Lecture 3 - Pavements.pdf

Department of Civil, Environmental and Mining Engineering Pavements Engineering Introduction Introduction to pavement engineering Pavement applications Pavements are a fundamental structure covering wide number of appli...

Department of Civil, Environmental and Mining Engineering Pavements Engineering Introduction Introduction to pavement engineering Pavement applications Pavements are a fundamental structure covering wide number of applications – Footpaths and cyclepaths – Residential streets – Minor rural roads – Major roads – Airports – Container terminals – Industrial floors – Mine haul Complex structures, poorly understood with developing design methodologies Subgrade Subgrade is what pavements are built on Subgrade is any natural soil that: – Exists in situ – Or is imported in an embankment situation – But is not a pavement quality material – Subgrade is a road term for foundation Types of pavement Rigid pavements – Most commonly concrete pavements jointed plain (unreinforced) concrete pavements (PCP) jointed reinforced concrete pavements (JRCP) continuously reinforced concrete pavements (CRCP) steel fibre reinforced concrete pavements (SFCP). – Applied extensively in New South Wales for roads – Often applied for heavy duty pavements Design method for roads and heavy duty are different Types of pavement (cont) Flexible pavement – May be many materials and forms Unbound granular – unsealed granular – spray seals on granular – thin asphalt on granular Bound – deep strength asphalt – full depth asphalt – cement stabilised – bitumen stabilised Segmented block pavements Rigid pavement terminology Austroads 2004 Flexible pavement terminology Austroads 2004 Load response of rigid pavement Austroads 2004 Subgrade stress spread over large area Load response of flexible bound pavement Austroads 2004 Subgrade stress spread over smaller area  Load dispersion flexible unbound pavements Austroads 2004 Granular material modulus depends on stress condition Modulus decreases with depth Typical pavement cross-sections Common in WA Boxed construction typical urban profile Kerb and channel Common in Victoria and NSW Original surface Formation construction typical rural profile Typical pavement cross-sections Cutting construction rural profile Slope construction rural profile Ground water and surface water controls very important Terminology Pavement – Collective material placed on subgrade to support (vehicular) traffic Subgrade – formation on which pavement is constructed, may be natural, modified or selected fill Formation – surface of finished earthworks Shoulder – part of carriageway beyond traffic lanes, but contiguous and flush with pavement surface Verge – unpaved parts of road reserve, forms service corridor Terminology (cont) Sub-base – first layer of structural pavement material required to prevent excessive subgrade deformation, often low quality material Basecourse – upper part of the pavement structure on which the surfacing layer if used is placed. This may be granular, bound or rigid. Surface/wearing course – to resist abrasion from traffic and or minimise entry of water. May be: asphalt or bituminous spray seal or in low volume rural roads granular or modified granular material. Terminology (cont) Asphalt: Mix of bitumen, aggregate and filler manufactured in a specific plant and laid hot. (All asphalt plants heat aggregate) – Dense Grade Asphalt: continuously graded to provide maximum density – Gap Grade Asphalt: one aggregate size removed to allow higher binder content – Stone Mastic Asphalt: coarse aggregate and filler to allow very high binder content – Open Grade Asphalt: coarse aggregate and bitumen with high voids and permeability  Asphalt Bitumen tanks Conveyor Heating and Asphalt paving Aggregate hoppers mixing drum Control room Asphalt compaction Aggregate bins Spray Seal Hot bitumen or bitumen emulsion sprayed on surface Single sizes aggregate spread over bitumen Department of Civil, Environmental and Mining Engineering Pavements Engineering Pavement drainage Pavement drainage Pavement drainage is a vital engineering consideration Drainage is required to control – Water that fall onto the pavement as rain – Water that may drain onto the pavement from higher ground – Water that may cross the road as a water course – Water that may flow underground and affect lower pavement layers Rural roads Table drains: – Intercept water from high ground – Drain water from pavement away from structure Effective Ineffective Rural Roads Mitre drains – Drain water from table drains