Introduction to Subsurface Exploration

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Morgan Harmon
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1 Introduction to Subsurface Exploration Introduction to Subsurface Exploration Objectives Planning Test Pits Soil Borings Soil/Rock Sampling In Situ Tests

2 Site Characterization Define objectives of exploration Background study Design subsurface exploration program Boring # and depth Sampling # and depth In-situ testing methods and # Characterize Soil and Rock Develop idealized soil profile Perform monitoring instrumentation Objectives

3 Objectives Get Stratigraphy and G.W.T Determine engineering properties In Situ Tests Disturbed samples, index tests Undisturbed samples, Lab tests Background study

4 Background study Planning Boring # and Depth Related to (a) knowledge of site conditions, (b) Type of foundation In general, clay deposit produce more well defined strata and sand can be more locally variable Allow for cross sections

2B (Strength concern) Depth D1 at (Δσ v / q) < 0.1 Depth D2 at (Δσ v / σ v0 ) < 0.

5 Planning (cont d) # of borings Rule of thumb: 1 boring per 2500 ft 2 of building area Approximate spacing of boreholes Planning (cont d) Depth of borings Depth > 2B (Strength concern) Depth D1 at (Δσ v / q) < 0.1 Depth D2 at (Δσ v / σ v0 ) < 0.05 Minimum Depth = min(d1, D2) In deep excavations, depth > 1.5 depth of excavation

6 建築技術規則建築技術規則 65 條規定 : 地基鑽探孔應均勻分佈於基地內 600m 2 鑽一孔, 但每孔基地至少一孔, 如基地面積超過 5000m 2 時, 當地主管建築機關得視實際情形規定孔數鑽探深度如用版基時, 應為建築物最大基礎版寬之兩倍以上, 或建築物寬度之 1.5~2 倍 ; 如為樁基或墩機時至少應達預計樁長加 3m 各鑽孔至少應有一孔之鑽探深度為前項鑽孔深度之 1.5~2 倍

7 Test Pits Examine soil strata Ground Water Table Seepage Condition Retrieve disturbed and undisturbed samples Perform density and strength tests in situ Identify organic soil, bedrock ripability, potential borrow soils, etc.

8 古地震研究 - 車籠埔斷層新城斷層

9 Test Pits Examine fault zone for evidence of recent movement. Identify slickensides evidence of slope movement Limitations: Depth limit (10 ~20 ) Stability of Walls G. W. T.

10 Soil Borings Hand Augur, Power Augur Continuous Flight Augur Hollow Stem Augur Rotary Drilling (Rotary Wash) Percussion Drilling Wireline System Hand Augur, Power Augur Remove disturbed samples Determine soil profile Locate G.W.T. Limitations: Above G.W.T. in granular soils Below G.W.T. must be med-stiff clay Difficult to penetrate dens sand and stiff-hard clay. Practical limit ~ 10

11 Hand Augur and Core Sampler

12 Continuous Flight Augur Rapid drilling and disturbed samples In soils with some cohesion Hole will collapse in granular and soft soils

Rotary Drilling (Rotary Wash) Can obtain all types of samples in soil & rock, undisturbed,

13 Hollow Stem Augur Hollow stem serve as a casing to keep hole open Can get SPT test results Cannot penetrate very strong soil or rock. Problem when sampling below G.W.T. Rotary Drilling (Rotary Wash) Can obtain all types of samples in soil & rock, undisturbed, disturbed, cores. Require relatively large expensive equipment. Require pumps for circulating fluid (mud) Holes requires stabilization

Rotary Drilling Except circular water, there are

14 Rotary Drilling Except circular water, there are two ways to keep stabilization Casing Used in sands and gravels, and soft clay, esp. below G.W.T. Installation very slow, and removed can be very time consuming. Mud Slurry Slurry may be from natural soils or slurry by adding Bentonite Can t determine G.W.T.

