Documents published in Scipedia

• 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 [...]

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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 [...]

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• Importance of Calibration Laboratories in In-situ Test Methods

  N. Parasie*, G. Sinjorgo, L. Vrielink, J. Peuchen

  ISC2024.

  
  

  Abstract

  Geotechnical in-situ test methods provide valuable data for asset development, operation, and decommissioning. For confidence in test results, industry typically requires calibration and verification of in-situ test sensors to be conducted in a calibration laboratory. A calibration laboratory typically operates according to ISO/IEC 17025 (2017) ‘general requirements for the competence of testing and calibration laboratories’, or similar standard. This paper summarises observations from practice, with focus on the following challenges (1) evaluation of measurement uncertainty of key parameter values for which no standardised methods or verification materials are available, (2) validation of test methods with no backup from a formal standard published by a standardisation organisation and (3) field test site and interlaboratory comparisons within a competitive industry setting. Specific examples are presented for the (1) estimation of calibration uncertainty for sleeve friction of a subtraction-type cone penetrometer and (2) method validation for thermal conductivity of soil derived from in-situ heat flow measurements.

  Abstract
  Geotechnical in-situ test methods provide valuable data for asset development, operation, and decommissioning. For confidence in test results, industry typically requires [...]

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• Offshore shear wave velocity measurements for the assessment of soil sampling quality

  A. Deu*, A. Gens, A. Viana Da Fonseca, M. Devincenzi, D. Tarragó

  ISC2024.

  
  

  Abstract

  Shear wave velocity Vs is a critical soil parameter for several geotechnical and geophysical engineering applications including seismic site response analysis, liquefaction risk assessment and design of shallow and deep foundations. Moreover, the comparison of shear wave velocity between laboratory and in situ measurements has become a standard acceptance criterion for the assessment of sampling quality. Offshore in situ shear wave velocity testing is considerably more challenging than onshore, due to the difficulties in the correct deployment of the instrumentation as well as of the wave source, in absence of direct visibility of the ground level below water. This paper describes the methodology employed for offshore shear wave velocities (Vs) measurements in the harbour of Barcelona in September 2022. Medusa SDMT tests were performed in sea depths ranging between 15-17 m from a jackup and employing a drill rig to penetrate the probe down to 40 m below the seafloor. The paper includes examples of recorded S-wave seismograms, analyses of Vs repeatability for the same depth measurements and Vs profiles with depth. In the same test locations, carefully prepared specimens of undisturbed samples were tested after reconsolidation to the estimated in situ stress states in stress path triaxial cells with bender elements transducers. The obtained lab shear wave velocities were compared with the in situ values obtained with the Medusa SDMT tests to assess sample quality.

  Abstract
  Shear wave velocity Vs is a critical soil parameter for several geotechnical and geophysical engineering applications including seismic site response analysis, liquefaction [...]

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• Influence of hydrogeological conditions on estimation of undrained shear strength by CPTu tests: case study in a tropical soil

  I. Rigatto*, L. Ferreira

  ISC2024.

  
  

  Abstract

  CPTu tests have gained prominence in the geotechnical characterization of materials, registering a significant increase in their application in the Brazilian context, especially due to requirements to consider undrained resistance in analyses guided by more recent regulations. However, the interpretation of these tests often lacks a detailed and personalized approach, as they disregard specific nuances of each location. In this study, the foundation of a dam made up of tropical soil with a specific hydrogeological condition, characterized by bottom drainage with deep percolation, previously identified in other research campaigns, was evaluated. The interpretation of the CPTu test aimed to estimate the undrained resistance of the material through two different approaches: considering the dissipation tests carried out to model the insitu pore pressure according to the elevation versus a hypothetical hydrostatic condition, which could be misinterpreted in places where there is a predominance of SPT tests and insufficient geological knowledge. Multiple methodologies were evaluated to interpret undrained shear strength, including approaches that use Bq directly and that exclusively considers the laboratory characterization of a sample and the overconsolidation ratio at that point. In this case analyzed, it was observed that the change in pore pressure conditions resulted in a considerable variation in the undrained shear strength ratio, over 10% when pore pressures are considered in the equations. The results highlight the relevance of considering local hydrogeological conditions when interpreting field tests, especially for foundations of large structures.

  Abstract
  CPTu tests have gained prominence in the geotechnical characterization of materials, registering a significant increase in their application in the Brazilian context, especially [...]

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