Developing Subsurface Exploration & Testing Programs Considering Geophysics & In-Situ Testing Benjamin S. Rivers, P.E. Geotechnical Engineer FHWA Resource Center
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
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
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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
Rational Approach 4/16/2008 6
GEC #5 Figure 1: Soil & Rock Property Selection Flowchart 4/16/2008 7
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
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
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) 4/16/2008 10
Planning Exploration & Testing Program Exploration Tools Undisturbed Sampling Disturbed Sampling In-situ Testing Geophysical and Remote Sensing Methods 4/16/2008 11
In-Situ Geotechnical Tests for Soils 4/16/2008 12
Standard Penetration Test (SPT) 4/16/2008 13
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 4/16/2008 14
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 4/16/2008 15
Standard Penetration Test (SPT) Measured N-values Corrected N 60 4 0 10 20 30 40 50 4 0 10 20 30 40 50 Depth (mete ers) 6 8 10 12 55 ER = 34 (energy ratio) 45 60 40 56 41 63 41 63 39 63 47 Depth (met ters) 6 8 10 12 Donut Safety Trend 14 Donut Safety Sequence 64 69 56 14 16 16 4/16/2008 16
Cone Penetration Test (CPT)
Cone Penetration Testing (ASTM D 5778)
Geostratigraphy by Piezocone Tests, Blytheville, AR q t (MPa) f s (kpa) u 2 (kpa) 0 0 10 20 30 40 50 0 0 100 200 300 400 0 0 1000 2000 3000 Clayey Silt 5 5 5 Sand 10 10 10 15 15 15 Depth (m) 20 20 20 Clean Sand 25 25 25 30 30 30 35 35 35 Clay 40 40 40
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. 4/16/2008 20
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 4/16/2008 21
Vane Shear Devices Scandinavian Vanes McClelland Offshore Vane 4/16/2008 22
Vane Shear Test (VST)
Results from Vane Shear Tests Vane Strength, s uv (kpa) Sensitivity, S t 0 0 10 20 30 40 50 60 70 80 0 0 1 2 3 4 5 5 Peak Remolded 5 Depth (meters) 10 15 20 Depth (meters) 10 15 20 25 25 30 4/16/2008 24 30
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 4/16/2008 25
Flat Plate Dilatometer (DMT) Blade, Pressure Panel, Tubing, and Nitrogen 4/16/2008 26
Flat Plate Dilatometer Test (DMT)
0 DMT in Piedmont Residuum, Charlotte, NC 0 0 0 2 2 2 2 4 4 4 4 Depth (meters) 6 8 10 6 8 10 6 8 10 6 8 10 12 12 12 12 14 Po P1 14 Clay Silt 14 14 16 0 500 1000 1500 0 1 10 16 16 16 0 200 400 600 800 0 5 10 15 Pressure (kpa) Material Index I D Modulus E D (atm) Horiz. Index K D 4/16/2008 28
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 4/16/2008 29
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 4/16/2008 30
Pressuremeter Test (PMT)
Pre-Bored Pressuremeters (ASTM D4719) Menard Pressure Panel Texam Monocell Probe 4/16/2008 32
PMT Data - Utah DOT Project 5 4 Press sure (tsf) 3 2 1 0! 0 200 400 600 Volume Change (cc) 4/16/2008 33
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 4/16/2008 34
PMT Results Pressure vs. Volume or Volumetric Strain Soil Behavior Load/Volumetric Displacement Soil Properties E, G, m v, s u 4/16/2008 35
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 4/16/2008 36
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 4/16/2008 37
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 4/16/2008 38
Geophysical Properties P - Wave Velocities # *+ - ),-$ Resistivity Values (ConeTec & GeoProbe, 1997) *+ # *+ ) " # # 0 1000 2000 3000 4000 5000 6000 7000 8000 " # $%% & ' ( " *+ S - Wave Velocities 1 10 100 1000 10000 +* $ρ& ( # *+ ) " # # } V s = 0 0 1000 2000 3000 4000 # $%% & ' ( 4/16/2008 39
Results from Seismic Refraction 4/16/2008 40
Electrical Resisitivity Measurements 4/16/2008 41
Electromagnetic Conductivity (EM)
Televiewer Source: Eliassen, et al 4/16/2008 43
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 4/16/2008 44
GEC #5: Evaluation of Soil & Rock Properties http://www.fhwa.dot.gov/engineering/geotech /library_listing.cfm Publication No. FHWA-IF-02-034 Highly Recommended 4/16/2008 45
Reference on Geophysics Application of Geophysical Methods to Highway Related Problems (FHWA Manual DTFH68-02-P-00083; Sept. 2003) www.cflhd.gov/geotechnical 4/16/2008 46
Subsurface Investigation Courses NHI Course 132031: Subsurface Investigations Geotechnical Site Characterization NHI Course 132079: Subsurface Investigation Qualification QC/QA www.nhi.fhwa.dot.gov 4/16/2008 47