Gravel Road Building Myth versus Science

Transcription

1 Local Government Supervisors Association of Western Australia Gravel Road Building Myth versus Science Brendan Scott

2 Brendan Lecturer in Geotechnical Engineering, School of Civil, Environmental and Mining Engineering, The University of Adelaide PhD thesis on rolling dynamic compaction, used for ground improvement and thick lift compaction 10+ years of working on compaction projects around Australia Thanks for having me talk to the LGSA WA. 2

3 Road Pavements Common forms of road construction in rural areas: Sprayed seals on natural gravel Unsealed natural gravel roads WEARING SURFACE Spray seal (if present) Upper surface of base course is wearing surface for unsealed roads Durable layer that resists genera:on of loose surface material from tyres Prevents water from entering pavement Resistant to corruga:ons Is not excessively slippery in wet condi:ons Some plas:city to provide cohesion is needed, but too much plas:city leads to potholes and rubng.

4 Base Course Common forms of road construction in rural areas: Sprayed seals on natural gravel Unsealed natural gravel roads BASE COURSE Is the wearing surface for unsealed roads Structural support of traffic loading Constructed of higher quality materials (upper part of pavement where performance criteria are higher) Thickness of layer important to reduces stresses and strains on subgrade

5 Sub base Common forms of road construction in rural areas: Sprayed seals on natural gravel Unsealed natural gravel roads SUB BASE Is an inferior (cheaper) material than base course Aim is to reduce thickness of base course layer (cost) Typically wider grading limits and lower strength than base course layer Relaxed specifica:on dictates more materials are acceptable for use as sub base Reduces stresses and strains on subgrade

6 Subgrade Gravel pavement layers provide important role: Limit stresses and strains in subgrade due to wheel loading Prevent strength failures of subgrade Prevent densification and excessive deformation of subgrade SUBGRADE Material below sub base layer OOen is natural ground

7 Pavement Design The support provided by the subgrade is a critical factor in determining the pavement design thickness, composition and performance. The subgrade strength is dependent on the conditions at construction and during service and is governed by the: soil type; its density, and its moisture content. Typical measure of subgrade support: California Bearing Ratio, CBR;

8 California Bearing Ratio - CBR The test is usually carried out: on a remoulded specimen, compacted, in a standard mould, to the optimum moisture and density specified for the material to be placed in the field, and soaked for 4 days. The test can also be carried out on the undisturbed material in the laboratory or in the field.

9 California Bearing Ratio - CBR Note that the CBR need not be less than 100%. The design of pavements are usually based on the CBR value. Poor pavement soils, such as highly plastic clays, have a low CBR, e.g. 2%, Good pavement soils have a high CBR, e.g. 60% and over.

10 California Bearing Ratio - CBR

11 Material Properties that affect performance In general, granular materials, such as gravels, cobbles and crushed rock are the most suitable geotechnical materials for pavement construction. Good compaction characteristics Can achieve a high density Good drainage properties, pervious Durable Won t compress or expand with changes in moisture. Clays, silts, organic materials are far less suitable.

12 Material Properties that affect performance Material properties that control pavement performance include: Particle Size Distribution Plasticity (fines content) Moisture content Density All of the above factors influence the strength and stiffness

13 Material Properties that affect performance Strength and stiffness are influenced by: Wetting and drying history Compaction history Why is Strength and Stiffness are important: Ability to carry load Resist progressive failure due to repeated loading Retain strength over time and Retain strength due to changes in environmental conditions (good drainage needed) Adequate stiffness to avoid large strains (deformations)

14 How to test for strength and stiffness? California Bearing Ratio (CBR) Common Lab test Field version (much less common) Used to test subgrades, base course and sub base materials Widely used Need to selected target density for testing Test is sensitive to particle breakdown Large particles can be problematic (test is designed for 19 mm minus materials) Clegg Hammer Lab and field versions 4.5 kg hammer is dropped onto a soil from a height of 460 mm Deceleration is measured to give a Clegg Impact Value Larger the value the better the material (higher strength/stiffness)

15 How to test for strength and stiffness? Falling Weight Deflectometer (FWD) Commonly used field test to infer soil stiffness Typically used after construction Also used to assess existing road pavements Need to know layer thickness to obtain reliable stiffness estimates Dynamic Cone Penetrometer Commonly used field test Cheap and simple, no need for the gym! 9kg hammer falling 510 mm (Australian version) 8kg hammer falling 575 mm (South African version) Results typically used to infer CBR Can lack reproducibility Equipment can get damaged easily

16 Pavement Design Traffic Loading An essential part of any pavement design is the quantity, type and distribution of traffic which the pavement will carry. The required life of the pavement, as well as the anticipated growth of traffic, also influence pavement design. Features of traffic that largely determine pavement performance are: the number of axle passes, the axle loadings and the axle configurations.

17 Pavement Design Traffic Loading For all pavements, performance is influenced only by the heavy end of the traffic spectrum. In the design of road pavements, no account need be taken of cars and light commercial vehicles as far as loadings are concerned, though their existence may affect road capacity.

