Effects of Sampling on Soil Properties

Presented by : Ahmed Bayoumi

Outline

! Introduction! Disturbance Sources! Disturbance Definition! Disturbance Effect on Soil Properties! Disturbance Associated with Samplers! Recommendations: Minimization of

Disturbance

Introduction! Sampling

! Perfect samples sample which has not been disturbed by the boring, sampling,

and trimming but has experienced stress release - (Ladd & Lambe, 1963)

! In-situ stresses: no true undisturbed sample! Cohesive soils! Cohesionless soils

! Disturbance! Cohesive soils! Cohesionless soils

! Sampling Apparatus for Undisturbed Samples! Push-tube samplers: open tube and piston (free, fixed, and

retractable)! Core-barrel samplers: single, double, and triple-tube core barrel

(e.g. pitcher and denison)! Sand samplers (thin-walled fixed piston, open tube, in-situ

freezing)

Introduction Contd! Validity of investigations carried out in laboratory tests rests solely on

the quality of the samples and on how far they are representative of the stratum from which they are take (Cooling, 1949)

! Economy sampling cost (sample diameter)2

! Soft clay & sensitive clay undisturbed samples required! Cyclic loading on highly overconsolidated clay or liquefaction potential

of sand high quality undisturbed samples (Broms, 1980)! Disturbance effect is higher for undrained tests! Number of samples = f(site uniformity, cost)

Disturbance Sources ! During borehole drilling: water level in borehole! During sampling: friction cutting shoe! Sample brought to the surface: reduction of pore pressure! During transportation: shocking & vibrations! During storage ! During preparation and testing of the samples

Disturbance Definitions ! Several Definitions:

Ladd & Lambe, 1963

Okumura, 1971

Nelson et al, 1971s residual effective stress of samplep residual effective stress of prefect sample

" Major Deficiency: p depends on Ko and pore pressure coef. Au" Other Definitions:

Disturbance = volume change when sample is reconsolidated in thelaboratory at the same stress conditions as those in situ (Berre & Bjerrum, 1973)

Drnevich & Massarsch, 1978 and Tokimatsu and Hosaka, 1986

Hvorslev, 1949: Recover ratio Lr

s

pisturbanceD

=

s-pisturbanceD =

s

sp -isturbanceD

=

field

abl

GGisturbanceD =

sample recovered oflength actualLr = sample recovered oflength lTheoretica

Disturbance Soil Properties

! Strength

Non-uniform axial stress distribution disturbed zone at the perimeter has a large effect on stress distribution (Gerrard & Wardle, 1971)

Migration of pore water reduces effective stress and hence reduction in shear strength (about 15%) (Jakobson, 1954 and others)

Reduction in undrained shear strength is typically 20-50% (Ladd & Lambe, 1963)

Undrained shear strength is reduced by 15% and 30% for soils with plasticity index of 50 and 30 respectively (Kimura and Saitoh, 1982)

Disturbance has largest effect on soil fabric and defects. Sample size wrtseams/defects location and spacing (Rowe, 1970)

Disturbance Soil Properties (contd)

! Shear modulus Shear velocity in sandy silt and clayey silt determined from resonant column are

20-30% less than that from field tests (Stokoe & Richard, 1973) In-situ freezing of sand : Glab= 0.8-1.0 Gfield (Case study Shibuya et al., 1995) Clay sampled by fixed piston thin wall: Glab= 0.40 Gfield (Case study Shibuya et

al., 1995)

! Youngs modulus Effect of sample disturbance is not always clear (Broms, 1980). Usually decrease

with disturbance. Trends depend upon soil type and sampler. E(block) = 5E(tube)! Stress-strain relationship

Larger effect on brittle and sensitive soils (Broms, 1980)

! Dynamic Properties Undrained cyclic shear strength is adversely affected by disturbance (Tokimatsu

and Hosaka, 1986)

! Liquefaction Disturbance has substantial effect on relative density and thus liquefaction

potential is better estimated using field tests

Disturbance Soil Properties (contd)

! Consolidation Parameters and permeability

Reconsolidation stress and compression index of disturbed soils are about 70% and 50%, respectively, of those of the undisturbed soil.

(Shogaki and Kaneko, 1994)

For normally consolidated region, cv, mv, and K of remolded samples are about 70%, 40%, and 80% smaller than those of the undisturbed soil (Shogaki and Kaneko, 1994)

Settlement estimated for undisturbed samples is about 4-8 times the measured settlement field tests better (Broms, 1980)

Disturbance Sampling Device ! Two Canadian Sensitive Clays Sampling (Shelby, Piston, and Block)

(Milovic, 1970) Shelby and Piston: Do=7.302 cm Di=6.909 cm area ratio = 12% Cubic blocks 30 cm on each side Result compared with plate load test

Strength, moduli (elastic and bulk), and effective stress parameters obtained on Shelby and Piston specimens are consistently lower than that of the Block specimens

mv for both Shelby and Piston specimens is larger than that of the block specimen

Modulus of elasticity of laboratory test (even for block) is smaller that of the plate test.

