Self-boring Pressuremeter Research Articles

Rate effects of cylindrical cavity expansion in fine-grained soil

Soil responds to cavity expansion is inherently rate-dependent, especially in the case of fine-grained soils. To better understand such rate effects, self-boring pressuremeter tests were conducted on Kunming peaty soil within a strain rate range of 0.1%/min to 5.0%/min. The results showed a clear dependence of cavity pressure and excess pore pressure (EPP) on strain rates—both increased with higher rates for a given radial displacement. In light of the experimental results, three cases of cylindrical cavity expansion were investigated using the finite element method and analytical method, partially drained expansion in Modified Cam-Clay (MCC) soil, and undrained and partially drained expansion in elastoviscoplastic (EVP) soil. The EVP behavior was and modeled using the MCC model and the overstress viscoplastic theory. The results indicated that over the strain rate range of 0.0001%/min and 50%/min, the rate response of cavity pressure for the case of partially drained expansion in MCC soil (permeability coefficient ranging from 5×10-6 m/s to 2.5×10-11 m/s) is not obvious, while the EPP response during undrained expansion in EVP soil shows rate-independent. Only the partially drained solution for cavity expansion in EVP soil captured the rate-sensitive responses of both cavity pressure and EPP, confirmed by the pressuremeter tests on the Kunming peaty soil, Saint-Herblain clay, and Burswood clay. This suggests that the rate effect results from a combination of drainage-related and time-dependent soil behavior. Parametric studies further demonstrated that both viscous behavior and the overconsolidation ratio significantly influence cylindrical cavity expansion response, and the drainage conditions during expansion can be assessed using a nondimensional velocity.

Cylindrical cavity expansion-contraction solutions in undrained MCC soils with the auxiliary variable approach

Cylindrical cavity exhibits non-self-similarity during contraction process following expansion. Previous studies solve this problem with total strain approach and simple constitutive models, but the approach is not applicable when using an advanced constitutive model. This paper presents a semi-analytical solution for a cylindrical cavity undergoing expansion-contraction in undrained soils with auxiliary variable approach, incorporating the Modified Cam-Clay (MCC) model. The stress states around the cavity are formed by the superposition of initial and superimposed stress states. By treating superimposed effective stresses as self-similar, a semi-analytical solution is derived for solving the cavity expansion-contraction problem. The elastoplastic stress-strain relationship is formulated as a set of first-order differential equations, which can be solved as an initial value problem though Runge-Kutta (RK) method. Then the stress distribution around the cavity during expansion-contraction process can be determined. To validate the proposed approach, a series of well-conduced self-boring pressuremeter (SBP) tests are used to verify the proposed approach, which shows good agreements. Additionally, a FEM simulation incorporating the MCC model is performed, and the simulation results are presented to carry out parametric studies on soil parameters. A significant influence on the range of the plastic and reverse plastic zones is shown for overconsolidation ratio, while the in-situ coefficient of the earth pressure only quantitatively affects the stress distribution.

Field testing of shear strength of granite residual soils

The characteristics of residual soils are very different from those of sedimentary soils. Although the strength characteristics of sedimentary soils have been studied extensively, the shear strength characteristics of granitic residual soils (GRS) subjected to the weathering of parent rocks have rarely been investigated. In this study, the shear strength characteristics of GRS in the Taishan area of southeast China (TSGRS) were studied by field and laboratory tests. The field tests consisted of a cone penetration test (CPT), borehole shear test (BST), self-boring pressuremeter test (SBPT), and seismic dilatometer Marchetti test (SDMT). The shortcomings of laboratory testing are obvious, with potential disturbances arising through the sampling, transportation, and preparation of soil samples. Due to the special structure of GRS samples and the ease of disturbance, the results obtained from laboratory tests were generally lower than those obtained from situ tests. The CPT and scanning electron microscopy (SEM) results indicated significant weathering and crustal hardening in the shallow TSGRS. This resulted in significant differences in the strength and strength parameters of shallow soil obtained by the BST. Based on the SDMT and SBPT results, a comprehensive evaluation method of shear strength for TSGRS was proposed. The SBPT was suitable for evaluating the strength of shallow GRS. The material index (ID) and horizontal stress index (KD) values obtained by the SDMT satisfied the empirical relationship proposed by Marchetti based on the ID index, and were therefore considered suitable for the evaluation of the shear strength of deep GRS.

