)(Series 600)allows a fine graded material (6F1) and a coarser graded(6F2). The latter can be considered as relatively freedraining and is thus most suitable for sites with a3.16 Granular sub-base, Type 1 (seeSpecification (MCHW1), Series 800) is the

shallow water table. It should, however, be noted thatcapping is not required to be a drainage layer as long ascontained water does not prevent it from satisfying itsprimary function of load spreading. The specifiedgradings also do not guarantee adequate shear strengthand a demonstration area should normally be placed andtested to check on the material's characteristics bytrafficking with normal site vehicles and constructionplant.

3.12 Alternative permitted materials are cement andlime treated soil and, particularly when the removal andreplacement of unacceptable soil is the alternative,lime/cement or lime/PFA. Further details are given inHA 44/91 (DMRB 4.1.1).

3.13 Reuse of crushed excavated road pavementmaterials as capping may also be carried out providedthe compacted material complies with the Specification(MCHW1)(Series 600).

3.14 The design should allow as wide a range ofcapping materials as possible and particular materialsshould only be excluded if there are over-ridingengineering reasons for so doing. In pavements withcapping over subgrades with CBRs greater than 5%, itmay be necessary to lay a greater thickness than thatgiven in Figure 3.1 if large stone sizes are involved.

3.15 Some contamination from weak cohesive soilsinto granular capping, particularly with 6F2, can beexpected and the design thicknesses allow for this. Insome cases, a geosynthetic separator may also bebeneficial.

SUB-BASE MATERIALS

Granular Sub-bases

CELECTRONIC COPY - NOTJanuary 1994 PAPER COPIES OF THIS ELECTRO3.17 For rigid and rigid compositeconstruction a cemented sub-base is required tominimise the risk of water penetrating slabjoints and cracks, causing erosion andweakening the sub-base. Cement-boundsub-bases also aid compaction of the overlyingpavement concrete. An impermeable membraneis required over the sub-base to prevent suctionof water from the pavement concrete. This alsoacts as a slip layer for jointed concrete andshould be plastic sheeting. For CRCP andCRCR, the membrane should be sprayedbituminous. Strong cement bound material,CBM3, or wet lean concrete, C15, shall be usedexcept when the initial design life of thepavement is less than 12 msa, in which caseCBM2 or C10 are permitted (see Specification(MCHW1) Series 1000, for materials).

3.18 For flexible and flexible compositeconstruction cemented sub-bases may also beused. Weak cement bound material, CBM1 orCBM2, or a weak wet lean mix, C7.5 areadvised (see Specification (MCHW1) Series1000).

emented Sub-bases FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 3/3


sub-base lies directly on soil of less thanCBR (atthe time of construction). Constructionpractices on thin foundations may have to bemodified compared with normal proceduresdue to the reduced ability of the foundation tocarry construction traffic.

Non-Standard Sub-base Materials

3.19 With reducing availability of suitable sub-basematerials, there is pressure to use non-standardmaterials, such as crushed masonry, by-productaggregates and industrial residues. Because of greatervariability and the possibility of contamination of such3.23 It is permitted to replace some or all ofthe sub-base by bituminous material. Asubstitution rate of 30mm of bituminousroadbase to 100mm of Type 1 sub-base shall beused. This technique must not be applied tocapping, or to the lowest 150mm layer of sub-base where

materials, it may be necessary to increase the frequencyof control testing. In any event, the OverseeingDepartment must be consulted before non-standardmaterials are used.

3.20 Variants of Type 1, such as material having acoarser grading and thus increased permeability, mayalso be used subject to test and approval by theOverseeing Department. Aggregates with gradings thathave a pronounced gap or an excess of material passinga 0.075 mm sieve are probably unsuitable.

3.21 A `correct' Ten per cent Fines Value (TFV),according to BS812 Part 111, 1990, can only beobtained on samples from materials having 15 % ormore of their particles in the 10-14 mm size range. Acompaction trial may be carried out to check actualparticle damage under the type of roller to be used. Low TFV does not necessarily preclude use as grading,particle size and self-cementation, which occurs withsome materials, may counteract the weakness.

3.22 Some aggregates have self- cementingproperties. To detect self-cementing properties, trialareas could be tested in-situ at intervals of time. Alternatively, triaxial tests in the laboratory may beused to assess any improvement in stiffness modulusand/or shear strength. Care is required to distinguishbetween self cementing and suction effects.

