Pile Settlement - EnCE 4610
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Transcript of Pile Settlement - EnCE 4610
ENCE 4610
Foundation Analysis and Design
Static Load Tests
Pile Settlement
Pile Groups
Methods of Evaluation of Deep Foundations
• Full-scale static load tests on test piles
• Analytic methods, based on soil properties from laboratory and/or in-situ tests
• Dynamic Methods, based on dynamics of pile driving or wave propagation
Concepts to Review • Shaft and End Bearing Piles
• Resistance to load
– Shaft resistance (Qs )
– Toe resistance (Qt)
• End-bearing piles, toe resistance predominates
• Shaft Friction Piles, shaft resistance predominates
• For tension piles, shaft resistance predominates (toe cannot be in tension)
Ultimate vs. Allowable Capacity Ultimate capacity is the load required to
cause failure, whether by excessive settlement or irreversible movement of the pile relative to the soil
For driven piles, one must also consider the resistance to driving, which can be different from the ultimate capacity
Allowable capacity is the ultimate capacity divided by a factor of safety (ASD)
Important to distinguish between the two
Prediction and Verification
• At what load will the pile fail? (Bearing Capacity)
• How much will pile deflect under service loads?
(Settlement)
Prediction on basis of site investigation
and laboratory testing
Verification by some method of
load testing
Static Load Tests The most precise – if not
always the most accurate – method of determining the ultimate upward or downward load capacity of a deep foundation
Static load tests, however, are time consuming and expensive; must be used judiciously
Object of the test is to develop a load-displacement curve, from which the load capacity can be determined
Dead Load Test
• Considered a reliable static test method
• Slow and expensive
• Dangerous when done unsafely
• No longer commonly used in the U.S.; used where labor costs are lower
Reaction Piles
Static Load Procedure (ASTM D1143)
Two categories of tests Controlled stress tests – the
most common method
Controlled strain tests
For driven piles, need
to delay static load
test to allow pile set-up Granular soils – 2 days
Cohesive soils – 30 days
Procedure Set up reaction stand
Apply a test load to the pile
Record the load-settlement
history for each load applied
Apply the next load
Loads are generally applied in
increments of 25, 50, 75, 100,
125, 150, 175 and 200% of
proposed design load
Static Load Procedure (ASTM D1143)
Load test increments in
time Slow test – maintains load until
pile movement is sufficiently
small
Quick test – each load
increment is held for a
predetermined length of time,
for 2.5 – 15 minutes
Generally requires 2-5 hours
to complete
May be best method for
most deep foundations
Reaction Pile Test
Advantages and Disadvantages • Advantages
• Can be installed with same
equipment as production piles
• Test can be done on
inclination (batter)
Disadvantages Reaction piles may pull out
If not done properly, reaction
pile capacity may result
Flexible system stores energy
during tests
STATIC LOAD TEST
Advantages Gives reference
capacity Relatively slow
loading minimises dynamic components
Can customise to include creep effects
Can be instrumented to yield static resistance distribution & end bearing
Disadvantages
Time consuming
Expensive
Done on specially
designated piles
Often done
carelessly or
inaccurately
Static Load Test
Interpretation
At what point of the measured pile top load vs deflection curve do we define
the failure load Ru?
Loading method
and curve interpretation can make significant
differences in result.
Davisson’s Method
• Advantages – Somewhat Conservative
– Matches Dynamic Analysis
Failure Criterion (Quake)
– Relatively Independent of
Judgement (E-Mod. for
concrete, timber; diameter)
• Disadvantage – Capacity/Settlement a
Function of Pile Properties
NAVFAC DM 7.02
p. 7.2-229
Example of Davisson's Method Formulas for round pile; substitute actual
area for other shapes
1.5 Uplift Test
Equivalent Curve from Osterberg Data
Osterberg Test
Advantages and Disadvantages • No reaction load
needed
• Requires jack load only half of test load
• Shaft is loaded upward
rather than in
downward direction
• Tensile vertical strains
near toe will cause
cracking in soil
• Maximum movement is
at the pile toe rather
than pile top
• Only for specially prepared piles
Other Topics in Pile Capacity
Upward Load Capacity Most deep foundations derive
their upward load capacity
from the shaft resistance
Exceptions include belled drilled shafts (shown at left) and bulb piles (Franki)
For driven piles, ultimate
upward load = shaft capacity, but factors of safety/load-resistance factors can be different from compression
Pile Group Effects
Pile Settlement
Settlement Most methods for designing
deep foundations for bearing capacity insure that settlement does not exceed ½" (13 mm)
There are certain situations where it is necessary to know the settlement of a deep foundation
Structures sensitive to settlement
Toe bearing predominates
Downdrag loads are present
Compressible strata are present
Need an equivalent spring for finite element analysis
• In the case of “critical” structures, settlement analysis will be performed using a “t-z” method computer program o Example of one is in the wave
equation analysis routine at vulcanhammer.info
• What we need are quick methods of making preliminary estimates
Vesić’s Method for Settlement
• Steps for Vesić’s method o Compute ultimate shaft and toe
capacities using methods shown earlier o Compute elastic settlement o Load on the shaft is the smaller of two
loads: • the load applied to the pile • the shaft resistance