Discussion of “Measuring strength and consolidation properties in lacustrine clay using piezocone and self-boring pressuremeter tests”

Discussion of “Measuring strength and consolidation properties in lacustrine clay using piezocone and self-boring pressuremeter tests”

Displacement of piles from pressuremeter test results - a summary of French research and practice

This paper presents the ‘load-transfer functions’ t-z and p-y methods for determining the axial and lateral displacements of single piles. They are based on the results of pressuremeter tests. The methods from the results of the Ménard pre-borehole pressuremeter tests (MPM) and the self-boring pressuremeter tests (PAF, for Pressiomètre AutoForeur in French) are described. Especially, the t-z and p-y methods derived from the MPM test results are commonly used in the French practice. For both t-z and p-y curves, some theoretical background (usually FEM calculations in linear elasticity) is given. The results of the t-z and p-y analyses are compared to the measurements from loading tests on full scale piles. A proposal concerning barrettes is also presented.

Stiffness decay characteristics and disturbance effect evaluation of structured clay based on in-situ tests

The structure of the sedimentary clay influences its mechanical behavior in non-negligible ways. This paper proposes an effective approach for investigating the in-situ stiffness characteristics from shear modulus and strain decay curves (G–γ curves) based on self-boring pressuremeter and seismic dilatometer tests. To evaluate the excavation disturbance effects on the structured clay, the stiffness parameters from pre-bored pressuremeter tests are compared with the results of self-boring pressuremeter tests. The result indicates that the complete in-situ G–γ curves can be acquired by integrating the strain-dependent tangent shear modulus Gt from self-boring pressuremeter tests and the small-strain modulus G0 from seismic dilatometer tests. Simultaneous observations of the G–γ curves with hyperbolic shapes in semi-logarithmic coordinates at the same strain scale show the similarity of the stiffness decay mode of the soil at different depths. The increase in the measured values of Gt and G0 with depth can be attributed to the improved consolidation pressures and cemented strength in the structured clay. Additionally, the G/G0–γ curves measured by the in-situ tests generally agree well with the results predicted by the Stokoe model. The excavation disturbance weakens the stiffness of the structured clay, as evidenced by Gt from the pre-bored pressuremeter data being significantly smaller than that from self-boring pressuremeter tests at the same depth. Based on quantitative analysis, the disturbance degree computed from the measured results has a low sensitivity to the soil depth and a strong negative correlation with the strain level of the soil. This study provides an effective method for predicting the stiffness parameters of structured soil based on in-situ tests.

Measuring strength and consolidation properties in lacustrine clay using piezocone and self-boring pressuremeter tests

As a result of the growing population in Switzerland, new infrastructure is increasingly founded in areas of extremely soft clays, which are challenging to build on. Reliable characterization of such sediments is, therefore, essential to facilitate efficient and economical designs. For various reasons, it is often beneficial to measure soil properties in situ, using probes like the piezocone or self-boring pressuremeter. The main objective of this study was to explore the key differences of these two site investigation techniques in terms of measuring undrained strength and consolidation characteristics in varved lacustrine clay. Experimental data from a field-testing campaign carried out in Switzerland demonstrate that both probes yield comparable results. The quality of the predictions was further assessed by comparing the field measurements to results from laboratory tests and existing empirical relationships. Additionally, a new method based on Monte Carlo simulations was presented, to provide context to the disparity between the consolidation coefficients estimated in the field ( ch) and laboratory ( cv), and to aid prediction of field values of ch from laboratory measurements.

Excess Pore Water Pressure Generation, Distribution and Dissipation in Self-boring Pressuremeter Test: Numerical Analysis

Excess Pore Water Pressure Generation, Distribution and Dissipation in Self-boring Pressuremeter Test: Numerical Analysis

Undrained Elastoplastic Solution for Cylindrical Cavity Expansion in Structured Cam Clay Soil Considering the Destructuration Effects

Soil structure has significant influences on the mechanical behaviors of natural soils, although it is rarely considered in previous cavity expansion analyses. This paper presents an undrained elastoplastic solution for cylindrical cavity expansion in structured soils, considering the destructuration effects. Firstly, a structural ratio was defined to denote the degree of the initial structure, and the Structured Cam Clay (SCC) model was employed to describe the subsequent stress-induced destructuration, including the structure degradation and crushing. Secondly, combined with the large strain theory, the considered problem was formulated as a system of first-order differential equations, which can be solved in a simplified procedure with the introduced auxiliary variable. Finally, the significance and efficiency of the present solution was demonstrated by comparing with the previous solutions, and parametric studies were also conducted to investigate the effects of soil structure and destructuration on the cavity expansion process. The results show that the soil structure has pronounced effects on the mechanical behavior of structured soils around the cavity. For structured soils, a cavity pressure that is larger than the corresponding reconstituted soils when the cavity expands to the same radius is required, and the effective stresses first increase to a peak value before decreasing rapidly with soil structure degradation and crushing. The same final critical state is reached for soils with different degrees of the initial structure, which indicates that the soil structure is completely destroyed during the cavity expansion. With the increase of the destructuring index, the soil structure was destroyed more rapidly, and the stress release during the plastic deformation became more significant. Moreover, the present solution was applied in the jacking of a casing during the sand compact pile installation and in situ self-boring pressuremeter (SBPM) tests, which indicates that the present solution provides an effective theoretical tool for predicting the behavior of natural structured soils around the cavity.

