Documents published in Scipedia

• Interpretation of Cone Penetration Tests in Gravelly Soil

  K. Rollins*, J. Roy, A. Walburger, S. Amoroso, R. Dhakal, L. Minarelli, L. Marenghi

  ISC2024.

  
  

  Abstract

  Although gravelly soils have been observed to liquefy in 27 earthquakes in the past 120 years, many engineers believe that gravel cannot liquefy due to its high hydraulic conductivity. Gradations from gravel liquefaction case histories have shown these deposits typically contain 25 to 40% sand, reducing the hydraulic conductivity and enabling excess pore pressures to cause liquefaction. While cone penetrometers (CPT), typically used to evaluate liquefaction resistance in sand, may show increases in penetration resistance due to their small diameter relative to gravel particles, the CPT has successfully predicted gravel liquefaction for looser sandy gravels. Case histories in Wellington, New Zealand demonstrate the successful identification of gravel liquefaction hazards using CPT. Although some layers in the profile indicated high penetration resistance, most of the profile was correctly predicted to liquefy. The Soil Behavior Type (SBT) from the CPT did not consistently indicate a sandy gravel profile but was often classified as behaving like a sand or silty sand; likely influenced by higher sand percentages between gravel particles. To evaluate the ability of the CPT to characterize gravelly soils and their liquefaction potential, additional field case histories are desirable. This paper presents test results from two case histories, one in Wellington, New Zealand, and one in Petrinja, Croatia, where gravels have liquefied. In both cases, the CPT occasionally overestimated liquefaction resistance in gravel layers. The advantages of using a 74 mm diameter Dynamic Cone Penetrometer (DPT) are also highlighted with companion testing.

  Abstract
  Although gravelly soils have been observed to liquefy in 27 earthquakes in the past 120 years, many engineers believe that gravel cannot liquefy due to its high hydraulic [...]

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• Quantitative Assessment of Tip Saturation for High Quality Piezocone Testing

  I. Rocchi*, L. Tonni, G. Gottardi

  ISC2024.

  
  

  Abstract

  The research compares 2 CPTU profiles obtained with a same piezocone setup but having a different degree of saturation associated with the pore pressure measuring system. In the reference test, saturation was performed injecting 20cS silicon oil in the conduit connecting the porous stone to the pressure sensor and applying vacuum while submerged in oil for 15 minutes. The piezocone tip was then assembled with a saturated porous stone while submerged in oil. In the other test, the degree of saturation was purposely lowered by introducing air in the same conduct, whereas all other saturation steps were unchanged. The degree of saturation was compared quantitatively by measuring an analogue of the Skempton’s coefficient B, which is routinely used in laboratory testing to assess specimen saturation in a triaxial cell. The value associated with the saturation condition was measured employing a tool specifically designed for this purpose. The saturation procedures adopted were selected based on preliminary experimental activity in the laboratory, which provided target values of the pore pressure parameter corresponding to full or partial saturation. The CPTUs were performed at a test site presenting 10m clay unit followed by sand. The profiles measured were compared in terms of pore-pressure profiles, as well as the influence this had on corrected tip resistance, Soil Behaviour Type classification and mechanical properties. Additionally, a dissipation was performed for each test to compare consolidation parameters.

  Abstract
  The research compares 2 CPTU profiles obtained with a same piezocone setup but having a different degree of saturation associated with the pore pressure measuring system. [...]

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• Calibration Chamber Testing on Tailings for Interpretation of Partially Drained CPT

  M. Ghafghazi, L. Qi*, H. Zhao, W. Liu, M. Etezad, J. Sharp, M. Talesnick

  ISC2024.

  
  

  Abstract

  The Cone Penetration Test (CPT) is the primary site investigation tool in silt-rich tailings. The permeability and compressibility range of tailings often puts the standard CPT penetration rate in the partial drainage range where the tip resistance, pore water pressure, and sleeve friction become functions of not only the state, but also drainage conditions. This makes interpretation of the state parameter, which represents liquefaction susceptibility and residual strength of tailings particularly challenging. It is not clear whether existing CPT interpretation frameworks are capable of accounting for effects of partial drainage. Developing field correlations for interpreting partially drained CPT data in tailings is hampered by spatial variability and sampling disturbance. CPT tests were performed in a calibration chamber and in the field on a gold tailings material. Dissipation tests were performed, and the pore water pressures on the cone at the u2 position were monitored. The degree of partial drainage was estimated based on the coefficient of consolidation inferred from these dissipation tests. Similarities and differences of the calibration chamber and field CPTs were discussed.

  Abstract
  The Cone Penetration Test (CPT) is the primary site investigation tool in silt-rich tailings. The permeability and compressibility range of tailings often puts the standard [...]

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• Methodology for Determining Optimum Vibrating Wire Piezometer Pairing Relationships in VWP Monitoring Clusters

  C. Meyer*

  ISC2024.

