Undrained Penetration Research Articles

Novel hybrid XGBoost-based soft computing models for predicting penetration resistance of buried pipelines in cohesive soils

In this paper, both the finite element limit analysis (FELA) and soft computing techniques of four hybrid XGBoost (XGB) models, namely, GA-XGB, optimized with Genetic Algorithms; SMA-XGB, optimized with Slime Mould Algorithms; PSO-XGB, optimized with Particle Swarm Optimization; ACO-XGB, Ant Colony Optimization; and one Genetic Programing model (GP), are employed to develop surrogate models for predicting the undrained penetration resistance of buried pipelines embedded in clays. The penetration resistance of pipelines is caused by internal and external pressures, resulting in a force that acts on the pipeline in the downward and inclined directions at the same time. The penetration resistance factor (N) of a buried pipeline is determined based on four dimensionless variables, namely, the buried depth ratio (H/D), the inclination angle of the applied load (β), the soil strength or overburden ratio (γH/c), and the adhesion factor at soil-pipeline interfaces (α). The findings of this study are presented and summarized in the form of charts for dimensionless penetration resistance factors (N) and failure mechanisms of pipelines. Furthermore, the results from this study are compared with the results from previous studies and agrees well with those of previous studies. According to the results of the four developed hybrid XGBoost-based models, SMA-XGB outperforms GA-XGB, PSO-XGB, and ACO-XGB with no symptoms of overfitting, while the GP model resulted in an adequately accurate model with the advantage of a closed-form equation; its determination correlation coefficient of training and testing equals 0.98 (R2 = 0.980).

Penetration Behavior of the Footing of Jack-Up Vessel of OWTs in Thin Stiff over NC Clay

This study investigated the behavior of the spudcan foundation of jack-up vessels of offshore wind turbines during the undrained vertical penetration into thin stiff-over-normally consolidated clay. Large deformation finite element (LDFE) analyses were used to simulate the continuous spudcan penetration into the seabed surface. Detailed parametric analysis was performed to explore a range of normalized soil properties and layer geometry and roughness of the soil–spudcan interface. The results were validated against previously reported data. The LDFE results were consistent with those of centrifuge tests. The evolving soil-failure patterns revealed soil backflow and the trapping of stronger top-layer material beneath the spudcan. The plug shape was influenced by the top layer thickness, the strength gradient of the bottom layer, and the relative strength ratio, which also affected the penetration resistance of soils. In this study, an expression was derived to quantify the plug shape with the aim of providing a theoretical basis for the design of spudcan footings with penetration resistance suitable for thin stiff-over-soft clay.

Evaluation of Soil Strength from Variable Rate CPTU Tests in Silt

Abstract The interpretation of cone penetration test (CPTU), at the standard rate of 20 mm/s, results in clays assuming undrained penetration tests, whereas the interpretation in sands is carried out using a drained approach. However, for silts, the drainage conditions during a standard rate test are less well understood, and often partial drainage occurs. The main objectives of this work are to show the limitations of applying existing correlations for sand and clay to a silt material and to give guidance to practicing engineers on how to establish strength parameters for silts that allow a safe design. This paper presents the results of a comprehensive laboratory and field test study that was carried out on a silt site in Halsen-Stjørdal, Norway. Varied rate CPTU tests were conducted, and various soil samples were taken. Using a conventional Nkt approach to determine undrained shear strength, standard rate CPTU tests provided 20 % higher values compared with high-rate tests. Thus, the undrained strength may be overestimated from standard rate tests. Care needs to be taken when using undrained shear strength in silts. Nevertheless, this value represents a limiting strength value, which is often of interest when investigating the response as if it were undrained. The friction angle could be successfully determined from CPTU results by applying the modified Norwegian Institute of Technology (NTH) method to the measured data set. The results reveal a rate dependency of the friction angle, resulting in an underestimation of 20 % if a standard rate test is used compared with using low-rate CPTU tests, giving drained results that match the reference values from the laboratory. To overcome uncertainties in picking design strength values, it is highly recommended to carry out at least one fast and one slow CPTU test to evaluate the drainage situation and to adjust the design values accordingly.

