Developing Subsurface Exploration & Testing Programs Considering Geophysics & In-Situ

Transcription

1 Developing Subsurface Exploration & Testing Programs Considering Geophysics & In-Situ Testing Benjamin S. Rivers, P.E. Geotechnical Engineer FHWA Resource Center

2 Purpose of a Site Investigation Assess Suitability of Site for Proposed Project Enable Adequate and Economical Design No Failures - No Conservatism Foresee and Provide for Construction Problems that may Arise (Reduce Claims) 4/16/2008 2

3 Who looks at the results and do they get what they want? Geotech Engineers/Designers Better characterization of engineering properties, less uncertainty Contractors Better characterization of construction needs, less uncertainty Owners Better value from engineers and contractors Value is Economics Less expensive, longer lasting projects through better understanding of engineering properties and their distribution Less Uncertainty/Better Reliability in Characterization and Parameters Faster investigations time is money 4/16/2008 3

4 4/16/2008 4

5 If you do not know what you should be looking for in site investigation, you are not likely to find much of value. R. Glossop-8th Rankine Lecture 4/16/2008 5

6 Rational Approach 4/16/2008 6

7 GEC #5 Figure 1: Soil & Rock Property Selection Flowchart 4/16/2008 7

8 Planning Exploration & Testing Program Gather & Analyze Existing Information; Conduct Site Visit; Develop Preliminary Site Model Identify Material Properties required for Design & Construction; Estimate Scope of Field Program; Divide into Zones of Interest Develop Site Exploration Program Conduct Exploration & Field Testing Perform Descriptions and Laboratory Index Testing Summarize Data & Develop Subsurface Profile Are Results Consistent with Preliminary Model??? 4/16/2008 8

9 Planning Exploration & Testing Program Review Design Objectives and Initial Results; Identify Critical Areas Additional data needs??? Identify Representative Samples for Performance Testing. Conduct Performance Testing, Review Test Data, Summarize Are Results Consistant & Valid? Is a Phase II Investigation Necessary? Select Material Properties and Final Model Perform Design and Consider Constructability Issues 4/16/2008 9

10 Planning Exploration & Testing Program Identify Data Needs Identify Design & Constructability Requirements Identify Performance Criteria & Schedule Constraints Identify Areas of Concern on Site and Potential Variability Develop Likely Sequence and Phases of Construction Identify Engineering Analyses to be Performed Identify Required Engineering Properties & Parameters Evaluate Methods to Obtain Parameters Evaluate Number & Locations of Tests/Samples Needed (GEC #5: Table 1) 4/16/

11 Planning Exploration & Testing Program Exploration Tools Undisturbed Sampling Disturbed Sampling In-situ Testing Geophysical and Remote Sensing Methods 4/16/

12 In-Situ Geotechnical Tests for Soils 4/16/

13 Standard Penetration Test (SPT) 4/16/

14 Standard Penetration Test Advantages Disadvantages Obtain Sample + Number Simple & rugged device at low cost Suitable in many soil types Can perform in weak rocks Available throughout the U.S. (worldwide) Disturbed sample (index tests only) Crude number for analysis Not applicable in soft clays and silts High variability and uncertainty 4/16/

15 Test Results N SPT Resistance Value (blows/foot) Consistancy/Relative Density Soil Properties for sands to φ, E, liquefaction potential Must apply corrections for energy and overburden pressure for normalization 4/16/

16 Standard Penetration Test (SPT) Measured N-values Corrected N Depth (mete ers) ER = 34 (energy ratio) Depth (met ters) Donut Safety Trend 14 Donut Safety Sequence /16/

17 Cone Penetration Test (CPT)

18 Cone Penetration Testing (ASTM D 5778)

19 Geostratigraphy by Piezocone Tests, Blytheville, AR q t (MPa) f s (kpa) u 2 (kpa) Clayey Silt Sand Depth (m) Clean Sand Clay

20 Cone Penetration Test Advantages Disadvantages Fast and continuous profiling of strata Economical and productive Results not operatordependent Strong theoretical basis for interpretation Particularly suited to soft soils Electronics must be calibrated & protected No soil samples Unsuited to gravelly soils and cobbles. 4/16/

21 CPT/CPTu/SCPTu Results Common to all: Tip Resistance (Force/Area) Sleeve Resistance (Force/Area) CPTu: Pore-water Pressure SCPTu: Shear Wave Velocity Soil Properties: V s, G max, E max, ρ tot, e o Sands - φ, D r, σ ho, u o /water table elevation Clays - s u, σ p, c h, k h, OCR 4/16/

