Interpreting and Understanding Geotechnical Reports

Transcription

1 Interpreting and Understanding Geotechnical Reports February 2018 Utah Geo-Institute Chair: Ryan Cole, PhD, PE, M. ASCE Vice Chair & R8 LIC: Ryan Maw PE, GE, M. ASCE Introduction & Mission The Geo-Institute is a membership organization focused on advancing Geoprofessionals. Created by the American Society of Civil Engineers (ASCE) in October 1996 and has roughly 12,000 members. Mutual interest in protecting the public by enhancing the understanding of, and expertise in, the engineering behavior of earth materials. 1

2 Presentation Objectives & Outcomes Geotechnical Reporting Geotechnical Field & Laboratory Study Programs Management of Geotechnical Risks Ownership of Project Geotechnical Risks Improved understanding and more effective communication between Structural and Geotechnical Engineers of Record (SER & GER) What is Geotechnical Engineering? Urban Geology Earth Magazine (2016) A Structural Engineer, Foundation Contractor, Owner, and Site Civil Engineer. Geotechnical Engineering is Voodoo Magic Overly conservative What am I paying for (limitations) Expensive (problem finders) Limitations, limitations, & more limitations (all the risk to contractor) If you ask three GERs a question you will get five answers The only time they are sure about something is when it is not their own work 2

3 What is Geotechnical Engineering? Application of Civil Engineering technology to soil and Rock materials (Holtz & Kovacs) Soil & Rock Mechanics Mechanics: physical science of result of forces on bodies Engineering Properties Foundation Engineering Business of Risk Solutions to problems in the built environment What is Geotechnical Engineering? NCSEA Advocacy Committee Position Statement: The interface between the building owner, structural engineer, and geotechnical engineer is very important from both a technical and liability perspective on practically any building project. February 2018 Taiwan Earthquake LA Times (2018) 3

4 What is Geotechnical Engineering? 100% 90% Risk Exposure 80% Information ($$$) 70% 60% Risk 50% 40% 30% 20% 10% 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Data When is a Geotechnical Report Required? IBC Section 1803 geotechnical investigation shall be conducted when: Required by Building Official Questionable soil Expansive soil High groundwater table (within 5 feet of lowest floor level)) Founded oncompacted fill (>12-inches) / CLSM Deep foundations Irregular / inconsistent rock strata Restrictive setback / clearances (slopes) Underpinning / Shoring Slope stability Liquefaction 4

5 What is Questionable Soil? Classification, strength, or compressibility of the soil is in doubt or where a load-bearing value superior to that specified in the code is claimed IBC allowable Clay/Silt 1.5 ksf IBC allowable Sand 2.0 ksf Expansive Soil (all four have to be met) PI>15 More than 10% passing No. 200 Sieve More than 10% of soil particle less than 5 micrometers Expansion Index greater than 20 When is a Geotechnical Report Required? When is a Geotechnical Report Required? But what if. Das Park Hotel The code does not directly address every situation - The building official shall be permitted to waive the requirements for a geotechnical investigation where satisfactory data from adjacent areas is available that demonstrates an investigation is not necessary for any of the conditions in Sections through and Sections and At the end of the day the Building Official is the only person who can waive the geotechnical investigation 5

6 When is a Geotechnical Report Required? Dan Brown & Associates Who determines what is required? Registered design professional The scope of the geotechnical investigation including the number and types of borings or sounding, the equipment used to drill or sample, the in-situ testing equipment and the laboratory testing program shall be determined by a registered design professional. When is a Geotechnical Report Required? Who can do the field exploration? The investigation procedure and apparatus shall be in accordance with generally accepted engineering practice. The registered design professional shall have a fully qualified representative on site during all boring and sampling operations. 6

7 Geotechnical Reporting How should it be provided? What should the geotechnical report provide? Why do we need an Executive Summary? A tool used to communicate site conditions, design, and construction recommendation. At a minimum (IBC Chapter 18). Plot of Field Study Locations Complete record of test hole logs and soil samples Overview of soil profile Groundwater elevations Recommended foundation type / soil mitigation risks Mitigation of liquefaction, compressible, and expansive soil Expected total & differential settlement (give both magnitude and distance) Deep foundation information Geotechnical Reporting Compacted fill / CLSM Recommendations 7

8 Geotechnical Reporting Additional Requirements for Compacted Fill / CLSM Site preparation prior to fill placement Specification for material to be used Testing methods to assess density (what proctor) Maximum allowable thickness of lift placement Number and frequency of field compaction testing or CLSM compressive strength Geotechnical Reporting Deep Foundations / Ground Improvement Types and Capacities Suitability of deep foundation materials for site Center-to-Center Spacing Installation Procedures / Driving Criteria Field inspection and reporting requirements Load test requirements Designation of bearing elevation(s) Reductions for groups / Pmultipliers (row shading) 8