In-situ state of stress may not be realized in the laboratory (disturbance effects)

Disturbance Sampling Device (contd) ! Cemented Champlain Clays Sampling (fixed piston thin wall, Block)

(La Rochelle and Lefebure, 1970)

Unconfined compressive strength and modulus of elasticity on fixed piston samples are 50 - 60% lower than that of the block samples

Piston samples are highly affected by the area ratio (10%) Block samples are not affected appreciably by the stress release Preconsolidation stress is not highly affected by sampling method. However,

recompression index obtained on the fixed piston (disturbed) is almost twice that of samples obtained by the block for stresses smaller than the preconsolidation stress.

Disturbance Sampling Device (contd) ! Sands

(Tokimatsu and Hosaka, 1986): undrained cyclic shear strength of sand on tube sample is considerably lower than that of the sample by freezing

Disturbance Sampling Device (contd)

! Numerical Analysis Tubes in Clayey Soil(Budhu and Wu, 1992): soft clay axisymmetric FE Cam clay model

" Disturbance locations: central core of sample is subjected to 3 stages compression(irrecoverable) -extension-recompression. Disturbance concentrated at the top of the sample and near soil-sampler interface.

" Frictionless sampler: degree of disturbance is constant after a penetration of 75% of the tube diameter

" Frictional sampler: degree of disturbance increases as the penetration proceeds. Long samples not favorable.

" Tube Thickness: Thicker tubes causes more disturbance" Penetration rate: Large rates causes less disturbance, yet very large

rate could yield larger failure zone (Hvorslev penetration rate of 20 mm/s excellent compromise)

" Tube cutting angle: Should be large enough to cause separation" Sampling Method: sample compression for piston sampler is much

larger than the open tube. Choice depends on soil type and loading history

Disturbance Sampling Device (contd)

! Numerical Analysis Sampler Design in Clayey Soil(Clayton et al., 1998): axisymmetric FE effect of sampler parametric study

" Design parameters:

" AR: increasing AR causes significant increase in the peak compressive strain" ICR: increasing ICR causes an increase in the peak extension strain and slight

decrease in the peak compression strain" ICA: no effect on the peak compression stain but influence the peak extension

strain" OCA: increasing OCA causes appreciable increase in the peak compression strain

and to a lesser extent an increase in the peak extension strain

H2R-Rtan)angle edge-cutting sideuto(OCA

HR-Rtan)angle edge-cutting inside(ICA

RR-Rratio) clearance (inside ICR

RR-R ratio) (area AR

11

1

121

1

12

1

21

22

==

==

=

=

Disturbance Sampling Device Developments (contd)

! Experimental New Large Diameter Sampler for Sensitive Soils

(La Rochelle et al., 1980): Laval sampler 200 mm " Design:

No inside clearance Angle of cutting is very sharp Piston eliminated No suction during sampling Large diameter to reduce relative amount of

disturbed annuls around the internal core

" Quality: good sample quality very close to block sampling

Rock Samplers! Basically rotary core-barrels! High area ratio: possible disturbance! Washing and erosion of sample by drilling fluid is possible! Recovering intact fractures and shear zones is almost impossible

(Fang, 1975)

! During coring: friction is low as long as the core is unbroken. Friction is triggered when core is broken and rock fragments become wedged between the core and the barrel (ASCE, 1949)

! Integral coring method recommended to minimize disturbance and to retrieve intact samples including their discontinuities (Fang, 1975)

Sampling Recommendations

! For consolidation: Dsampler sampler Dring ring + 12 mm (Bowles, 1988)! Type of advancement: pushing sampling tube with fast, continuous, uniform motion (ASCE,

1999)

! Rate of penetration: moderate (FE and Hvorslev,1949)! Tubes: area ratio 10%-15% (Bowles, 1988 and ASCE, 1999)! Taper Angle: if area ratio is increased from 5 to 20% the taper angle should be decreased

from 15 to 9 degrees (ASCE, 1999)! Inside clearance ratio = 0.5-1.0% (Broms, 1980)! Inside Clearance ratio = 0-1.0% for very short samples, 0.5-3.0% for medium length

samples (Hvorslev, 1949). Suggested 0.75-1.5% for L/D=6-8 + soil condition (ASCE, 1999)! Outside clearance ratio < 2-3% for cohesive soil and 0% for sand (ASCE, 1999)! L/D ratio: 5-10 for cohesionless soil and 10-20 for cohesive soils (ASCE, 1999)! Inside diameter of the sampling tube must be > the inside diameter of the cutting

edge (Broms, 1980)

