Calibration Chamber Tests Research Articles

Evaluation of state parameter interpretation methods using CPT calibration chamber data

The cone penetration test (CPT) is widely used to determine the in situ state parameter of soils because it provides continuous data and excellent repeatability at a relatively low cost. Accurate interpretation of the state parameter from CPT is the basis for evaluating the strength of granular soils, including assessing liquefaction susceptibility in important structures such as tailings storage facilities. A few interpretation methods are used in practice. They use two different overburden stress normalisation schemes on tip resistance. These methods vary in how much information they utilise to differentiate among soils. This paper evaluates these methods by applying them to an extensive database of calibration chamber tests. Then, the state parameter interpreted by each method is compared with that determined from laboratory data. The database includes manufactured sands, natural sands, and clean sand tailings. The soils were selected such that both calibration chamber testing and triaxial compression data were available from the literature. This evaluation serves as a minimum requirement for applying these methods in engineering projects, especially those dealing with challenging soils such as fines-rich tailings. This study suggests that methods that account for soil properties and in situ horizontal stresses perform better than those that do not.

CPT calibration chamber testing on tailings

The cone penetration test (CPT) is a widely used technique for on-site investigations. In the field, interpretation correlations of CPT can be established using laboratory calibration chamber tests. Empirical and theoretical correlations were developed for clay and sand under undrained and drained conditions, respectively. However, interpreting these results can be more challenging for intermediate soils that exhibit partial drainage during penetration and lie between fully undrained and drained conditions. Tailings, which are the by-products of mining and are often rich in fine particles, are a typical example of such intermediate soils. To gain a deeper understanding of the behavior of the tailings during CPT, a calibration chamber test was performed at a standard rate of 2 cm/s and subsequently at a slow rate of 0.02 cm/s, and the obtained results were analyzed.

Cavity expansion-based interpretation of piezocone penetration test (CPTu) data in clays

A cavity expansion-based method is proposed in this paper to correlate the relevant findings with the piezocone penetration test (CPTu) data in clays. The rigorous and explicit solution for both spherical and cylindrical undrained cavity expansion is adopted based on a unified critical state constitutive model for clays and sands. The analogous model developed is comprehensively validated against two series of advanced numerical simulations and calibration chamber tests of the CPTu in fine-grained soils, along with bias analyses. The effects of the overconsolidation ratio and initial state parameter are extensively investigated for various clay types. Both theoretical and empirical correlations are then proposed for back-calculations of cone factor, undrained shear strength, overconsolidation ratio and initial state parameter of clays. Two case studies of CPTu tests are conducted in addition to examine correlations and contours of the in situ state parameter, demonstrating the applicability of the proposed theoretical approach for interpretation of the CPTu data.

Material point method simulations of displacement pile and CPT penetration in sand considering the effects of grain breakage

Full displacement installation dramatically alters the stress and strain fields applying around piles and Cone Penetrometer (CPT) probes at sand sites. Calibration chamber tests have indicated that severe particle crushing occurs near to the pile or cone tips and a distinct shear zone may develop that leaves crushed sand adhering to the pile or cone shaft. It is therefore important to investigate how particle breakage affects the stress and deformation fields developed as this process occurs. In this study, installation by monotonic jacking of conically tipped model piles or CPT probes into silica sand is simulated by material point method (MPM) analyses. A thermodynamically consistent model which considers competing grain crushing and dilation is employed to model the sand behavior, while an MPM approach is adopted to cater for the large deformations involved in the penetration process. Particle crushing is predicted beneath the pile tip and around the pile shaft. The numerical results are compared to directly related instrumented pile calibration chamber tests. The numerical results show that the computed stress and displacement fields match key aspects of the experimental results well, especially in reproducing the volumetric compression observed experimentally. The results confirm that combining MPM with suitably sophisticated constitutive models creates new capabilities for analyzing large-deformation problems.