9 Field Study Programs What drives costs in field studies? Comparing apples to apples what are we after? Right tool for right job Equipment access Traffic Control ($100 - $1000/day) Water ($100 - $4000/day) Materials & Supplies Test hole backfill (future construction issues) Groundwater measurements Test Hole Frequency Project Specific risk based Medium to heavy weight structures Guidance BOCA National Building Code, 1996) Subsurface Condition Poor Quality / Erratic 1,000 3,000 Average 2,000 4,000 High Quality / Uniform 3,000 10,000 A test hole per feet of wall 500 feet of embankments / cuts (AASHTO 2014 / UDOT 2017) Field Study Programs Structure Footprint Area For Each Exploration Test Hole (ft 2 ) 9

10 Field Study Programs AASHTO 2014 Test Hole Depth - General extend at least to a depth such that change in vertical stress from new construction is less than 10 percent of the initial vertical stress. (Coduto 2001) Below depth of potential liquefaction & significant settlements Below fill materials (undocumented) Two times embankment and wall heights Test Hole Depths - Shallow Foundations Buildings on spread footing or mat foundations (Sowers 1979) Subsurface Condition Poor / Erratic Average Good / Uniform Where, S=number of stories D=anticipated depth of foundation Field Study Programs Minimum Depth of Borings 20 S D 15 S D 10 S D Average site conditions, two story structure with 5-ft foundation depth ~ minimum depth of 30 feet 10

11 Field Study Programs Test Hole Depths - Deep Foundations & Ground Improvement General Guidelines Through unsuitable strata to dense material AASHTO (2014) Deeper of 20-ft below pile tip elevation Two times the maximum pile group dimension Shallow bedrocks deeper of the following A minimum of 10 ft of rock core Length of rock core at least three times diameter of shaft or shaft group Laboratory Testing Programs Laboratory Testing Index Moisture Grain-size Atterberg Limits ASTM D

12 Laboratory Testing Programs Laboratory Testing Strength & Consolidation UCS not much UU Triaxial testing - $ CD / CU Triaxial Testing $900-1,500 CBR - $ (per point) Corrosion / Resistivity - $ (Suite) One -Dimensional Consolidation $ GET THE DATA!!!!! One-Dimensional Consolidation What is the most relevant geotechnical design value for a SER of building structures in Utah? FHWA

13 One-Dimensional Consolidation Texas A&M University (1985) One-Dimensional Consolidation 0 ~7000 psf 0.05 RR~0.02 CR~ Strain ( H/H) Effective consolidation stress, s'v (psf) 13

14 One-Dimensional Consolidation One-Dimensional Consolidation 14

15 One-Dimensional Consolidation Sample Quality & Disturbance Material density & Water Content Sampler type Size of specimen (3 tubes) Sampling depth Drilling method Stress history / yield stress Rate of sample advance Sample soak time Shock and structural disturbance during transportation and storage Disturbance during extrusion and trimming operations Changes in temperature and state of saturation One-Dimensional Consolidation Ladd & DeGroot (1992) 15

16 One-Dimensional Consolidation One-Dimensional Consolidation USACE Effect of Sampling Disturbance on Laboratory-Measured Soil Properties (1992) 16

17 One-Dimensional Consolidation 0 ~Inundation with water Strain ( H/H) Effective consolidation stress, s'v (psf) One-Dimensional Consolidation ~Inundation with water Strain ( H/H) Effective consolidation stress, s'v (psf) 17

18 One-Dimensional Consolidation Tolerable Settlements How much movement is too much movement for the structures we design? Great Salt Lake Lucin Cutoff Railroad Trestle (Trestlewood Wood Products) What kind of savings can be realized to a project by allowing more than 1-inch of settlement? Geotechnical Risks What is Geotechnical Risk? the risk to building and construction work created by the site ground conditions (Clayton 2001) Risk: Likelihood + Consequence Cannot be avoided only managed, transferred, mitigated, and shared Three generalized types of Geotechnical Risks Technical / Design Project / Change management Contractual 18

19 Geotechnical Risks What is Geotechnical Risk? Alaska Way Viaduct (2001) 1:20 chance of structural loss due to an earthquake within a decade Cypress Street Viaduct (1989) Seattle Times Geotechnical Risks Gov. Chris Gregoire (2010) - It s a hypothetical; it has no bearing. There s no reason to expect cost overruns. What is Geotechnical Risk? City Councilor Richard Conlin (2010)- We have done our due diligence and a majority of the Council believes strongly, that the risk of overruns has been distorted and overblown in order to kill the project. We are working in partnership with the state to ensure that the project is completed on budget and on time. Only with true collaboration and working in partnership can large projects be completed on budget. Legislature passed state Law ( ): The state's contribution shall not exceed two billion four hundred million dollars. If costs exceed. Any costs in excess of two billion eight hundred million dollars shall be borne by property owners in the Seattle area who benefit from replacement of the existing viaduct with the deep bore tunnel. Seattle Times 19

