Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Soils and Geotechnical EngineeringAreas of Frequent MisunderstandingAreas We Could Better Work TogetherThe “right” exploration program?Lateral earth pressuresUncertainty in settlement estimatesThe relationship between bearing pressures, settlement and subgrade modulus valuesSeismic assessmentsBalancing uncertainty, risk and costs, and how to avoid excessive conservatism in interpreting recommendations.
Low Rise Buildings
T
• Institutions• Commercial• Industrial
Renaissance Project• 1.1 million sf Hospital• 4 levels of underground garage
High Rise
Copley Place• 10 Acres of Bldgs.• 5 to 35 stories• Air rights ( I-90, Amtrak, 2 subway
lines, ramp, etc.
International Place• Five 35 to 45 story towers• 5 levels of underground • Architect: Phillip Johnson
Transportation
5 mile long bridge8.0 magnitude earthquake
Great Salt Lake Causeway1984
Project Manager - $3.4 billion section of “Big Dig”
Geotechnical Design Process
?
Likely Design Problems
Proposal
Analyses
Monitoring
Experience
Recommendations
Efficient Design & Minor Construction Adjustments
Expected Site Geology
Interpreted Site Geology
Lab Testing
Subsurface explorations
Geotechnical Issues Driven by Geology
Sands: generally good, seismic settlement
Saturated Clays: settlement; difficult site work
Clays above WT: difficult site work, expansive?Silts: seismic settlement, difficult site work, collapse?
Rock: Difficult excavationWater:
Major!!Shallow or deep? (or both?)Varies with Season
Geohazards (expansive or collapsible soils, faults, earthquakes, landslides, etc.)
Geotechnical Scope vs. Risk
Too un-conservative(failure)
Too conservative(wasted money)
Failure with loss of life
The right balance ?
Definitely OK
$ $ $ $$$
Uncertainties with soils
MaintenanceRisks
Very rarelyissues
$$ $
The Right Geotechnical Scope?
Geotechnical Scopes Saving on Geotechnical Services May Increase Project Costs Dramatically
Boring Spacing (Utah Practice Numbers)
Definition
Decreasing # BoringsWhat has Changed in Past 35 years?
Borings?Geology?
1 How much area is covered by each boring?
= total area# borings
SandSiltClay
Mix
Gravel & Cobbles
Boring 1
Boring 2Boring 3
Geology – How it Appears in Borings
Does it matter that we are sampling a small percentage of the site?
Exploration Methods
Auger BoringsMud‐Rotary Borings (for liquefaction)Cone Penetrometer Testing (CPT)Test PitsGeophysicsOther In‐Situ Methods
Vane ShearDilatometerPressuremeter
What is the “right” exploration program?
No one right exploration program or method.
structuresoilsexpected issues
Customized Best Program
Architect/Structural Engineer/Client Defined Scope – Is it Helpful?
Bldg. Footprint, No. Stories, Basements, Grading, Loads ? Yes!Existing Subsurface Information? YesNo. Borings – OK but not optimalDepth Borings – No will get less than needed Pavement Design, Corrosion? YesWhat Should Be in Report Does not limit low end reportBottom Line: Bidding Low Scope (to Get the Work) Conservative Design Higher Building Costs
Borings vs. CPT
Should Depend On:Soil Conditions The Design Challenges
There Are Pros And Cons Of Each ApproachFor Soft Sites, a Mix is the Most Optimal.
Borings vs. CPT ?Soil Borings CPT
Pros ProsYou get samples of the soils, so you are sure of what you have. Continues soil profile (does excellent job of seeing the detailed
switching between sands and clays that is common in Utah).A good approach and less expensive in most circumstances. Faster
Excellent for clarifying when the liquefaction “borderline soils” will or will not liquefy (because you really need lab tests on the soils to do this.)
Continuous soil profile is very good for: time rate of settlement (if a surcharge is needed, wick drains
and spacing, and for how long; how much post construction settlement);
refining totals on liquefaction settlements.
Rigs are readily available. In some cases saving on lab testing can make CPT + testing costs lower.
You get soil samples so can get conventional lab tests done (which in many cases may provide the most accurate analysis results) Data is reliable, reproducible and accurate within +/‐ 5%.
Cons Cons5 ft.. or 10 ft.. Spacing on samples misses the alternating sand/clay/silt
Availability is typically 4 to 6 weeks out (and many clients say they can’t wait)
Wide sample spacing requires more judgment in estimating liquefaction settlement.
Replaces soil lab tests for direct measurement of soil behavior which requires expertise to interpret.
Wide sample spacing requires more judgment in estimating time rate of settlement.
Since there are no samples, soil type is by correlation and will be wrong sometimes.
Sands can heave leading to erroneous SPT blowcounts. Cost is higher per foot.Blowcounts have been proved at times to be inaccurate up to +/‐50%.