Bituminous Replacement

A

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lm

sdn

aso

3faswELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO3/4.01

.005

.002

.001

10 1000 10000100

Permissive CompressiveSubgrade Strain

Cumulative Traffic (Standard axles)

NALYTICAL FOUNDATION DESIGN

.24 An analytical foundation design requires thetiffness modulus of the subgrade, capping and sub-baseo be determined, assumptions made regarding Poisson'satio and a linear elastic calculation made using aayered system analysis. From such a computation, theaximum compressive strain in the subgrade under a

tandard axle load may be calculated and related to rutevelopment. If this method is followed, a considerableumber of sensitivity analyses must be carried out tossess the effects of material variability. The aimhould be to provide a design with an 85 % probabilityf achieving the required design life.

.25 The allowable subgrade strain may be takenrom Figure 3.2. Unless information to the contrary isvailable, a construction traffic loading of 1000tandard axles should be used; this is suitable for a siteith access points at 1 km spacing. For very short FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED January 1994

rain


3.26 If an analytical pavement design is to beconsidered, approval is required from theOverseeing Departments at the preliminarydesign stage. Submissions seeking approvalshall include a justification for the choice ofnon-standard materials and/or thicknesses,

lengths this loading may be too high or for very long may be used when those layers are constructed of finesections too low. Guidance for such situations may be soil or fine capping. The drainage layers so formedfound in LR1132 (1984). Since such a design is for may be treated as capping for structural designconstruction traffic only, it will have to be followed by purposes.proposals for the pavement as a whole.supporting calculations and an indication of anyadditional specification requirements or testingregime which may be necessary for theirvalidation.

3.dDpbpth

DRAINAGE

3.27 It is of vital importance to keep water out of thesub-base, capping and subgrade, both duringconstruction and during the service life of the pavement. This is achieved by excluding incoming water andproviding an escape route for water already in thefoundation (Figure 3.3). During construction, everyeffort should be made to protect the subgrade by placingaggregate before rain can soften it. Wherever possiblethe foundation drainage should be kept separate frompavement run-off drainage in all new construction and 3.30in reconstruction work. There should always be a can downslope route from the sub-base to the drain. Further ingrdetails are in HA 44/91 (1991) (DMRB 4.1.1). In surcreconstruction and widening projects it is necessary to givemaintain the continuity of drainage from existing estimcapping and sub-base materials to adjacent new On tmaterials, using appropriate thicknesses and crossfalls. perm

3.28 When the water table is high and the subgrade impois moisture sensitive (Plasticity Index < 25) a subgrade (198drain is beneficial. A granular aggregate drainageblanket (see Specification (MCHW1), Series 600) of 3.31thickness at least 150mm and not more than 220mm of ththick may be used. In order to stop pore clogging by on thfines from other adjacent layers, geosynthetic separators hydr

the v

is deELECTRONIC COPY - NOTJanuary 1994 PAPER COPIES OF THIS ELECTROupperpavementsub - base

+capping

subgrade

todrain

seepage

29 Where a drainage blanket is not used,rains as detailed in Highway Constructionetails (MCHW3) shall be used. The drain islaced below the bottom of capping, notecause sub-base and capping need to beermeable, but so that they will be drained ifey are permeable.

It is useful to check the speed at which waterdrain out of a granular sub-base, as a result ofess due, perhaps, to a faulty pavement or aharging drain. A procedure for calculating this isn in Jones & Jones (1989a) along with a means ofating ingress through cracks in the bound layers.

his basis it may be possible to specify aeability value. Care should be taken to ensure that

sed by a specified grading, see Jones & Jones9 b).

If it is necessary to determine the permeabilitye sub-base or capping material, this must be donee full grading, at the correct density under a lowaulic head. A suitable permeameter and procedure

FIGURE 3.3 Foundation Drainage

alue required does not conflict with any limitations

scribed in HA 41/90 (1990) (DMRB 4.1.1). FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 3/5


3.32 Drainage of the sub-base may beomitted only if the underlying materials(capping and subgrade) are more permeablethan the sub-base, and the water table neverapproaches the formation closer than 300mm.

3.33 For routine cases all material within450mm of the road surface shall be nonfrost-susceptible as required by theSpecification (MCHW1)(Series 700) and testedaccording to BS812 : Part 124 : (1989).

FROST PROTECTION

Example.