20 Geotechnical Risks What is Geotechnical Risk? July 2013: $1.1 Billion, 14 months, World s Largest TBM Seattle Times Geotechnical Risks December 2013: Maybe not Fragment of steel pipe (deep well left in places after 2002 groundwater study) between spokes of Bertha s Cutting Face Seattle Times (January 2013) 20

21 Geotechnical Risks Seattle Times Geotechnical Risks What is Geotechnical Risk? View 100-ft down in 120-ft recovery pit Seattle Times 21

22 Geotechnical Risks What is Geotechnical Risk - $$$ and Schedule 2017 STP sued WSDOT Court ruled STP waived right in Contract read the fine print WSDOT we bought it you broke it STP sued soil testing company for Steel Casing Completion date: April 4, 2017 Legal Battles End???? Geotechnical Risks: Technical/Design Sources of Technical / Design Risks Geological Parameters / Properties Analytical 22

23 Geotechnical Risks: Technical/Design Technical / Design Risks - Geological Site Variability Geologic Reconnaissance Frequency of test holes / methods Site history Actual Geologic hazards Landslides (extent and activity) Fault (extent and activity) Hazardous materials / contaminants Variable not addressed in investigation of geological details was inadequate or impractical Geotechnical Risks: Technical Design Insufficiency of site geologic/geotechnical data Shallow groundwater / dewatering (Artesian) Resisting unusually large forces (uplift / lateral loads) Schedule (dewatering / settlement) Clays with undrained shear strengths below 750 psf Historic Fills (especially undocumented) Shoring near settlement sensitive structures Clay thicknesses > 10 ft (PI>50 CH / MH) Collapsible / expansive soils Mapped nearby landslides or faults Liquefaction & Site Response (structures with periods > 0.5 seconds) 23

24 Geotechnical Risks: Technical/Design Technical / Design Risks Parameters & Properties An inadequate understanding of ground / site conditions and field and laboratory testing unreasonable choice of design parameters / values Geotechnical Risks: Technical/Design Technical / Design Risks - Analytical Dynamic Pile Testing - $2 3k Full Scale Static Pile & Shaft Load Tests Static piles & shafts (<36 ) FHWA (2016) $15-35k (based on max test load) Large Diameter Drilled Shafts (>36 ) $100k (Osterberg Load Cells) Documented savings on a local project of up to $600k 24

25 Geotechnical Risks: Technical/Design Technical / Design Risks - Analytical An inadequate understanding of ground / site conditions and analytical methods unreasonable choice of analytical methods Geotechnical Risks: Technical/Design Technical / Design Risks - Analytical Get the data Communicate project history and risk upfront, early, and often Environmental issues or special site constraints Communicate project needs / loads Do you have all the data you need? Get it in writing / stamped before going to construction Review Project Plans / Definitions Make them, make a site visit 25

26 Geotechnical Risks: Management Project / Change Management Likelihood of experiencing a cost or time over-run on a major project is somewhere between 20 and 50% (Baynes 2010) Likelihood of physical failure is much less 1 or 2% for civil projects (20% for mining projects) (Baynes 2010) Bingham Canyon Mine (2013) The odds of dealing with a cost or time over run on a quarter to half of our projects are real So, how do we manage these risks as SERs, GERs, Owners, and Architects? Geotechnical Risks: Management Project / Change Management Planning Design Delivery / Procurement (Specifications) Construction 26

27 Avoid / Ignore Geotechnical Risks: Management Project / Change Management: Planning Engage the GER as early as possible Identification of Risk / Risk Register Accept / Own RISK Transfer Defined scope / target data Get a well thought out & detailed proposal Get high-quality data Reduce Communication Experienced individuals completing the field work Geotechnical Risks: Management Project / Change Management: Design Errors Negligence Personnel / Staff Analyses Plan to address risk register Communication Documentation Quality 27

28 Geotechnical Risks: Contractual Geotechnical Risks: Contractual Selection of the contactor What did I just buy? High risk lead time items Performance specifications Specifications Contract mechanism that allows the contractor to continue work under potential claim conditions Contract flexibility that allows the contactor to be paid for work that needs to be done Fairly allocates geotechnical risk between the owner and contractor 28

29 Who Owns the Risk? US6 Thistle Landslide (1983) Does the owner ultimately own the risk? On a challenging site how can risk be shared or managed? Managing or reducing the risk to structural engineers? Contracts & Limitations What are you paying for? Any Questions? 29