Can’t go through dense sand/gravel layers in/near benches and may not make it through thick sands.
Case History – 1Low Cost Investigation & No Review Of Grading Plan
Soil Borings Graded Surface
Original Site Grade Line
Test pits
Cross Section
Geotechnical Report Savings~ $ 20,000
Repair Costs~ $ 300 – 500K(& Bldg. Risk)
Case History – 2 :The Right Exploration Method?
Added Explor. & Analysis Costs
~ $ 5,000
Savings~ $ 1.4 million
or Risk
Auger Boring
5’
5’
5’
5’
5’
5’2.5’
clay
clay
clay
bad
bad
good
bad
good
bad
good
silt
sand
clay
sand
clay
sand
clay
silt
sand
clay
sand
clay
sand
clay
4‐9 ” 0‐1 ”$1.7 million $0 CPT
?
2‐6 ”$325 K
silt
sand
clay
sand
clay
sand
clay
Auger Boring& CPTCPT can misclassify some soil types,
which can lead to misinterpretation.
Seismic Effects on Structures
Forces on StructuresLiquefaction Bearing FailureLiquefaction SettlementLateral Spread
Shaking
Liquefaction – What is The Threat?
Liquefiable Soils
Non-Liquefiable
Crust
Water
Non-Liquefiable
Soils
LooseTyp. Sand or SiltLittle ClayBelow WaterNot too Deep
Requirements
Liquefiable Soils
Non-Liquefiable
Crust
Water
Non-Liquefiable
Soils
SettlementLateralSpread
Liquefiable Soils
Non-Liquefiable
Soils
Water
BearingFailure
Case History – 3Geotechnical Interaction w. Structural Engineer
132,000 sf 1‐story BuildingLiquefiable Soils?
Case History – 3 : The Problem
Settlement: 2” to 6”Horizontal Movements: 1 ft. to 5 ft.Goal: Only “Life Safety”
Case History – 3 : Alternate Approach
Structural and Geotechnical Analysis Costs
~ $ 6,000
Cost Savings$1,375,000
Skip Team MeetingIterative Design (Life Safety)Time to Discuss Soil/Structure Interaction
Liquefiable Soils and Site Class:Misunderstandings
Ignoring Effect of Liquefaction on Site ClassLiquefaction Always Site Class F ?
Liquefaction Site Class F , Right?
Period < 0.5 sec. Can use <F
Col
laps
e C
ondi
tions
?
Not F• Deep Water?
or• Stiff Crust?
Class FLiq. Near Footing ?
If in doubt, ask!
Case History – 4 : Sometimes Things Are Different
Structural and Geotechnical Analysis Costs
~ $ 6,000
Cost Savings$1,375,000
Case History – 4 : Sometimes Things Are Different
Structural and Geotechnical Analysis Costs
~ $ 6,000
Cost Savings$1,375,000
?
Settlement : Refresher on Types & Timing
ElasticConsolidationSecondary compressionLiquefaction
As the Fill and Building Load is Applied
As the water is squeezed out of the clay (weeks to years)After consolidation occurs (relatively small)
Immediately after earthquake
usually
Settlement : Refresher on Types & Timing
ElasticConsolidationSecondary compressionLiquefaction
Saturated ClaySands & Clay Above WT
Uncertainty in Estimates
Variable SoilsVariable Soil Properties
Variable Timing (Sand Layers)Deeper Influence w. Foundation Size
Misunderstandings in Estimates
Settlement Zone ~ 2 Width (fdn. & Fill) !Clays < Max. Past Pressure Still Settle
Case History – 4 :
1‐Story Steel frame / CMU3’ to 8’ Fill = 2” to 8 “Footing Loads = 0 to 0.5”Structural Engr. Max. 1” = 1” 250 daysQuestion is . . .
Acceptable ?
Which Matters?
Settlement : Which parts matter?
Elastic SettlementConsolidation Settlement (clay)
Secondary Compression
(clay/organics)
Site Fill and Building Construction EarthquakePost-Construction
Liquefaction Settlement
(sands/silts)
Consolidation Settlement
(clay)
Site Fill and Building Construction
ConnectionsConnections& Finishes
Life
Saf
ety?
13 yrs.
Settlement : Which parts matter?
Secondary Compression
(clay/organics)
Post-Construction
That said . . . . . .
Questions For the Geotechnical Engineer
Did they Have Site Grading?Did Structure Change?“Settlement is 0 to 2 in.” ‐ Meaning?
Uniform Settlement at a level less than 2 in.?Differential Settlement of 2” Across Building?Differential Settlement of 2” Over a Column Bay?
How did they estimate settlement without knowing any loads?
Check their assumed load vs. your service loads
Bearing Pressures & Footing Size
Report : “Allowable Bearing Pressure = ____ psf”I have not sized any footings, so how was the ____ psf developed? Is it applicable?
