Lesson 08-Chapter 8 Shallow Foundations.pdf

SOILS AND FOUNDATIONS Lesson 08 Chapter 8 – Shallow Foundations

Testing

Theory

Experience

Topics g Topic

1 (Section 8.0, 8.1, 8.2, 8.3, 8.4)

g Topic

2 (Section 8.5, 8.6, 8.7, 8.8, 8.9)

g Topic

3 (Section 8.10)

- General and Bearing Capacity - Settlement - Spread footings on embankments, IGMs, rocks - Effect of deformations on bridge structures - Construction

Shallow Foundations Lesson 08 - Topic 1 General and Bearing Capacity

Section 8.0 to 8.4

Learning Outcomes g At

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

Stresses Imposed by Structures

g Abutment

and piers may have shallow or deep foundations

General Approach to Foundation Design g Duty

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

g Rational

method of design

- Evaluate various foundation types

Recommended Foundation Design Approach g Step

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


2: Evaluate subsurface investigation and laboratory testing data for reliability and completeness Choose design method consistent with quality and quantity of subsurface data


3: Consider alternate foundation types

Foundation Alternatives g Shallow

Foundations g Deep Foundations

- Piles, shafts

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

Foundation Cost g If

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

Loads and Limit States g Loads

- Permanent and Transient - Codes specify load combinations

g Foundation

limit states

- Ultimate

• Bearing capacity, eccentricity, sliding, global stability, structural capacity

- Serviceability

• Excessive settlement, excessive lateral displacement, structural deterioration of foundation

Types of Shallow Foundations g Isolated

Spread Footings

- Length (L) to width (B) ratio, L/B < 10

Types of Shallow Foundations g Combined

Strip Spread Footings

- Length (L) to width (B) ratio, L/B ≥ 10

Shallow Foundations for Bridge Abutments

Shallow Foundations for Retaining Walls

Combined Footings

Abutment Fill 2 1

Original Ground

Toe of Side Slope

Toe of End Slope

Mat Foundations

REINFORCED CONCRETE MAT

Spread Footing Design Procedure g Geotechnical

design of spread footing is a two part process

g First

Part:

- Establish an allowable stress to prevent shear failure in soil

g Second

Part:

- Estimate the settlement under the applied stress

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

Bearing Capacity Chart Ultimate Bearing Capacity, qult Allowable Bearing Capacity,

Allowable Bearing Capacity, ksf (kPa)

q q all = ult FS

Contours of Allowable Bearing Capacity for a given settlement S1 S2 S3

Effective Footing Width, ft (m)

Design Process Flow Chart g Figure

8-10

Bearing Capacity g Bearing

capacity failure occurs when the shear strength of foundation soil is exceeded g Similar to slope stability failure Q L= ∞ A

ψ

q E

B

III

I II C

D

(a) GENERAL SHEAR

LOAD SETTLEMENT

g General

(b) LOCAL SHEAR

LOAD SETTLEMENT

shear g Local shear g Punching shear

LOAD SETTLEMENT

Bearing Capacity Failure Mechanisms

(c) PUNCHING SHEAR

TEST AT GREATER DEPTH SURFACE TEST

Footing Dimension Terminology g Bf

= Width of footing

- Least lateral dimension Df

g Lf g Df

Bf

= Length of footing

= Depth of embedment of footing

Lf

Basic Bearing Capacity Equation g Equation

8-8

q ult = c (N c ) + q (N q ) + 0.5 ( γ )(B f )(N γ ) c = cohesion q = surcharge at footing base Nc, Nq, Nγ = Bearing capacity factors γ = unit weight of foundation soil

Assumptions of Basic Bearing Capacity Equation (Section 8.4.3) g Strip

(continuous) footing g Rigid footing g General shear g Concentric loading (i.e., loading through the centroid of the footing) g Footing bearing on level surface of homogeneous soil g No impact of groundwater