Geostatic stress in oil-sand tailings

Horizontal geostatic stress estimated from self-bored pressuremeter (SBP) data using the ‘lift off’ method is uncertain because of even small deficiencies in self-boring, but that uncertainty can be minimised by modelling the complete test. Iterative forward modelling based on large-strain cavity expansion in frictional dilating (non-associated Mohr–Coulomb) soil, with correction for finite SBP geometry, is both easily implemented in a spreadsheet and computes quickly. Such modelling of a campaign of SBP tests in oil-sand tailings shows a baseline geostatic stress ratio K0 = 0.6 for those tailings that are truly normally consolidated. Other geological history factors, including compaction by tracking and wetting–drying cycles, adds about a Δσh ≈ 60 kPa ‘locked-in’ stress to this normally consolidated trend; an alternative view is that these factors produce K0 ≈ 1. The modelling spreadsheet is provided as a downloadable Excel application in the online supplementary material.

Site Investigations of the Lacustrine Clay in Taihu Lake, China, Using Self-Boring Pressuremeter Test

Site investigations of the soils are considered very important for evaluation of the site conditions, as well as the design and construction for the project built in it. Taihu tunnel is thus far the longest tunnel constructed in the lake in China, with an entire length of over 10 km. However, due to the very insufficient site data obtained for the lacustrine clay in the Taihu lake area, a series of self-boring pressuremeter (SBPM) field tests was therefore carried out. Undrained shear strengths were deduced from the SBPM test, with the results showing generally higher than those obtained from the laboratory tests, which may be attributed to the disturbance to the soil mass during the sampling process. Degradation characteristics of the soil shear modulus (Gs) were mainly investigated, via a thorough comparison between different soil layers, and generally, the shear modulus would cease its decreasing trends and become stable when the shear strain reaches over 1%. Meanwhile, it was found that a linear relationship between the plasticity index and the shear modulus, and between the decay rate of the shear modulus and the plasticity index as well, could be developed. Further statistical analysis over the undrained shear strength and shear modulus distribution of the soils shows that the undrained shear strength of the soils follows a normal distribution, while the shear modulus follows a log-normal distribution. More importantly, the spatial correlation length of the shear modulus is found much smaller than that of the undrained strength.

Prediction of Unloading Failure Strain and Undrained Shear Strength of Saturated Clays by Limit Pressure from Prebored Pressuremeter

Abstract Saturated clays mostly fail in lateral unloading by the reduction of undrained shear strength (Su) because of an increase in lateral unloading strain (εu), which is the most predominant factor affecting the soil mass in excavated structures. An attempt is made to extend the work of Houlsby and Withers (1988) on the self-boring pressuremeter (SBP) and Ferreira and Robertson (1994) for the full-displacement pressuremeter (FDP) and SBP, for unloading curves of the prebored pressuremeter (PBP) test for large strain (up to 41.5 %) tests in a specific type of soil (i.e., saturated lean clay, which so far have been rarely analyzed). This article presents a study to examine and explain the magnitude of strains developed in loading and unloading curves of PBP to predict the ratio of compression to unloading strain (εc/εu) and Su from unloading curves, for which open field experimentation is conducted on saturated clay deposits with a plasticity index of 10 to 16 %. The results of εc/εu and Su that were determined from the undrained PBP tests were compared with anisotropically consolidated undrained triaxial compression tests, which showed reasonable agreement. Correlations are proposed for net limit pressure versus εc/εu and Su from PBP, which offer a new approach to quantifying εu and Su by solely performing compression curves. The proposed correlations are validated by performing PBP tests at two separate sites and are further supported by comparison with the pressuremeter curves of previous studies. The variation in experimental versus predicted values for two separate sites (using the proposed correlation) is within ±10 % for a confidence interval of 95 %.