Number of days freezing 12Amount of frost 3 Co___________________________________

Penetration H = 4 %36= 24 cm= 240 mm

3.36 From meteorological data the maximum depthof frost penetration over a given historical period can bereadily assessed. The method should show areas wherefrost is clearly no problem at all, or conversely, aserious problem. A return period analysis wouldestablish the probability of a winter of a certainintensity occurring within the nominal life of thepavement. However, highly precise assessment of frost

netration is not advised due to unquantifiablecroclimate effects.3.34 This requirement can be over- severe insome places (e.g. coastal areas) and may bereduced to 350mm if the mean annual frostindex of the site is less than 50. Advice on thefrost index for any particular area may beobtained from the Meteorological OfficeAdvisory Services, Building ConstructionSection, and further information from RR45(1986).

3.35 The frost index, I, is defined as theproduct of the number of days of continuousfreezing and the average amount of frost (indegrees Celsius) on those days. It is related tothe depth of frost penetration, H.

H = 4 %%I , cm

pemiELECTRONIC COPY - NOPAPER COPIES OF THIS ELECTR3/6T FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED January 1994

Volume 7 Section 2 Chapter 4Part 2 HD 25/94 In-situ Testing4. IN-SITU TESTING

4.1 The two reasons for testing pavement rafoundation layers are to check compliance with the dedesign during construction and in pavement assessmentsee HD30 (DMRB 7.3.3). The Specification(MCHW1)(Series 800) gives a method of constructionto be followed. An inadequate test result may indicateeither that the method was not followed, that thematerial was sub-standard, that abnormal conditionsrequiring a variation in procedure were encountered, orthat damage has occurred. The following paragraphsintroduce some of the tests which are available, most ofwhich are specified in BS1377 (1990). They are forgeneral information and advice only and do notcomprise part of the Overseeing Departments'requirements.

Moisture Condition Value (MCV)

4.2 The test takes about half an hour and involvescompaction of soil or fine aggregate using a handoperated device. The amount of compactive effort isplotted against the density so that the test gives theamount of effort needed to obtain the specified density.The effort can be compared to that needed at OptimumMoisture Content and a rapid indirect assessment madeof whether the material is at the desired moisturecontent. The size of the apparatus limits its use to fillfiner than 20mm maximum particle size.

Density Testing (Figure 4.1)

4.3 The sand replacement test involves excavatingand weighing material removed from a small hole andrefilling the hole with a uniform sand. The hole volumeis calculated from the mass of sand used. The waterreplacement test is similar except that a plastic linerfilled with water is used to determine the volume. Theequipment for either is transported by vehicle. The testsare time consuming (up to 1 hour) and thus expensive,and operator sensitive. However they do give a directmeans of measuring density, which can then becompared with values obtained in the laboratory or intrials.

4.4 An alternative is nuclear density testing. Aradiating source is applied to the material. The amountof radiation detected decreases in proportion to the bulkdensity of the material between source and receiver. To

caIf rede5 m

so

4.de'badehe`Tde

Ca

4.insureplvetesperepeagwistiELECTRONIC COPY - NOTJanuary 1994 PAPER COPIES OF THIS ELECTR

determine the moisture content another source sends outSand Replacement Nuclear Density(Transmission

Mode)(Backscatter

Mode)

diation intercepted by hydrogen atoms. The drynsity is

FIGURE 4.1 Density Testing Apparatus

lculated from the bulk density and moisture content.the material being tested is carbonaceous, care isquired in interpreting the moisture content and drynsity obtained. Testing is extremely rapid (less thanminutes) and a reading may be repeated readily. Theachine is portable. Calibration is required for eachil or aggregate tested.

5 It should be noted that two modes of nuclearnsity test are possible. The quickest and easiest isckscatter' mode which is influenced only by thensity of the top 100 - 150 mm of material and is mostavily influenced by material very near the surface. ransmission' mode provides a more representativensity result.

lifornia Bearing Ratio (Figure 4.2)

