SOILS AND FOUNDATIONS Lesson 08

Transcription

1 SOILS AND FOUNDATIONS Lesson 08 Chapter 8 Shallow Foundations Testing Theory Experience

2 Topics gtopic 1 (Section 8.0, 8.1, 8.2, 8.3, 8.4) - General and Bearing Capacity gtopic 2 (Section 8.5, 8.6, 8.7, 8.8, 8.9) - Settlement - Spread footings on embankments, IGMs,, rocks - Effect of deformations on bridge structures gtopic 3 (Section 8.10) - Construction

3 Shallow Foundations Lesson 08 - Topic 1 General and Bearing Capacity Section 8.0 to 8.4

4 Learning Outcomes gat the end of this session, the participant will be able to: - Identify different types of shallow foundations - Recall foundation design procedure - Contrast factors that influence bearing capacity in sand and clay - Compute bearing capacity in sand and clay - Describe allowable bearing pressure for rock foundations

5 Stresses Imposed by Structures gabutment and piers may have shallow or deep foundations

6 General Approach to Foundation Design gduty of Foundation Designer - Establish the most economical design that safely conforms to prescribed structural criteria and properly accounts for the intended function of the structure grational method of design - Evaluate various foundation types

7 Recommended Foundation Design Approach gstep 1: Determine: - Direction, type and magnitude of foundation loads - Tolerable deformations - Special constraints Underclearance requirements Structure type, span lengths Time constraints on construction Extreme event loading Construction load requirements

8 Recommended Foundation Design Approach gstep 2: Evaluate subsurface investigation and laboratory testing data for reliability and completeness Choose design method consistent with quality and quantity of subsurface data

9 Recommended Foundation Design Approach gstep 3: Consider alternate foundation types

10 Foundation Alternatives gshallow Foundations gdeep Foundations - Piles, shafts

11 Foundation Cost g Express foundation capacity in terms of $ g TOTAL cost of foundation system divided by the load supported by the foundation in tons g TOTAL cost of a foundation must include ALL costs associated with the foundations - Need for excavation support system, pile caps, etc. - Environmental restrictions - All other factors as applicable

12 Foundation Cost gif estimated costs of alternative foundation systems during design are within 15%, the alternate foundation designs should be considered for inclusion in contract documents

13 Loads and Limit States gloads - Permanent and Transient - Codes specify load combinations gfoundation limit states - Ultimate Bearing capacity, eccentricity, sliding, global stability, structural capacity - Serviceability Excessive settlement, excessive lateral displacement, structural deterioration of foundation

14 Types of Shallow Foundations gisolated Spread Footings - Length (L) to width (B) ratio, L/B < 10

15 Types of Shallow Foundations gcombined Strip Spread Footings - Length (L) to width (B) ratio, L/B 10

16 Shallow Foundations for Bridge Abutments

17 Shallow Foundations for Retaining Walls

18 Combined Footings Abutment Fill 2 1 Toe of End Slope Original Ground Toe of Side Slope

19 Mat Foundations REINFORCED CONCRETE MAT

20 Spread Footing Design Procedure ggeotechnical design of spread footing is a two part process gfirst Part: - Establish an allowable stress to prevent shear failure in soil gsecond Part: - Estimate the settlement under the applied stress

21 Allowable Bearing Capacity g Allowable bearing capacity is lesser of: Applied stress that will result in shear failure divided by FS - Ultimate limit criterion OR Applied stress that results in a specified amount of settlement of the structure - Serviceability criterion

22 Bearing Capacity Chart Ultimate Bearing Capacity, q ult Allowable Bearing Capacity, ksf (kpa) Allowable Bearing Capacity, q all = qult FS Contours of Allowable Bearing Capacity for a given settlement S1 S2 S3 Effective Footing Width, ft (m)

23 Design Process Flow Chart gfigure 8-108

24 Bearing Capacity gbearing capacity failure occurs when the shear strength of foundation soil is exceeded gsimilar to slope stability failure Q L = q A ψ I B III E C II D

25 Bearing Capacity Failure Mechanisms (a) GENERAL SHEAR SETTLEMENT LOAD ggeneral shear glocal shear gpunching shear (b) LOCAL SHEAR SETTLEMENT LOAD LOAD SETTLEMENT TEST AT GREATER DEPTH (c) PUNCHING SHEAR SURFACE TEST

26 Footing Dimension Terminology gb f = Width of footing - Least lateral dimension D f gl f = Length of footing B f gd f = Depth of embedment of footing L f