Bearing Capacity Factors Bearing Capacity Factors

1000

Figure 8-15 Table 8-1

100

Nc

10

Nq

Nγ

1 0

5

10

15

20

25

Friction Angle, degrees

30

35

40

45

Example 8-1 γT = 125 pcf

d = D = 5′

B = 6′

γsub = 63 pcf

φ = 20° c = 500 psf

Example 8-1 g Solution

Effect of Variation of Soil Properties and Footing Dimensions (Table 8-2) Properties and Dimensions γ = γa = effective unit weight γb = submerged unit weight Df = embedment depth Bf = footing width (assume strip footing) A. Initial situation: γ = 120 pcf, Df = 0', Bf = 5', deep water table B. Effect of embedment: Df = 5', γ=120 pcf, Bf = 5', deep water table C. Effect of width: Bf = 10' γ = 120 pcf, Df = 0', deep water table D. Effect of water table at surface: γ = 57.6 pcf, Df = 0', Bf = 5'

Cohesive Soil

Cohesionless Soil

φ=0 c = 1000 psf qult (psf) 5140

φ = 30o c=0 qult (psf) 6720

Effect of Variation of Soil Properties and Footing Dimensions (Table 8-2) Properties and Dimensions γ = γa = effective unit weight γb = submerged unit weight Df = embedment depth Bf = footing width (assume strip footing) A. Initial situation: γ = 120 pcf, Df = 0', Bf = 5', deep water table B. Effect of embedment: Df = 5', γ=120 pcf, Bf = 5', deep water table C. Effect of width: Bf = 10' γ = 120 pcf, Df = 0', deep water table D. Effect of water table at surface: γ = 57.6 pcf, Df = 0', Bf = 5'

Cohesive Soil

Cohesionless Soil

φ=0 c = 1000 psf qult (psf) 5140

φ = 30o c=0 qult (psf) 6720

5740

17760

5140

13440

5140

3226

Student Exercise 5 g Find

the allowable bearing capacity assuming a FS=3 for the condition shown below for a 10’x50’ footing with rough base Final Grade 4′ 30′

10′

Sand γ = 115 pcf φ = 35° C=0

Bearing Capacity Correction Factors g Footing

shape

- Adjusted for eccentricity

g Depth

of water table g Embedment depth g Sloping ground surface g Inclined base g Inclined loading

Student Exercise 5 g Solution

Modified Bearing Capacity Equation Equation 8-11 q ult = cN c s c b c + qN q C Wq s q b q d q + 0.5γ B f N γ C Wγ s γ b γ g

sc, sγ, sq

shape correction factors

g

bc, bγ, bq base inclination correction factors

g

Cwq, Cwγ

groundwater correction factors

g

dq

embedment correction factor

g

Nc, Nγ, Nq bearing capacity factors as function of φ

Estimation of φ for Bearing Capacity Factors (Table 8-3) Description Corrected N-value N160

Very Very Loose Medium Dense Loose Dense 0

4

10

30

50

Friction angle φ Degrees

25 – 30

27 – 32

30 – 35

35 – 40

38 – 43

Moist unit weight (γ) pcf

70 – 100

90 – 115

110 – 130

120 – 140

130 – 150

Shape Correction Factors g Basic

equation assumes strip footing which means Lf/Bf ≥ 10

g For

footings with Lf/Bf < 10 apply shape correction factors

g Compute

the effective shape of the footing based on eccentricity

Effective Footing Dimensions

B′f = Bf – 2eB ; L′f = Lf – 2eL ; A′= B′f L′f

Pressure Distributions Structural design

Sizing the footing

Shape Correction Factors Factor

Shape Factors, sc, sγ, sq g In

Friction Angle

φ=0 φ>0

Cohesion Term (sc) ⎛ Bf 1 + ⎜⎜ ⎝ 5L f ⎛ Bf 1 + ⎜⎜ ⎝ Lf

⎞ ⎟⎟ ⎠

⎞⎛ N q ⎞ ⎟ ⎟⎟⎜⎜ ⎟ ⎠⎝ N c ⎠

Unit Weight Term (sγ)

Surcharge Term (sq)

1.0

1.0

⎛ Bf 1 − 0.4⎜⎜ ⎝ Lf

⎞ ⎟⎟ ⎠

⎞ ⎛B 1 + ⎜⎜ f tan φ ⎟⎟ ⎠ ⎝ Lf

routine foundation design, use of effective dimensions in shape factors is not practical

Location of Groundwater table g To

correct the unit weight

DW CWγ CWq 0 0.5 0.5 Df 0.5 1.0 > 1.5Bf + Df 1.0 1.0 Note: For intermediate positions of the groundwater table, interpolate between the values shown above.