Maximising the efficiency of Menard pressuremeter testing in cohesive materials by a cookie-cutter drilling technique

Menard pressuremeter testing has been widely used in geotechnical engineering applications for 40 years and is an important technique in determining in-situ horizontal stress distributions. In this study, Menard pressuremeter testing is combined with a “cookie-cutter” insertion technique to determine horizontal stresses for a soft-rock in an operational Australian mine. The method presents an alternative to the Self-Bored Pressuremeter, with cookie-cutter drill rods allowing for sample recovery and further laboratory testing. The method accommodates for the presence of gravel and hard layered materials that present a risk of damage to cutting shoes of Self-Bored Pressuremeter devices. The combination of a sonic drill rig, coupled with the cookie cutter rods produces a close tolerance pocket resulting in “pseudo self boring pressuremeter tests”. The undrained shear strength, unload-reload shear modulus and in-situ horizontal stress are presented from pressuremeter tests conducted in the region for the first time. The undrained shear strength was observed in the range of 0.47–0.57 MPa, the unload-reload shear modulus between 17.43 and 18.25 MPa, the lift-off pressure in the range of 0.35–0.61 MPa. The K0 of coal was equal to 1, with interseam materials ranging from 2.1 to 3.5. Results of the cookie-cutter insertion method are compared with conventional drilling methods, with the cookie-cutter insertion test providing results in good agreement with both advanced triaxial laboratory tests and FEM numerical analysis. Cookie-cutter pressuremeter tests were conducted on cohesive soils at Australia's second-largest open-pit mine, with pressuremeter test results presented for Victorian brown coal for the first time.

A pressuremeter-based evaluation of structure in London Clay using a kinematic hardening constitutive model

The self-boring pressuremeter (SBP) test was designed to measure in situ engineering properties of the ground with a relatively small amount of disturbance. The properties that may be inferred from the test depend on the mechanical model used for its interpretation and on the significance given to other previously available information. In this paper, numerical modelling using the advanced kinematic hardening structure model (KHSM) for natural soils has been performed to investigate the influence of the initial structure and the degradation of structure on the SBP cavity pore pressures and expansion curves within London Clay. The validation of the KHSM against well-known analytical solutions and the calibration procedure used to identify the material parameters are presented. The numerical analyses reveal that the simulations of the SBP tests using the KHSM model provide a very close match of the expansion curves to the experimental data, but underestimate the pore pressures at the initial stage of the SBP expansion test. A parametric study has been carried out to determine the effects induced by the parameters of the destructuration model along with the disturbance experienced during the SBP installation, which is difficult to estimate in situ. Two disturbance scenarios were considered where the initial structure was assumed to vary linearly across an area close to the wall of the cavity. These simulations indicate that accounting for installation disturbance leads to a substantial improvement in pore pressure predictions for the SBP.

An optimization strategy for evaluating modified Cam clay parameters using self-boring pressuremeter test data

This paper presents an efficient, practical, and automated strategy for deriving modified Cam clay parameters from undrained self-boring pressuremeter (SBPM) data. A mixed approach involving a parametric sweep and numerical optimization is used, with a focus on parameter groups that dominate the modified Cam clay response in undrained cavity expansion. The proposed technique is illustrated using data from SBPM tests carried out in soft estuarine clay. The resulting parameters are used to back-analyse large-scale foundation load tests and are shown to provide an excellent match to the measured foundation response.

Stiffness parameters from cone pressuremeter tests at Changi East, Singapore

A large-scale land reclamation project was carried out in the eastern part of Singapore involving 2000 hectares of land at a foreshore location from 1991 to 2005. The land was reclaimed, by placing sand hydraulically on the seabed comprising soft marine clay. Prefabricated vertical drains with surcharge preloading and deep compaction techniques, were used to improve the soft marine clay and reclaimed sand fill. Several specialist in situ tests were conducted during the project, such as with the cone penetration test (CPT), cone pressuremeter test (CPMT), dilatometer test and self-boring pressuremeter tests. The CPMT is an in situ testing method used to measure both the soil strength and stiffness parameters. The CPMT combines the advantages of a conventional pressuremeter test and a CPT. The CPT provides a continuous profile of the soil by measuring the stress on the cone tip, the sleeve friction and the pore-water pressure. The CPMT enables the assessment of the soil strength and stiffness at selected depths. CPMT tests were conducted on both the reclaimed sand and underlying soft marine clay layers during the Changi East reclamation project.