22 Vane Shear Devices Scandinavian Vanes McClelland Offshore Vane 4/16/

23 Vane Shear Test (VST)

24 Results from Vane Shear Tests Vane Strength, s uv (kpa) Sensitivity, S t Peak Remolded 5 Depth (meters) Depth (meters) /16/

25 Vane Shear Test Advantages Disadvantages Assessment of undrained shear strength of clays Simple test and equipment Measure inplace sensitivity Long history of use in practice for embankments, foundations, & cuts Limited to soft to stiff clays & silts with s uv < 200 kpa Raw s uv needs empirical correction Can be affected by sand seams and lenses 4/16/

26 Flat Plate Dilatometer (DMT) Blade, Pressure Panel, Tubing, and Nitrogen 4/16/

27 Flat Plate Dilatometer Test (DMT)

28 0 DMT in Piedmont Residuum, Charlotte, NC Depth (meters) Po P1 14 Clay Silt Pressure (kpa) Material Index I D Modulus E D (atm) Horiz. Index K D 4/16/

29 DMT Results Pressure Readings (A, B, C) Soil Properties: Sands φ, E, D r, m v, Clays σ p, K o, s u, m v, E, c h, k h 4/16/

30 Dilatometer Test (DMT) Advantages Disadvantages Simple and Robust Equipment Repeatable and Operator- Independent Quick and Economical Theoretical Derivations for elastic modulus, strength, stress history Difficult to push in very dense materials and not in gravels. Primarily established on correlative relationships Needs calibration for local geologies 4/16/

31 Pressuremeter Test (PMT)

32 Pre-Bored Pressuremeters (ASTM D4719) Menard Pressure Panel Texam Monocell Probe 4/16/

33 PMT Data - Utah DOT Project 5 4 Press sure (tsf) ! Volume Change (cc) 4/16/

34 Pressuremeter Test (PMT) Advantages Theoretically sound in determination of soil parameters Tests larger zone of soil mass than other in-situ tests Develop stress-strainshear curves Disadvantages Complicated procedures requires high level of expertise Time consuming Delicate easy to damage 4/16/

35 PMT Results Pressure vs. Volume or Volumetric Strain Soil Behavior Load/Volumetric Displacement Soil Properties E, G, m v, s u 4/16/

36 Geophysical Investigations Initial Site Exploration/Preliminary Surveys Assist with Placement of Borings/In-Situ Tests Difficult Locations Gravels, Cobbles, Boulders, Debris Difficult Terrain Contaminated Sites Supplementary Exploration Observe Variations Between Borings/Soundings/Outcrop, etc. Locate Anomalies 4/16/

37 Common Geophysical Methods Surface Methods Siesmic Refraction Spectral-Analysis-of- Surface-Waves (SASW) Electrical Resistivity Electromagnetics (EM) Ground Penetrating Radar (GPR) Microgravity Borehole Methods Crosshole/Downhole Suspension Logger Electrical Logging Nuclear Logging Optical and Acoustical Televiewer 4/16/

38 Geophysical Investigations Stratification of Subsurface Materials Profile Top of Bedrock Depth to Groundwater Limits of Types of Soil Deposits Rippability of Hard Soil and Rock Locate Voids, Buried Utilities, Substructures Shear Velocity and Modulus Properties 4/16/

39 Geophysical Properties P - Wave Velocities # *+ - ),-$ Resistivity Values (ConeTec & GeoProbe, 1997) *+ # *+ ) " # # " # $%% & ' ( " *+ S - Wave Velocities * $ρ& ( # *+ ) " # # } V s = # $%% & ' ( 4/16/

40 Results from Seismic Refraction 4/16/

41 Electrical Resisitivity Measurements 4/16/

42 Electromagnetic Conductivity (EM)

43 Televiewer Source: Eliassen, et al 4/16/

44 Value Reduce Uncertainty Increase Reliability Quicker Rational Approach Summary Develop Preminary Model Identify Data Needs Develop & Execute Appropriate Exploration & Testing Program Re-evaluate Data Needs Evaluate Data Select Properties and Finalize Subsurface Model 4/16/

45 GEC #5: Evaluation of Soil & Rock Properties /library_listing.cfm Publication No. FHWA-IF Highly Recommended 4/16/

46 Reference on Geophysics Application of Geophysical Methods to Highway Related Problems (FHWA Manual DTFH68-02-P-00083; Sept. 2003) 4/16/

47 Subsurface Investigation Courses NHI Course : Subsurface Investigations Geotechnical Site Characterization NHI Course : Subsurface Investigation Qualification QC/QA 4/16/