to soak away remote from road Material selection Batters should resist erosion – material selection and batter slope Culverts A good culvert passes water to carry a design flow event An excellent culvert passes water to carry a design flow event and allows for aquatic life Culverts And even better, allows for land animal crossings too Even if there is no water Subsoil Drainage Where groundwater is high – Soil suction draws water into pavement – Subsoil drainage is designed to lower water levels – But must have a natural outlet or pumping system Always remember that they need maintenance Cut off drains Cut off drains may be required to control slope scour Subsoil drains may need to be provided to control capillary rise Urban drainage Catch pits divert rainwater to pipe systems Department of Civil, Environmental and Mining Engineering Pavements Engineering Subgrade Geotechnical investigation Geotechnical and pavement design Pavement design - part of geotechnical engineers role including: – selection of road alignment/location of structures – fixing road profile and minimum batter slopes – materials selection – designing the road pavement – designing foundations for structures – design of earth retaining structures – designing subsurface drainage systems – designing of ground improvement works – predicting magnitude and rate of settlement – Feeding all data to assist geometric design Subgrade support The most critical part of pavement design – determines pavement thickness Characterisation of subgrade strength – California Bearing Ratio (CBR) rigid pavements empirical flexible pavement design CBR is a measure of shear resistance – Elastic properties: modulus, shear strength, Poisson’s Ratio mechanistic flexible pavement design Factors affecting subgrade strength subgrade variability sequence of earthworks construction field density and moisture content achieved moisture changes during service life subsurface drainage; depth to the water table weak layers below the design subgrade level. Subgrade variability On large projects, subgrade strength will vary – Project should be divided into homogeneous sections A B A Fill C B D Fill CBR C CBR A CBR CBR B CBR B + CBR C CBR A + CBR B CBR C + CBR D CBR Fill + CBR B CBR Fill + CBR B Compaction conditions – Variation of CBR with density/moisture content for clayey sand – CBR test must specify density, moisture content and surcharge – CBR testing is usually undertaken under soaked condition 95% of the standard maximum dry density Moisture changes during service Unsealed pavements – effected by rainfall, evaporation, water table – rate of change depends on permeability of base – magnitude of change depends on subgrade properties Sealed pavements – central zones reach stable equilibrium unless effected by seasonal water table variations – outer zones effected by seasonal climate conditions – magnitude of change determined by subgrade properties, roadside geometry Weak layers below subgrade Presence of weak layers below top of subgrade must be accounted for – CBR testing should continue at least 1m below top of subgrade where doubt exists – Japan method is commonly used to determine effective subgrade CBR: ∑ ℎ 𝐶𝐵𝑅𝑖. – CBRmax = ≤ 20 when cement stabilised ∑ ℎ𝑖 Layers must be decreasing CBR Layers must be greater than 0.2m thick Total hi = 1.0m Example Japan Method Assume 3 layered subgrade, – layer 1: h1 = 0.2m CBR1 = 8% – layer 2: h2 = 0.5m CBR2 = 4% – layer 3: h3 = 0.3m CBR3 = 2% ∑ ℎ𝑖𝐶𝐵𝑅𝑖. 𝐶𝐵𝑅 = ∑ ℎ𝑖 0.2 × 8. + 0.5 × 4. + 0.3 × 2. = = 3.8% 0.2 + 0.5 + 0.3 CBR with layers Example of CBR 200 mm CBR 10% 500 mm CBR 5% Use Japan method – layer 3 = 1000-500-200 = 300 900 mm CBR 2% 400 mm CBR 2% CBR = 2% 500 mm CBR 5% 200 mm CBR 10% 500 mm CBR 5% Use Japan method - Top 2 layers only 900 mm CBR 12% Stabilised subgrades & Japan method When a weak subgrade is stabilised, the following method applies: – the maximum value of CBR is limited to 20% – the lower 0.2m of stabilised is taken as the average of the original CBR and the stabilised CBR – the remaining material below the stabilised layer up to 1m depth is assigned the original CBR – the Japan method is then applied as before Example Japan Method with stabilisation Assume deep subgrade CBR 2% stabilised to 0.4m with lime to a CBR of 12% – layer 1: h1 = 0.4m - 0.2m = 0.2m CBR1 = 12% – layer 2: h2 = 0.2m CBR2 = (12+2)/2 = 7% – layer 3: h3 = 1.0m – 0.4m = 0.6m CBR3 = 2% 𝐶𝐵𝑅 ∑ ℎ𝑖𝐶𝐵𝑅𝑖. = ∑ ℎ𝑖. 0.2 × 12 + 0.2 × 7. + 0.6 × 2. = = 2.9% 0.2 + 0.2 + 0.6 Japan method for heavy duty pavements For road pavements – effective depth is