CPT Guide 5 th Edition. Cone Penetration Test (CPT) Quality control (QC) Gregg Drilling & Testing, Inc. Dr. Peter K. Robertson Webinar #

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1 Gregg Drilling & Testing, Inc. Site Investigation Experts Cone Penetration Test (CPT) Quality control (QC) Dr. Peter K. Robertson Webinar # CPT Guide 5 th Edition Download FREE copy from: Robertson & Cabal (Robertson) 5 th Edition

data you have? What to do with what you have?

2 CPT Quality Control Two basic components: How to get good CPT (CPTu) data? Consultants - Specifications (Standards) Contractors - Equipment & procedures How to evaluate the data you have? What to do with what you have? Evaluate, correct and process data Basic Cone Parameters Friction ratio = (f s /q c )100 % Sleeve Friction f s = load / 2 rh Pore Pressure u 2 Tip Resistance q c = load / r 2 2

3 CPT Quality Control How to get good CPT (CPTu) data? In general, CPT is very reliable, since it s essentially operator independent. However, there are some things to ensure excellent data Consultants - Specifications (Standards) Contractors - Equipment & procedures Standards There are many Standards for CPT (& CPTu): ISSMGE - (IRTP) 1999/2001 USA - ASTM D: European - CEN TC341 CPT and CPTu (CEN/ISO :2012) Other National standards and codes Offshore 3

Standards - key elements Equipment cone design, dimensional tolerances Calibration accuracy (capacity FSO) Procedures zero load readings, saturation, push rate, temperature Corrections unequal area

4 Standards - key elements Equipment cone design, dimensional tolerances Calibration accuracy (capacity FSO) Procedures zero load readings, saturation, push rate, temperature Corrections unequal area effects - q c to q t Checks Cone apex angle : 60 deg. Equipment Diameter : 35.7 mm for 10 cm 2 (up to 43.7mm for 15 cm 2 ) CEN: from 5 to 20 cm 2 Friction sleeve area: 150 cm 2 for 10 cm 2 (225 cm 2 for 15 cm 2 ) Prefered filter location for CPTu: behind cone (u 2 ) Limits on dimensional tolerances 4

5 Range in measurements Dimensionless Dimensionless SBT Chart SAND f s /p a = 0.1 f s /p a = 0.01 f s /p a = 1.0 CLAY Soft fine-grained soils Challenge to measure accurate values in soft soils but penetrate dense soils that are >10x stronger! In most soft fine-grained soils: q c < 1 MPa (10 tsf) 1 kpa < f s < 50 kpa Modified from Robertson, 2010 p a = atmospheric pressure = 0.1 MPa ~ 1 tsf Example CPT in mixed soils q c > 60 MPa f s > 800 kpa u 2 ~ -100 kpa Large range in values! 3 orders of magnitude for q c & f s q c < 1 MPa f s < 5 kpa u 2 > 2,000 kpa 6-7 orders of magnitude for u 2 5

6 Example CPT in soft sediments In upper 5m: q c < 200 kpa f s < 5 kpa q c < 0.5 MPa < 500 kpa f s < 10 kpa In very soft soils cones often work at the very low end of the measurement range Unequal end area effects on q c q t = q c + u 2 (1-a) a = 0.60 to 0.85 a = tip net area ratio ~ A n /A c In sands: q t = q c In very soft clays: correction to q t is important Cones should have high net area ratio a > 0.8 6

7 Unequal end area effects on f s f t = f s (u 2 A sb u 3 A st )/A s 10 cm 2 cones A s = 150 cm 2 15 cm 2 cones A s = 225 cm 2 Equal end area cone, A sb = A st (ideally small) Pore pressure ratio, a = u 3 /u 2 Typically: 0.50 < a < 0.80 In very soft clays: Important to have equal end areas Error in f s due to end areas Company cone size A sb A st A sb /A st Df s Error (cm 2 ) (mm 2 ) (mm 2 ) (kpa) (f s = 10 kpa) u 2 = 300 kpa (f s /s v = 0.1) (Lunne, 2006) u 3 = 210 kpa Soft soil (Du 2 /s v =3) Fugro % Fugro % AP Berg % Pagani % Envi % Gregg % In very soft fine-grained soils important to have equal end area sleeve with small area *ASTM requires equal end area sleeves 7

8 Calibration Most modern cones have strain gauge load cells that are temperature compensated, with excellent repeatability and little non linearity and hysteresis. Cones come in different capacities (FSO): Zero load drift critical in very soft soils Common high capacity cones - Tip FSO ~ 100 MPa (i.e 1,000 tsf) 10 cm 2 10 ton 15 cm 2 15 ton Load cell design F Q + F Q Q Separate load cells (better accuracy for f s ) Subtraction (slightly more robust) 8

9 Application Classes Some Standards specify Application Class IRTP 2001 Very soft soils Mixed soils Dense-stiff soils Application Classes ISO CEN TC341 CPTu Very soft soils Mixed soils CPTu CPT CPTu CPT Dense-stiff soils 9

