understanding & interpreting geotechnical reports

Trent Parkhill

NCSEA ‐ October 2017

Understanding & Interpreting Geotechnical

Reports

TopicsSoils and Soil MechanicsAreas of Frequent MisunderstandingAreas We Could Better Work Together

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.

MY PERSPECTIVE

Trent ParkhillSenior Principal Geotechnical Engineer38 years

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

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

SOILS/GEOLOGY IN MY EXAMPLES

Legend

Sand

Silt

Clay

Depends On Where You Are

Very Simple Geology

SandSand ClaySilt Silt

Oquirrh Mts.

WasatchMts.

SandSiltClay

Mix

Gravel & Cobbles

Typical Local Geology

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 – IMPACT ON PROJECTS

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


~10%

0.000014%

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?

Geology – Comparison

Interpreted From Borings Actual

WHAT IS THE “RIGHT” EXPLORATION PROGRAM?

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.

VERTICAL AND LATERAL MOVEMENT DURING SEISMIC EVENTS

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 : The Conventional Solution

Expected Cost: $600,000Bid: $1,700,000

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

LIQUEFACTION = SITE CLASS F?

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!

BUT AT THE ADJACENT SITE . . .

Simplified Local Geology

SandSand ClaySilt Silt

Oquirrh Mts.

WasatchMts.

Case History – 4 : Sometimes Things Are Different


~ $ 6,000




~ $ 6,000


?


100’

?Very Soft Clay!

UNCERTAINTY IN SETTLEMENT ESTIMATES . . .

“Why Can’t You Guys Just Give Us One Number?”

+

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

WHAT IS “ACCEPTABLE SETTLEMENT”?

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 : What is Acceptable?

How much Settlement Can a Building Take?1” ?

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

HOW TO AVOID EXCESSIVE CONSERVATISM

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

How To Avoid Excessive Conservatism

SUBGRADE MODULUS: MISUNDERSTANDINGS

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

PROBLEMS WITH SLOPE STABILITY ANALYSES

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 ‘

WHAT CAN YOU DO ?

Risk ? / Unnecessary Conservatism?

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

LESSON FROM CASE HISTORIES

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

QUESTIONS?

BACKUP SLIDES TO ANSWER QUESTIONS

The Code

IBC ASCE 7‐10

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.

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