Study on the Characteristics of Effective Internal Friction Angles of Silty Clay from the Yellow River Delta and the Inverse Method of CPTu Testing

This study focuses on the silty clay of the Yellow River Delta, conducting laboratory experiments to explore the strength characteristics of typical silty clay in the Yellow River Delta. The study utilizes CPTu calibration chamber tests to systematically reveal the features of cone tip resistance (qt), side friction resistance (fs), and pore water pressure (u2) of the silty clay. This research provides a theoretical basis for Delta investigation in the Yellow River region. The study compares the applicability of existing CPTu inversion methods and highlights the NTH method’s advantages in evaluating silty clay’s effective internal friction angle. Supported by indoor calibration chamber tests, the study confirms the reliability of the NTH method in estimating the effective internal friction angle under normally consolidated conditions while also identifying its limitations. These research findings offer data support for the in situ rapid and accurate estimation of design parameters, like the internal friction angle and undrained shear strength of the silty clay in the Yellow River Delta. Moreover, they provide insights for obtaining other crucial in situ data.

DEM study of the stress fields around the closed-ended displacement pile driven in sand

Three-dimensional discrete element method (DEM) is employed to model the published calibration chamber test of a closed-ended displacement pile driven in sand. To achieve large chamber-to-pile and pile-to-particle size ratios for unbiased predictions, a high-performance graphics processing unit-powered DEM code is utilised in conjunction with the axisymmetric assumption and particle refinement method. The DEM parameters are first calibrated against the drained triaxial compression test data on Fontainebleau sand. Then, the DEM modelling of pile driving demonstrates quantitative agreement with the experimental counterpart in terms of cone resistance, stress contours, and distributions.

Addressing complexities in MPM modeling of calibration chamber cone penetrometer tests in homogenous and highly interlayered soils

Cone penetrometer tests (CPTs) are used to characterize soil for a variety of geotechnical engineering applications, including earthquake-induced liquefaction triggering assessment. Numerical modeling of CPTs is frequently used to better understand soil behavior, soil-penetrometer interaction, and engineering estimates made from CPT data. However, calibrating and validating numerical CPT simulations with experimental calibration chamber (CC) data can be challenging. Specifically, uncertainties in the interpretation of laboratory strength and compression data compound with uncertainties in the CC testing and the assumptions made when developing the numerical model. This article provides a comprehensive review of uncertainties in the calibration and validation of CPT numerical simulations performed in homogenous sand, homogenous clay, and layered sand-clay soil profiles, comparing numerical results with well-documented experimental calibration chamber tests performed at Deltares. In particular, the Material Point Method (MPM) is used to perform the numerical analyses. A framework is presented to assess how uncertainty in the numerical model output is attributed to each input parameter. It is demonstrated that uncertainties can be explored numerically. Finally, recommendations for future experimental and numerical studies of CPTs are provided.

Compressibility-Based Interpretation of Cone Penetrometer Calibration Chamber Tests and Corresponding Boundary Effects

Abstract Calibration chambers have been used to calibrate in situ testing devices (e.g., standard penetration test, cone penetration test, and flat plate dilatometer) in laboratory settings. Researchers have long suggested, however, that penetration resistance measured in a calibration chamber differs from what would be measured under similar conditions in the free field due to the finite chamber size and chamber boundary conditions. Using a database of 847 cone penetration tests performed in sand-filled calibration chambers, this paper reexamines assumptions used to interpret tests and boundary effects, analyzes the efficacy of published calibration chamber correction factors, and provides a novel framework for interpreting calibration chamber data based on soil compressibility. This study suggests that (1) trends in cone tip resistance are relatively unaffected by boundary effects; (2) factors used to correct calibration chamber cone tip resistance to an equivalent free-field value only marginally improved the correlations/trends; and (3) cone tip resistance is similar for soils of equally similar compressibility, density, and effective confining stress. The authors suggest that calibration chamber data may not require corrections for chamber size or boundary conditions so long as a ratio of chamber diameter to cone penetrometer diameter of 20 is satisfied.

