Road Soil. Curtis F. Berthelot Ph.D., P.Eng. Department of Civil Engineering. Road Soil Introduction

Transcription

1 Road Soil Characterization ti By: Curtis F. Berthelot Ph.D., P.Eng. Department of Civil Engineering Road Soil Introduction Roads are constructed of layered heterogeneous multiphase geo-materials that exhibit internal friction, cohesion, dilation, and viscoelastoplastic behavior under load. Behavior is often influenced by confinement because the material is non-linear stress dependent. Several methods have evolved to quantify performancerelated properties or road soils. 2 1

2 Granular Pavement Structure Asphalt wearing coarse Granular Base Subbase Subgrade Subgrade: in-situ material used to construct grade. Subbase: select material (usually low quality granular) that is borrowed from a pit and placed over the subgrade. Base: high quality granular layer placed directly under asphalt concrete. Wearing coarse : asphaltic or hydraulic bound aggregate 3 PCC Cross Section Portland Cement Concrete Granular Base Subgrade Subgrade: in-situ material used to construct grade. Subbase: none. Base: high quality coarse crush with high air voids for drainage. Wearing coarse : Portland Cement Concrete 4 2

3 Road Soil Characterization Physical properties: Grain size analysis (AASHTO T27, ASTM C136) Sand equivalent (AASHTO T176) Classification (ASTM D3282): Unified Soil Classification System AASHTO 5 Road Soil Characterization Phenomenological properties: Atterberg limits and plasticity index (ASTM D4318, AASHTO T89, AASHTO T90) Proctor water-density (AASHTO T99, ASTM D 698) California bearing ratio (AASHTO T193, ASTM D1883) Hveem Resistance 6 3

4 Grain Size Analysis 100 Percent Finer Than Grain Size (mm) Clay Subgrade (Hydrometer Sieve) Clay Subgrade (Mechanical Sieve) Clay Till Subgrade (Hydrometer Sieve) Clay Till Subgrade (Mechanical Sieve) Subbase (Mechanical Sieve) Granular Base (Mechanical Sieve) 7 Atterberg Limits Introduction Water significantly affects behavior of fine grained soils Different fine grained soils behave differently How can different fine grained soils be categorized? 8 4

5 Soil Classification Most common methods: USCS: Unified Soil Classification System. AASHTO: American Association of State Highway and Transportation Officials. 9 USCS Soil Classification Four main groups of soils: 1. Coarse-grained 2. Fine-grained 3. Organic soils 4. Peat 10 5

6 USCS Soil Classification Coarse-grained: less than 50% of soil passes No. 200 sieve includes sands and gravels Sands: more than 50% passes No. 4 sieve Gravels: more than 50% retained on No. 4 sieve With or without fines Little or no fines: clean Well graded/poorly graded Well graded: distribution of all sizes Poorly graded: one sized particles 11 USCS Soil Classification Fine-grained: more than 50% of soil passes No. 200 sieve Further subdivided by liquid limit, organic content, and silt or clay behavior Liquid limit: greater than or less than 50? Silty behavior, or clayey behavior? Organic soils and peat: visually identifiable 12 6

7 USCS Soil Classification Letter system for soil classification: G Gravel S Sand M Silt C Clay W Well Graded P poorly graded U uniformly graded L low liquid limit H high liquid limit 13 Group symbol GW GP GM GC SW SP SM SC ML CL OL MH CH OH Pt USCS Soil Classification ASTM D 2487 Group name well graded gravel, fine to coarse gravel poorly graded gravel silty gravel clayey gravel well graded sand, fine to coarse sand poorly-graded sand silty sand clayey sand silt clay organic silt, organic clay silt of high plasticity, elastic silt clay of high plasticity, fat clay organic clay, organic silt peat 14 7

8 USCS Soil Classification CV Plasticity Index CI CH 10 0 CL-ML ML CL Liquid Limit (%) 15 AASHTO Soil Classification Group Index (GI): Used to further evaluate soils within a group Based on service performance of soils Range from 0 to 20 GI = (Fines-35)[ (LL-40)]+0.01(Fines-15) (PI-10) Fines = % passing 0.075mm sieve LL = Liquid limit PI = Plasticity Index 16 8

9 Sand Equivalent Introduction: A small amount of clay can impact soil behavior significantly Clay slippery, cohesive Important to quantify amount of clay in aggregate 17 Sand Equivalent Used as a rapid field test to determine relative amount of plastic fines in an aggregate. g A specific amount of material (< 5mm maximum size) mixed with water/calcium chloride solution and allowed to settle for a set time. Clay level and sand level are measured and SE value is calculated. SE < 40: poor aggregate. 18 9

10 Sand Equivalent Ref: Hot Mix Asphalt Materials, Mixture Design and Construction, Proctor Moisture-Density Compaction improves engineering properties of soils Minimizes settlements in road surface. Increases soil strength. Increases bearing capacity. Helps control volume change. Amount of moisture in the soil affects compaction

11 Proctor Moisture-Density Using specific amount of compaction effort, density and moisture content varies. Test used to determine optimum moisture content to get maximum soil density for a level of compaction energy. Standard Proctor is minimum acceptable. Modified is preferred 21 Proctor Moisture-Density 22 11

12 Proctor Moisture-Density 23 Proctor Moisture-Density Dry Density (kgs/m3) Gravimetric Moisture Content(%) GranularBase Sand Clay Till Silt Clay 24 12

13 Developed in late 1920 s. California Bearing Ratio (CBR) Used by highway departments for evaluation of road soils. Bearing ratio of soils determines amount of load that soil can carry. Saskatchewan Highways uses bearing ratio for pavement design. Higher soil bearing ratio = thinner pavement, base, or sub-base layers = less $$$. 25 California Bearing Ratio (CBR) Soil sample prepared according to Proctor compaction procedure. Sample is soaked for 4 days. Swelling during soaking is measured

14 California Bearing Ratio (CBR) 27 California Bearing Ratio (CBR) Determined by pushing a cylindrical piston with an area of 1935 mm 2 into a prepared soil sample at a standard rate of f mm/min. Applied pressure required to maintain the penetration rate is recorded at 2.5mm intervals up to a penetration depth of 12.5mm. Measures the relative shearing resistance of soil. Does not directly relate to field loading conditions, but it does provide a repeatable relative measure of soil strength

15 California Bearing Ratio (CBR) 29 California Bearing Ratio (CBR) P 49.6 mm Surcharge 200 mm 15 mm 1 Compacted Soaked Sample Crushed Rock CBR Penetration Pressure (mm) (MPa) mm dia

16 California Bearing Ratio (CBR) CBR is calculated as ratio of pressure at a given penetration increment during the soil test to that of a standard high-quality crushed rock. CBR P P 2.5mm to12.5mm penetration Crushed 2.5mm to12.5mm penetration 31 SDHT GI-CBR Correlation y = -3E-05x x x x x R 2 = CBR Group Index 32 16

17 Discussion Don t hold back. You have a great deal to contribute. Join in the spirit and camaraderie of making something exciting ii happen. There are many ways and places to make a living. We must do that, but each of us also desires to accomplish something more exhilarating and create lasting value through our efforts