I Subgrade Soils.pptx

Subgrade Soils

1. 1.SUBGRADE SOILS General 1.1 Soil Classification 1.2 Field Investigation 1.2.1 Depth of Investigation 1.2.2 In-place Tests 1.2.3 Groundwater table 1.2.4 Field Log 1.3 Laboratory Tests

2. 1. SUBGRADE SOILS Integral part of the pavement structure Desirable properties Stability Incompressibility Resistant to property change in adverse climatic conditions Good drainage Ease of compaction

3. 1.1 Soil Classification Objective Determining suitability for use Creating common understanding among professionals Unified Soil Classification System (USCS) Classifies granular soils on grain size characteristics Classifies fine-grained soils on plasticity properties

4. Divides soils into three general areas : USCS.pptx Coarse-grained soils (G,S) Fine-grained soils (M,C) Peaty soils (pt) Uses gradation coefficients( C U , C g ) to describe coarse grained soils. Subdivides fine grained soils using the plasticity chart, with PI given as : PI=0.73(LL-20)

5. B. AASHTO System Determines the relative quality of soils for use in embankments, sub grades, sub base and base course. Uses grain size distribution and Atterberg limits for classification Evaluates soils within each group using the group index formula as: GI=(F-35)[0.2+0.05(LL-40)] +0.01(F-15)(PI-10)

6. General Classification Granular materials (35 % or less passing No. 200) Silt-Clay materials (more than 35% passing No 200) Group Classification A-1 A-3 A-2 A-4 A-5 A-6 A-7 A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2 -7 A-7-5 A-7-6 Sieve analysis, per cent passing No 10 (2 mm) 50 max - - - - - - - - - No 40 (0.425 mm) 30 max 50 max 51min - - - - - - - No 200 (0.075 mm) 15 max 25 max 10 max 35 max 35 max 35 max 35 max 36 min 36 min 36 min 36 min Characteristics of fraction passing No 40 LL - - 40 max 41 min 40 max 41 min 40 max 41 min 40 max 41 min PI 6 max NP 10 max 10 max 11 min 11 min 10 max 10 max 11 min 11 min Usual types of significant Stone fragments, fine gravel, and sand Fine sand Silty or clayey gravel or sand Silty soils Clayey soils constituent materials General rating as subgrade Excellent to good Fair to poor AASHTO CLASSIFICATION TABLE

7. 1.2 FIELD INVESTIGATION Objectives Identification of engineering properties of sub grade materials Determination of construction material sources Estimation of seepage rates Information collected: Depth, thickness and properties of each soil layer Location of ground water table Depth to bed rock

8. Surface investigation( R/A) Geophysical methods Test pits or trenches Hand augers Boring test holes and sampling drill rigs

9. 1.2.1 Depth of investigation Design depth(D) The depth from the finished level to the depth that the load bearing strength has no effect on the pavement’s performance in relation to traffic load D≤ 1.50m for pits and borings

10. 1.2.2 in-place tests(R/a) SPT Vane Shear test Pressuremeter test

11. 1.2.3 groundwater table Determined by excavating holes Identified by installing piezometer when it is not initially determined .

12. Minimum materials testing frequency( Tanzania pavement design Manual) Road type Min number of indicative tests per km Min number of strength tests per km Paved trunk roads 4 2 Other paved roads 2 1 Gravel roads 2 1 per 2 km

13. 1.2.4 field log Must include Sample number, depth and type Field tests, depth and results Depth to layer thickness Field soil description

14. Test hole log

15. 1.3 LABORATORY TESTS Test Sample required Soil Disturbed/undisturbed Undisturbed Cohesive Granular Moisture content X x X Grain size X x X Relative density X x X Atterberg limits x x Unit weight X x X Unconfined compression X x X Direct Shear X x X Consolidation X x X CBR X X X Permeability X X

16. A. PI test The range of moisture content over which the soil is in plastic state B. Compaction/Proctor/ Test Gives the moisture-density relation of soils

17. Proctor apparatus ZAV CURVE

18. C. Field Density Test RC= ρ field /MDD*100% (> 80 %)

19. D. CBR Test Used to evaluate the strength of sub grade soil for pavement thickness design Calculated as: CBR % =B/A *100 % where, B = Load or pressure sustained by the specimen at 2.54 or 5.08 mm penetration, read from load- penetration curve, of the specimen. A = Load or pressure sustained by standard aggregates at the corresponding penetration level = 6.9 MPa for 2.54 mm penetration = 10.34 MPa for 5.08 mm penetration Measured after 4 days of soaking for wet or moderate climatic zones

20. A road section for which a pavement design is undertaken should be subdivided into sub grade areas( max 5) where the sub grade CBR can be reasonably expected to be uniform, i.e. without significant variations

21. Design CBR as the 90% ile value