Penetrometer testing in a calcareous silt to explore changes in soil strength

This paper describes a centrifuge study using a range of penetrometer tests (T-bar, piezocone and free-fall piezocone) to explore strength changes in a reconstituted, normally consolidated, natural calcareous silt. Various penetrometer test procedures were applied to measure the penetration resistances including monotonic, cyclic and twitch-type movements as well as pauses for pore pressure dissipation. These mobilised combinations of partial or full remoulding, strain softening, consolidation and viscous rate effects. The penetrometer resistance – representing a proxy for strength – reduced by a factor of 4·1 from drained to undrained conditions (at the lowest fully undrained penetration rate). In undrained conditions, viscous enhancement of the penetration resistance raised the tip and shaft resistance in free-fall piezocone tests by ∼2·8 and ∼3·6 times, respectively. The ‘restart’ resistance immediately after the dissipation tests was ∼2·5 times higher than the resistance prior to dissipation, giving an indication of consolidation-induced strength gain. The ‘twitch’ test (using sequential steps decreasing the velocity) captured drainage and viscous rate effects, and also gave a ‘restart’ resistance that showed even greater consolidation effects than from a dissipation test. Overall, the different penetrometer test types and procedures measured resistances in the same soil sample that varied by a factor exceeding 20 from highest to lowest, resulting from different penetration rates and history, due to strain rate, strain level (or remoulding) and consolidation. An expression for the monotonic penetration resistance combining drainage and viscous rate effects was fitted to the response of all tests, spanning >7 orders of magnitude in strain rate.

Effective Stress Strength Parameters of Clays from DMT

Abstract In this study, an established effective stress limit plasticity solution for piezocone penetration tests (CPTu) in clay developed by the Norwegian Institute of Technology (NTH) for evaluating the effective stress friction angle (ϕ′) is extended to flat plate dilatometer tests (DMT) readings. A nexus between CPTu and DMT is built through spherical cavity expansion solutions for undrained penetration to link the cone tip resistance (qt) and shoulder porewater pressure (u2) from CPTu to the measured contact pressure (p0) and expansion pressure (p1) obtained from DMT in soft to firm intact clays. Data from 49 paired sets of CPTu-DMT soundings in a variety of clays are used to support and validate the links. A variety of soils ranging from lean to plastic clays and clayey silts from marine, alluvial, lacustrine, deltaic, and glaciofluvial origins are subjected to flat DMTs, and the measurements are utilized to evaluate the effective stress friction angle (ϕ′). Data from 46 clays are compiled to examine the DMT-interpreted ϕ′ values in comparison with laboratory benchmark ϕ′ values obtained from CAUC or isotropically consolidated undrained triaxial compression tests on high-quality samples. An approximate inversion of the theoretical solution is derived to allow ϕ′ profiles to be determined with depth. Four well-documented DMT examples, including two natural clay sites and two chamber test series (one pressurized and the other unpressurized), are presented to illustrate the NTH procedures.

Adhesive Contact Algorithm for MPM and its Application to the Simulation of Cone Penetration in Clay

Contact between bodies is one of the most challenging problems to solve, especially when combined with large deformations. For MPM, several methods have been developed to simulate frictional contact, i.e. shear stress is proportional to the normal stress via a friction coefficient; however, for cohesive soils under undrained conditions the interface shear stress is more likely a function of the undrained shear strength and independent of the normal stress (adhesive contact). This paper presents the extension of one of the most widely used contact algorithms in MPM for adhesive contact. This enhanced formulation is validated with a sliding block benchmark and applied to the simulation of cone penetration testing (CPT) in clay. CPT is an in situ test commonly used in geoengineering to determine the subsoil's stratigraphy and to estimate soil parameters. It is shown that the adhesion at the cone-soil interface affects significantly the measured cone resistance. Numerical results are compared with available analytical and experimental studies, showing the effectiveness of the proposed method to describe undrained penetration in clay.

Interpretation of Cone Penetration Testing in Silty Soils Conducted under Partially Drained Conditions

The standard penetration rate used in cone penetration tests (CPTs) is 20 mm/s, regardless of soil type, which yields fully drained penetration in sand and fully undrained penetration in clay. However, for silty soils that represent an intermediate grain size composition and unique characteristics compared with sand and clay, the standard rate of penetration results in partially drained penetration, often leading to misinterpretation of this soil type. In this study, 15 CPTs, with penetration rates varying from 0.5 to 60 mm/s, were performed at a test site in northern Denmark, where the subsoil consisted primarily of sandy silt with clay bands. The results illustrated that when the penetration rate is reduced, the cone resistance increases, but the pore pressure decreases. The transition between undrained and fully drained penetration was determined by converting the results into a normalized penetration rate. Also evaluated and presented in this paper is how cone resistance obtained under partially drained conditions underestimates the interpreted relative density Dr and friction angle ϕ. Triaxial test results on undisturbed silt samples were applied for this analysis.