Relating cone penetration resistance to sand state using the material point method

Cone penetration tests (CPTs) can quantitatively inform about the mechanical state of a sand. However, relating measured cone resistance values to sand state requires complex back-analysis of the processes occurring in the soil during the test. This paper provides new evidence of the value added in this area by modern large-deformation modelling based on the material point method (MPM). It is shown that accurate simulation of the relationship between cone resistance and sand state can be achieved, on condition that the constitutive behaviour of the soil – and especially its critical state features – is adequately modelled over a wide range of confining pressures. This study relies on the predictive capabilities of the critical state NorSand model, and shows how previous calibrations endeavours from the literature (based on triaxial test results) can support the MPM simulation of unrelated CPT results obtained through calibration chamber tests. MPM CPT simulations of ever-increasing quality will positively impact the state of the art of CPT interpretation procedures, to date still largely based on simplified cavity expansion theories.

Improved cone penetration test predictions of the state parameter of loose mine tailings

The most widely used in situ testing instrument for tailings storage facility (TSF) is the cone penetration test (CPT), which uses calibration chamber (CC) test data as the preferred method to correlate the soil's state with the CPT acquired data. Many, if not most, mine tailings are different from the soils historically used in CC studies (mainly sands). Moreover, these tests were conducted at denser states than typically found in TSFs. Additionally, CC tests cannot practically be performed for every new soil-specific project. For these reasons, practitioners usually perform numerical simulations to approximate these correlations. One of the most popular techniques for this is the spherical cavity expansion approach. This study compares novel small CC testing of two different silty mine tailings, and their respective spherical cavity expansion simulations. Since these simulations use the NorSand constitutive model, this study also presents the calibration process that includes the triaxial laboratory testing to calibrate the NorSand properties, and the corresponding iterative process to infer the additional plastic and elastic parameters of the model. Finally, an adjustment equation is introduced for the contractive state of these silty materials.

Verification of Seismic Cone Penetration Test Calibration Chamber Tests on a Sand

Abstract Calibration chambers are experimental apparatuses that allow the physical modeling of an in situ testing method, such as the cone penetration test (CPT), by performing laboratory experiments on soil samples with known stress, strain, and density characteristics. Over a series of development and improvements, more recent calibration chambers and laboratory cone penetrometers have become compact and less laborious than their predecessors. This includes the focus of this study where a miniature cone penetration testing (MCPT) calibration chamber previously designed and used at Western University is upgraded to measure shear wave velocity (VS) and enable anisotropic consolidation of soil specimens using a hydraulic piston. In addition to VS, MCPT can also measure cone tip resistance, sleeve friction, and excess pore water pressure developed behind the cone tip. A full test plan using the newly upgraded MCPT calibration chamber is presented in this study. These tests are performed on Fraser River sand at various relative densities and effective stresses. Results of this testing program are validated by comparison with a series of in situ seismic CPTs in the Fraser River delta, existing correlations between VS and cone tip resistance, and several other calibration chamber tests on different sands. Through these comparisons, the overall viability of the initial MCPT calibration chamber design, modifications made to the MCPT setup, and ultimately the results produced from this device are confirmed.

Numerical study on the installation effect of a jacked pile in sands on the pile vertical bearing capacities

The installation effect of a jacked pile in silica sand is investigated numerically. A simplified state-dependent dilatancy model is used to model the sand behavior and the sand grain crushing effect is considered by modifying the critical state line at large confining pressures. The Arbitrary Lagrangian-Eulerian (ALE) technique is adopted to model the pile penetration and the load-displacement behavior at the operational loading stage is simulated using standard finite element method with the initial states being the equilibrium of the pile-jacking-disturbed soil stress and void ratio fields and a free pile head. The numerical procedure has been verified against some calibration chamber tests and centrifuge tests. Simulations of wished-in-place piles are performed for the quantitative study of the installation effect. The results show that the capacity ratios between the jacked and wished-in-place piles become higher for long piles in dense sand. Linear fittings have been applied to obtain the capacity ratios as functions of the relative density and the penetration length-to-diameter ratio, which can be used for a quick estimation of the bearing capacity difference between jacked and wished-in-place piles.