usually taken as 1 m below top of subgrade – wheel loads are relatively light – axle loads are regulated For industrial pavements – effective depth may be 2 to 3 m below top of subgrade – wheel loads can be very high – no regulations for maximum axle loads Department of Civil, Environmental and Mining Engineering Pavement Engineering Subgrade classification Methods to section subgrade Subgrades are a factor of natural soils – not constant – changes in soil classification and properties are to be expected Ensure changes in subgrade are noted. This may be achieved by: – reference to geological mapping – systematic sampling to at least 2m below finished subgrade – close space sampling to identify boundaries in soil types – identifying sources of fill and characterising materials – identifying water table and effects on subgrade Required properties Testing should include – Particle Size Distribution – Atterberg Limits Liquid Limit (Cone penetrometer) Plastic Limit Linear shrinkage – California Bearing Ratio (CBR) Methods to determine subgrade CBR Insitu tests – Insitu CBR Test (surface level only) – Dynamic Cone Penetrometer (up to 2.5m) – Static Cone Penetromer (up to 1m) – Electric friction cone penetrometer (good for depth) – Clegg (impact) hammer (surface level only) – Falling Weight Deflectometer Laboratory tests – Undisturbed CBR – Remoulded CBR (unsoaked, soaked, EMC) – Triaxial test at range of moisture contents Insitu and Laboratory CBR tests CBR test method Principle – sample compacted in a 152mm diameter 175mm high mould at design density and moisture – for soaked test, sample is soaked 4 days – a surcharge mass (with hole for piston) is placed equivalent to pavement mass – a 49.6 mm diameter piston is driven into a soil sample at 1mm/min – load readings are recorded at 0.5mm intervals or continuously if automated CBR test calculation CBR Determination – plot load vs penetration – if curve initially concave up, correct by extending straight line to axis – determine corrected loads at 2.5mm and 5.0mm penetration in kN – CBR2.5 = P2.5/13.2kN – if CBR5.0 = P5.0/20.0kN > CBR2.5, repeat test – if CBR5.0 = P5.0/20.0kN > CBR2.5, on repeat – then CBR = CBR5.0 otherwise CBR2.5 Large variability between laboratories testing same samples is common – Engineers must apply judgement  No correction CBR Determination required Considering curve 3: Load on piston (kN) Requires adjustment due to initial shape CBR 5.0 CBR 2.5 Corrected for shape No correction CBR = max (CBR 2.5/13.2kN: required CBR 5.0/20.0kN) Penetration (mm) Corrected 0 Corrected 5.0 Corrected 2.5 Test reports A laboratory CBR test report should state: – Maximum Dry Density standard or modified – Optimum Moisture content – Relative density of test sample this is the density specified for subgrade compaction – Moisture ratio at which test is undertaken – Surcharge mass this is the equivalent to the depth of cover over the layer – Period of soak sample is prepared and usually soaked for a period – Moisture ratio after test – Swell of material during test CBR test precautions Some polymer materials need a drying cycle before they “set up” – Allow the material to dry back before testing – Test dry and soaked Some examples are Polycom, PolyRoad and soiloc Soak period Laboratory CBR are soaked for a period – too often a 4 day soaked is used without thought Static cone penetrometer Direct dial gauge reports Qc (kg/cm²) Qc ≈ (2.5 to 3.3)CBR Dynamic Cone Penetrometer 9kg weight Consists of 16mm rod with 20mm tapered cone – A 9kg weight set to drop 510mm – Penetration is measured in mm/blow mm – CBR = 315.2x-1.14 where x = mm/blow drop Applicable to fine grained soils (silts and clays) 16mm shaft  20mm cone Electric Friction Cone Penetrometer 36mm cone hydraulically driven into soil Measurments – tip resistance qc – sleeve friction fs Calculations – Friction ratio Rf = qc/fs x 100 – Friction ratio gives soil type – CBR ≈ 4.5qc (qc in kN) (36mm cone) Soil types by EFC Clegg Hammer 4.5 kg hammer with accelerometer to measure impact deceleration Measurements – Clegg Impact Value CIV Calculations – CBR ≈ 0.06CIV² + 0.52CIV + 1 Gives surface value only Falling Weight Defectometer Falling Weight Deflectometer Measures half deflection bowl by dropping weight onto pavement and measuring deflection at intervals Backcalculation programs determine modulus of layers including subgrade – EFromD3 (ARRB Group) – ELMOD (Dynatest) – Rubicon (South Africa) CBR ≈ Modulus/10 – Treat results with caution – location of boundaries? Calculated CBR from grading Applicable to fine grained materials only (75% passing 2.36mm) Represents soaked CBR @ 95% MDD (modified) In following: – P2.35 = %age passing 2.35mm sieve – P0.425 = %age passing 0.425mm sieve – P0.075 = %age passing 0.075mm sieve – LS = Linear shrinkage – PI = Plasticity index Calculating CBR from grading (NAASRA) Calculate 1st estimate log 𝐶 𝐵𝑅 = 1.688 − 0.00506𝑃. + 0.00186𝑃. − 𝐿𝑆 0.0168 + 0.000385𝑃. Calculate 2nd estimate log 𝐶 𝐵𝑅 = 1.886 − 0.00372𝑃. − 0.0045𝑃. 