DEEP FOUNDATIONS. Lesson 09 - Topic 4 Drilled Shafts

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1 DEEP FOUNDATIONS Lesson 09 - Topic 4 Drilled Shafts

2 Learning Outcomes gat the end of this session, the participant will be able to: - 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

3 Definitions Figure 9-56

4 Driven Piles vs Drilled Shafts gdrilled shaft is installed in a drilled hole, unlike the driven pile gwet concrete is placed in the drilled hole and cures directly against the soil forming the walls of the borehole - Side-support (casing and/or slurry) may be necessary for stabilization of the open hole and may be left in place ginstallation method and equipment varies with the subsurface conditions

5 Advantages of Drilled Shafts gconstruction equipment is mobile and construction can proceed rapidly gexcavated geomaterials can be examined gfor end-bearing bearing situations, the soil beneath the tip may be examined or probed for weaker materials gchanges in shaft size may be made during construction

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

7 Advantages of Drilled Shafts gapplicable 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 guse of a large single drilled shaft (without pile cap) is possible gextensive data bases documenting load-transfer information are available

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

9 Special Considerations for Drilled Shafts gconstruction procedures are critical to the quality of the drilled shaft gknowledgeable inspection is required gnot normally used in deep deposits of soft clay or in situations where artesian pressures exist gstatic load tests to verify ultimate capacity of large diameter shafts are very costly

10 Effect of Subsurface Conditions on Drilled Shafts gcaving soils - Temporary casing or other side support gflowing groundwater - Leaching of concrete - Use of slurry gartesian water conditions - Could cause collapse of the shaft excavation gcobbles and boulders - Sometimes require special tools

11 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

12 Estimating Ultimate Axial Capacity of Shafts in Soils gultimate capacity, Q ult, in compression Q ult = Q S + Q T W gultimate capacity, Q ult, in uplift Q ult 0.7Q S + W

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

14 Computation of Geotechnical Axial Capacity gcohesive soils - Total stress for undrained conditions Similar to Tomlinson method for driven piles - Effective stress for drained conditions gcohesionless soils - Effective stress method for drained loading conditions

15 Cohesive soils Side Resistance gside resistance (Eq( Eq ) 9 Q S = πd N i= 1 α i S ui Δz gα is the adhesion factor as follows: gultimate unit side load transfer i α = 0.55 for S p 1. 5 ( p 1.5) u a 1.5 Su pa α = Su a for 2. 5 f si = α i S ui

16 Non-contributing zones

17 Side Resistance Figure 9-58 Mobilization in Cohesive Soils

18 Cohesive soils Tip Resistance gtip resistance (Eq( Eq ) 9 Q T = q T A T = N c s ut A t gα is the adhesion factor as follows: N c = 6.0[1+0.2(z/D)]; N c 9

19 Unit Tip Resistance in Cohesive Soils q TR = (2.5/[aD/ b]) q T where D is the diameter of shaft in inches, a = (z/d) with a 0.015, and b = 0.45(s ut ) 0.5 with 0.5 b 1.5

20 Tip Resistance Mobilization Figure 9-59 in Cohesive Soils

21 Cohesionless soils Side Resistance gside resistance (Eq( Eq ) 9 Q S = πd N i= 1 γ / i z i β i Δz gβ is the adhesion factor as follows: where: i i β = z with 1.2 > βi > gultimate unit side load transfer ( 4( ksf) f si = β i σ / vi i

22 Side Resistance Mobilization in Cohesionless Soils Figure 9-60

23 Cohesionless soils Tip Resistance gtip resistance (Eq( Eq ) 9 Q T = q T A T For N 60 75: q T = 1.2N 60 in ksf For N 60 > 75: q T = 90 ksf greduced tip resistance for large size shafts (D is shaft diameter in inches) q TR = [50/(12D)] q T

24 Tip Resistance Mobilization in Cohesionless Soils Figure 9-61

25 Axial Shaft Capacity in Layered Soils gdivide subsurface profile into layers gin each layer use the appropriate method gsum the resistances from each layer