Laboratory-scale seismic CPT calibration chamber tests on Fraser River sand

Physical modeling of cone penetration testing (CPT) can be carried out under known stress, deformation, and density conditions using laboratory calibration chambers. This study presents the results of reduced-scale model CPTs (MCPT) on Fraser River sand using a calibration chamber that allows anisotropic consolidation of soil specimens, as well as the measurement of shear wave velocity (VS), using bender elements. In addition to VS, MCPT can also measure cone tip resistance (qc), sleeve friction (fs), and pore water pressure developed behind the cone tip (Δu2). The CPT model tests are performed on Fraser River sand at different effective stresses and relative densities. The CPT calibration chamber test results are comparable with those of in situ seismic cone penetration tests (SCPT) in the Fraser River delta, existing correlations between VSand qc, and calibration chamber tests on other sands. This confirms the overall feasibility of the MCPT calibration chamber design for the measurements of qc, fs, and VSin sandy soils.

Simplified method for evaluation of liquefaction based on pressuremeter tests (PMT)

Liquefaction is of major concern in many countries subjected to seismic hazard. In practice, prediction of liquefaction is performed using simplified methods based on in situ tests such as the standard penetration test (SPT) or the cone penetration test (CPT). For some applications such as large projects (tunnels, high rise buildings), the Ménard pressuremeter is preferred as it provides a well-established method for foundation design. However, no simplified method to evaluate liquefaction potential has been fully developed for the pressuremeter. Based on pressuremeter calibration chamber and field tests on sands, a chart is proposed for estimating the liquefaction susceptibility of saturated sands. In addition, a method to correct the measured limit pressure from pressuremeter tests is also proposed for tests in sands with fines content. The method was evaluated using extensive data obtained from tests performed in a calibration chamber using clean sand and sand with fines content against 45 pressuremeter tests performed in situ in various soil conditions. Based on these tests, a relationship was obtained between the field data and the chart developed from the calibration chamber tests. The paper also provides suggestions to correct the limit pressure to account for the fines content.

Interpretation of CPT in unsaturated sands under drained conditions: A numerical study

AbstractA finite difference‐based numerical model simulating the cone penetration process in unsaturated sands is presented. Mohr–Coulomb model (MCM) with simple modifications and Sun model (SM) were implemented to capture the unsaturated sand behaviour. It was shown that the cone tip resistance values resulting from the two models were fairly comparable. Predicted cone tip resistance values in dry, saturated and unsaturated sands using MCM were validated by the results of field and calibration chamber tests. Sensitivity analyses were performed, and the influence of parameters including relative density, mean effective stress and apparent cohesion due to suction on the tip resistance was investigated. Based on a large number of numerical analyses, a simple relationship was suggested to predict the cone tip resistance in dry, saturated and unsaturated sands. The accuracy of the proposed relationship was evaluated by a large database of Cone penetration test (CPT) results performed in various calibration chambers containing different sands. Finally, the applicability of the suggested equation in the estimation of the soil relative density and friction angle from CPT results was shown through useful and practical charts and illustrative examples.

Comparison of performance of single- and double-core prefabricated vertical drains for thick reclaimed ground improvement

The performance of a double-core prefabricated vertical drain (PVD), which could reduce the well resistance via its large cross-sectional area and structural stability, was investigated experimentally and numerically. The double-core and the conventional single-core PVD, which has been widely adopted in reclaimed ground improvement, were compared in this study. In the experimental investigation, modified Delft tests and a calibration chamber test were carried out in a laboratory to determine the discharge capacities of the PVDs. Thereafter, single- and double-core PVDs were installed in the Busan New Port site, which had a 40 m thickness of clay deposit, and the performances of the PVDs estimated through surface settlements were compared. In addition, a numerical program named ILLICON was applied with field measurements, and a parametric study was conducted for different discharge capacities of the PVDs. The result of the laboratory tests and field measurements indicated that there was no difference in the performance between the two types of PVDs for improving the soft ground, considering actual ground conditions. Similarly, the parametric study results also implied that both PVDs could be satisfactorily applied in thick reclaimed ground, and there is no significant difference in the performance of single- and double-core PVDs.