Centrifuge experiments on the settlement of circular foundations on clay

Centrifuge experiments were conducted to investigate the mechanisms governing the settlement of shallow circular foundations on clay. Model tests were performed on both soft and firm beds of overconsolidated kaolin clay. Foundation loads were applied as dead-weight and also through pneumatic loading. A Perspex window in the centrifuge package allowed digital images to be captured of a vertical cross section during and after the application of loading. Soil displacements deduced by particle image velocimetry (PIV) allow deformation mechanisms to be presented for undrained penetration, creep, and consolidation caused by transient flow. A technique is presented for discriminating consolidation settlements from the varying rates of short- and long-term creep. The mobilizable strength design (MSD) method is shown to make useful predictions of the undrained penetration using an estimated stress–strain behaviour of the clay with allowances made for anisotropy and rate effects. Subsequent creep and consolidation settlements were then determined using established correlations.

A parkable piezoprobe for measuring cv at shallow depths for offshore design

The coefficient of consolidation (cv) at shallow depth is an important parameter for the design of many offshore facilities, particularly pipelines. This paper sets out the concept, operation and interpretation of a new simple tool for measuring cv in situ at shallow depths. This tool is a large cylindrical penetrometer with a hemispherical tip, which is embedded statically under self-weight, for example from a winch. After embedment, the tool is ‘parked' – not requiring support from a drill-string – and the pore pressure dissipation is monitored. This ‘parkable' feature allows the tool to be added to conventional drilling or penetrometer systems and used in parallel. The operation of the tool has been simulated using large-deformation finite-element analysis with the Cam Clay plasticity soil model. Results in the form of the normalised undrained penetration response and pore water pressure dissipation–time history are presented. The penetration response indicates the weight requirement of the device. The dissipation curves allow cv to be determined from field measurements of pore pressure.

Comparing CPTU Q–F and Q–Δu2/σv0′soil classification charts

This paper extends a framework for classifying soil using normalised cone resistance (qt) and porewater pressure (u2) data from piezocone tests (CPTU) to include the friction ratio. Analytical predictions of normalised cone tip resistance ([Formula: see text]) and friction ratio (F = fs/qn) for a range of overconsolidation ratios are compared to databases of piezocone measurements during undrained penetration at clay sites, as well as to data from sands, silts and mixed soil types having partial to full drainage. Aspects of soil behaviour that result in data plotting in different zones of the complementary charts are discussed. It is concluded that Q–F classification charts are useful in the absence of high-quality pore pressure data. However, when attempting to separate the effects of yield stress ratio from partial consolidation during penetration in soils that may have some degree of sensitivity, structure or cementation, additional site-specific calibration may be required.

Influence of Partial Consolidation during Cone Penetration on Estimated Soil Behavior Type and Pore Pressure Dissipation Measurements

Estimation of soil behavior type from cone penetration testing, and the interpretation of dissipation tests, is complicated in in- termediate soil types, such as silty sands, sandy silts, etc., where partial consolidation occurs during penetration. This issue is investigated in this paper using results from cavity expansion and finite element analyses as well as field and centrifuge piezocone data. The implications for soil classification are examined using analytical expressions to explore the effect of normalized shear strength, rigidity index, and over- consolidation ratio relative to the influence of partial consolidation and viscous effects under fully undrained conditions. It is shown that partial drainage conditions can affect where data plots on soil behavior charts, thus complicating soil classification. The effect on dissipation tests following partial consolidation during cone penetration is shown to create errors in interpretation using experimental and numerical data. A new approach is developed based in part on manipulation of solutions for pore pressure dissipation (following undrained penetration) to account for these errors when interpreting dissipation tests. Errors can become significant during standard cone penetration testing when the t50 dissipation time is less than about 50 s. Guidelines, including equations and a chart, are presented for practical use. Finally, imple- mentation of this approach is demonstrated in a brief case study. DOI: 10.1061/(ASCE)GT.1943-5606.0000646. © 2012 American Society of Civil Engineers. CE Database subject headings: Soil consolidation; Cone penetration tests; Soil properties; Pore pressure; Sand (soil type); Silts. Author keywords: Cone penetration test; CPT; Cone penetrometer; Penetration rate; Consolidation; Dissipation test; Soil behavior type; Coefficient of consolidation; Intermediate soils.