27 Basic Bearing Capacity Equation gequation 8-88 q ult = c (N c ) + q (N q ) ( γ)(b f )(N γ ) c = cohesion q = surcharge at footing base N c, N q, N γ = Bearing capacity factors γ = unit weight of foundation soil

28 Assumptions of Basic Bearing Capacity Equation (Section 8.4.3) gstrip (continuous) footing grigid footing ggeneral shear gconcentric loading (i.e., loading through the centroid of the footing) gfooting bearing on level surface of homogeneous soil gno impact of groundwater

29 Bearing Capacity Factors Bearing Capacity Factors N c Figure 8-15 Table N q N γ Friction Angle, degrees

30 Example 8-18 d = D = 5 B = 6 γ T = 125 pcf γ sub = 63 pcf φ = 20 c = 500 psf

31 Example 8-18 gsolution

32 Effect of Variation of Soil Properties and Footing Dimensions (Table 8-2) 8 Properties and Dimensions γ = γ a = effective unit weight γ b = submerged unit weight D f = embedment depth B f = footing width (assume strip footing) A. Initial situation: γ = 120 pcf, D f = 0', B f = 5', deep water table B. Effect of embedment: D f = 5', γ=120 pcf, B f = 5', deep water table C. Effect of width: B f = 10' γ = 120 pcf, D f = 0', deep water table D. Effect of water table at surface: γ = 57.6 pcf, D f = 0', B f = 5' Cohesive Soil φ = 0 c = 1000 psf q ult (psf) Cohesionless Soil φ = 30 o c = 0 q ult (psf)

33 Effect of Variation of Soil Properties and Footing Dimensions (Table 8-2) 8 Properties and Dimensions γ = γ a = effective unit weight γ b = submerged unit weight D f = embedment depth B f = footing width (assume strip footing) A. Initial situation: γ = 120 pcf, D f = 0', B f = 5', deep water table B. Effect of embedment: D f = 5', γ=120 pcf, B f = 5', deep water table C. Effect of width: B f = 10' γ = 120 pcf, D f = 0', deep water table D. Effect of water table at surface: γ = 57.6 pcf, D f = 0', B f = 5' Cohesive Soil φ = 0 c = 1000 psf q ult (psf) Cohesionless Soil φ = 30 o c = 0 q ult (psf)

34 Student Exercise 5 gfind the allowable bearing capacity assuming a FS=3 for the condition shown below for a 10 x50 footing with rough base Final Grade Sand γ = 115 pcf φ = 35 C = 0

35 Bearing Capacity Correction Factors gfooting shape - Adjusted for eccentricity gdepth of water table gembedment depth gsloping ground surface ginclined base ginclined loading

36 Student Exercise 5 gsolution

37 Modified Bearing Capacity Equation Equation 8-11 q ult = cn s b + qn C s b d + 0.5γ B N C s b c c c q Wq q q q f γ Wγ γ γ g s c, s γ, s q shape correction factors g b c, b γ, b q base inclination correction factors g C wq, C wγ g d q groundwater correction factors embedment correction factor g N c, N γ, N q bearing capacity factors as function of φ

38 Estimation of φ for Bearing Capacity Factors (Table 8-3) 8 Description Very Loose Loose Medium Dense Very Dense Corrected N-value N Friction angle φ Degrees Moist unit weight (γ) pcf

39 Shape Correction Factors gbasic equation assumes strip footing which means L f /B f 10 gfor footings with L f /B f < 10 apply shape correction factors gcompute the effective shape of the footing based on eccentricity

40 Effective Footing Dimensions B f = B f 2e B ; L f = L f 2e L ; A = B f L f

41 Pressure Distributions Structural design Sizing the footing

42 Shape Correction Factors Factor Friction Angle Cohesion Term (s c ) Unit Weight Term (s γ ) Surcharge Term (s q ) Shape Factors, s c, s γ, s q φ = φ > 0 B 5L f f B Nq B 1 B 1+ f + tan φ f Lf Nc f Lf Lf gin routine foundation design, use of effective dimensions in shape factors is not practical

43 Location of Groundwater table gto correct the unit weight D W C Wγ C W q D f > 1.5B f + D f Note: For intermediate positions of the groundwater table, interpolate between the values shown above.