10 Application Classes Application class 1: soft to very soft soil. Not for mixed soil profiles with soft to dense layers. Tests can only be performed with CPTu. Application class 2: For mixed soil profiles with soft to dense layers, in terms of profiling, material identification and engineering properties, with limitations/restrictions in the soft layers. Penetrometer type (CPT or CPTu) depends on project requirements. Application class 3: For stiff and dense soils, in terms of profiling, material identification and engineering properties. Penetrometer type (CPT or CPTu) depends on project requirements. Accuracy Most strain gauge load cells have an accuracy of about 0.1% to 0.5% FSO 100 MPa (1,000 tsf) FSO capacity on tip (q c ) ~ accuracy of 0.1 to 0.5 MPa (i.e. 1 to 5 tsf) Dense sand/stiff clay (with q c > 10 MPa) excellent accuracy (> 1 to 5%) Soft clay (q c < 1 MPa) need lower capacity (FSO) Key factor is zero load drift Zero load drift must not exceed expected accuracy (i.e. very small for soft soils) 10

In general: Accuracy - Repeatability Tip (q t ) is more accurate & repeatable than sleeve (f s ) Prefer separate load cells to improve accuracy of f s Equal end area sleeves to minimize water effects

11 In general: Accuracy - Repeatability Tip (q t ) is more accurate & repeatable than sleeve (f s ) Prefer separate load cells to improve accuracy of f s Equal end area sleeves to minimize water effects on f s Check dimensional tolerance on sleeve Tip (q t ) is more accurate & repeatable than u 2 Except in very soft fine-grained soils (where q c can be very small and u 2 can be very large) Potential loss of saturation in stiff dilative soils (negative values for u 2 ) Repeatability - example Loss of saturation can produce sluggish pore pressure response 11

12 Repeatability of pore pressures data? Why is pore pressure data so complex and often lacks repeatability? complex stress and strain field around cone strongly dilative soils can produce negative pore pressures at u 2 location Pore pressure data can be very good in soft finegrained soils with high GWL high positive pore pressures throughout short depth to saturated soils Complex distribution of pore pressures Modified from Campanella et al

13 Repeatability of f s? Sleeve friction measurements often considered unreliable? use separate load cell design use equal end area sleeves (with small areas) check dimensional tolerance (avoid over sized sleeve) Repeatability q t within 20 kpa f s within 2 kpa u 2 within 20 kpa Very soft, NC Clay Excellent accuracy & repeatability is possible with good equipment & procedures 13

14 Influence of oversized sleeve Excellent repeatability in q t 5 kpa Excellent repeatability in u 2 Very soft, NC Clay New Oversized sleeve Oversized sleeve can produce more variable f s measurements due to variable wear Check List (Contractors) equipment & procedures 14

15 Zero load readings Ideally 3 zero load readings (with cone vertical): 1. immediately before test (clean cone) 2. immediately after test 3. after test with cone cleaned Zero load drift immediately before and after test, i.e. readings 1 & 2): used for interpretation of test results (captures impact of test on data zero load correction) Zero load stability from before and after cleaning of cone (i.e. readings 1 and 3): measure of correct functioning of equipment Causes of zero load drift? Most common cause: Although most cones are temperature compensated, large temperature changes can cause small zero load drift hot day (~25 0 C) to cool groundwater (~5 0 C) dense sand over soft clay Ensure cone is powered for > 15 mins before test Take zero load reading before test at ground temp e.g. bucket of water to keep cone cool 15

16 Consultants/clients Specify required standard (e.g. ASTM) check for compliance Specify Application Class, if possible Supervise and ask for field check list educate field supervisors Evaluate results in the field, if possible Check zero load drift and stability, if testing soft soils Client/consultant Check List 16

17 CPT Quality Control How to evaluate the data you have? Evaluate, correct and process data CPeT-IT very useful software What to do with what you have? what to do if data is not ideal? examples Check for: Evaluate data Data spikes (e.g. rod breaks, etc.) that should be removed Zero or negative readings (very soft soil with larger zero load drift) Overlay comparison to check consistency at site Loss of saturation in pore pressure Dissipation test(s) to estimate piezometric level (GWL) 17

18 Example data spikes Data spike in qc Data spike removed Occasional (single data) spike in one channel can be due to electrical noise or depth triggering system Raw data Example data spikes Corrected data Add hand auger Data spikes in q c Data spikes removed Missing data in f s No missing data 18

19 Example negative values Raw data Corrected data Negative f s values in very soft soils Small values added to f s (1 or 2 kpa) Data spikes in q c Data spikes removed Zero drift can cause some f s values to go negative in very soft soil Impact on interpretation Gaps in interpretation Negative values makes SBT identification (soil type) difficult, resulting in gaps in interpretation. 19

20 Impact on interpretation Missing gaps completed Adjust f s values to get correct SBT to fill missing gaps - often + 1 or 2 kpa Check interpretation values Worked Examples CPeT-IT (see petit) John Th. Ioannides 20

21 Summary In most cases CPT data is reliable and representative of ground conditions e.g. much more reliable than SPT! Reduce chances of poor data by: sound specification of standards evaluate data in the field (train field staff) use good data processing software (e.g. CPeT-IT) Questions? 21

Estimating in-situ soil permeability from CPT & CPTu

Estimating in-situ soil permeability from CPT & CPTu P.K. Robertson Gregg Drilling & Testing Inc., Signal Hill, California, USA ABSTRACT: Engineers often want to estimate profiles of soil permeability