Comparison of diaphragm wall movement prediction and field performance for different construction techniques

The demand for deep excavation for basement construction is growing rapidly in Bangkok due to the limited availability of land. Recently, the maximum depth of excavation has increased up to approximately −25 m. Deep excavation in Bangkok subsoils requires consideration of the soil conditions and the real piezometric level of ground water. In general, Bangkok subsoil consists of 13~16 m of thick, dark grey soft-to-medium clay, followed by stiff, silty clay, before the first sand layer at 21~30 m. The piezometric level of ground water has risen to −13 m from its previous level of −23 m due to the control of deep well pumping for twenty years by the Thai Government. A diaphragm wall is normally used as a soil retaining structure. The behavior and performance of a diaphragm wall is usually predicted by the finite element method (FEM) using Mohr–Coulomb soil modeling as failure criteria, with the soil modulus parameters for soft and stiff Bangkok clay from the self-boring pressuremeter test. This paper presents a comparison between the diaphragm wall movements from FEM analysis predictions and field measurements for two projects, the new Bank of Thailand head office and the Rosewood Hotel, which were constructed with different techniques. The predicted movement agreed well with the field measurements.

Halden research site: geotechnical characterization of a post glacial silt

This paper describes the geology and geotechnical engineering properties of the Halden silt; a 10–12 m thick deposit of fjord-marine, low plasticity clayey silt. Over the last six years, the test site has been well characterized by combining the results from a number of geophysical and in situ tools, including; electrical resistivity tomography, multi-channel analysis of surface wave surveys, cone penetration testing, dissipation testing, in situ pore pressure measurements, seismic flat dilatometer testing, field vane testing, self-boring pressure meter testing, screw plate load testing and hydraulic fracture testing. The results from these investigations assist the interpretation of layering and in situ soil properties. Soil sampling and advanced laboratory testing have provided data for interpretation of geological setting and depositional history, soil fabric, strength, stiffness and hydraulic properties. However, interpretation of the stress history, based on oedometer tests and clay-based correlations to the cone penetration test, are unreliable. They contradict the depositional history, which suggests that the soil units at the site are near normally consolidated, except for some surface weathering and desiccation. Further, undrained shear strength interpretations are complex as the in situ tests are potentially influenced by partial drainage, and conventional undrained triaxial tests do not provide a unique (peak) undrained shear strength. Despite certain interpretation challenges the paper presents important reference data to assist in the interpretation and assessment of similar silts, and provide some guidance on important geotechnical properties for projects where limited design parameters are available.

Characterization and engineering properties of the NGTS Onsøy soft clay site

In the 1960s, NGI investigated numerous potential research sites and established the first Onsoy soft clay site in 1968. In 2016, the NGTS project established five research sites, one of which is the NGTS Onsoy soft clay site. The purpose of this paper is to provide a reference for further in-situ and laboratory testing at the site and easy access to high-quality soil investigation data. The paper interprets the soil conditions providing engineering soil properties, comments on data reliability and quality, and correlations between different soil parameters where appropriate. The lightly overconsolidated soft clay site has been characterized based on extensive in-situ and laboratory testing. In-situ methods include electrical resistivity tomography, multichannel analysis of surface waves, total soundings, rotary pressure soundings, cone penetration testing with and without seismic and electrical resistivity measurements, dilatometer with seismic measurements, self-boring pressuremeter, electric piezometers, thermistor string, hydraulic fracture testing and earth pressure cells. Soil samples were obtained using Sherbrook block sampler, Geonor ϕ 72 mm piston sampler, Geonor ϕ 54 mm composite piston sampler and hydraulic piston and push samplers with liners. Laboratory tests include multi-sensor core logging, index testing, cyclic and monotonic, consolidated and unconsolidated triaxial tests, bender element tests, oedometer tests with constant rate of strain and incremental loading and direct simple shear tests.

A proposed method to determine in-situ shear modulus and shear strain decay curves in different structured soil

This paper illustrates the application of the self-boring pressuremeter test and the seismic dilatometer test to acquire the in-situ decay curves of stiffness with shear strain level (G-γ decay curves) of three types of structural soil, which are granite residual soil, structural soft soil and expansive soft rock. The proposed approach in combines the functions of SBPT and SDMT to provide the high standard of accuracy for the small-strain stiffness (from SDMT) and the major attenuation stage of stiffness (from SBPT). Using the proposed mathematical model can properly describe the tendency in typical in-situ G-γ decay curves based on the data of tests. To analyse the suitability of the proposed approach, the G-γ curve obtain from the resonant column test of granite residual soil is also employed to compare with the in-situ curves. The shear modulus G obtained from laboratory tests is found to be smaller and the stiffness attenuation rate is found to be faster than the curve of the in-situ test, which reflects the process of sampling, transporting and preparation of soil samples could cause unrecoverable damages in soil.