  
  

  Abstract

  Current trends in the mining sector and specifically tailings storage facilities have seen a significant increase in monitoring frequency, instrumentation installed on site and field tests conducted. Monitoring methodologies are also shifting away from analog and towards digital electronic systems. These instruments are also being integrated with online dashboards. Owing to all these factors, the instrument that is now most commonly being installed to meet these requirements are VWP’s (Vibrating Wire Piezometers). However interpreting VWP results and deriving the phreatic surface from these are not as straight forward as initially assumed, it requires engineering judgement and a methodology to determine and verify optimum pairing of VWP clusters. Obtaining the correct phreatic surface from VWP’s is critical as this will have a direct impact on the trigger levels and TARP’s of the online dashboard. Inaccuracies in calculating the phreatic surface can lead to the triggering of incorrect levels, which may result in flawed assessments of stability. The primary approach relied on phreatic surface and hydraulic gradients from CPTu testing being compared to the phreatic surface and hydraulic gradient determined from various combinations of VWP’s in a cluster at the time of CPTu testing. In cases where no historical VWP data is available at the time of CPTu testing, a methodology was also investigated using standpipe piezometers only. Piezometric head was converted to pressure and linear regression used to determine the phreatic surface. Results from the primary approach showed that certain pairs of VWP’s yield phreatic surfaces and hydraulic gradients that match the CPTu findings. Standpipe interpretation provided a good starting point and correlates with primary identified pairs. This methodology provides a verification tool to provide confidence when selecting VWP combinations for dashboard reporting.

  Abstract
  Current trends in the mining sector and specifically tailings storage facilities have seen a significant increase in monitoring frequency, instrumentation installed on site [...]

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• Restrictions of CPTu Based Interpretation Methods and Impact Thereof on Limit Equilibrium Modelling

  J. Visagie*, E. du Plessis

  ISC2024.

  
  

  Abstract

  There is a large reliance on piezometric cone penetration testing (CPTu) in the tailings industry to estimate strength parameters for assessing the stability of tailings storage facilities (TSFs). It is common practice to assess the post-liquefied stability of a TSF using a residual undrained shear strength ratio (USSR). In such an assessment the residual USSR is typically applied to all saturated tailings. The current methods available to analyse CPTu data, are largely based on correlations and assumptions, and therefore contain limitations. Due to the limitations of the methods, this could lead to either over- or un-conservative estimates of stability. In this study, three methods for determining liquefaction potential and residual undrained shear strength ratio (USSR) are compared, namely: Robertson (2022), Been and Jefferies (2016), and Olsen and Stark (2002). These methods differ in the way in which they apply cone resistance, sleeve friction, and dynamic pore pressure response to estimate residual USSR. Their limitations are highlighted and discussed. A hybrid method is proposed for applying the results to post-liquefaction stability analyses. The hybrid interpretation approach uses a combination of the methods to account for different ranges of effective overburden stress and to identify weaker and stronger layers in the tailings profile based on state parameter, residual USSR, and pore pressure response. This hybrid method was applied to a stability assessment of a decommissioned platinum tailings storage facility in South Africa. The results indicated that the hybrid interpretation resulted in a more realistic phreatic surface location and a more accurate failure plane than conventional interpretations.

  Abstract
  There is a large reliance on piezometric cone penetration testing (CPTu) in the tailings industry to estimate strength parameters for assessing the stability of tailings storage [...]

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• Technical Note on Calibration for Cone Penetration Testing in Soft Soils

  G. Scholey*

  ISC2024.

  
  

  Abstract

  Even the most experienced geotechnical engineer is likely to assume that the results of cone penetration tests are unquestionably accurate, reliable and repeatable. There are, however, multiple factors, some that have nothing to do with the soil properties, that need to be carefully addressed prior to testing if the equipment is to return results that can be relied on for design purposes. In soils which are very soft or soft, cone penetration test results can be particularly sensitive to the method of calibration. A high degree of rigour to the calibration process is required, otherwise there is a risk that the results obtained could be inaccurate and adversely impact on the reliability of the interpretation of design soil strength profiles. In this technical note sources of error in cone calibration are discussed. Reference is made to ISO 22476-1 which was revised in 2022, with the addition of a defined approach to calibration. Examples are used to demonstrate the typical errors that could be introduced during calibration.

  Abstract
  Even the most experienced geotechnical engineer is likely to assume that the results of cone penetration tests are unquestionably accurate, reliable and repeatable. There [...]

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• Portable Pressiocone System for Carrying Out CPTU+FDP (Full Displacement Pressumeter) Tests

  M. Sacchetto*, G. Vinco

  ISC2024.

  
  

  Abstract

  Geotechnical investigations in the subsoil of existing buildings have always been challenging due to limited space and difficult access. URETEK has developed a portable integrated system for simultaneously carrying out a CPT and a pressumeter test with Full Displacement Pressuremeter (FDP). The 30 kN thrust penetrometer to be used is very small. The reaction is given by two “microanchors”. The cone is a standard 10 cm2 digital memory cone (no cable), capable of measuring qc, fs, u every centimetre. Above the cone there is the FDP equipment with a rubber sheath covered by steel plates and connected, by a tube filled with water, to a device for creating pressure to inflate the sheath and measure pressure-volume curves as in a standard pressumeter test. The pressure-volume device and the depth transducer are connected to a microcomputer that is programmed to carry out CPT+FDP tests in an easy-to-use/user-friendly way.