26 Group Action, Group Settlement, Downdrag and Lateral Loads gsimilar to driven piles grefer to FHWA (1999) publication for guidance

27 Example 9-59 gusing FS=2.5, size a shaft for resisting 170 tons of vertical design load N 60 -values N 60 = 11 N 60 = 14 N 60 = 14 N 60 = 22 N 60 = 12 N 60 = 19 N 60 = 21 N 60 = 37

28 Example 9-59 gfs=2.5 gultimate axial load = (2.5)(170) = 425 tons gassume a 3-ft 3 diameter straight shaft gthus, circumference = πd = 9.42-ft gassume a shaft length of 60-ft guse β formulation as follows Q S = πd N i= 1 γ / i z i β i Δz i where: β = z with 1.2 > βi > i i

29 Example 9-59 gcompute side resistance with depth Depth Interval, Δz, ft Surface Area per depth interval, Δz(π)(D), ft 2 Avg effective vertical (overburden) stress, γ / z i, tsf β β = i z i with 1.2 > βi > ΔQ S Tons Q S

30 Example 9-59 g Compute tip resistance g At 60-ft, N 60 = 21 g q T = 1.2 N 60 = 25.2 ksf = 12.6 tsf g Tip area, A T = 7.07 sq. ft. g Q T = q T A T = 7.07(12.6) = 89.1 tons g Total axial resistance, Q ult = Q S + Q T g Q ult = tons tons = 440 tons g Okay

31 Example 9-69 N 60 = 20 N 60 = 25 N 60 = 50

32 Axial Capacity in Rocks gside resistance (Eq( Eq , ) 9 Q = πd SR R L R q SR qsr = 0.65αEpa ( ) 0.5 ( ) 0. 5 qu pa < 0.65pa fc pa guse information in Chapter 5 to evaluate the elastic modulus of rock mass

33 Axial Capacity in Rocks gtip resistance (Eq( Eq , ) 9 Q = TR A T q q TR = 2.5 q u TR

34 Intermediate GeoMaterials (IGMs) gcohesive IGM - S u value of 2.5 to 25 tsf gcohesionless IGM - N 60 values > 50 blows/ft grefer to FHWA (1999) publication for further information and design procedures for shafts in IGMs

35 Construction Methods gdry method gwet method gcasing method gcleaning of the shaft excavation is the most important step in construction of drilled shafts

36 Dry Method Drill Clean Position Cage Place Concrete

37 Wet Method Drill Slurry Clean Position Place Cage Concrete

38 Casing Method Drill Case Clean Position Place Cage Concrete

39 Effect of Shaft Cleaning During Construction

40 Quality Assurance and Integrity Testing gdrilled shafts are manufactured at the site goften anomalies develop during construction gan anomaly is deviation from an assumed geometry of the shaft and/or shaft properties (e.g., homogeneity) gnhi day course

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

42 Non Destructive Tests (NDTs( NDTs) ) for Detection of Anomalies gndts are geophysical tests gexternal - Sonic echo - Impulse response - Ultra-seismic ginternal - Crosshole Sonic Logging (CSL) - Gamma Density Logging (GDL) - CSL Tomography (CSLT) - Perimeter Sonic Logging (PSL) - Neutron Moisture Logging (NML)

43 Crosshole Sonic Logging

45 Gamma Density Logging

47 Load Testing of Drilled Shafts gstatic Load Tests - Similar to driven piles - Osterberg Load Cell test gstatnamic test gmust perform caliper logging and NDTs before load testing

48 Osterberg Load Cell Test

50 Osterberg Cell gtable 9-11, 9 Table 9-129

51 Cage Centralizers O-cells between two steel plates Instrumentation (strain gages) CSL tubes

52 Statnamic Load Test

53 Statnamic Load Tests

54 Learning Outcomes gat the end of this session, the participant will be able to: - 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

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

56 Inspector Qualification Courses

CHAPTER 8 CALCULATION THEORY

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