Lesson 09-Chapter 9 Deep Foundations - Part 4 (Shafts).pdf

DEEP FOUNDATIONS 

Lesson 09 - Topic 4 Drilled Shafts

Learning Outcomes  At the end of this this session, the  participant will be able to:

g

-

Contrast driven piles and drilled shafts Compare mobilization of base (tip) and side (shaft) resistance - Describe drilled shaft construction  processes - Discuss the need for quality control for drilled shaft construction

Learning Outcomes  At the end of this this session, the  participant will be able to:

g

-

Contrast driven piles and drilled shafts Compare mobilization of base (tip) and side (shaft) resistance - Describe drilled shaft construction  processes - Discuss the need for quality control for drilled shaft construction

Definitions Figure 9-56

Driven Piles vs Drilled Shafts Drilled shaft is installed in a drilled hole, unlike the driven pile g Wet concrete is placed in the drilled hole and cures directly against the soil forming the walls of the borehole g

- Side- support support (casing and/or slurry) may be

necessary for stabilization of the open hole and may be left in place g

Installation method and equipment varies with the subsurface conditions

 Advantages of Drilled Drilled Shafts Construction equipment is mobile and construction can proceed rapidly  g Excavated geomaterials can be examined  g For end -  -bearing b   earing situations, the soil beneath the tip may be examined or  probed for weaker materials g Changes in shaft size may be made during construction g

 Advantages of Drilled Shafts Heave and settlement at the ground is normally small  g Personnel, equipment and materials for construction are readily available g Noise and vibration level from the equipment is less than other forms of construction for deep foundations (e.g., driven piles) g

 Advantages of Drilled Shafts  Applicable to a wide variety of subsurface conditions, e.g., can be constructed through cobbles and for many feet into hard rock as well as frozen ground  g Use of a large single drilled shaft (without pile cap) is possible g Extensive data bases documenting load -  -transfer t  ransfer information are available g

 Advantages of Drilled Shafts Smaller footprint than a footing and can thus be constructed near railroad, existing structures and in constricted areas g Shafts may be more economical than spread footing, particularly when the foundation support layer is deeper than 10 -  -ft f  t below the ground or at water crossings g

Special Considerations for Drilled Shafts Construction procedures are critical to the quality of the drilled shaft  g Knowledgeable inspection is required  g Not normally used in deep deposits of soft clay or in situations where artesian  pressures exist  g Static load tests to verify ultimate capacity of large diameter shafts are very costly  g

Effect of Subsurface Conditions on Drilled Shafts g

Caving soils

g

Flowing groundwater 

- Temporary casing or other side support  - Leaching of concrete - Use of slurry 

 Artesian water conditions

g

- Could cause collapse of the shaft

excavation g

Cobbles and boulders

- Sometimes require special tools

Effect of Subsurface Conditions on Drilled Shafts g

Presence of existing foundations and structures - Loss of ground volume into the exacation

g

Landfill material that cannot be excavated  - e.g., an old car body 

g

Rock  - Specialized drilling tools

g

Weak stratum below base of shaft  - May need to extend shaft through the weaker

layer 

Estimating Ultimate Axial Capacity of Shafts in Soils g

Ultimate capacity, Q ult , in compression Q ult  = Q S  + Q T  – W 

g

Ultimate capacity, Q ult , in uplift  Q ult  ≤  0.7Q S  + W 

Geotechnical Allowable Shaft Load, Q all  Q all  = Q ult  / FS  FS is the factor of safety  g Usually FS = 2.5 assuming a normal level of field quality control during shaft construction. “Normal” is based on the minimum recommendations of FHWA g If a static load test is performed, FS=2.0 may be used  g

Computation of Geotechnical  Axial Capacity  g

Cohesive soils - Total stress for undrained conditions •

Similar to Tomlinson method for driven piles

- Effective stress for drained conditions g

Cohesionless soils - Effective stress method for drained

loading conditions

Cohesive soils – Side Resistance g

Side resistance ( Eq  Eq . 9- 36) 36)  N

QS = πD∑ α iS ui Δz i i =1

g

g

is the adhesion factor as follows: α = 0.55

for  S u  p a ≤ 1.5

α = 0.55 − 0.1(S u  p a − 1.5)  

for

1.5 ≤ S u  p a ≤ 2.5

Ultimate unit side load transfer 

f si = αi Sui

Non- contributing contributing zones

Side Resistance Mobilization in Cohesive Soils

Figure 9-58

Cohesive soils – Tip Resistance g

Tip resistance ( Eq  Eq . 9- 39) 39)

QT = qT AT = NcsutAt g

is the adhesion factor as follows: c

= 6.0[1+0.2(z/D)]; Nc ≤ 9

Unit Tip Resistance in Cohesive Soils qTR  = (2.5/[aD/12 + 2.5b]) qT where

D is the diameter of shaft in inches,

a = 0.0071 + 0.0021 (z/D) 0.5

 b = 0.45(sut)

with a ≤ 0.015, and with 0.5 ≤ b ≤ 1.5

Tip Resistance Mobilization in Cohesive Soils

Figure 9-59

Cohesionless soils – Side Resistance g

Side resistance ( Eq  Eq . 9- 44) 44)  N

QS = πD∑ γ z iβ i Δz i / i

i =1

g

is the adhesion factor as follows: where: β i = 1.5 − 0.135 z i   with 1.2 > β i > 0.25

g

Ultimate unit side load transfer ( ≤   ) ≤4   ksf  f si =

/

βi σ vi

Side Resistance Mobilization in Cohesionless Soils Figure 9-60

Cohesionless soils – Tip Resistance g

Tip resistance ( Eq  Eq . 9- 47) 47)