15 Percussion drilling Fast No samples Gravel Wireline System

16 Soil/Rock Sampling Disturbed samples Split spoon sampler Standard penetration test Soil sampler (sand, silt, peat) Undisturbed Samples Shelby tube (Thin wall tube) Piston sampler Rock Cores Split Spoon Sampler Standard Penetration Test Split-Spoon Sampler ~ 2 in length 2 O.D. 1.5 I.D. w/o liner I.D. w/ liner Area ratio = (D o2 -D i2 )/D i2 ~ 110%. 140#, 30, (6, 6, 6 ) Typically 4 in top 10, then every 5 Area ratio < 10% considered undisturbed

18 Soil sampler Undistrubed Sampler Shelby Tube 2.5 and 3 are most common Area ratio ~ 10% For sand, put a spring core catcher at the end of shelby tube Piston Sampler Thin wall tube with a piston, 50 mm~120 mm For sensitive soils, this is better The presence of the piston prevent distortion by not admitting excess soil Use piston tube to achieve vacuum in sampler for extraction of sample

19 Shelby Tube Piston Sampler

20 Sample disturbance Undisturbed sampling of sands Freezing Piston with circulation tubes with nitrogen gas. Impregnation A substance that would harden (gel) with little to no expansion.

21 Coring of rock Core barrel + Coring bit Single tube Double tube Recovery Ratio (length of core recovered/theoretical length of rock cored) Rock Quality Designation (RQD) Sum(length of recovered pieces >= 4 )/(theoretical length of rock cored) Rock drilling

22 In-Situ Geotechnical Tests for Soils

23 SPT Representative SPT Profile Downtown Memphis Depth (meters) SPT-N (bpf) B B B5 Soil Profile Fill Silty Sand Sandy Silt Gravelly Sand Desiccated OC Clay Clayey Sand OC Clay Gravelly Sand

24 SPT Method Standardization N values are very dependent on Equipment used (E m ) Size of hole (C b ) Type of spoon lined or unlined (C s ) Length of rods (C r ) Operator To standardized find N 60 60% of theoretical energy N 60 =E m C b C s C r N/0.6 SPT Correction for overburden stress to standard of 1 tsf (~100 kpa) N c = C n N C n =0.77 log 10 (20 P a /σ v )=f(σ v ) (N 60 ) 1 = C n N 60, which is used in estimating many engineering parameters, particular for seismic design work.

25 SPT Correction for ground water e.g. above G.W.T., N=30 for medium-dense silty fine sand, below G.W.T. N=45 because the soil is dilative and SPT cause negative Δu. Terzaghi recommended (N 60 ) 1 =15+((N 60 ) 1 15)/2, for (N 60 ) 1 >15 and silty sands or find sands below G.W.T. SPT Applications Development of engineering properties Granular soil: Dr, φ, E, Liquefaction Cohesive Soil: not much Site specific correlations w/ lab test results is about all that can be done, although Cu and OCR have been related to SPT results. Settlement and bearing capacity of granular soils

27 SPT Advantage Inexpensive Availability Sample obtained Huge database Able to penetrate local hazard Disadvantage Operate dependent Accuracy is poor Not good for gravel No continuous profile No good correlation for clay c u = undrained strength γ T = unit weight I R = rigidity index φ' = friction angle OCR = overconsolidation K 0 = lateral stress state e o = void ratio V s = shear wave E' = Young's modulus C c = compression index q b = pile end bearing f s = pile skin friction k = permeability q a = bearing stress CLAY Is One Number Enough??? N SAND D R = relative density γ T = unit weight LI = liquefaction index φ' = friction angle c' = cohesion intercept e o = void ratio q a = bearing capacity σ p ' = preconsolidation V s = shear wave E' = Young's modulus Ψ = dilatancy angle q b = pile end bearing f s = pile skin friction

28 Cone Penetrometers Electronic Steel Probes with 60 Apex Tip ASTM D 5778 Procedures Hydraulic Push at 20 mm/s No Boring, No Samples, No Cuttings, No Spoil Continuous readings of stress, friction, pressure CPT

Cone Penetration Tests (CPT) Cone Trucks Mobile

(kpa) -200 0 200 400 600 800 0 4 4 4 f s Depth

29 Cone Penetration Tests (CPT) Cone Trucks Mobile 25-tonne rigs with enclosed cabins to allow testing under all weather conditions CPT Profile q t (MPa) f s (kpa) u b (kpa) f s Depth (meters) u b q t