Hydrate-Bearing Sediments from the Krishna−Godavari Basin: Physical Characterization, Pressure Core Testing, and Scaled Production Monitoring

Gas hydrate-bearing sediments recovered by pressure coring from the Krishna−Godavari Basin offshore India during the 2006 National Gas Hydrate Program (NGHP) expedition were characterized using the instrumented pressure testing chamber (IPTC). The IPTC studies provided longitudinal profiles of P- and S-wave velocities, electrical conductivity, and undrained penetration resistance. The formation consisted of fine-grained clayey sediments of high specific surface and high plasticity. X-ray images showed horizontal layering, pronounced heterogeneity from milli- to centimeter scales, with the presence of high-density nodules and both horizontal and sub-vertical gas hydrate lenses. Relative to reference values for sediments without hydrates, regions of high P- and S-wave velocities, low electrical conductivity, and high undrained shear strength were identified and correlated with regions of apparent high hydrate volume fractions in all tested specimens. Physical properties were monitored during the depressuriz...

Analysis of Factors Influencing Soil Classification Using Normalized Piezocone Tip Resistance and Pore Pressure Parameters

This paper discusses the development of a framework for classifying soil using normalized piezocone test (CPTU) data from the corrected tip resistance ( qt ) and penetration pore-water pressure at the shoulder ( u2 ) . Parametric studies for normalized cone tip resistance (Q= qcnet ∕ σ v0 ′ ) and normalized excess pressures (Δ u2 ∕ σ v0 ′ ) as a function of overconsolidation ratio (OCR= σ vy ′ ∕ σ v0 ′ ) during undrained penetration are combined with piezocone data from clay sites, as well as results from relatively uniform thick deposits of sands, silts, and varietal clays from around the globe. The study focuses on separating the influence of yield stress ratio from that of partial consolidation on normalized CPTU parameters, which both tend to increase Q and decrease the pore pressure parameter ( Bq =Δ u2 ∕ qcnet ) . The resulting recommended classification chart is significantly different from existing charts, and implies that assessment of data in Q–Δ u2 ∕ σ v0 ′ space is superior to Q– Bq space when...

Limits in determining permeability from on-the-fly uCPT sounding

Limits are defined for the transition from partially drained to undrained penetration during uCPT testing. These limits prescribe the range of cone metrics for which coefficient of permeability magnitudes may be recovered from peak pore pressure data, on the fly. The transition from partially drained to undrained behaviour is defined through the traditional non-dimensional metrics of cone resistance Qt, sleeve friction Fr and pore pressure ratio Bq, together with the undrained shear strength Su, normalised by shear modulus G and in situ effective stress, σ′v0. For plausible ranges of Su/σ′v0 and G/Su, the lower bound for transition from partial drainage is defined by the uCPT metric products and ratios of BqQt = 1·2, QtFr = 0·3, and Bq/Fr = 4, with the first and last proving the best determinants of drainage condition. Standard (2 cm/s and 10 cm2 face area) uCPT data together with independently measured permeabilities identify the transition from partially drained to undrained conditions at permeability magnitudes of the order of 10−5 m/s. These results are used to define limits of partial drainage where peak tip pore pressures may be used to recover in situ permeability profiles. For conditions of partial drainage, a non-dimensional permeability KD is defined independently in terms of cone metrics as KD = 1/BqQt (with BqQt < 1·2), enabling permeability to be recovered during standard penetration for K > 10−5 m/s. Where undrained data are excluded, non-dimensional permeability KD is optimally defined as KD = 0·62/(BqQt)1·6.

Velocity effects on Piezocone measurements in normally and over consolidated clays

Cone penetration tests (CPTs) performed under controlled conditions in a centrifuge can improve understanding and interpretation of CPT data obtained in the field. However, the cone diameter and cone penetration velocity employed in the centrifuge differ significantly from their respective full scale values and the effect of these scale differences need to be understood to allow extrapolation to the field. This paper examines the influence of cone velocity on tip resistance and penetration pore pressures, and also investigates trends during pore pressure dissipation when cone penetration is halted. Piezocone penetration tests were performed at different rates in normally consolidated and overconsolidated specimens of clay and silty clay in a beam centrifuge. The penetration rate was varied by over four orders of magnitude to provide information on partially drained and undrained penetration resistance and on the associated pore pressure response. The overconsolidation ratio and silt content are shown to have significant effect on the rate dependency of cone tip resistance and pore pressures.

