Abutment Design Example Chris Byrum Doug Parmerlee
Example Bridge
Evaluate Existing Test Hole Data
Not much before 1940 MDOT Housel Soil Mechanics 1940-80s ASTM SPT N-modified values
Evaluate Existing Test Hole Data
Not much before 1940 MDOT Housel Soil Mechanics 1940-80s ASTM SPT N-modified values
Evaluate Existing Test Hole Data
Not much before 1940 MDOT Housel Soil Mechanics 1940-80s ASTM SPT N-modified values
New Test Holes
In-Situ Vane-shear and 3” Shelby Tubes
Design Shear Strength Profiles 4* Sc…old MDOT
Vane only Final Selection UC only
Used for: Global Stability, Bearing Capacity, Piling Side Resistance, Pile Lateral…..
Used for Piling Tip Resistance
External Stability •
Task 1: Lateral Squeeze
•
Task 2: Global Stability
•
Task 3: Settlement Analyses
•
Task 4:Bearing Capacity
LATERAL SQUEEZE ANALYSIS FS =
2*Cu + gama*Ds*tan(theta)
(4.14*Cu) H*gama
Also used in the past… H*gama < 3Cu, or 4Cu Where,
Cu = Undrained shear strength of soft layer, psf Ds = Thickness of soft soil layer, ft gama = Unit weight of fill soil, psf theta = Angle of fill slope, degrees H = Height of fill, ft
Primary Shear
“Secondary/Sympathy” Shear
Edge of Recent Fill Laterally Squeezed Bulging Uplift Area
Previous Scenario
H = 6 to 7 feet
Utility Contractor Adds Weight
Reaches F.S. = 1.0 Condition Road Centerline
E.O.M.
FS =
2*Cu + gama*Ds *t an(t heta)
(4.14*Cu) H*gama
Failure Backcalculation Co m p o n e n t 1 Co m p o n e n t 2
For FS = 1
Cu = 180 Ds = 30 gama = 125 theta = 45 H = 6.5
ps f ft pc f degrees ft
0. 10
0. 92
FS 1.01
3*Cu = 540 4*Cu = 720 gam a*H = 812.5
Example Bridge
FS =
2*Cu + gama*Ds *t *tan(theta)
(4.14*Cu) H*gama Co m p o n e n t 1 Co m p o n e n t 2
Cu = 2100 Ds = 100 gama = 120 theta = 88 H = 46
Cu 1600 1800 2000 2200 2400
FS 1.91 2.15 2.39 2.63 2.87
psf ft pc f degrees ft
0.01
1.58
1.59
3*Cu = 6300 4*Cu = 8400 gam a*H = 5520
Lateral Squeeze 3 2.75 . 2.5 S . F 2.25 d e t 2 a l u 1.75 c l a 1.5 C 1.25 1 1500
Abut. B
Abut. A
1700
1900
2100
2300
Undrained Shear strength, Cu, psf
Where,
FS
Cu = Undrained shear strength of soft layer, psf Ds = Thickness of soft soil layer, ft gama = Unit weight of fill soil, psf theta = Angle of fill fill s lope, degrees degrees H = Height of fill, ft
2500
Abutment A
GLOBAL ST STABI ABILITY LITY ANA ANALLYSIS
Abutment A
Set 2% strain force > 1.1 FS force
Abutment A
Set ultimate force > 1.3 FS force
Abutment A
Set Grid Limits > 1.54 FS force
Change in Plan
Decided to add-a-span, use short pile-supported abutment, on 25+ feet of fill instead of the tall full-height abutment
Abutment A
Abutment B
Abutment B
Abutment B
Abutment B
ARE SPREAD FOOTINGS OK? •
Bearing Capacity
ARE SPREAD FOOTINGS OK? •
Bearing Capacity
……Factored B.C. = approx. 5000 - 6000 psf Approach Embankment Weight next to Abutment: Abut. A = 48*125 = 6000 psf not likely! Abut. B = 30 * 125 = 3750 psf maybe
ARE SPREAD FOOTINGS OK? •
•
•
•
Settlement Management
Need to estimate settlement of footings caused by approach embankments And Footing pressures causing settlement serviceability-limit (1-inch and 1.5-inch limits) Pre-loads? Lightweight Fills? Pile Downdrag?
