Class Principles of Foundation Engineering CEE430/530

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1 Class Principles of Foundation Engineering CEE430/

2 General Information Lecturer: Scott A. Barnhill, P.E. Lecture Time: Thursday, 7:10 pm to 9:50 pm Classroom: Kaufmann, Room 224 Office Hour: I have no office. Contact me to meet before class. sabarnhill@geronline.com (ODU not working) Cell Phone: Text: Principles of Foundation Engineering, 7th Edition, Braja M. Das NOT the SI Version 1-2

3 Class Web Site Class Dates Information to Be Covered Homework Answers Any Handouts Other Reading Sources 1-3

4 Homework Homework is to have an answer sheet as the first page. Problem Number Answers with any sub problems answers also shown. Show data given at top of each problem. Show every calculation step. No Excel or MathCad Period!!!. 1-4

5 Grading Homework - 20% Mid Term Exam - 30% Final Exam - 30% Project - 20% Class Presentation 10% Report 10% Your Personal Calculations 20% Peer Review 50% My Opinion 10% Range Letter Grade 0 54 F D D D C C C B B B A A 1-5

6 Purpose of the Class Familiarize you with soil properties Learn how subsurface soils are tested Learn how to apply soil properties to foundation design Learn about analyzing various foundation alternatives Learn about retaining walls Learn about how to improve the ground Apply what you have learned to actual projects 1-6

7 Chapter 1: Geotechnical Properties of Soil 1-7

8 Your Knowledge You are already suppose to know everything covered in this chapter. 1-8

9 Strength What is the strength of steel? What is the strength of concrete? What if their strengths varied wildly even with a single column or beam. How would you design? Strength 1-9

10 Soil What is the strength of soil? 1-10

11 Strength of Soil 0 20 CPT-1 CPT-2 0 CPT-1 CPT-2 CPT-3 10 CPT-3 CPT-4 CPT-5 CPT Geomean Elevation (feet) Depth (feet) Undrained Shear Strength (tsf) Undrained Shear Strength - Su (tsf) 1-11

12 Lesner Bridge Elevation (feet) CPT 80 Borings Corrected SPT (N60) bpf 1-12

13 Classification Schemes 1-13

14 Weight-Volume 1-14

15 Unit Weight Relationships 1-15

16 Common Relationships W W ( 1 s w ) e V V ( 1 e) n s d d m 1 w sat w G s w 1 e e n 1 n e 1 e wg s S w S w sat w 1 w sat d G s W Total Weight V Total Volume w water content Ws Weight of Solids Vs Volume of Solids e void ratio d Dry Unit Weight m Moist Unit Weight w Unit Weight of Water Bouyant Unit Weight Gs Specific Gravity n porosity S Saturation w sat Saturated Moisture Content sat d ( 1 w sat ) 1-16

17 Specific Gravity 1-17

18 Atterberg Limits Liquid Limit (LL), Plastic Limit (PL) & Plasticity Index (PI) =LL-PL Soils at the plastic Limit are 100 times Stronger than at the liquid limit. 1-18

19 Atterberg Limit Relationships Soils with moisture content near or at the liquid limit are usually normally consolidated. As the moisture content moves towards the plastic limit, preconsolidation increases. Soils with moisture contents exceeding the liquid limit, the soils can be underconsolidated. Must know site history. Cohesive strength increases as moisture content moves towards the plastic limit. 1-19

20 Atterberg Limits Chart 1-20

21 Empirical Correlations Atterberg Limits are used in numerous empirical correlations Preconsolidation Undrained Strength Constrained Modulus Permeability Moist Unit Weight Dry Unit Weight Submerged Unit Weight Cc & Cr Swell Pressure Cv Void ratio Critical State Soil Mechanics Parameters,,, o Use correlations to compare to more sophisticated tests. Look for how consistent the soil. 1-21

22 Unified Soils Classification 1-22

23 Coarse Grained Soils 1-23

24 Fine Grained Soils 1-24

25 Organic Soils 1-25

26 Effective Stress If you cannot master the concept of effective stress and cannot calculate it accurately, you will not get a good grade in this class. All foundation design requires it. 1-26

27 Calculation of Effective Stress You must understand the concept of effective stress. It is so fundamental to foundation engineering that you will simply not be able to complete almost every design problem. 1-27

28 Effective Stress No Seepage In Figure, pore pressure at Point A is u=h 2 w Where w (62.4 pcf) is the unit weight of water -u = (h 1 m + h 2 sat ) - h 2 w = h 1 m +h 2 ( sat - w ) = h 1 m + h 2 Where = effective, or submerged, unit weight of soil sat G s w 1 e e w G s e w w - w - w 1 e sat w ( Gs 1 - e 1 ( 1-28