18 Roads in WA Vast rural road network Small population in rural areas Need for cost-effective road construction Arid or semi-arid climate (most of WA) Road materials affected by local geology Pedocretes (lateritic gravel, ferricrete, calcrete, silcrete)

19 Roads in WA Natural gravel material Produced without crushing Some removal or breakdown of over-size material Cost effective source for base course or subbase pavement layers Greater variability in material composition Risks associated with variability can be mitigated by quality control, or modifying materials with addition of sand, lime or cement. Crushed Rock material Hard rock material produced from manufactured crushing process Consistent quality Materials behave in predictable and well understood manner More costly than naturally occurring gravel material 19

20 Road Construction Materials Examples of a few common base course and sub base materials: Laterite gravels So-called as they have been enriched by aluminium and iron oxides during weathering process Well graded Similar strength characteristics as crushed rock Good durability Typically sought after material for road construction Can vary in quality

21 Road Construction Materials Examples of a few common base course and sub base materials: Crushed Lateritic Caprock Crushing massive lateritic caprock is common in south-west region of WA Ripping caprock with dozer, breaking up boulders using a rock breaker before material is crushed Need to ensure roots and organic matter are removed from caprock prior to crushing

22 Road Construction Materials Examples of a few common base course and sub base materials: Tamala Limestone Coastal limestone found from Augusta to north of Geraldton. Typically ripped by a dozer and crushed under the dozer tracks (or in a jaw crusher) to produce material grading suitable for road construction. Widely used as a sub base material; often bitumen stabilised for use as a base course Can gain strength when compacted at optimum moisture content and dried back

23 Physical Properties that affect material performance Based on their grain size, soils can be divided into 2 classes: Coarse grained; and Fine grained soils. Examples of each? Coarse grained sand, gravel, cobbles, boulder; Fine grained clay, silt, organic.

24 Fine-grained Soil Particle size smaller than mm The behaviour of fine grained soils depends on: the minerals present in the soil mass, and also the amount of water present

25 Fine-grained Soil Fines provide cohesion Having clay fines content in road pavements is desirable (as it helps to seal the surface) Too many fines, material will lose strength rapidly with increased moisture content Organic material is highly undesirable High silt content also undesirable

26 Fine-grained Soil

27 Coarse-grained Soils Particle size larger than mm The behaviour of coarse grained soils is governed by: shape; size; relative density and distribution of particles; and only rarely by its mineral composition.

28 Coarse-grained Soils Both soils and rocks are highly variable A soil mass is generally a 3 phase system that consists of solid particles, liquid (e.g. water) and gas (e.g. air) The strength and behaviour of a soil is governed by the state of the soil University of Adelaide 28

29 Physical Properties that affect material performance

30 Coarse-grained Soils Sieves:

31 Particle Size Distribution (Grading) Grain Size Distribution: Fines (Clay + Silt) Sand Gravel + Cobbles + Boulders

32 Particle Size Distribution (Grading) Well graded Smaller par:cles fit in between larger par:cles (smaller voids, dense soil) Poorly graded Deficiency in smaller par:cles will create a harsh and permeable 32

33 Particle size and shape Largest particle size affects workability and stability. Large particles help provide strength, but if particles are too large then surface finish is compromised. Angular shaped particles are more desirable than rounded as there is greater friction between particles.

34 Compaction Compaction is the densification of soils by the application of mechanical energy. The objectives of compaction include: Increase strength; Reduce settlement; Reduce permeability; and Control the potential for swelling and shrinkage.

35 Compaction Theory Max. Dry Density (MDD or ρ d max ) (Max. Dry Unit Weight, γ d max ) Op:mum Moisture Content (OMC or w opt )

36 Physical Properties that affect material performance

37 Compaction Laboratory Proctor Testing

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39 Compaction Theory MDD = 18.8 kn/m 3 PASS FAIL EXAMPLE SPEC = 98% of Standard MDD =18.8 * 0.98 =18.4 kn/m 3

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41 Compaction Impact Rollers Deeper influence zone than conventional circular drum rollers due to dynamic effects and higher energy output Can place materials efficiently due to thicker lift compaction and km/hr operating speed Can break down surface rock

42 Road Maintenance Periodic replenishment and maintenance grading of unsealed roads is required to reinstate shape loss 42

43 Road Maintenance Primary goal is to keep water drained away from the road. Standing water is a major reason for distress and failure. Gravel roads can fail (ruts or pot holes form) when exposed to heavy loads in wet conditions. 43

44 References Cocks et. al (2015). The use of naturally occurring materials for pavements in Western Australia, Australian Geomechanics Journal, Volume 50, No. 1, March (OUTSTANDING REFERENCE PAPER) Main Roads WA (2003). A guide to the selection and use of naturally occurring materials as base and subbase in roads in Western Australia, Main Roads Western Australia Report 18M, Issue 13 June U.S. Department of Transportation (2015). Gravel roads Construction & Maintenance Guide, Federal Highways Administration. 44

45 Acknowledgements Thank you for listening The University of Adelaide Broons 45