30 Comparison CPT and SPT Downtown Memphis Depth (meters) SPT-N (bpf) and q c (MPa) B B B5 CPT-qc (MPa) Soil Profile Fill Silty Sand Sandy Silt Gravelly Sand Desiccated OC Clay Clayey Sand OC Clay Gravelly Sand CPT Method Mechanical Dutch cone Electric cone Measurements Tip resistance Sleeve friction Water pressure Others

31 CPT Applications Soil identification Granular soil: Dr, φ, E, Liquefaction Cohesive soil: Su, OCR SPT-N vs. CPT-q c Robertson and Campanellas correlation (1983) between q c, F r, and soil type

32 q c -σ 0 '-D r For NC quartz sand q c -σ 0 '-ψ For NC quartz sand CPT Advantage Continuous profile Accurate Pore water pressure Inexpensive Fast Disadvantage Doesn't work in gravel No sample Limited penetration depth

Seismic Piezocone Test Obtains Four Independent Measurements with Depth: V s Cone Tip Stress, q t Penetration Porewater Pressure, u Sleeve Friction, f s

33 Seismic Piezocone Test Obtains Four Independent Measurements with Depth: V s Cone Tip Stress, q t Penetration Porewater Pressure, u Sleeve Friction, f s Arrival Time of Downhole Shear Wave, t s u 2 f s u 1 60 o q c Downhole Shear Wave Velocity Anchoring System Automated Source Polarized Wave Downhole V s

34 Vane Shear Test Vane Shear Test

35 Vane Shear Test Vane Shear Test Used primarily to access the undrained strength of soft clay. Method Borehole, pipe, Push and rotate Relate peak strength to undrained strength, Su Rotate continued for 10~25 revolution to remold soil and then the residual strength is measured

36 Vane Shear Test Assumptions in evaluation Undrained Isotropic No disturbance due to insertion No progressive failure (perfect plasticity) Su = k T k: correction factor Vane Shear Test Advantage Fast and economical Reproducible in homogeneous deposits Significant data base Very good for estimating sensitvity Disadvange Su is the only application

37 Pressuremeter Test (PMT) Method Pre-bore PMT Self-boring PMT Measurement Pressure-deformation relationship Pressuremeter Test (PMT)

38 Pressuremeter Test (PMT) Pressuremeter Test (PMT)

39 Evaluation of Pressuremeter test Pressuremeter Test (PMT) Applications Esitmating soil strength parameters A better approach is to use the PMT results directly for foundation design

40 Pressuremeter Test (PMT) Advantage Stress-strain response obtained Ko is obtained (SBPMT better in this regard) Excellent tool for pile (esp. lateral load) Disadvantage Soil stratigraphy must be known in advance Excess pore water pressure not known Dependent on borehole disturbance More time consuming and expensive Misleading if soil is highly anisotropic Dilatometer Test (DMT)

41 Dilatometer Test (DMT) Method Measurements Thrust A-pressure ( 0.05 mm) B-pressure ( 1.1 mm) C-pressure (0.05 mm ) Corrections for readings Dilatometer Test (DMT)

43 Dilatometer Test (DMT) Applications SAND: Classification, Stratigraphy, Liquefaction, Dr, State parameter, φ Clay: Su, Kh, Coeff. Of consol., Stress history, M, E, G

44 Determination of soil description and unit weight (Schmertmann,1986) Dilatometer Test (DMT) Advantage Simple and rapid, rugged, less disturbed Good for horizontal stress, OCR Nearly continuous profile Disadvantage Limited field exposure Availability Difficult in hard soil Thrust measurement complicates the system No sample obtained

45 Plate Load Test (PLT) Evaluation of PLT (Sand)

46 Evaluation of PLT (Clay) Screw plate test

Chapter 12 Subsurface Exploration

Page 12 1 Chapter 12 Subsurface Exploration 1. The process of identifying the layers of deposits that underlie a proposed structure and their physical characteristics is generally referred to as (a) subsurface