  Abstract
  Geotechnical investigations in the subsoil of existing buildings have always been challenging due to limited space and difficult access. URETEK has developed a portable integrated [...]

  • 0
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• Finite element analysis of self-boring pressuremeter tests in clay

  Y. Dong*

  ISC2024.

  
  

  Abstract

  Self-boring pressuremeter (SBPM) tests are widely used in site investigations, due to their distinct advantage to measure the shear stress-strain-strength properties of the surounding soil with minimum disturbance. The measured pressuremeter curve can be interpreted using analytical solutions based on the long cylindrical cavity expansion theory with relatively simple constitutive models. However, SBPM tests are strongly affected by the soil behavior and details of installation procedure. In addition, the derived parameters for clays (e.g. undrained shear strength, and shear modulus) are affected by a number of state variables such as overconsolidation ratio, and stress level. In this paper, SBPM tests are investigated using finite element analysis and the MIT-S1 model, to consider complex soil behavior more realistically. SBPM tests in K0-consolidated Boston Blue Clay at different OCRs are simulated in axial symmetric and plain strain conditions, consistent with the assumptions used in analytical solutions. The derived undrained shear strength from both contraction and expansion curves are compared with theoretical values from stress-strain curves, to evaluate the reliability of the derived parameters from the SBPM tests.

  Abstract
  Self-boring pressuremeter (SBPM) tests are widely used in site investigations, due to their distinct advantage to measure the shear stress-strain-strength properties of the [...]

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• Development of a Calibration Chamber System for Testing at High Confining Pressures

  M. Haynes, D. Reid, R. Fanni, K. Smith

  ISC2024.

  
  

  Abstract

  Cone penetration testing with pore pressure measurement (CPTu) represents a state of practice tool to assess the in situ state parameter, strength, and liquefaction susceptibility of sandy soils and mine tailings. Many techniques for the interpretation of CPTu data are based on the results of calibration chamber test programs on sand and, more recently, mine tailings. While these efforts have led to the current methods to interpret CPTu data, two factors relevant to CPTu interpretation require consideration: (i) the available calibration chamber data is dominated by tests with consolidated mean effective stresses < 200 kPa; and (ii) tailings storage facilities are being constructed to heights such that in situ effective stresses are far higher than those of the available calibration chamber test database. While much of CPTu interpretation is carried out in a dimensionless framework, there is evidence that existing relationships between stressnormalised tip resistance and state parameter are dependent on effective stress. This stress-dependence has been attributed to a variation in shear rigidity with effective stress, which is not accounted for in many interpretation techniques. However, at high stresses, other factors such as the curvature of the critical state line in an e-log(p’) plane may contribute. To assess CPTu of sands at high stresses, a novel small-scale calibration chamber employing a miniature cone capable of testing soils consolidated to a mean effective stress up to 2,000 kPa is outlined. Test results are presented for tests carried out over a range of mean effective stresses up to 1,000 kPa.

  Abstract
  Cone penetration testing with pore pressure measurement (CPTu) represents a state of practice tool to assess the in situ state parameter, strength, and liquefaction susceptibility [...]

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• Does CPT Reference Value Drift Really Inform CPT Correctness?

  A. McConnell*, M. Chapman

  ISC2024.

  
  

  Abstract

  The two authors’ company (IGS) is an in situ testing and sampling contractor. Approximately 40% of the company’s business is cone penetration testing (CPT). The CPT cones they use are good quality commercial units supplied by the Dutch company Geomil, with qc capacities ranging from 3MPa to 100MPa. Both compression-type and subtraction-type cones are used. IGS undertakes their own in-house calibrations on all cones, using externally calibrated load cells, and a combination of dead weights and hydraulic load application. Calibration and adjustment is undertaken on every cone on an unusually frequent basis, explained in the paper, far more frequently than current standards or manufacturer recommendations require. At each calibration, the reference readings (sometimes known as baseline readings) of each cone’s tip qc, sleeve fs, and pore pressure U sensors are noted. And the slope of the applied-load/pressure-vs-cone-readout for each of these sensors (ie accuracy) is measured and adjusted to give as close as reasonably possible 100% accurate output. All of this is recorded for each cone. Thus the authors have a database of reference reading drift that can be compared to slope adjustments (ie calibration adjustments) that have been needed to achieve the desired cone accuracy. This paper graphically reports the data for tip and sleeve of eight typical CPT cones of the day-to-day types used by the company.

  Abstract
  The two authors’ company (IGS) is an in situ testing and sampling contractor. Approximately 40% of the company’s business is cone penetration testing (CPT). The CPT [...]

  • 13
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