Embedment Depth g To

account for the shearing resistance in the soil above the footing base

Friction Angle, φ (degrees) 32

Note: The depth correction 37 factor should be used only when the soils above the footing bearing elevation are as competent as the soils 42 beneath the footing level; otherwise, the depth correction factor should be taken as 1.0. See Note

Df/Bf

dq

1 2 4 8 1 2 4 8 1 2 4 8

1.20 1.30 1.35 1.40 1.20 1.25 1.30 1.35 1.15 1.20 1.25 1.30

Sloping Ground Surface g Modify

the bearing capacity equation as follows:

q ult = c (N cq ) + 0.5 ( γ )(B f )(N γq ) g Useful

in designing footings constructed within bridge approach fills

Footing in Slope

Footing Near Slope

Inclined Base g Footings

with inclined base should be avoided or limted to angles less than 8-10º g Sliding may be an issue for inclined bases

⎛ ⎛ α1 − ⎞b q ⎞ ⎟ ⎟ b1q− −⎜ ⎜⎜ ⎟ .3tan ⎝ 147 ⎠ N φ ⎠ ⎝ c

Factor

Cohesion Friction Term (c) Angle bc

Unit Weight Term (γ)

Surcharge Term (q)

bγ 1.0

bq 1.0

Base φ=0 Inclination Factors, φ>0 (1-0.017α tanφ)2 (1-0.017α tanφ)2 bc, bγ, bq φ= friction angle, degrees; α = footing inclination from horizontal, upward +, degrees

Inclined Loading g If

shear (horizontal) component is checked for sliding resistance, the inclination correction factor is omitted g Use effective footing dimensions in evaluation of the vertical component of the load

Comments on Use of Bearing Capacity Correction Factors g For

settlement-controlled allowable bearing capacity, the effect application of correction factors may be negligible

g Application

of correction factors is secondary to the adequate assessment of the shear strength characteristics of the foundation soil through correctly performed subsurface exploration

Local or Punching Shear c* = 0.67c φ*=tan-1(0.67tanφ) g Loose

sands g Sensitive clays g Collapsible soils g Brittle clays

Bearing Capacity Factors of Safety q ult q all = FS g qall

= allowable bearing capacity g qult = ultimate bearing capacity g Typical FS = 2.5 to 3.5 g FS is a function of

- Confidence in shear strength parameter, c and φ - Importance of structure - Consequences of failure

Overstress Allowances g For

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

Practical Aspects of Bearing Capacity

Presumptive Allowable Bearing Capacity g NOT

recommended for soils g See Tables 8-8, 8-9 and 8-10 for rocks



- 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

Any Questions? THE ROAD TO UNDERSTANDING SOILS AND FOUNDATIONS

Shallow Foundations Lesson 08 - Topic 2 Settlement, footings on embankments, IGMs, rocks, effect of deformations on bridge structures

Section 8.5 to 8.9



- Calculate immediate settlements in granular -

soils Calculate consolidation settlements in saturated fine-grained soils Describe tolerances and consequences of deformations on bridge structures

Settlement of Spread Footings g Immediate

(short-term) g Consolidation (long-term)

Immediate Settlement g Hough’s

method

- Conservative by a factor of 2 (FHWA, 1987)

g Schmertmann’s

method

- More rational - Based on nonlinear theory of elasticity and measurements

Charts Figure 2-11 g Ds

= 4B to 6B for continuous footings where Lf/Bf ≥ 10

g Ds

= 1.5B to 2B for square footings where Lf/Bf = 1

Trend of Analytical Results and Measurements

Square footings where Lf/Bf =1 Continuous footings where Lf/Bf ≥ 10

Depth below Footing

Legend:

Vertical Strain, %

2B

4B

Schmertmann Method n

S i = C1C 2 Δp ∑ ΔH i i =1

⎛ po ⎞ C1 = 1 − 0.5⎜⎜ ⎟⎟ ⎝ Δp ⎠ g Iz

gE gX g C1 g C2

≥ 0 .5

⎛ Iz ⎞ ΔH i = H c ⎜ ⎟ ⎝ XE ⎠ ⎛ t (years) ⎞ C 2 = 1 + 0.2 log10 ⎜ ⎟ ⎝ 0.1 ⎠

Strain Influence Factor Elastic Modulus, Table 5-20 Modification factor for E Correction factor for strain relief Correction factor for creep deformation

⎛ ⎜ Δp I zp = 0 . 5 + 0 . 1⎜ ⎜ p op ⎝

⎞ ⎟ ⎟ ⎟ ⎠

0 .5

see ( b ) below Axisymmetric Lf/Bf =1

Lf = Length of footing Bf = least width of footing Bf

Δp = p − po po

Plane Strain Lf/Bf ≥ 10

Bf /2 (for axisymmetric case) Bf (for plane strain case) Depth to Peak Strain Influence Factor, Izp

p op

Example 8-2 g Given:

6’x24’ footing on soil profile shown below. Determine settlement at end of construction and 10 years after construction Ground Surface

3 ft

γt = 115 pcf; N160 = 8

3 ft

γt = 125 pcf; N160 = 25

Coarse Sand

5 ft

γt = 120 pcf; N160 = 30

Sandy Gravel

25 ft

γt = 128 pcf; N160 = 68

Clayey Silt Sandy Silt

Bf = 6 ft

Draw Strain Influence Diagram

Axisymmetric Lf/Bf =1

Depth below footing

0

0.1

0.2

Influence Factor (Iz) 0.3 0.4

0.5

0.6

0.7

0

0

4

4

B

8

8

2B

12

16

12

16

3B

20

20 0.0

Plane Strain Lf/Bf ≥ 10

0.1

0.2

0.3 0.4 Influence Factor (Iz)

g Calculate

0.5

0.6

peak Iz = 0.64

0.7

Strain Influence Diagram Divide into layers 0

0.1

0.2

Influence Factor (Iz) 0.3 0.4

0.5

0.6

0.7

0

0

Depth below footing (ft)

Layer 1 Layer 2

4

4

Layer 3

8

8

12

12 Layer 4

16

16

20

20 0.0

0.1

0.2

0.3 0.4 Influence Factor (Iz)

0.5

0.6

0.7

Determine Elastic Modulus, Es g Use

Table 5-20, Page 5-90

Layer 1: Sandy Silt: E = 4N160 tsf Layer 2: Coarse Sand: E = 10N160 tsf Layer 3: Coarse Sand: E = 10N160 tsf Layer 4: Sandy Gravel: E = 12N160 tsf g Calculate

X-factor, X = 1.42

Setup Table for Settlement Computation Layer

Hc

N160

(inches) 1 2 3 4

36 12 48 96

25 30 30 68

E

XE

Z1

(tsf)

(tsf)

(ft)

100 300 300 816

142 426 426 1,159

1.5 3.5 6 12

IZ at Zi

IZ Hi = Hc XE

(in/tsf) 0.31 0.56 0.55 0.22

0.0759 0.0152 0.0599 0.0176

Σ Hi=

0.1686

Compute Correction Factors C1 , C2 ⎛ po ⎞ ⎛ 3 ft ×115 pcf ⎟⎟ = 1 − 0.5⎜⎜ C1 = 1 − 0.5⎜⎜ ⎝ 1655 psf ⎝ Δp ⎠