o Load on the toe is the smaller of two loads: • (The load applied to the pile) –
(the shaft resistance) (= 0 if negative)
• The toe resistance o Settlement is computed by the
equation S = Sf + Ss + Sp • where
o S = total settlement o Sf = elastic settlement o Ss = shaft settlement o Sp = toe settlement
• Vesić’s method can give a single number or a load-settlement curve o Method given as described in EI
02C067, Design of Deep Foundations • Method described in Murthy
15.29 leaves out many steps and confuses shaft and elastic settlement
o Can be used with either drilled shafts or driven piles
• Drilled shafts can be analyzed with load-transfer curve methods o “Old drilled shaft method,” as
presented in book o “New drilled shaft method,”
which we will cover in LRFD
Vesić’s Method for Settlement
• Elastic Settlement Sf = (Qp+α*Qf)L/(A*Ep) o Qp = toe resistance = Q – Qs >
0
o Qf = ultimate shaft resistance (or working load if Q < Qf
o Q = applied or working load
o α = load distribution factor
o 0.5 < α < 0.7, generally assume 0.6
o L = pile length
o A = pile cross-sectional area
o Ep = elastic constant of pile material
• Toe Settlement
o B = pile toe size/diameter
o qpu = unit toe resistance
o Cw = (0.93+0.16*(L/B)1/2)Cs
o Assume zero if Qp < 0
pu
pw
pBq
QCS
Vesić’s Method for Settlement
• Shaft Settlement
Value of Cs shown in
table at the right for both driven and bored
piles
• If Q < Qf, use Q
Soil Driven Piles
Bored Piles
Sand (dense to loose)
0.02-0.04 0.09-0.18
Clay (stiff to soft)
0.02-0.03 0.03-0.06
Silt (dense to loose)
0.03-0.05 0.09-0.12
pu
fs
sLq
QCS
Settlement Example
Applied Load = 50 kips
Key Variables and Solution
Coefficients Use Cs = 0.03
Cw =
(0.93+0.16*(30/1.5)1/2)(0.03) =
0.049
α = 0.6
Solutions Sp = (0.049)(0)/((1.5)(7.529)) = 0
Sf =
(0+(0.6)(50))(30)/((0.191)(43200
00)) = 0.001’ = 0.013”
Ss = (0.03)(50)/((30)(7.529) =
0.007’ = 0.080”
• Results from Dennis and Olson Analysis
• Total Shaft Friction = 103.8 kips
• Total Toe Capacity = 13.3 kips
• Toe Unit Capacity qpu = 7.529 ksf
• Mobilized Shaft Friction Qf = 50 kips < 103.8 kips
• Mobilized Toe Capacity Qp = 0
• Other Key Variables • Pile diameter B = 1.5’
• Pile Length L = 30’
• Pile Cross-Sectional Area A = 0.191 sq. ft.
• Pile Modulus of Elasticity E = 4,320,000 ksf
Solution: S = 0” + 0.013” + 0.080” = 0.093”
Group Effects • Piles are generally used
in groups; drilled shafts are less frequently so
• Group capacity can be less than the sum of the individual capacities of the piles, depending upon a number of factors
• Group settlements can also be driven by different considerations than settlements of single piles
Stress Zones in
Supporting Soils
Basic Relationships in Group Capacity
• Cohesionless Soils o Centre-to-Centre spacing of the
piles/shafts should be > 3d
o Driven Piles
• Group capacity can be greater than the sum of the individual capacities, so Eg = 1
• Pile group should not be underlain by a weak deposit, in which case the settlement of the weak deposit will drive the performance of the group
• Jetting or predrilling should be avoided
o Drilled Shafts
• Use Eg = 2/3 for spacings = 3B; this increases linearly to Eg = 1 for spacing = 6B and is 1 above this
Basic relationship
Qgu
= allowable axial (down or up) capacity of group
Eg = group efficiency factor
ΣQu = allowable axial (down or
up) capacity of single pile
Considerations Pile Spacing
Drilled shafts vs. driven piles
Cohesive vs. cohesionless soils
Individual vs. block failure
u
gu
gQ
QE
Effect of Pile Spacing
Individual vs. Block Failure
Group Capacity for Cohesive Soils
Smallest of four options: Drilled shaft method for
cohesionless soils (always good for drilled shafts)
If undrained shear strength < 2 ksf (95 kpa) and the pile cap is in firm contact with the g round, Eg = 1
If undrained shear strength > 2 ksf (95 kpa), Eg = 1
Use block failure criterion to the left
As always, Centre-to-Centre spacing of the piles/shafts should be > 3d
95Z
15B
15
B+
D+=N*
c
Group Settlements Quick Methods Immediate
settlements – group settlement factor
Long-term consolidation – equivalent mat method
• Cohesionless Soils: Group Settlement Factor o Fg = Sg/S = (Hw/B)½
o Sg = group settlement
o Fg = group settlement factor o Hw = width of pile group o B = pile diameter o S = settlement of single pile
Pile group settlements can be treated in a similar manner to those of shallow foundations
Settlements can be divided into two types
Immediate settlements – those shortly after foundation loading, especially in sands
Consolidation settlements – in clays, same mechanism as with shallow foundations
Ultimately, for more accurate computation of group settlements, computer programs using the t-z methods should be employed
Cohesive Soils: Equivalent Mat Method
Replace group with a mat along the embedded pile length L; this depth is 2/3 of L for friction piles and L for end bearing piles
Distribute the load from the mat to the underlying soil by the 2:1 method
Calculate settlement of soil layers below the mat by one-dimensional consolidation theory; any soil above the mat is assumed to be incompressible
Cohesive Soils:
Equivalent Mat
Method
Group Settlement Example
Compute group settlement factor for sands (use sands for settlement calculations, since they are at the base of the group)
gf = (H
w/B)½ = (22.5/1.5)½ = 3.87
Compute group settlement
δg = (0.093”)(3.87) = 0.36"
This method is to be used with immediate settlements; for long term consolidation, use equivalent mat method with Terzaghi's consolidation theory
Find: Immediate settlement
of 3 x 3 pile group, Hw = 22.5'
Individual settlement at 50 kip load = 0.093"
Questions