Cone penetration tests in dry and saturated Ticino sand

It is commonly assumed that dry and saturated sands exhibit similar cone resistance–relative density relationships. Some studies pointed out that partial saturation and calcareous sands with considerable fines content are potential factors affecting these relationships. However, there is experimental evidence in Shaqour Bull Eng Geol Environ 66:59-70, (2006) that clean uncemented quartz sand may exhibit lower cone resistance in saturated conditions. The present study aims on contributing towards better understanding the effect of water saturation on cone resistance in sand. For this purpose, Ticino sand samples were prepared dry and saturated in a calibration chamber and cone penetration tests were performed over a wide range of relative densities and at two consolidation stresses. Overall, it was observed that dry and saturated samples exhibited similar cone resistances. Only slightly higher cone resistances were observed for dry samples at the lower consolidation stress. Two anomalous samples, which were tested dry at medium relative density, were found to exhibit way higher cone resistances than expected from published cone resistance–relative density relationships. The Young's modulus was observed to be proportional to cone resistance and independent of whether a sample was tested dry or saturated, being therefore considered as more robust soil property for cone resistance relationships.

Assessment of the efficacies of correction procedures for multiple thin layer effects on Cone Penetration Tests

Multiple interbedded fine-grained layers in a sand deposit have a “smoothing” effect on the measured Cone Penetration Test (CPT) tip resistance (qc), resulting in a significant underestimation of the predicted liquefaction resistance of the sand layers. Trends identified by De Lange [14] through calibration chamber tests on stratified sand-clay profiles are used herein to develop a new thin-layer correction procedure for qc (the “Deltares” procedure). The efficacies of the Deltares and the independently-developed Boulanger and DeJong [6] procedures are both directly assessed using CPT data from calibration chamber tests and indirectly inferred from CPT-based liquefaction case histories in Christchurch, New Zealand. The results highlight limitations of the assessed thin-layer CPT qc correction procedures for layers less than 40 mm thick. Multiple, interbedded thin layers also influence the measured CPT sleeve friction (fs), but in a more complex way than they influence qc. To-date, no procedures have been proposed to address all the thin-layer-effects phenomena on the measured fs, with errors in properly characterizing the fs of a layer inherently influencing the accuracy of predicting the liquefaction susceptibility and potential of the layer. In totality, the thin-layer-effects correction procedures proposed to-date generally result in slightly less accurate predictions of the observed liquefaction severity for cases having highly stratified profiles, opposite of what would be expected and desired.

Experimental investigation on the installation and loading performance of model-scale deep helical piles in very dense sand

The use of helical piles has grown over the years owing to different advantages, such as large uplift capacity due to the anchor effect of the helix and installation torque-capacity correlation. Although the use of helical foundations is expanding worldwide, some key aspects fundamental to the design are not well understood to date. Practical experience indicates that the installation forces and loading performance of helical piles in sand are dependent on the helices characteristics and confining stresses. Therefore, for a better understanding of these dependencies, the effects of the helix-to-shaft diameter ratio (wing ratio) and vertical confining stress on the installation torque and forces, and on the uplift and compression capacities of helical piles, were evaluated from nine calibration chamber tests, conducted on instrumented single-helix piles in very dense sand. Among other findings, this study indicated that for a certain shaft diameter (i) the wing ratio influences the installation torque, but does not affect the installation vertical force; (ii) the ultimate uplift pressure mobilized on the helix decreases with the increase of the wing ratio; (iii) the growth rate of the helix bearing resistance with the confining pressure, for vertical stresses higher than 100 kPa, becomes reduced for piles with a larger wing ratio.

Application of a state-dependent sand model in simulating the cone penetration tests

A critical-state sand model with density- and the third stress invariant-dependence is combined into a large deformation finite element method to numerically study the cone penetration process in sands. A fully implicit stress integration algorithm is developed to implement the constitutive equations. The large deformation simulations are achieved through the adaptive remeshing technique based on the Arbitrary Lagrangian-Eulerian (ALE). A series of CPT calibration chamber tests are simulated to validate the soil model and numerical approach, followed by discussions on the effect of the soil density on the cone penetration resistance as well as the changes of the sand mechanical properties induced by the penetration process. Lastly, the validated numerical technique is applied to study the extent of sand disturbed by the process of pile driving.