⎞ ⎟⎟ = 0.896 ⎠

⎛ t (years) ⎞ C 2 = 1 + 0.2 log10 ⎜ ⎟ ⎝ 0.1 ⎠ g At

end of construction, t=0.1 year

⎛ 0.1 ⎞ C 2 = 1 + 0.2 log10 ⎜ ⎟ = 1.0 ⎝ 0.1 ⎠ g At

t=10 years

⎛ 10 ⎞ C 2 = 1 + 0.2 log10 ⎜ ⎟ = 1.4 ⎝ 0.1 ⎠

Determine Immediate Settlement g At

end of construction, t = 0.1 year

S i = C1C 2 Δp∑ H i ⎛ ⎞ ⎜ 1655psf ⎟⎛ in ⎞ S i = (0.896)(1.0)⎜ ⎟⎜ 0.1686 tsf ⎟ psf ⎠ ⎜ 2000 ⎟⎝ tsf ⎠ ⎝ S i = 0.125 inches

g At

t = 10 years

⎛ 1.4 ⎞ S i = 0.125 inches⎜ ⎟ = 0.175 inches ⎝ 1.0 ⎠

Consolidation Settlement g Same

procedures as in Chapter 7 (Approach Roadway Deformations)

Example 8-3 g

Calculate consolidation settlement for following case: 130 kips

4′ 10′ 10′

Gravel γT = 130 pcf Normally consolidated clay γsub = 65 pcf, e0 = 0.75, Cc = 0.4 Rock

Example 8-3 p0 = (14′ × 130 pcf) + (5′ × 65 pcf) = 2,145 psf 130 kips 130kips Δp = = = 0.208 ksf = 208 psf 2 625 ft (10 ft + 15 ft) ⎛ p 0 + Δp ⎞ Cc ⎟⎟ ΔH = H log 10 ⎜⎜ 1 + e0 ⎝ p0 ⎠ ⎛ 2145 psf + 208 psf ⎛ 0.4 ⎞ ΔH = 10ft ⎜ ⎟log10 ⎜⎜ 2145 psf ⎝ 1 + 0.75 ⎠ ⎝

ΔH = 0.09′ = 1.1″

⎞ ⎟⎟ ⎠

Student Exercise 6 g Find

footing settlement (immediate + consolidation) for the following case 5′ 25′ Sand and Gravel Avg. N′ = 40

Clayey Silt CC = 0.25 e0 = 0.90

45′ (Normally Consolidated)

Student Exercise 6

Depth – ft.

Pressure - psf

Spread Footings on Embankments g Section

8.6 g If 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)

Settlement of Footings on Structural Fills g In

absence of other data, use N160 = 32 for the structural to estimate settlement of footings on compacted structural fill

Vertical Stress Distribution 0

Bridge Bridge Pier Pier

20

Earth Earth Embankment Embankment

Depth

40 h=40’ h=40’

h=20’ h=20’

60 80 100 00

11

22

33

44

Vertical Vertical Stress Stress

55

Footings on IGMs and Rocks g Use

theory of elasticity 2

Cd Δp Bf (1 − ν ) δv = Em where: δv Cd Δp Bf ν Em

= = = = = =

vertical settlement at surface shape and rigidity factors (Table 8-12) change in stress at top of rock surface due to applied footing load footing width or diameter Poisson’s ratio (refer to Table 5-23 in Chapter 5) Young’s modulus of rock mass (see Section 5.12.3 in Chapter 5)

Effect of Deformations on Bridge Structures g Section

Tilt (Rotation)

8.9 Differential Settlement

Differential Settlement

Tolerable Movements for Bridges (Table 8-13) Limiting Angular Distortion, δ/S 0.004 0.005

Type of Bridge 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.

Construction Point Concept for Evaluation of Settlements g Divide

the loadings based on sequence of construction g Key construction point is when the final load bearing member is constructed, e.g., when a bridge deck is constructed g Table

8-14

- Put in a slide



- Calculate immediate settlements in granular -

soils Calculate consolidation settlements in saturated fine-grained soils Describe tolerances and consequences of deformations on bridge structures


Shallow Foundations Lesson 08 - Topic 3 Construction

Section 8.10



- Discuss elements of shallow foundation construction/inspection

Key Elements of Shallow Foundation Construction g Table

8-15

g Contractor

set-up

g Excavation g Shallow

foundation g Post installation

- Monitoring

Structural Fill g Tests

for gradation and durability of fill at sufficient frequency to ensure that the material meets the specification g Compaction tests g If surcharge fill is used for pre-loading verify the unit weight of surcharge