QT = qT AT For N60 ≤ 75: qT = 1.2N60

in ksf

For N60 > 75: qT = 90 ksf g

Reduced tip resistance for large size shafts (D is shaft diameter in inches) qTR  = [50/(12D)] qT

Tip Resistance Mobilization in Cohesionless Soils

Figure 9-61

 Axial Shaft Capacity in Layered Soils Divide subsurface profile into layers g In each layer use the appropriate method  g Sum the resistances from each layer  g

Group Action, Group Settlement, Downdrag and Lateral Loads Similar to driven piles g Refer to FHWA (1999) publication for guidance g

Example 9- 5  5  g

Using FS=2.5, size a shaft for resisting 170 tons of vertical design load  N60-values N60 = 11 N60 = 14 N60 = 14 N60 = 22 N60 = 12 N60 = 19 N60 = 21 N60 = 37

Example 9- 5  5  FS=2.5  g Ultimate axial load = (2.5)(170) = 425 tons g

 Assume a 3- ft ft diameter straight shaft  g Thus, circumference = d = 9.42 -  -ft  f  t  g

 Assume a shaft length of 60 -  -ft  f  t  g Use formulation as follows g

 N

QS = πD∑ γ z iβ i Δz i / i

i =1

where: β i = 1.5 − 0.135 z i   with 1.2 > β i > 0.25

Example 9- 5  5  g

Compute side resistance with depth

Depth

Surface Area

Avg effective vertical

Interval,

per depth

(overburden) stress,

z, ft

 / 

interval, z( )(D), ft

zi, tsf

2

QS

β i = 1.5 − 0.135 z i

Tons

with 1.2 > β i > 0.25

0–4

37.7

0.115

1.20

5.20

4 – 30

245.0

0.572

0.94

131.70

30 – 60

282.7

1.308

0.59

218.20 QS

355.10

Example 9- 5  5  Compute tip resistance g At 60 -  -ft,  ft, N 60  = 21 g

q T = 1.2 N 60  = 25.2 ksf = 12.6 tsf  g Tip area, AT  = 7.07 sq. ft. g

g

Q T  = q T  AT  = 7.07(12.6) = 89.1 tons

Total axial resistance, Q ult  = Q S  + Q T  g Q ult  = 355.1 tons + 89.1 tons = 440 tons g

g

Okay 

Example 9- 6  6 

N60 = 20

N60 = 25

N60 = 50

 Axial Capacity in Rocks g

Side resistance ( Eq  Eq . 9- 50, 50, 9- 51) 51)

Q SR  = πD R L R q SR  q SR  = 0.65α E p a (q u  p a )0.5 < 0.65 p a (f c′  p a )0.5 g

Use information in Chapter 5 to evaluate the elastic modulus of rock mass

 Axial Capacity in Rocks g

Tip resistance ( Eq  Eq . 9- 52, 52, 9- 53) 53)

Q TR  = A T q TR  qTR  = 2.5 qu

Intermediate GeoMaterials ( IGMs IGMs ) g

Cohesive IGM  - S u  value of 2.5 to 25 tsf 

g

Cohesionless IGM  - N 60  values > 50 blows/ft 

Refer to FHWA (1999) publication for further information and design  procedures for shafts in IGMs

g

Construction Methods Dry method  g Wet method  g

g

Casing method 

g

Cleaning of the shaft excavation is the most important step in construction of drilled shafts

Dry Method  Drill 

Clean Position Place Cage Concrete

Wet Method  Drill 

Slurry Clean Position Cage

Place Concrete

Casing Method  Drill 

Case

Clean Position Place Cage Concrete

Effect of Shaft Cleaning During Construction

Quality Assurance and Integrity Testing  Drilled shafts are “manufactured” at the site g Often anomalies develop during construction g An anomaly is deviation from an assumed geometry of the shaft and/or shaft properties (e.g., homogeneity) g

g

NHI 132070 2.5 -  -day d   ay course

Types of Anomalies in Drilled Shaft  Necking  g Bulbing  g Soft -  -bottom b   ottom g Voids or soil intrusions g Poor quality concrete g Debonding  g Lack of concrete cover over reinforcement  g Honey -  -combing  c   ombing  g

Non Destructive Tests ( NDTs NDTs ) for Detection of Anomalies NDTs are geophysical tests g External  g

- Sonic echo - Impulse response - Ultra- seismic  seismic 

g

Internal  -

Crosshole Sonic Logging (CSL) Gamma Density Logging (GDL) CSL Tomography (CSLT) Perimeter Sonic Logging (PSL) Neutron Moisture Logging (NML)

Crosshole Sonic Logging 

Gamma Density Logging 

Load Testing of Drilled Shafts g

Static Load Tests -

Similar to driven piles Osterberg Load Cell test 

g

Statnamic test 

g

Must perform caliper logging and NDTs before load testing 

Osterberg Load Cell Test 

Osterberg Cell  g

Table 9- 11, 11, Table 9- 12  12 

Cage Centralizers

O-cells between two steel plates

CSL tubes

Instrumentation (strain gages)

Statnamic Load Test 

Statnamic Load Tests

Learning Outcomes  At the end of this session, the  participant will be able to:

g

-

Contrast driven piles and drilled shafts Compare mobilization of base (tip) and side (shaft) resistance - Describe drilled shaft construction  processes - Discuss the need for quality control for drilled shaft construction

 Any Questions?   THE RO A D TO G UNDERS T A NDIN SOIL S  A ND FOUND A TIONS