𝑃. 𝑃. + 5.15 − 0.0456 10 − 0.0143𝑃𝐼 𝑃. 𝑃. CBR ≈ (3CBRmin + CBRmax)/4 Repeat load triaxial testing Repeat load triaxial testing gives modulus values under a range of stress conditions – vertical load and confining stress vary throughout test – modulus is reported at range of stress conditions – undertaken at range of moisture conditions – applicable to mechanistic pavement designs Department of Civil, Environmental and Mining Engineering Pavement Engineering Subgrade working Moisture balance Edge effects on outer wheel path can be critical Based on Austroads 2004 Selecting CBR for moisture conditions Moisture conditions – Equilibrium: balance of all factors where conditions remained constant most likely towards centre of pavement – In-service: moisture regime affected by seasonal changes not constant applicable to OWP – Design: the critical in-service moisture content applicable to OWP Selection of design CBR (cont) CBR testing should be undertaken by 3 different methods – Field testing should be undertaken under worst climate conditions (end of wet) – Look for anomalies and try to explain. – If anomalies cannot be explained, take lowest value – If values appear too extreme, use conservative judgment Sectioning pavement Divided into sections based on: – soil types – similar CBR test results Design CBR for sections may be determined by: – statistical: for low traffic pavements CBR = average CBR – k x standard deviation – lowest reasonably confirmed CBR for heavy duty pavements Refer MRWA Road Note 8 for statistical methods Subgrade strength strong subgrades – various agencies adopt maximum CBR – maximum allowable values range from 10 to 15 – recommendation sands CBRmax = 10 sub-base quality subgrades CBRmax = 15 soft subgrades – CBR < 5 at construction, compaction of subsequent layers may be compromised – CBR < 3 at construction, pre-treatment or a working platform is required. Presumptive CBR values may be applicable to low volume roads or preliminary design Typical presumptive design CBR values (Austroads 2004) Description of subgrade Typical CBR values (%) Material USC class good drainage fair to poor drainage Heavy plastic clay CH 5 2-3 Silt ML 4 2 Silty clay CL 5-6 3-4 Sandy clay SC 5-6 3-4 Sand SW, SP 10-15 5-10 Treatment of soft subgrades Methods for constructing on soft subgrades: – draining and drying subsoil drains open drains – excavate and replace with stable material – stabilisation of top of existing subgrade – use of geotextile Lime stabilisation Improves stiffness Improves permeability and reduces water sensitivity Lime demand test used for determining application rate – Test method RMS T144 Hydrated lime for road construction materials (Lime demand test) – Lime added to sample to stabilise at pH 12.4 Design method for subgrade equivalency: – Determine CBR of fully stabilised material Assign top of stabilised thickness less 200 mm to this CBR – Assign lower 200 mm of stabilised layer as average of stabilised CBR and existing CBR – Assign lower 1000 mm – stabilised layer as existing CBR – Apply Japan method Geotextiles Geofabrics: – provide separation to prevent mixing of fines from subgrade with pavement materials – provide tensile reinforcement low modulus but must be pre rutted to develop the tensile strain Geogrids – provide reinforcement only and used in conjunction with gravel or rock capping high modulus gravel interlocks in the spaced of the grid activates with minimal strain Composites – geofabric and geogrid provide separation and tensile reinforcement. Drainage layer Drainage layers are used to separate moisture exchanges between pavement and subgrade – can be purpose made geocomposite – constructed with geofabrics and coarse aggregates – need to have drainage outlet – recent testing shows crushed glass is a good capillary rise blocker Expansive subgrades Cause significant shape loss to flexible pavements Cause structural damage to rigid pavements Guide to Classification of Expansive Soils (Austroads 2004) Expansive Liquid Limit Plasticity PI x %70 >45 >3200 >5 high >70 >45 2200-3200 2.5-5 moderate 50-70 25-45 1200-2200 0.5-2.5 low 6, 1< Cc