6 The California Bearing Ratio (CBR) testvolves the insertion of a small plunger into the groundrface at a rate of 1 mm per minute, whilst the load iscorded. Surcharge rings can be placed around theunger to simulate an overburden. A laboratoryrsion of the same test is available in which the sampleted is constrained within a small mould. The stress atnetrations of 2.5 and 5 mm is compared with thesult for a standard aggregate and the ratio given as arcentage. The test is not suitable for coarsegregates because the plunger and aggregate particlesll be of similar size. The test measures neitherffness modulus nor shear strength directly - giving a FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED 4/1

somewhat combined measure of both. It takes around

Chapter 4 Volume 7 Section 2In-situ Testing Part 2 HD 25/94

half an hour on site and between 1 and 2 hours in the coarser materials than other penetrometers. The rate oflaboratory and there is a large body of experience of its penetration into the ground can then be related

use. a

4.7 There are several variants on the CBR test;laboratory, field, with surcharge, saturated, etc. In thecontext of this document the laboratory CBR with asurcharge to simulate the appropriate vertical stress ofthe case being considered should be taken as thestandard method used. The appropriate moisturecontent and wetting or drying condition is alsoimportant. Laboratory CBR results for granular soils areoften higher than those in the field due to mouldconfinement effects. The test is penetration controlledand so does not model the stress level imposed bytraffic. The time of loading is also much longer thanthat due to traffic. CBR is an empirical test and is bestmeasured as initially intended although other testdevices such as the Clegg Impact Hammer, variousstatic and dynamic cone penetrometers and the platebearing test can be used to determine approximateestimates of CBR.

Clegg Hammer (Figure 4.2-Clegg, 1976)

4.8 In some respects this is a dynamic CBR andsuffers from similar scale problems. Thehammer/plunger is lifted and dropped, and thedeceleration on impact is recorded. The equipment isportable and the test extremely rapid (20 seconds). Except on stiff aggregates and subgrade soils, the testcauses some local shear failure and thus is not a directmeasure of stiffness modulus. The stress applied ishigh and the time of loading short so that the stresspulse due to traffic is not modelled accurately. For soilsgenerally dry of optimum moisture content the `CleggImpact Value' has been related approximately to CBR. It is useful as a guide tool and is able to detect soft spotson a subgrade or fine capping and can differentiatebetween material types.

Cone Penetrometers (Figure 4.2)

4.9 Various sizes of field cone penetrometer existfor the rapid approximate assessment of CBR. Ingeneral they can only give values of up to about 5 or6%, and are therefore applicable for soft and mediumfine grained subgrades.

4.10 The dynamic cone penetrometer is similar toother field cone penetrometers except that it is driveninto the ground under the action of a weight droppedonto an anvil. It is therefore suited to stronger and

P

4(1Sm

v

4mELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO4/2Measured Quantity

Stress Deceleration Forceat Impact

Displacement

CBR CleggHammer

Cone

pproximately to CBR.

FIGURE 4.2 Strength Tests

late Bearing Test (Figure 4.3)

.11 This test is described in detail in BS1377990) and its use for testing is described in The

pecification (MCHW1)(Series 600). For pavementaterials no removal of surface material or non-

ibratory compaction is needed.

FIGURE 4.3 Plate Bearing Apparatus

.12 As a variation to the standard method the plateay be unloaded and reloaded until a relatively FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED January 1994

E ' B pr (1&v2)

2y

Conversion factor toapply to obtain k762

1.0

0.8

0.6

0.4

0.2

F=0.00124D + 0.0848r=0.996

Volume 7 Section 2 Chapter 4Part 2 HD 25/94 In-situ Testing

constant elastic modulus is observed (eg. 3 times). The results are interpreted using the equation

where E is the elastic modulus, p is the stress applied, yis the plate deflection and r its radius. AssumingPoisson's ratio (v) typical of granular material (.3) thisapproximates toE ' 1.45 pry

4.13 The effectiveness of compaction can also beassessed by comparing the elastic modulus on first andlast loading. If the ratio (last/first) is greater than 2.0,compaction is probably inadequate.

4.14 An approximate empirical correlation withCBR can be made, as follows:-

CBR = 6.1 x 10 x (k ) %-8 1.733762where k is the modulus of subgrade reaction (equal to762p/y in units kN/m /m at normally a plate penetration y2of 1.25mm) from a plate of 762 mm (30 inch) diameter. Figure 4.4 allows conversion for other plate sizes.

Example

At y = 1.25 mm, p = 70 kN/m2Plate diameter = 300 mm____________________________________

k = 0.43 x 70 x 10762 3 1.25

= 2.41 x 10 4 CBR = 24%

4.15 The test is laborious to set up and carry out,and requires a lorry or excavator to provide reaction. The speed of loading is slow giving poor simulation oftraffic loading.

