Pore Pressure Measurements Research Articles

An integrated approach to study of strata behaviour and gas flow dynamics and its application

This paper presents an advanced and integrated research approach to longwall mining-induced strata movement, stress changes, fractures, and gas flow dynamics with actual examples of its application from recent studies for co-extraction of coal and methane development at Huainan Mining Group in China, in a deep and multi-seam mining environment. The advanced approach takes advantage of the latest techniques in Australia for mine scale geotechnical characterisation, field measurement, monitoring and numerical modelling. Key techniques described in this paper include coal mine site 3D geotechnical characterisation methods, surface deep downhole multi-point extensometers and piezometers for overburden displacement and pore pressure measurements during mining, tracer gas tests for goaf gas flow patterns, and advanced numerical modelling codes for coupled coal mine strata, water and gas simulations, and longwall goaf gas flow investigations. This integrated approach has resulted in significant insights into the complex dynamic interaction between strata, groundwater, and gas during mining at Huainan Mining Group in recent years. Based on the findings from the extensive field monitoring and numerical modelling studies, a three-dimensional annular-shaped overlying zone along the perimeter of the longwall panel was identified for optimal methane drainage during mining.

Using piezocone dissipation test to estimate the undrained shear strength in cohesive soil

This paper describes a method developed to link the measured piezocone dissipation excess pore-water pressure (Δu) to the soil undrained shear strength (su). In cohesive soils, both Δu and suare dependent on the same variables (compressibility, stress state, stress history), which allows them to be related by the theoretical cavity expansion – critical state framework. A mathematical derivation is presented to demonstrate that the ratio of normalized maximum excess pore pressure and the normalized undrained shear strength fluctuates around a mean value, being affected by soil strength and compressibility and independent of stress history. The predicted suvalues obtained from the proposed approach are calibrated against field vane shear strength in both normally consolidated (monotonic dissipation tests) and overconsolidated soils (dilatory dissipation tests). Reported results are consistent and encourage the use of the method in engineering practice. On a routine basis, suestimated from pore pressure can be compared with values predicted from penetration resistance, adding desired redundancy for purposes of error detection when interpreting cone penetration testing with pore pressure measurement (CPTU) data.

Bulk modulus dispersion and attenuation in sandstones

We report experimental data on the frequency dependence of bulk elastic modulus in porous sandstones. A new methodology was developed to investigate the dispersion/attenuation phenomena on a rock’s bulk modulus [Formula: see text] for varying confining pressures in the range of 1–50 MPa and fluids of varying viscosities (i.e., air, glycerin, and water). This methodology combined (1) ultrasonic (i.e., [Formula: see text]) P- and S-wave velocity measurements, leading to the high-frequency (HF) [Formula: see text], (2) stress-strain measurements from forced periodic oscillations of confining pressure at low-frequency (LF) ranges (i.e., [Formula: see text]), leading to [Formula: see text] and [Formula: see text], and (3) pore-pressure measurement to document the induced fluid-flow in the LF range (i.e., [Formula: see text]). The stress-strain method was first checked using three standard samples: glass, gypsum, and Plexiglas samples. Over the frequency and pressure range of the apparatus [Formula: see text] was stable and accurate and the lowest measurable LF attenuation was [Formula: see text]. The methodology was applied to investigate Fontainebleau sandstone samples of 7% and 9% porosity. The [Formula: see text] and [Formula: see text] exhibited correlated variations, which also correlated with an experimental evidence of frequency-dependent fluid-flow out of the sample. Attenuation peaks as high as [Formula: see text] and [Formula: see text] are measured. The attenuation/dispersion measured under glycerin saturation was compared to Biot-Gassmann predictions. The overall behavior of one sample was consistent with a dispersion/attenuation characteristic of the drained/undrained transition. On the reverse, the other sample exhibited exotic behaviors as the measurements were underestimated by the drained/undrained transition and indicated a direct transition from drained to unrelaxed domain. These different behaviors were consistent with the values of the critical frequencies expected for the drained/undrained (i.e., [Formula: see text]) and relaxed/unrelaxed (i.e., [Formula: see text]) transitions.

