Estimation of intact and remoulded undrained shear strengths from penetration tests in soft clays

Low, Han Eng; Lunne, Tom; Andersen, Knut H.; Sjursen, Morten; Li, X.; Randolph, Mark

doi:10.1680/geot.9.p.017

Cited by 144 publications

(51 citation statements)

References 19 publications

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“…Notably, the net piezocone penetration resistance, q cnet , is close to the net T-bar resistance; this is somewhat surprising given that cone resistance is generally associated with higher bearing factors to translate net penetration resistance to undrained shear strength. At depths >7 m, the piezoball resistance is~10% higher than the T-bar resistance, similar to observations from previous studies (DeJong et al, 2011;Low et al, 2010).…”

Section: Penetration Testssupporting

confidence: 90%

Experience with a dual pore pressure element piezoball

Colreavy

O’Loughlin

Randolph

2016

International Journal of Physical Modelling in Geotechnics

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Piezoballs, which are full-flow ball penetrometers incorporating pore pressure measurements, are an attractive soft soil characterisation tool as they allow measurement of the intact and remoulded strength and the consolidation coefficient in a single test. The merit of full-flow penetrometers as a reliable tool that is superior to the cone in quantifying the strength of soft clay is gaining acceptance. Much of the recent focus on piezoballs has been on the pore pressure measurement location. Prompted by recent studies that highlight the merit of measuring pore pressure concurrently at more than one measurement location, this paper considers a new centrifuge-scale piezoball, with simultaneous pore pressure measurement at the equator and mid-face positions. Results from centrifuge tests in normally consolidated kaolin clay form the basis for the examination and yield coefficients of consolidation that are consistent with values derived using a number of different methods. Although the mid-face position appears to be the more sensible pore pressure measurement position for dissipation tests, consideration of the values measured at both positions provide strong indications of the drainage response during penetration that is shown in the paper to be important for deriving coefficients of consolidation from subsequent dissipation phases.

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Section: Penetration Testssupporting

confidence: 90%

Experience with a dual pore pressure element piezoball

Colreavy

O’Loughlin

Randolph

2016

International Journal of Physical Modelling in Geotechnics

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“…where σV0 is the in situ total overburden stress, Nkt is the empirical cone penetration resistance factor, and qt,q-s & qt,ref are the corrected cone penetration resistance (Lunne et al 1997, Low et al 2010. The additional subscripts for qt reflect the type of instrument ("q-s" for the dynamic-CPTU instrument and "ref" for the static-CPTU device).…”

Section: In Situ Intact Undrained Shear Strengthmentioning

confidence: 99%

“… Low et al (2010). combined a worldwide high-quality database for the assessment of su,qt and remolded undrained shear strength, and recommend Nkt in the range of 11.5 to 15.5 based on the average undrained shear strength (su,ave) measured in direct simple shear, triaxial compression and extension experiments (see alsoLunne et al 2011).…”

mentioning

confidence: 99%

Cone penetration testing to assess slope stability in the 1979 Nice landslide area (Ligurian Margin, SE France)

et al. 2015

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In the landslide-prone area near the Nice international airport, southeastern France, an interdisciplinary approach is applied to develop realistic lithological/geometrical profiles and geotechnical/strength sub-seafloor models. Such models are indispensable for slope stability assessments using limit equilibrium or finite element methods.Regression analyses, based on the undrained shear strength (su) of intact gassy sediments are used to generate a sub-seafloor strength model based on 37 short dynamic and eight long static piezocone penetration tests, and laboratory experiments on one Calypso piston and 10 gravity cores. Significant strength variations were detected when comparing measurements from the shelf and the shelf break, with a significant drop in su to 5.5 kPa being interpreted as a weak zone at a depth between 6.5 and 8.5 m below seafloor (mbsf). Here, a 10% reduction of the in situ total unit weight compared to the surrounding sediments is found to coincide with coarse-grained layers that turn into a weak zone and detachment plane for former and present-day gravitational, retrogressive slide events, as seen in 2D chirp profiles. The combination of high-resolution chirp profiles and comprehensive geotechnical information allows us to A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2 compute enhanced 2D finite element slope stability analysis with undrained sediment response compared to previous 2D numerical and 3D limit equilibrium assessments.Those models suggest that significant portions (detachment planes at 20 m or even 55 mbsf) of the Quaternary delta and slope apron deposits may be mobilized. Given that factors of safety are equal or less than 1 when further considering the effect of free gas, a high risk for a landslide event of considerable size off Nice international airport is identified. . This decrease in shear strength is related to the compressibility of the gas, structural differences according to the gas concentration (i.e. ACCEPTED MANUSCRIPT Highlights An interdisciplinary approach is used comprisntaining morphological, 2D chirp seismic, sedimentological, geotechnical laboratory and in situ datasets. Dynamic piezocone penetration tests (CPTU) provide a powerful, time-and costefficient in situ technique. A large number of coring, static-and mainly dynamic-CPTU tests are used to develop an area-widea regional sub-seafloor strength model near the Nice airport (southeastern France). Detailed 2D finite element slope stability assessments are performed, according to which a.  A potentially tsunamigenic submarine landslide event with a failure depth of >up to 1050 m could may occur.

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“…Adopt appropriate values for the bearing capacity factors N c,bA and N c,bF accounting for the geometry and the softening or (Low et al, 2010) and 7.5 (Skempton, 1951) can be considered for N c,bA and N c,bF , respectively due to the geometrical similarity noting that the effect of softening is small for torpedo anchor installation (Kim et al, 2015 (Einav and Randolph, 2006;Steiner et al, 2014;Chow et al, 2014).…”

Section: Bearing Resistance Methodsmentioning

confidence: 99%

Numerical investigation of dynamic installation of torpedo anchors in clay

Kim¹,

Hossain²,

Wang³

et al. 2015

Ocean Engineering

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a b s t r a c tThis paper reports the results from three-dimensional dynamic finite element analysis undertaken to provide insight into the behaviour of torpedo anchors during dynamic installation in non-homogeneous clay. The large deformation finite element (LDFE) analyses were carried out using the coupled EulerianLagrangian approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength using the Herschel-Bulkley model. The results were validated against field data and centrifuge test data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of anchor shaft length and diameter; number, width and length of fins; impact velocity and soil strength. The anchor velocity profile during penetration in clay showed that the dynamic installation process consisted of two stages: (a) in Stage 1, the soil resistance was less than the submerged weight of the anchor and hence the anchor accelerated; (b) in Stage 2, at greater penetration, frictional and end bearing resistance dominated and the anchor decelerated. The corresponding soil failure patterns revealed two interesting aspects including (a) mobilization of an end bearing mechanism at the base of the anchor shaft and fins and (b) formation of a cavity above the shaft of the installing anchor and subsequent soil backflow into the cavity depending on the soil undrained shear strength. To predict the embedment depth in the field, an improved rational analytical embedment model, based on strain rate dependent shearing resistance and fluid mechanics drag resistance, was proposed, with the LDFE data used to calibrate the model.

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Estimation of intact and remoulded undrained shear strengths from penetration tests in soft clays

Cited by 144 publications

References 19 publications

Experience with a dual pore pressure element piezoball

Experience with a dual pore pressure element piezoball

Cone penetration testing to assess slope stability in the 1979 Nice landslide area (Ligurian Margin, SE France)

Numerical investigation of dynamic installation of torpedo anchors in clay

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