29 Effective Stress Seepage In this problem, there is upward seepage of water. For this case, the effective stress at Point A is = h 1 w + h 2 sat u = (h1 + h2 +h) w -u = (h 1 w + h 2 sat ) - (h 1 + h 2 + h) w = h 2 ( sat - w ) - h w = h 2 - h w or h h 2 w h h 2 i w 2 i is the hydraulic gradient. If i is very high so that -i w = 0, the effective stress = 0. There Will be no contact between soil particles. This is referred to as the quick condition (quick sand), or failure by heave. Critical Gradient i=i cr = / w =(Gs-1)/(1+eo) 1-29

30 Variation of effective stress in a soil profile A 4 m 3 Dry Sand d = 14.5 kn/m Water Table B 3 Clay sat = 17.2 kn/m 5 m C 1-30

31 Effective Stress Solution Point Depth (m) (kn/m3) u (kn/m2) ' (kn/m2) A B 4 (4)(d) = (4)(14.5) = = 58 C (5)(sat) = (5)(17.2) = 144 (5)(w) = (5)(9.81) = = A 4 m 3 Dry Sand d = 14.5 kn/m Water Table B 3 Clay sat = 17.2 kn/m 5 m C 1-31

32 Effective Stress #2 A 3 Dry Sand d = 14.5 kn/m 3 Dry Sand sat = 15.2 kn/m 4 m Water Table B C 3 Clay sat = 17.2 kn/m 5 m D 1-32

33 Effective Stress Solution #2 A B 3 (3)(d) = (3)(14.5) = = 43.5 C (1)(sat1) = (1)(15.2) = 58.7 (1)(w) = (1)(9.81) = = 48.9 D (5)(sat2) = (5)(17.2) = (5)(w) = 9.81+(5)(9.81) = = 86.1 A 3 Dry Sand d = 14.5 kn/m 3 Dry Sand sat = 15.2 kn/m 4 m Water Table B C 3 Clay sat = 17.2 kn/m 5 m D What happens of the groundwater changes in the future? 1-33

34 Preconsolidation Profile Estimated Preconsolidation Profile GB-06 GB-05 0 GB-02 Depth (meters) Liquidity Index SPT UC Tests Consolidation Tests Overburden Fill Pressure 1335 kn Column GB-04 GB-03 GB GEOTECHNICAL PROFILE - ELEVATION 3515.GPJ GEOENV RESOURCES.GDT 6/29/ Preconsolidation Pressure (kpa) Environmental Groundwater Hazardous Materials Geotechnical Industrial CONSULTING ENGINEERS GeoEnvironmental Resources, Inc Southern Boulevard, Suite 101 Virginia Beach, VA Preconsolidation DD(X) Land Based Test Facility Wallops Island, Virginia PROJECT NUMBER DRAWING NUMBER

35 Preconsolidation Profile - CPT Elevation (feet) CPT-1 Clay CPT-2 Clay Preconsolidation Pressure (tsf) CPT-3 Clay Consolidation Tests CPT-1 Sands CPT-2 Sands CPT-3 Sands Triaxial Tests Estimated Overburden 1-35

36 Consolidation Consolidation is the movement of pore water out of the soil. Initially the applied load is carried by the pore water. This creates an increase in pore water pressure. As pore water moves out of the soil which is controlled by the permeability, pore pressure dissipates and the soil matrix begins to carry the load. The soil then compresses. 1-36

37 Principles of Consolidation hi = 1-37

38 Consolidation Testing 1-38

39 Normally Consolidated Clay 1-39

40 Overconsolidated Clay 1-40

41 Real World Note unload-reload cycle to remove disturbance 1-41

42 NAVFAC LI Versus P c LI = (w-pl)/pi Note: p a = atmospheric pressure [~100 KN/m2 (1 U.S. ton/ft2)] 1-42

43 One Dimensional Consolidation 1-43

44 Primary Consolidation Settlement Equation 1.65 Cr ( Cs H) S 1 e log c ( Cc H) o 1 e =0 for normally consolidated soil log o c For normally consolidated soils c= o Therefore log(c/ o)=0 and the first quantity goes to zero as well. 1-44