Comparative performance of the international piezocone and China CPT in Jiangsu Quaternary clays of China

The cone penetration test (CPT) and piezocone penetration test (CPTU) with measurement of pore pressure are one of the more powerful site investigation tools especially for soft clay. Most of the correlations with geotechnical data were developed based on 10cm2 international CPT and CPTU results, but in China there are 15cm2 and 20cm2 CPT devices. However, little has been reported on the effects of size of device, both diameter and area of friction sleeve, on the measured results. Firstly, the difference between the Chinese CPT and the international modern multifunctional digital CPT techniques are compared. Results are presented comparing various sizes and configurations of China double bridge CPT and international CPTU in Jiangsu clays of China. According to the CPT and CPTU tests at five sites, parameters measured by the 10cm2 CPTU are presented and compared to China double bridge 15cm2 CPT equipments. The effects of the increased penetration rate on the measured qt and fs resistances showed some overall decrease while the pore pressures were significantly increased. Cone tip resistance qc or qt and sleeve friction fs measurements were similar for 10 and 15cm2 penetrometers at the standard rate of 20mm/s. The findings give us the added confidence to the use of 10cm2 international piezocone penetrometers in China geotechnical engineering practice.

Determination of the continuous stress-dependent permeability, compressibility and poroelasticity of shale

The continuous changes of the stress-dependent permeability, compressibility and poroelasticity of a tight oil and gas shale were characterized by using the Constant Rate of Strain (CRS) consolidation test. The CRS consolidation test compresses a thin disk test specimen at a given constant rate of strain under one-dimensional consolidation with one-sided drainage condition and measurement of excess pore pressure at the undrained end. Permeability is calculated from the one-dimensional consolidation equation assuming oedometric loading, incompressible solid grains, and an idealized excess pore pressure distribution in the sample. The CRS test method has been widely used for the determination of the stress-dependent permeability of soft sediments, but has not been utilized for very stiff and low permeability shale as well as to obtain their stress-dependent compressibility and poroelasticity parameters. To test its appropriateness to shale, CRS tests were performed on thin disk samples of Mancos shale using a high pressure and high temperature triaxial cell under high isotropic confining stresses. Two modifications were done to make the CRS test applicable to determination of the continuous stress-dependent hydro-mechanical properties of tight shale: (1) the test method was modified for isotropic loading, and (2) nonlinear poroelastic effects were accounted for in the solution of the pore pressure dissipation equation. The permeability values from the CRS tests with the modified analytical solution were found to be in agreement with those obtained from the Constant Pressure Gradient Permeability test, and Pressure-pulse Decay Permeability test using nitrogen as pore fluid and corrected for Klinkenberg and non-Darcy flow effects.

A piezocone dissipation test interpretation method for hydraulic conductivity of soft clays

An alternative approach is developed in order to estimate the hydraulic conductivity of soft fine grained soils, based on numerical simulation of the full penetration and dissipation process for piezocones. Unlike previous methods of analysis, the process of penetration and dissipation has been explicitly simulated, thus eliminating several of the simplifications inherent in existing interpretation methods such as geometric approximations, predefined stress fields or neglecting material compressibility. The presented method is not established upon a particular set of data leading to limited applicability, but is rather developed using a more general approach and can be extended to other datasets if intended. Given the time to 50% consolidation and a number of influencing soil parameters, a single estimate of the soil horizontal permeability can be obtained via a single-run piezocone sounding using pore pressure measurements taken at the shoulder filter element (u2) located immediately behind the cone.The proposed interpretation method embodies many of the key parameters (namely the soil shear strength, soil rigidity, and soil confining stresses) likely to influence the soil behaviour and thus the parameter to be interpreted. Numerical analyses demonstrated that the rate of dissipation increases as the soil rigidity or the soil confining pressure increases, which is a consequence of higher excess pore pressure gradient at higher depths or at larger rigidities. The method, which involves a new excess pore pressure normalisation technique, is applicable to both monotonic and dilative dissipation data. The proposed interpretation method is compared to a series of experimental data including two recent field tests. Although the method was calibrated against only a select few cases, its applicability to a wide range of clayey soils was verified.