44 Embedment Depth gto account for the shearing resistance in the soil above the footing base Note: The depth correction factor should be used only when the soils above the footing bearing elevation are as competent as the soils beneath the footing level; otherwise, the depth correction factor should be taken as 1.0. Friction Angle, φ (degrees) D f /B f d q See Note

45 Sloping Ground Surface gmodify the bearing capacity equation as follows: q ult = c (N cq ) ( γ)(b f )(N γq ) guseful in designing footings constructed within bridge approach fills

46 Footing in Slope

47 Footing Near Slope

48 α1 b b1 q Nc tan φ Inclined Base gfootings with inclined base should be avoided or limted to angles less than º qg Sliding may be an issue for inclined bases Factor Base Inclination Factors, b c, b γ, b q Friction Angle Cohesion Term (c) Unit Weight Term (γ) Surcharge Term (q) b c b γ b q φ = φ > 0 ( α tanφ) 2 ( α tanφ) 2 φ= friction angle, degrees; α = footing inclination from horizontal, upward +, degrees

49 Inclined Loading gif shear (horizontal) component is checked for sliding resistance, the inclination correction factor is omitted guse effective footing dimensions in evaluation of the vertical component of the load

50 Comments on Use of Bearing Capacity Correction Factors gfor settlement-controlled allowable bearing capacity, the effect application of correction factors may be negligible gapplication of correction factors is secondary to the adequate assessment of the shear strength characteristics of the foundation soil through correctly performed subsurface exploration

51 Local or Punching Shear c* = 0.67c φ*=tan -1 (0.67tanφ) gloose sands gsensitive clays gcollapsible soils gbrittle clays

52 Bearing Capacity Factors of Safety q all = q ult FS gq all = allowable bearing capacity gq ult = ultimate bearing capacity gtypical FS = 2.5 to 3.5 gfs is a function of - Confidence in shear strength parameter, c and φ - Importance of structure - Consequences of failure

53 Overstress Allowances gfor short-duration infrequently occuring loads, an overstress of 25 to 50 % may be allowed for allowable bearing capacity

54 Practical Aspects of Bearing Capacity

55 Presumptive Allowable Bearing Capacity gnot recommended for soils gsee Tables 8-8, 8 8, and for rocks

56 Learning Outcomes gat the end of this session, the participant will be able to: - Identify different types of shallow foundations - Recall foundation design procedure - Contrast factors that influence bearing capacity in sand and clay - Compute bearing capacity in sand and clay - Describe allowable bearing pressure for rock foundations

57 Any Questions? Any Questions? THE ROAD TO UNDERSTANDING SOILS AND FOUNDATIONS

58 Shallow Foundations Lesson 08 - Topic 2 Settlement, footings on embankments, IGMs, rocks, effect of deformations on bridge structures Section 8.5 to 8.9

59 Learning Outcomes gat the end of this session, the participant will be able to: - Calculate immediate settlements in granular soils - Calculate consolidation settlements in saturated fine-grained soils - Describe tolerances and consequences of deformations on bridge structures

60 Settlement of Spread Footings gimmediate (short-term) term) gconsolidation (long-term)

61 Immediate Settlement ghough s method - Conservative by a factor of 2 (FHWA, 1987) gschmertmann s method - More rational - Based on nonlinear theory of elasticity and measurements

62 Charts Figure gd s = 4B to 6B for continuous footings where L f /B f 10 gd s = 1.5B to 2B for square footings where L f /B f = 1

63 Trend of Analytical Results and Measurements Legend: Square footings where L f /B f =1 Continuous footings where L f /B f 10 Depth below Footing Vertical Strain, % 2B 4B

64 Schmertmann Method S i = C 1 C 2 Δp n i= 1 ΔH po C1 = Δp i ΔH i = H c I z XE C2 = log10 t ( years) 0.1 g I z Strain Influence Factor g E Elastic Modulus, Table g X Modification factor for E g C 1 Correction factor for strain relief Correction factor for creep deformation g C 2

65 Depth to Peak Strain Influence Factor, I zp I zp = Δ p 0.1 p op 0.5 see (b) below Axisymmetric L f /B f =1 L f = Length of footing B f = least width of footing B f Δ p = p p o Plane Strain L f /B f 10 B f /2 (for axisymmetric case) B f (for plane strain case) p o p op

