Soil Exploration

1. Soil Exploration COSC 323: Soils in Construction

2. Steps of Soil Exploration • Reconnaissance • Map of aerial photographs • Site visit • Boring • Auger boring • Wash boring • Test pit • ____________ • Sampling • Testing • _____________

3. Core Borings • Drill into and through formations • Use a core barrel • Remained core  Laboratory testing • Type of rock (granite? sandstone?) • Texture (Coarse-grained? Fine-grained?) • Degree of stratification (laminations) • Orientation of rock formation(vertical?, horizontal?) • Presence of weathering, fractures, faults • _______________ = the length of core obtained / the distance drilled

4. Question • How many borings do we need? • How deep the borings should be?

5. How many borings do we need? • The more the better, but what about the cost? • Rule of thumb • _______ (50ft~100ft) for multi-story buildings • _______ (100ft~200ft) for one-story building, earth dams • __________ (500ft~1000ft) for highway

6. How deep the borings should be? • Until acceptable _______________ is obtained • Need to extend to _______ for very heavy structure (tall building)

7. More serious question • What would be the minimum depth of test boring when • An 8ft square footing is subjected to a contact pressure of 4000lb/ft2 • The wet unit weight of the soil supporting the footing is estimated to be 120lb/ft3 • The water table is estimated to be 30ft beneath the footing • We need more scientific ways

8. Core Boring Submerged Unit Weight ? Wet Unit Weight ? For Infinite Strip Loading

9. Core Boring For Square Loading

10. Core Boring For Square Loading (Low Pressure)

11. Solution • Water table = 30ft < 8ft (=footing’s width)  use wet unit weight (=120lb/ft3) 22ft

12. Standard Penetration Test • Useful in determining properties of cohesionless soil • Simple and inexpensive • Use a split-spoon sampler • Method • Use a ___________ falling _____ • Drive the split-spoon sampler _____ into the soil • Standard penetration resistance value (N-Value) • Number of blows required to penetrate the last _____ • Number of blows per foot.

13. Standard Penetration Test

14. Standard Penetration Test • SPT results are influenced by overburden pressure • How to correct N-values po is effective overburden pressure

15. Standard Penetration Test

16. Example • An SPT was performed at a depth of 20ft in sand of unit weight 135 lb/ft3. The blow count was 40. • What is the corrected N-value? 20ft

17. Example

18. Example

19. Example

20. Relative Density of Sand and SPT

21. SPT for Clay Relation of Consistency of Clay, and Unconfined Compressive (qu)

22. How to use SPT Bearing capacity factors and f f vs. SPT N-value

23. Cone Penetration Test • Popular in Europe, Gaining favor in US • Rapid subsurface exploration without taking soil samples • Base area = 1000mm2 • Methods • Static cone test  when pushed • Dynamic cone test when driven Mechanical cone penetrometer

24. Cone Penetration Method • How to measure • Push rods to advance the penetrometer’s tip • Push the inner rods to extend the tip • Repeat these steps • What to measure • ________________________ • Depth of soil penetrated

25. Cone Penetration Method Cone Resistance Friction Resistance Friction Ratio

26. Cone Penetration Test

27. Cone Penetration Test • Animation

28. Vane Test • Determine in-place shear strength for soft clay soil • Good for ____________ that lose part of their strength when disturbed • How to? • Push the vane tester into the soil and apply a torque to the vertical shaft c = cohesion of the clay T = torque required to shear the soil d = diameter of vane tester h = height of vane tester

29. Vane Test Correction factor for vane shear test

30. Report Format • Scope and purpose • Introduction • Geologic setting • Field studies • Laboratory tests • Analysis • Conclusions and recommendations • Appendix

33. Record of Soil Exploration

37. Boring log network SB3 SB2 SB1 SB6 SB5 SB4