The ability of rock physics models to infer marine in situ pore pressure

AbstractPore fluid pressure is an important parameter defining the mechanical strength of marine sediments. Obtaining high spatial resolution in situ pore pressure measurements in marine sediments, however, is a challenge, and as a result, only a handful of in situ pore pressure measurements exist at scientific drill sites. Integrating rock physics models with standard IODP/ODP measurements provides a potentially widely applicable approach for calculating in situ pore pressure. Here we use a rock physics approach to estimate in situ pore pressure at two Scientific Ocean Drill Sites where in situ pressure is well constrained: ODP Site 1173, used as reference for normal (hydrostatic) fluid pressures, and ODP Site 948, where previous studies infer high fluid pressures (λ* ∼ 0.45–0.95, where the pore pressure ratio λ* is defined as the pore pressure above hydrostatic divided by the difference between the largest principal stress and hydrostatic stress). Our analysis indicates that the rock physics method provides an accurate, low‐precision method for estimating in situ pore pressure at these drill sites, and sensitivity analysis indicates this method can detect modestly high (λ* > 0.6) pore pressure at the 95% confidence level. This approach has broad applicability because it provides an inexpensive, high‐resolution (meter‐scale) method for retrospectively detecting and quantifying high pore pressure at any drill site where quality wireline logs and ocean drilling data exist.

A new geotechnical method for natural slope exploration and analysis

Geotechnical investigation of natural slopes is challengeable especially when natural slopes having higher gradients and access is difficult. Also, it is even more prob- lematic to find the shear strength parameters spatially to evaluate the stability of slopes as most of the methods available to find the shear strength parameters in the literature are uneconomical or such methods cannot be applied in vegetated slopes. Recently, authors have conducted a series of in situ investigations based on the newly developed lightweight dynamic cone penetrometer to examine its applicability in analyzing the slopes covered with weathering remnants of decomposed granite. Six patterns were identified based on the penetration resistance varies with the depth. Spatial variability analysis conducted on different grid spaces showed that the coefficient of variation of cone resistance varies from 0 to 35 %. Semi-variogram analysis showed that the Spherical Models can be used to evaluate the spatial variability of weathering remnants of decomposed granite. A series of laboratory calibration tests based on the lightweight dynamic cone penetration tests and direct shear tests with pore pressure measurements were conducted at different void ratios and degrees of saturation. Based on the laboratory calibration test results, a method to determine the void ratio, e, from the data of qd was presented. Based on this, two formulas to evaluate the shear strength parameters, apparent cohesion and friction angle, were established with the cone resistance and degree of saturation. Slight modification was proposed in evaluating the apparent cohesion with respect to the different fine content in the soils. As a whole, the proposed method can be successfully applied to individual slopes to determine the profile thickness and to evaluate the shear strength parameters spatially. Based on this, hazard assessment of individual slopes can be made.

Spatial correlation length of normalized cone data in sand: case study in the north of Denmark

The main topic of this study is to assess the anisotropic spatial correlation lengths of a sand layer deposit based on cone penetration testing with pore pressure measurement (CPTu) data. Spatial correlation length can be an important factor in reliability analysis of geotechnical systems, yet it is rarely estimated during routine site investigations. Results from two different sites in the north of Denmark are reported in this paper, indicating quite strong anisotropy due to the depositional process, with significantly shorter spatial correlation lengths in the vertical direction. It is observed that the normalized cone resistance is a better estimator of spatial trends than the normalized friction ratio.