66 Example 8-28 ggiven: 6 x24 footing on soil profile shown below. Determine settlement at end of construction and 10 years after construction Clayey Silt Ground Surface 3 ft γ t = 115 pcf; N1 60 = 8 Sandy Silt B f = 6 ft 3 ft γ t = 125 pcf; N1 60 = 25 Coarse Sand 5 ft γ t = 120 pcf; N1 60 = 30 Sandy Gravel 25 ft γ t = 128 pcf; N1 60 = 68

67 Draw Strain Influence Diagram 0 Influence Factor (Iz) Axisymmetric L f /B f =1 Plane Strain L f /B f 10 Depth below footing B 2B 3B Influence Factor (Iz) gcalculate peak Iz =

68 Strain Influence Diagram Divide into layers 0 Influence Factor (Iz) Layer 1 Depth below footing (ft) Layer 2 Layer 3 Layer Influence Factor (Iz)

69 Determine Elastic Modulus, E s guse Table 5-20, 5 Page Layer 1: Sandy Silt: E = 4N1 60 tsf Layer 2: Coarse Sand: E = 10N1 60 tsf Layer 3: Coarse Sand: E = 10N1 60 tsf Layer 4: Sandy Gravel: E = 12N1 60 tsf gcalculate X-factor, X X = 1.42

70 Setup Table for Settlement Computation Layer H c N1 60 E XE Z 1 I Z at Z i H = i I Z XE H c (inches) (tsf) (tsf) (ft) (in/tsf) , Σ H i =

71 Compute Correction Factors C 1, C 2 p o 3ft 115 pcf C1 = = Δp = 1655 psf t C2 = log10 gat end of construction, t=0.1 year C = log10 gat t=10 years ( years) = C = log =

72 Determine Immediate Settlement gat end of construction, t = 0.1 year S S S i i i = C1C2Δp H 1655psf = ( 0.896)( 1.0) 2000 psf tsf = 0.125inches gat t = 10 years i in tsf 1.4 S i = 0.125inches = 0.175inches 1.0

73 Consolidation Settlement gsame procedures as in Chapter 7 (Approach Roadway Deformations)

74 Example 8-38 g Calculate consolidation settlement for following case: 130 kips 4 10 Gravel γ T = 130 pcf 10 Normally consolidated clay γ sub = 65 pcf, e 0 = 0.75, C c = 0.4 Rock

75 Example 8-38 p 0 = ( pcf) + (5 65 pcf) = 2,145 psf 130 kips 130kips Δp = = = 0.208ksf = 2 (10ft + 15ft) 625ft 208psf ΔH Δ = Cc H 1+ e 0 log = 10ft log H 10 p 0 + p ΔH = 0.09 = Δp 2145psf psf 2145psf

76 Student Exercise 6 gfind footing settlement (immediate + consolidation) for the following case 5 Sand and Gravel Avg. N = Clayey Silt C C = 0.25 e 0 = 0.90 (Normally Consolidated) 45

77 Student Exercise 6 Pressure - psf Depth ft.

78 Spread Footings on Embankments gsection 8.6 gif spread footings are placed on embankments, structural fills that include sand and gravel sized particles should be used that are compacted properly (minimum 95% of standard Proctor energy)

79 Settlement of Footings on Structural Fills gin absence of other data, use N1 60 = 32 for the structural to estimate settlement of footings on compacted structural fill

80

81 Vertical Stress Distribution 0 Bridge Pier 20 Earth Embankment 40 Depth 60 h=20 h= Vertical Stress

82

83

84 Footings on IGMs and Rocks guse theory of elasticity δ v = C d Δp B E f m (1 ν 2 ) where: δ v = vertical settlement at surface C d = shape and rigidity factors (Table 8-12) Δp = change in stress at top of rock surface due to applied footing load B f = footing width or diameter ν = Poisson s ratio (refer to Table 5-23 in Chapter 5) E m = Young s modulus of rock mass (see Section in Chapter 5)

85 Effect of Deformations on Bridge Structures Tilt (Rotation) gsection 8.9 Differential Settlement Differential Settlement

86 Tolerable Movements for Bridges (Table 8-13) 8 Limiting Angular Type of Bridge Distortion, δ/s Multiple-span (continuous span) bridges Single-span bridges Note: δ is differential settlement, S is the span length. The quantity, δ/s, is dimensionless and is applicable when the same units are used for δ and S, i.e., if δ is expressed in inches then S should also be expressed in inches.