45 Pore Pressure Dissipation 1-45

46 Drainage Conditions H = 0.5 Hc 1-46

47 Field Data PZ-2:A PZ-2:B No Longer Responding Pressure (kpa) Pressure (kpa) Background /17 4/8 5/28 7/17 9/5 10/25 12/14 Date Background 75 2/17 4/8 5/28 7/17 9/5 10/25 12/14 Date PZ-2:C PZ-2:D Pressure (kpa) Presure (kpa) Background 220 Background 165 2/17 4/8 5/28 7/17 9/5 10/25 12/14 Date 215 2/17 4/8 5/28 7/17 9/5 10/25 12/14 Date 1-47

48 Constant Cv Modeling 0 Pore Pressure Response Using Single Layer Theory Initial Instantaneous Loading 10 Depth (feet) Time (days) Multilayer Model Indicates Quicker Dissipation Excess Pore Pressure (psf) 1-48

49 Range of C v (after U.S. Dept. of Navy) Provided by consolidation test and varies with pressure. Use pressure at effective stress. 1-49

50 Tv Versus %U Time factor against average degree of consolidation (u0 = constant) T v 4 U% For U= 0 to 60% T log( 100 U% ) v For U>60% T v 1 4 U% 100 U% For U = 0 to 100% 1-50

51 Tv Versus %U 1-51

52 Tv Versus %U 1-52

53 Time for Compression Tv t ( Cv t) H 2 Tv H2 Cv Typical client wants to know how long he has to wait before starting construction. For U% = 90% Tv = With H = 10 feet & double drainage H/2 = 5 feet Cv = 0.2 ft 2 /day t= 0.849(5) 2 /0.2 = 106 days For single drainage H=10 feet t = 0.849(10) 2 /0.2 = days 1-53

54 Ramp or Construction Loading A new layer of structural fill or building structure cannot be loaded instantaneously on the ground. For this reason, the increase in loading gradually rises to the maximum load. This gives time for excess pore water pressure to begin dissipating. 1-54

55 One-dimensional consolidation due to single ramp loading 1-55

56 Ramp Loading Parameters T c C v t c H 2 Remember double or single drainage Where do we get t c? Construction Schedule. 1-56

57 Olson s Ramp Loading 1-57

58 Ramp Loading Example T c C v t c H 2 Example 1.10 With H = 10 feet & double drainage H/2 = 5 feet Cv = 0.2 ft 2 /day tc = 15 days, what %U at 50 days Tc = 0.2(15/(5) 2 ) = 0.12 Tv = 0.2(50/(5) 2 ) = 0.4 From chart U% = 70%. 1-58

59 Shear Strength S=c + tan( ) Shear Strength Tests Unconfined Compression Tests Direct Shear Tests Direct Simple Shear Triaxial Shear Tests Unconsolidated Undrained (UU) u=0 Consolidated Undrained w/ppm (CU) Consolidated Drained (CD) Each test will yield a different value of Su Drained Strength typically > Undrained Strength 1-59

60 Direct Shear Test 1-60

61 Typical Values of 1-61

62 Triaxial Shear Test 1-62

63 Sequence of Stress Application 1-63

64 Unconfined Shear Test (a) soil specimen; (b) Mohr s circle for the test; (c) variation of q u with the degree of saturation S u = c u = q u /2 1-64

65 Triaxial Test in Progress 1-65

66 Versus Void Ratio 1-66

67 Versus Plasticity Index Note scatter = PI PI

68 Deviator stress vs. axial straindrained triaxial test Strain Softening 1-68

69 Peak & Residual Strength Envelopes for Clays 1-69

70 Variation of r with CF 1-70

71 Empirical Correlations 1-71

72 Variation of r with liquid limit for some clays (after Stark, 1995) 1-72

73 Variation With Depth - Clay Deposit In normally consolidated clays undrained shear strength increases almost linearly with effective overburden pressure S u / ratio 1-73

74 S u / Relationships Use consolidated-undrained triaxial tests at different levels of stress to determine Su/ ratio. Can be estimated by: ( ( ( ( NC OC ( ( NC OCR 0.8 Critical State Pore Pressure Parameter o o 1 Varies with Soil C s C c 1-74

75 Variation of Cu with LI C u/ o = LI LI LI LI (based on Bjerrum and Simons 1960) LI = (w-pl)/pi 1-75

76 Homework From Chapter 1 CE 420 CE 520 All of CE 420 plus Read Chapter

Geotechnical Properties of Soil

Geotechnical Properties of Soil 1 Soil Texture Particle size, shape and size distribution Coarse-textured (Gravel, Sand) Fine-textured (Silt, Clay) Visibility by the naked eye (0.05 mm is the approximate