SETTLEMENT MANAGEMENT
SETTLEMENT MANAGEMENT
0 615 t f , n o i t a v e l E 590
Pressure, psf 3000 6000 9000
12000
Groundwater Table
P0, Overburden
565
540
515
Today’s Condition
0
Pressure, psf 3000 6000 9000
12000
615 t f , n o i t a v e l E 590 P0, Overburden
Glacial Lake Stanley
565
540
515
Groundwater Table
Lake Stanley Dry Period
0
Pressure, psf 3000 6000 9000
12000
Preconsolidation
615 t f , n o i t a v e l E 590
Pseudo-Pc
P0, Overburden Glacial Lake Stanley Pc Assumption
565
540
Ice Weight on Hard Till 515
0
Pressure, psf 3000 6000 9000
12000
615 t f , n o i t a v e l E 590
565
540
515
P0, O verburden Glacial Lake Stanley Footing Pressure Pc Assumption P0 + DP
Load Effects
0
Pressure, psf 3000 6000 9000
12000
0
615
615
t f , n o i t a v e l E 590
t f , n o i t a v e l E590
P0, O verburden Glacial Lake Stanley Footing Pre ssure Pc Assumption P0 + DP
2
Settlement, in 4 6
8
Uc = 100% Uc = 75% Uc = 50% Uc = 25%
565
565
540
540
515
515
SETTLEMENT MANAGEMENT
Wick Drains
Wick Drains
Wick Drains
Example Bridge
14”
10”
(5-20 years w/o wicks)
6”
Settlement Estimates – Soil Only, no footing pressures
8”
Change in Plan
Wick Drains Installed through Sand Drainage Layer
Change in Plan
Pre-Load to this Elev.
7”
(6 month wait for T90)
Settlement Estimates
Change in Plan
EPS Block
H-Piles
Less than 0.4” remains (ZERO Downdrag!!!!!) Settlement Estimates
Placement of EPS and Geogrid behind sheeting.
Example Bridge
Wick Drains Installed through Sand Drainage Layer
Pre-Load to Full Height
(2 month wait) Settlement Estimates
7”
Remove Pre-Load, Piles, and Partial EPS
EPS Block
Est. Settlement =60% of “all-sand” pre-load
Piling Analyses •
Axial Resistance
•
Lateral Resistance: batter vs COM624P
•
Bridge Approach Fill Settlement
•
Downdrag Negative Skin Friction
Axial Capacity: Driven into Shale Rock With about …… 400 Kip Side Resistance 100+ kip Tip/Bottom Resistance
HP14x73
0.4” settlement
0
Pressure, psf 3000 6000 9000
0.4"
12000
0
615
615
t f , n o i t a v e l E 590
t f , n o i t a v e l E590
565
P0, O verburden Glacial Lake Stanley Footing Pre ssure Pc Assumption P0 + DP
2
Settlement, in 4 6
8
Uc = 100% Uc = 75% Uc = 50% Uc = 25%
565 250 kip
540
540
320 kip 350 kip 360 kip
515
515
0
Pressure, psf 3000 6000 9000
0.4"
12000
0
615
615
t f , n o i t a v e l E 590
t f , n o i t a v e l E590
565
P0, Overburden Glacial Lake Stanley Footing Pressure Pc Assumption P0 + DP
0.5"
2
Settlement, in 4 6
8
Uc = 100% Uc = 75% Uc = 50% Uc = 25%
565
220 kip 250 kip
540
540
310 kip 320 kip 340 kip 350 kip 355 kip 360 kip
515
515
An extra 0.1” allowance for elastic pile shortening….
For R ndr = 500 kip HP14x73, 25% settlement remaining: R n = 500 – 220(250/400) = 362.5 kips Q p = 0.75(362.5) – 220 = 52 kips/pile OUCH!!!! Drive 500 kip pile, only 52 kip available for bridge weight!!! NO GO!!
PDA with Dynamic Signal Matching Dynamic Static Rs = 400 kips
Rsdd =
250/400( DD)…reduced side resistance during driving
0
Pressure, psf 3000 6000 9000
0.4"
12000
0
615
615
t f , n o i t a v e l E 590
t f , n o i t a v e l E590
565
P0, Overburden Glacial Lake Stanley Footing Pressure Pc Assumption P0 + DP
0.5"
2
Settlement, in 4 6
8
Uc = 100% Uc = 75% Uc = 50% Uc = 25%
565
220 kip 250 kip
540
540
310 kip 320 kip 340 kip 350 kip 355 kip 360 kip
515
515
An extra 0.1” allowance for elastic pile shortening….
Pile Lateral Resistance -0.5 •
•
COM624P LPILE
Lateral Deflection, in 0 0.5 1
1.5
0 20 Lateral Load
40 60 n i , g n i t 80 o o F w100 o l e B 120 h t p e D140 160
9-kip 15-kip 20-kip 25-kip 30-kip
From “LPILE” Technical Manual
P = soil pressure
Q V W
M
p
From “LPILE” Technical Manual
List of Recs Given To Bridge Engineer •
Global Stability
•
Settlement Amounts and Rates
•
Spreads versus Deep Foundations
•
Lateral Resistances
•
Special Provisions/Materials Specifications
•
Construction Considerations –
Water control
–
Surface preparation
–
Temporary Walls
–
Vibrations
–
Geotechnical Instrumentation needed?
Doug Parmerlee •
Overview of Abutment Design Concepts
Geotechnical Engineering During Construction
Geotechnical Field Monitoring •
Pile Axial Capacity
•
Settlement Rates and Amounts
•
Geosynthetics: Limits/Continuity/Splicing
•
Lightweight Fills: Limits/Materials
PDA with Dynamic Signal Matching
Static Pile Load Tests
Soil Pore Pressure Dissipation