87 Construction Point Concept for Evaluation of Settlements gdivide the loadings based on sequence of construction gkey construction point is when the final load bearing member is constructed, e.g., when a bridge deck is constructed gtable Put in a slide

88 Learning Outcomes gat the end of this session, the participant will be able to: - Calculate immediate settlements in granular soils - Calculate consolidation settlements in saturated fine-grained soils - Describe tolerances and consequences of deformations on bridge structures

89 Any Questions? Any Questions? THE ROAD TO UNDERSTANDING SOILS AND FOUNDATIONS

90 Shallow Foundations Lesson 08 - Topic 3 Construction Section 8.10

91 Learning Outcomes gat the end of this session, the participant will be able to: - Discuss elements of shallow foundation construction/inspection

92 Key Elements of Shallow Foundation Construction gtable gcontractor set-up gexcavation gshallow foundation gpost installation - Monitoring

93 Structural Fill gtests for gradation and durability of fill at sufficient frequency to ensure that the material meets the specification gcompaction tests gif surcharge fill is used for pre-loading verify the unit weight of surcharge

94 Monitoring gcheck elevations of footing, particularly when footings are on embankment fills gperiodic surveying during the service life of the footing, particularly if the subsurface has soft soils within the depth of influence gimpacts on neighboring facilities guse instrumentation as necessary

95 Learning Outcomes gat the end of this session, the participant will be able to: - Discuss elements of shallow foundation construction/inspection

96 Any Questions? Any Questions? THE ROAD TO UNDERSTANDING SOILS AND FOUNDATIONS

97 Interstate 0 Apple Freeway Note: Scale shown in Station Form S.B. Apple Frwy N.B. Apple Frwy Baseline Stationing Interstate 0 Proposed Toe of Slope Proposed Final Grade 2 1 Existing Ground Surface Proposed Abutment

98 Apple Freeway Exercise gappendix A - Section A.7 Subsurface Investigations Basic Soil Properties Laboratory Testing Slope Stability Terrain reconnaissance Site inspection Subsurface borings Visual description Classification tests Soil profile P o diagram Test request Consolidation results Strength results Design soil profile Circular arc analysis Sliding block analysis Lateral squeeze analysis Approach Roadway Settlement Design soil profile Magnitude and rate of settlement Surcharge Vertical drains Spread Footing Design Design soil profile Pier bearing capacity Pier settlement Abutment settlement Surcharge Vertical drains Driven Pile Design Design soil profile Static analysis pier Pipe pile H pile Static analysis abutment Pipe pile H pile Driving resistance Lateral movement - abutment Construction Monitoring Wave equation Hammer approval Embankment instrumentation

99 APPLE FREEWAY PIER BEARING CAPACITY Assumptions: Footing embeded 4 below ground Footing width = 1/3 pier height = 7 Footing length = 100 L/W = 100/7 > 10 9 Continuous BAF - 2 N Sand Clay

100 Compute N1 values 60 Depth (ft.) p 0 (psf) p 0 (tsf) N (bpf) Hammer Efficiency (E f ) E f / 60 N 60 (bpf) C n N1 60 (bpf) Average corrected blow count = 36

101 APPLE FREEWAY PIER SETTLEMENT SAND CLAY -1 CLAY-2 Time (days) Δ H 2 3 Δ H = 2.85

102 APPLE FREEWAY EAST ABUTMENT SETTLEMENT 0 Pressure (psf) Sand P f P abut 10 P o Clay Depth (ft) P c Gravel Layer Time (days) Δ H 1 2 Δ H = 2.59

103 APPLE FREEWAY EAST ABUTMENT SETTLEMENT TREATMENT Time days Δ H 5 10 Assume Wick Drains Installed *0.25 ΔRemaining 30 days after abutment loaded Begin Abutment Footing Construction emb. Δ 15 ΔH ABUT Emb. + Abut Time Days days 400 days ΔH Total 5 30 Fill to 10 Surcharge *Assume 10 Surcharge Used t Total ΔH

104 SPREAD FOOTING DESIGN Design Soil Profile Strength and consolidation values selected for all soil layers. Footing elevation and width chosen. Pier Bearing Capacity Q allowable = 3 tons/sq.ft. Pier Settlement Settlement = 2.8", t 90 = 220 days. Abutment Settlement Settlement - 2.6", t 90 = 433 days. Vertical Drains t 90 = 60 days - could reduce settlement to 0.25" after abutment constructed and loaded. Surcharge 10' surcharge: t 90 = 240 days before abutment constructed.

105 Any Questions? Any Questions? THE ROAD TO UNDERSTANDING SOILS AND FOUNDATIONS