1. SOIL MECHANICS AND CLASSIFICATION

2. FACTORS WHICH HOLD SOIL TOGETHER o Cohesion  Internal Friction

3. COHESION COHESION – the tendency of two particles to stick together – dependent upon surface area of particles – influenced by amount of water present

4. SOIL PARTICLE COHESION

5. Cohesive soils may exhibit plasticity Plasticity: the ability of soil to be deformed or molded without significant cracking or appreciable volume change Plasticity: the ability of soil to be deformed or molded without significant cracking or appreciable volume change

6. INTERNAL FRICTION INTERNAL FRICTION – Determined by geometric shape of soil particles – Influenced by weight of soil pressing down upon particles

7. SOIL FAILURE CAUSES o Gravity and Shear Forces o Trench Depth o Layered Systems o Water / Frost o Vibration o Soil Disturbance o Surcharge Loads

8. GRAVITY AND SHEAR FORCES

10. UNCONFINED COMPRESSIVE STRENGTH (UCS) The load per unit area at which soil will fail in compression May be determined by laboratory methods or field techniques

11. TRENCH DEPTH

12. LAYERED SYSTEMS

14. SOIL FAILURE CAUSES o Gravity and Shear Forces o Trench Depth o Layered Systems o Water / Frost o Vibration o Soil Disturbance o Surcharge Loads

15. CAVE-IN WARNING SIGNS –Bulging at bottom or center of trench wall –Appearance of tension (stress) cracks –Subsistence of soil near edge of trench

16. CAVE-IN WARNING SIGNS (cont.) –Water seepage –Spalling and Ravelling –Partial trench wall collapse

17. SOIL TYPES Four Texture Categories GRAVELSANDSILTCLAY

18. GRAVEL  Half of particles larger than 3/16” o No cohesiveness or plasticity o Does not hold water o Shape of particles may be angular or rounded

19. SAND  Particle size from 0.05mm to 2.0mm o Shape may be angular or rounded o Little or no plasticity o May exhibit apparent cohesion when wet

20. SILT  Particle size from 0.005mm to 0.05mm o Minimal plasticity o Minimal to moderate cohesion o Good shear strength under ideal conditions

21. CLAY  Particle size less than 0.005mm o High degree of cohesion, shear strength, and plasticity under ideal conditions o May vary from slurry to rock-hard o Best soil type for trench wall stability stability

22. OTHER SOIL CLASSIFICATIONS LOAM - Varying amounts of clay, sand, & silt - May contain organic matter HARDPAN and CALICHE - Cemented soils impervious to water MIXED SOILS - More than one classification type

23. SOIL MOISTURE STATE (Appendix A) DRYMOISTWETSATURATEDSUBMERGED

24. SOIL TESTING Determination of soil strength by manual & visual meansDetermination of soil strength by manual & visual means Design of protective systems dependent upon soil typeDesign of protective systems dependent upon soil type

25. SOIL TYPES STABLE ROCK TYPE A TYPE B TYPE C

26. STABLE ROCK Natural solid mineral material Natural solid mineral material Can be excavated with vertical sides intact Can be excavated with vertical sides intact No protective systems required No protective systems required

27. TYPE A SOIL Cohesive soil with Unconfined Compressive Strength (UCS) of 1.5 tsf or greater Cohesive soil with Unconfined Compressive Strength (UCS) of 1.5 tsf or greater High clay content, plasticity, and shear strength High clay content, plasticity, and shear strength Requires low level of protection Requires low level of protection

28. Limitations ( Cannot be classified as Type A if:) Soil is fissuredSoil is fissured Subject to vibrationSubject to vibration Previously disturbedPreviously disturbed Saturated or submergedSaturated or submerged Part of a layered system with layers sloping towards trenchPart of a layered system with layers sloping towards trench Judged by competent person to be unsafeJudged by competent person to be unsafe

29. TYPE B SOIL: UCS between 0.5 tsf and 1.5 tsf UCS between 0.5 tsf and 1.5 tsf Moderate clay content Moderate clay content Granular cohesionless soils such as angular gravel Granular cohesionless soils such as angular gravel Unstable or fractured rock Unstable or fractured rock

30. TYPE B SOILS (cont.): Flawed soils with UCS greater than 1.5 tsf, such as: Flawed soils with UCS greater than 1.5 tsf, such as: - Previously disturbed soils - Previously disturbed soils - Fissured soils - Fissured soils - Subject to vibration - Subject to vibration Some layered systems (slope less than 1V : 4H) Some layered systems (slope less than 1V : 4H)

31. TYPE C SOIL Least stable and most likely to collapse! Least stable and most likely to collapse! UCS less than 0.5 tsf UCS less than 0.5 tsf Minimal clay content, cohesion, Minimal clay content, cohesion, or internal friction Granular or sandy soils Granular or sandy soils Submerged soil or unstable rock Submerged soil or unstable rock

32. TYPE C SOIL (cont.): Signs of freely seeping water Signs of freely seeping water Disturbed soils not classified as Type B Disturbed soils not classified as Type B Some layered systems (slope greater than Some layered systems (slope greater than 1V : 4H)

33. LAYERED SYSTEMS Example 1:

34. LAYERED SYSTEMS Example 2:

35. SOIL TESTING At least one visual and one manual test is required - Soil should be tested from various depths and locations - Soil should be tested from various depths and locations - Test chunks of spoil pile or trench wall - Test chunks of spoil pile or trench wall

36. SOIL TESTING ( cont.): - Should be done at beginning of each work day and after each hazard-increasing event - Document results - Competent Person must never enter unprotected trench!

37. VISUAL TESTS Competent Person must observe for: Range of particle size Range of particle size Cracks, ravelling or spalling Cracks, ravelling or spalling Layered systems Layered systems Water effects Water effects Evidence of under- ground utilities or structures Evidence of under- ground utilities or structures Signs of previously disturbed soil Signs of previously disturbed soil Sources of vibration Sources of vibration

38. MANUAL TESTS PenetrometerPenetrometer Shear vaneShear vane Thumb penetration testThumb penetration test Plasticity test Plasticity test Ribbon Test Ribbon Test Thread Test Thread Test Sedimentation TestSedimentation Test

39. THUMB PENETRATION TEST SOIL TYPE THUMB PENETRATION A Soil indented with difficulty B Soil indented to cuticle with effort C Soil indented to knuckle easily

40. MANUAL TESTS PenetrometerPenetrometer Shear vaneShear vane Thumb penetration testThumb penetration test Plasticity test Plasticity test Ribbon Test Ribbon Test Thread Test Thread Test Sedimentation TestSedimentation Test

41. VIDEO SEGMENT: SOIL FIELD-TESTING METHODS (Coastal Safety)

42. SOIL MECHANICS AND CLASSIFICATION