Monitoring g Check

elevations of footing, particularly when footings are on embankment fills g Periodic surveying during the service life of the footing, particularly if the subsurface has soft soils within the depth of influence g Impacts on neighboring facilities g Use instrumentation as necessary



- Discuss elements of shallow foundation construction/inspection


Interstate 0 – Apple Freeway Note: Scale shown in Station Form S.B. Apple Frwy

Baseline Baseline Stationing Stationing

90 90

91 91

N.B. Apple Frwy

92 92

93 93

Interstate Interstate 00

Proposed Proposed Toe Toe of of Slope Slope Proposed Proposed Final Final Grade Grade 2

Existing Existing Ground Ground Surface Surface

1

Proposed Proposed Abutment Abutment

Apple Freeway Exercise g Appendix

A

- Section A.7

Subsurface Investigations

Terrain reconnaissance Site inspection Subsurface borings

9

Basic Soil Properties

Visual description Classification tests Soil profile

9

Laboratory Testing

Po diagram Test request Consolidation results Strength results

9

Slope Stability

Design soil profile Circular arc analysis Sliding block analysis Lateral squeeze analysis

9

Approach Roadway Settlement

Design soil profile Magnitude and rate of settlement Surcharge Vertical drains

9

Spread Footing Design

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

Design soil profile Pier bearing capacity Pier settlement Abutment settlement Surcharge Vertical drains

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″

4 6 11 21 22 40 37 33

4′ 7′

10′

15′

Sand

Clay

Compute N160 values Depth (ft.) 5 7 8 10 12 14

p0 (psf)

p0 (tsf)

N (bpf)

Hammer Efficiency (Ef)

550 770 880 1100 1195 1290

0.275 0.385 0.440 0.550 0.598 0.645

11 21 22 40 37 33

65 65 65 65 65 65

Ef / 60

N60 (bpf)

Cn

1.083 12 1.43 1.083 23 1.32 1.083 24 1.28 1.083 43 1.20 1.083 40 1.17 1.083 36 1.15 Average corrected blow count =

N160 (bpf) 17 30 30 52 47 41 36

APPLE FREEWAY PIER SETTLEMENT

SAND

CLAY -1

CLAYCLAY-2

Time (days) 50

100

150

200

250

1″

ΔH

2″

3″

Δ H = 2.85″

APPLE FREEWAY EAST ABUTMENT SETTLEMENT Pressure (psf) 1000

0

2000

3000

Sand

4000

5000

4470

Pf

5550

6000

Pabut

10′

Depth (ft)

Po

4920

Clay

20′

5850

Pc

30′

5650

40′

50′

Gravel Layer Time (days) 0

100

200

300

400

1″

ΔH 2″

Δ H = 2.59″

500

6200

APPLE FREEWAY EAST ABUTMENT SETTLEMENT TREATMENT Time – days 100

200

300

400

0 Assume Wick Drains Installed 5″

ΔH

10″

*0.25″ Δ Remaining 30 days after abutment loaded Begin Abutment Footing Construction 12.66″ emb. Δ

ΔHABUT

15″

15.25″ Emb. + Abut

Time – Days 100

0

200

0.83″

300 240 days

5″

30′ Fill to 10′ Surcharge

ΔH – Total 10″ 13.7″ t90 15″

15.25″Total ΔH

400

500

400 days

*Assume 10′ Surcharge Used

SPREAD FOOTING DESIGN Design Soil Profile Strength and consolidation values selected for all soil layers. Footing elevation and width chosen. Pier Bearing Capacity Qallowable = 3 tons/sq.ft. Pier Settlement Settlement = 2.8", t90 = 220 days. Abutment Settlement Settlement - 2.6", t90 = 433 days. Vertical Drains t90 = 60 days - could reduce settlement to 0.25" after abutment constructed and loaded. Surcharge 10' surcharge: t90 = 240 days before abutment constructed.


Lesson 08-Chapter 8 Shallow Foundations.pdf

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