Sampling Recommendations Contd

! Pistons soft and sensitive clays (Das, 1990, and Broms, 1980)! Retractable inner tubes ! Recover samples ASAP and minimize storage (Broms, 1980)! Influence of structural defects (e.g. fissures) decreases with confining stress! Organic soils (e.g. peat) stationary piston samplers and sharp cutting edge! Cohesionless soils: freezing - questionable for deformation properties!!! Cohesionless soils: Swedish foil sampler + seismic/plate load/penetrometer! Cohesive soils: reduce disturbance by trimming and testing ASAP and by

reconsolidation under in-situ (anisotropic) stress conditions

Sampling Recommendations Contd

! Sampling tube: smooth, clean

! Undisturbed sample: (Broms, 1980 and ASCE, 1999)! do not test top and bottom portions (2Dsample)! Length/Diameter:

! Clay (St > 30), 20! Clay (St = 5-30), 12! Clay (St< 5), 10! Loose frictional soil, 12! Medium frictional soil, 6

! In case of dissolved air: recompress sample before testing for 12 h. Dissolved air reduces strength and modulus. (Broms, 1980)

Sampling Recommendations Contd

! Stiff and hard clay are difficult to sample. Block samples are normally required (Broms, 1980 and several researchers)

! Open-tube samplers are generally not recommended for undisturbed operations (ASCE, 1999)

! Retractable-piston and foil samplers are not recommended for undisturbed

! Stabilization of borehole: drilling mud/steel casing (ASCE, 1999)! Thin-walled tubes with compressed air for sand sampling are not

recommended (ASCE, 1999)! Hand trimming and freezing are generally considered the highest

quality techniques for sampling cohesionless soils (ASCE, 1999)! Preliminary guide for selecting sampler for obtaining high quality

undisturbed samples (table 2-3, ASCE, 1999)

References! La Rochelle and Guy Lefebure (1970). Sampling disturbance in Champlain Clay, ASTM sp. tech. Publ. 483. Symp.

Seventy-third annual meeting ASTM, Toronto, Canada.! Rowe, P.W. (1970). Representative sampling in location, quality, and size, ASTM sp. tech. Publ. 483. Symp. Seventy-

third annual meeting ASTM, Toronto, Canada.! Broms, B. (1980). Soil sampling in Europe: state-of-the-art,J. of Geotech. Engrg. Div., ASCE, (106) (GT1), p 65-97! Shibuya, S., Mitachi, T., and Tanaka, H. (1995). Effects of sample disturbance on Gmax of soils A case study,

Earthquake Geotech. Engrb., Ishihara, Balkema, Rotterdam.! Budhu, M., and Wu, C.(1992).Numerical analysis of sampling disturbances in clay soils, Int. j. for numerical and

analytical methods in geomechanics, vol. 16, p 467-492.! Kimura, T., and Saitoh, K(1982).The influence of disturbance due to sample preparation on the undrained strength of

saturated cohesive soil, Soils and Foundations, vol.22, No. 4, p 109-120.! Shogaki, T., and Kaneko, M. (1994).Effects of sample disturbance on strength and consolidation parameters of soft clay,

Soils and Foundations, vol.34, No. 3, p 1-10.! Tokimatsu, K., and Hosaka, Y.(1986).Effects of sample disturbance on dynamic properties of sand, Soils and

Foundations, vol.26, No. 1, p 53-64.! Clayton, C., Siddique, A, and Hopper, R. (1998). Effects of sampler design on tube disturbance -numerical and analytical

investigation, Geotechnique 48, No. 6, p 847-867.! Larochelle, P, Sarrailh, J., Tavenas, F., Roy, M, and Leroueil, S. (1980). Causes of sampling disturbance and design of a

new sampler for sensitive soils, Canadian Geotech. journal, vol. 18, no. , p 52-66.! ASCE (1999). Soil sampling,Technical engineering and design guides as adapted from the US army corps of engineers,

no. 30, 214.

Recommended References! Idel, K., Muhs, H., and von Soos, P (1969). Proposal for quality-classes in soil sampling in

relation to boring methods and sampling equipment, 7th int. conf. on soil mech. And found. Engrg., proceed. Of the specialty session no. 1, Mexico, p 11-14.

! Jakobson, B. (1954). Influence of sampler type and testing method on shear strength of clay samples, Proceed. of the Swedish Geotech. Inst., No. 8, Stockholm, 59 p.

! Okumura, T. (1971). The variation of mechanical properties of clay samples depending on its degree of disturbance, 4th Asian Conf., Int. Soc. For soil mechanics and foundation engrg, p 73-81.

Effects of Sampling on Soil PropertiesOutlineIntroductionIntroduction ContdDisturbance SourcesDisturbance DefinitionsDisturbance Soil PropertiesDisturbance Soil Properties (contd)Disturbance Soil Properties (contd)Disturbance Sampling DeviceDisturbance Sampling Device (contd)Disturbance Sampling Device (contd)Disturbance Sampling Device (contd)Disturbance Sampling Device (contd)Disturbance Sampling Device Developments (contd)Rock SamplersSampling RecommendationsSampling Recommendations ContdSampling Recommendations ContdSampling Recommendations ContdReferencesRecommended References