D

4.pltostDw

m

rathstexdem

teasELECTRONIC COPY - NOTJanuary 1994 PAPER COPIES OF THIS ELECTRO0 100 200 300 400 500 600 700Plate Diameter (mm)

FIGURE 4.4 Correction for Smaller Plates

ynamic Plate Tests

16 These tests involve dropping a weight onto aaten. Usually a damping mechanism is incorporated control the loading time. Thus the area of loading,ress and speed of loading may be controlled. Theynaplaque measures the rebound of its sprungeights. The Falling Weight Deflectometer (FWD)easures the stress applied and the deflection at severaldial positions. Interpretation is generally in terms ofe stiffness modulus of each layer but is notraightforward and should be carried out by anperienced pavement engineer. If only the centralflection is used to determine a composite stiffnessodulus for the foundation (as for the plate bearingst), then interpretation is simple and can be carried out described above. FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 4/3

Volume 7 Section 2 Chapter 5Part 2 HD 25/94 References and Bibliography

BLIOGRAPHY

dated

889; Black WPM and Lister NW, "The Strength ofy Fill Subgrades: Its Prediction in Relation to Road

cification for Highway Works, (MCHW 1).

hway Construction Details (MCHW 3).

liography

95. REFERENCES AND BI

References Un

1981

BS5930; "Code of Practice for Site Investigations",BSI.

1984

LR1132; Powell W D, Potter J F, Mayhew H C andNunn M E, "The Structural Design of Bituminous LRRoads", TRRL. Cla

1987

Blood J D and Lord J A, "Formation Earthworks,Capping and Drainage Design and Specification", Proc RRNational Workshop on Design and Construction of TePavement Foundations, pp 47-66. TR

1989 198

BS812; Part 124; "Method for Determination of Frost CRHeave", BSI. "R

1990 198

BS1377; Part 4; "Compaction Tests", BSI. Jon

BS1377; Part 9; "In-situ Tests", BSI. Ag

BS812; Part 111; "Methods for determination of ten per Joncent fines value (TFV)", BSI. Pav

BS812; Part 113; "Methods for Determination of theAggregate Abrasion Value (AAV)", BSI.

HA41 (DMRB 4.2) "Permeability Testing ofAggregates".

1991

HA44 (DMRB 4.1.1); "Earthworks: Design andContract Documents".

1994

HD26 (DMRB 7.2.3) Pavement Design.

Spe

Hig

Bib

197

Per

198

Co

AgELECTRONIC COPY - NOTJanuary 1994 PAPER COPIES OF THIS ELECTRO

HD30 (DMRB 7.3.3) Structural Assessment Procedure.45; Sherwood P T and Rowe P G, "Winter Airmperatures in Relation to Frost Damage in Roads",RL

7

72; Brown S F, Loach S C and O'Reilly M P,epeated Loading of Fine Grained Soils", TRRL.

9

es H A and Jones R H, "Horizontal Permeability of

gregates in Roads, Nottingham.

es R H and Jones H A, "Granular Drainage layers inement Foundations", Proc Int Symp Unbound

formance", TRRL.

6

mpacted Aggregates", Proc Int Symp Unbound

gregates in Roads, Nottingham. FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 5/1

Volume 7 Section 2 Chapter 6Part 2 HD 25/94 Enquiries

ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYJanuary 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 6/1

6. ENQUIRIESAll technical enquiries or comments on this Part should be sent in writing as appropriate to:-

Chief Highway EngineerThe Department of TransportSt Christopher HouseSouthwark Street T A ROCHESTERLONDON SE1 OTE Chief Highway Engineer

The Deputy Chief EngineerThe Scottish Office Industry DepartmentRoads DirectorateNew St Andrews House J INNESEDINBURGH EH1 3TG Deputy Chief Engineer

The Director of HighwaysWelsh OfficeY Swyddfa GymreigGovernment BuildingsTy Glas RoadLlanishen K J THOMASCARDIFF CF4 5PL Director of Highways

Chief Engineer - Roads ServiceDepartment of the Environment for Northern IrelandRoads Service HeadquartersClarence Court10-18 Adelaide Street W J MCCOUBREYBELFAST BT2 8GB Chief Engineer - Roads Service

CONTENTSINTRODUCTIONSUBGRADE ASSESSMENTCAPPING AND SUB-BASEIN-SITU TESTINGREFERENCES AND BIBLIOGRAPHYENQUIRIES

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