Bearing Pressures & Footing Depth
Report : “Allowable Bearing Pressure = ______ psf”
I have footings at different elevations?Is ____ psf applicable?
~ ~
Settlement
BearingCapacity
Things The Geotech Has Not Been Told
Site Grading/Bearing Soil?Basements?Building location on site?Footing bearing elevations? Loads/Size?
Final Site Grade Line ?
Structural Fill? ?
Bearing Soil?
?? ?
Load/ Size?
these unknowns force conservatism
Subgrade Modulus Common Misunderstandings
Report : “0 to 1 in. of settlement”But with K value ‐ 2 in. under my loadWhat to do?
Subgrade Modulus Common Misunderstandings
Is Value K1x1? Or scaled?How Developed?
“Typical” Value?From Analysis?Iterate w. SE?
MatSand & Gravel
Soft Clay
W
Limited Slope Stability Analyses
“There is not a slope stability analysis in the report, is one needed?” Is the slope stability analysis adequate?
Case History – 6Low Cost Investigation Major Failure
Geotechnical Report Savings~ $ 12,000
Failure Costs> $ 3 million
30 ‘
More interaction.Before geotech is hiredAfter you get the geotechnical report
Improve The Cost Effectiveness Of The Design And Reduce Risk By:
More interaction between the geotechnical and structural engineer
before the geotechnical investigationDuring geotechnical work (if building starts to change)
Improve The Cost Effectiveness Of The Design And Reduce Risk By:
Restate your understandingExplain site grading, basements, building location, and footing bearing elevations Provide loadsBreak out live loads: persistent vs. transientExplain the range of footing sizes
Meet w. Geotechnical Engineer : After Report Review
Combined footings/mat ‐ give elevation and size, (and ask if the subgrade modulus is applicable or needs to be modified)
Request geotechnical engineer check recommended bearing pressures (now that they have loads and know what soils the footings will bear on)
If the geotechnical engineer does not have scope to revisit the project, speak with the owner (explain reduced costs and/or risk)
Meet w. Geotechnical Engineer : After Report Review
Initial geotechnical design. Discussion/meeting with the structural engineer.Geotechnical revisions: either an addendum letter or revised geotechnical report
Suggest That The Owner Includes The Following Stages Into The Geotechnical Work Scope:
Challenging conditions OR sophisticated owner engage the owner and geotechnical engineer in a conversation about uncertainty/risk/costs Be careful when telling the owner what to expect for geotechnical issues
Anything Else?
Get the geotechnical design firm involved during construction Help convince the owner that they lose money when they hire low cost geotechnical services
Anything Else?
Case History – 7Low Cost Geotechnical Services
Cross Section
Geotechnical Report Savings
~ $ 5,000
Added Costs~ $ 200K to 1.5
million
6’Water
Original Conclusions:• Collapse• Liquefaction = Site Class F• Deep foundations, or GI, or geogrid reinforced soil mat• 1.2” liq. Settlement• Acceleration from Outdated DSHA
Lateral Spread <6”
Site Class D
0 ‐ 2” liq. settlementIF Site Class F SRA; But not F
Balancing
Factors Determining Risk:# Of ExplorationsLab TestingTime (Do & Check Analyses)Time (Create And Check A Good Report)Contracted To Monitor Construction?
Geotechnical Budget . . . Uncertainty Increase Conservatism Building Costs More construction claims Claim Costs Serious Failure? ($ and possibly lives)
Case HistoriesBenefits from Added Scope
ExampleAdded
Geotechnical Costs
Project Savings
1 $20,000 > $300 K2 $5,000 $1.4 million3 $6,000 $1.4 million5 $8,000 $700 K6 $15,000 $3 million7 $5,000 $1.5 million
Geotech Early (Before Grading, Building Position, Loads, etc.)
Lack of Meetings
“I’ve Heard . . .”, “But on the adjacent project . . “
Lack Of Communication About Risk Decisions Oversimplified Conversations With Owner
Owner Not Willing To Engage in Conversation
Low Scope to Get Project
Misunderstandings Higher Building Costs and Claims
Uses of Site Class
Site Class = Approximate representation of how a soil column will amplify or dampen ground motion
Site Class Fa & Fv SDS & SD1SDS & SD1 + Occupancy Seismic Design Category (SDC)
Site Class E & F + Seismic Design Category (SDC) D, E or F Tie foundations together
Two InterpretationsASCE 7‐10: Liquefaction + Period < 0.5 sec. Exception
Example:Site Class D Fa & Fv SDS & SD1; SDS & SD1 + Occupancy SDC E
SDC E & Site Class is still DFor IBC 1809.13
Site Class D Back to F
Site Class F & SDC E ties
Liq. > say 1” ?
Tie foundations(grade beams or slab)
Yes
No Ties Reqd.
No
Stiff Crust & No Collapse
Design to Address Movements (ties, GI, etc.)(soil provides support)
Collapse Or Liq. Near Ftg.