Effect of Rate-Dependent Soil Strength on Cylinders Penetrating Into Soft Clay

This paper presents a predictive model for undrained penetration of cylinders into soft seafloor soils. The penetration depth will depend on the velocity of the cylinder as it touches down at the seafloor, and the net deceleration of the cylinder as it is acted on by forces of self-weight, soil buoyancy, and soil-shearing resistance. The soil-shearing resistance force increases as a function of penetration depth and, due to the dependence of undrained shear strength on strain rate, penetration velocity. This paper presents finite element (FE) simulations that quantify both effects and form the basis of a simplified soil-resisting force model. Strain-rate effects are modeled within a framework of rate-dependent plasticity, with shearing resistance increasing semilogarithmically with increasing strain rate above a certain threshold strain rate. With all forces acting on the cylinder, estimated penetration depths are predicted from simple equations of motion for a single particle. Comparisons to laboratory tests involving penetration of cylinders into soft reconstituted marine clay show reasonable agreement between model predictions and measurements.

Pore Pressure Response Following Undrained uCPT Sounding in a Dilating Soil

The generation and dissipation of pore fluid pressures following standard piezocone sounding (uCPT) sounding in silty sands are observed to exhibit many of the characteristics of undrained penetration in dilatant materials; steady excess pore pressures may be subhydrostatic, or may become subhydrostatic during dissipation, and are slow to decay. Enigmatic pore pressure dissipation histories which transit from sub- to supra- and again to subhydrostatic before equilibrating at hydrostatic are consistent with a response where undrained pressures are maximally negative remote from the penetrometer tip. This surprising distribution of induced pore fluid pressures is accommodated in cavity expansion models for a dilating soil. A Mohr-Coulomb constitutive model is established for undrained loading of a soil with pore pressure response defined by Skempton pore pressure parameters. Defined in terms of effective stresses, this allows undrained stresses and pore pressures to be determined following cavity expansion ...

Extending the strain path method analogy for modelling penetrometer installation

The Strain Path Method (SPM) is an approximate framework for simulating the disturbance caused by piles or penetrometers in soil. The key conceptual assumption of the SPM is that the deformation and strain fields caused during these penetration processes are strongly kinematically constrained (especially during undrained penetration of clays) and can be estimated independently from the actual constitutive properties of the surrounding soil. Previous applications of SPM have estimated strain fields for a variety of penetrometer geometries using velocity fields of ideal inviscid fluids. This paper refines the strain field for penetrometers with 60° conical tips using numerically computed velocity fields in viscous fluids with a variety of boundary conditions imposed on the penetrometer shaft. Following a parametric study, a set of flow conditions is selected which provides a best fit between computed soil deformations and physical displacement measurements made in three separate experiments. The approach is simple and rapid and, while highlighting some of the inaccuracies associated with the existing SPM solution, may also be used for comparative purposes to assist the development of other approaches to the deep penetration problem.

Deformation analysis of shallow penetration in clay

A new method of analysis is described for estimating the deformations and strains caused by shallow undrained penetration of piles and caissons in clay. The formulation combines previous analyses for steady, deep penetration, with methods used to compute soil deformations due to near-surface ground loss, and is referred to as the Shallow Strain Path Method (SSPM). Complete analytical solutions for the velocity and strain rates are given for a planar wall, an axisymmetric, closed-ended pile and unplugged, open-ended pile geometries. In these examples, the analyses consider a single source penetrating through the soil at a constant rate, generating a family of penetrometers with rounded tips, referred to as simple wall, pile and tube geometries. Soil deformations and strains are obtained by integrating the velocity and strain rates along the particle paths. The transition from shallow to deep penetration is analysed in detail. Shallow penetration causes heave at the ground surface, while settlements occur only in a thin veneer of material adjacent to the shaft and in a bulb-shaped region around the tip. The size of this region increases with the embedment depth. Deformations inside an open-ended pile/caisson are affected significantly by details of the simple tube wall geometry. © 1997 by John Wiley & Sons, Ltd.

Prediction of the undrained shear strength and standard penetration test using the static cone penetration test data

Prediction of the undrained shear strength and standard penetration test using the static cone penetration test data