Performance of cyclic cone penetration tests in chalk

This paper presents results from a preliminary testing campaign to explore the feasibility and benefits of performing cyclic cone penetration tests with measurement of pore pressure (CPTu) in chalk. The CPTu's cone is a miniature driven pile and this technique permits the monitoring of degradation of sleeve friction and pore pressure development during cyclic loading. It was found that while consistent degradation of measured sleeve friction takes place within a few cycles, this was not associated with an observed pore pressure build up. However, uncertainties about the pore pressure measurements arose because the remoulded chalk next to the cone was probably unsaturated and also the pore pressure transducer was positioned close to the tip of the cone. Analysis of the results suggests that the technique may have promising potential to increase understanding of the underlying mechanisms and aid the design of axially loaded driven piles in chalk. Nevertheless further validation and development would be necessary in the future.

Soil Liquefaction–Induced Uplift of Underground Structures: Physical and Numerical Modeling

AbstractUnderground structures located in liquefiable soil deposits are susceptible to floatation following a major earthquake event. Such failure phenomenon generally occurs when the soil liquefies and loses its shear resistance against the uplift force from the buoyancy of the underground structure. Numerical modeling accompanied with centrifuge experiments with shallow circular structures has been carried out to investigate the floatation failure at different buried depths of the structure. The influence of the magnitude of input sinusoidal earthquake shaking was also studied. Both numerical and experimental results showed matching uplift response of the structures and acceleration and pore-pressure measurements in the liquefied soil deposit. A higher uplift displacement of the structure was observed for shallower buried depth, thereby indicating the influence of overlying soil weight against floatation. Results also showed that the structures commenced floatation in the presence of high excess pore pr...

Estimating the hydraulic properties of an aquitard from in situ pore pressure measurements

A workflow is described to estimate specific storage (Ss) and hydraulic conductivity (K) from a profile of vibrating wire piezometers embedded into a regional aquitard in Australia. The loading efficiency, compressibility and Ss were estimated from pore pressure response to atmospheric pressure changes, and K was estimated from the earliest part of the measurement record following grouting. Results indicate that Ss and K were, respectively, 8.8 × 10−6 to 1.2 × 10−5 m−1 and 2 × 10−12 m s−1 for a claystone/siltstone, and 4.3 × 10−6 to 9.6 × 10−6 m−1 and 1 × 10−12 to 5 × 10−12 m s−1 for a thick mudstone. K estimates from the pore pressure response are within one order of magnitude when compared to direct measurement in a laboratory and inverse modelled flux rates determined from natural tracer profiles. Further analysis of the evolution and longevity of the properties of borehole grout (e.g. thermal and chemical effects) may help refine the estimation of formation hydraulic properties using this workflow. However, the convergence of K values illustrates the benefit of multiple lines of evidence to support aquitard characterization. An additional benefit of in situ pore pressure measurement is the generation of long-term data to constrain groundwater flow models, which provides a link between laboratory scale data and the formation scale.

Undrained Pore Pressure Prediction in Clayey Soil under Cyclic Loading

AbstractPrediction of pore-water pressure is important for understanding the behavior of soils under both static and cyclic loading and also for estimation of their effective stresses. Undrained cyclic pore pressure models available in the literature are for some typical soils and are hardly applicable for other soils. In this paper, a generalized hyperbolic model for prediction of undrained pore-water pressure has been proposed as a function of cyclic stress ratio, frequency, and plasticity of soils using data reported by a number of researchers. The proposed model has been validated by a chi-square goodness-of-fit test. The correlation between the predicted and measured pore pressure ratio at various confining pressure regimes has also been statistically examined to discern the model. The model output has been found to be correlated to an extent of up to 85% with the literature data.

Pressure-induced forces and shear stresses on rubble mound breakwater armour layers in regular waves

This paper presents the results from an experimental investigation of the pressure-induced forces in the core material below the main armour layer and shear stresses on the armour layer for a porous breakwater structure. Two parallel experiments were performed which both involved pore pressure measurements in the core material: (1) core material with an idealized armour layer made out of spherical objects that also allowed for detailed velocity measurements between and above the armour, and (2) core material with real rock armour stones. The same core material was applied through the entire structure i.e. no additional filter layers were applied. For both experiments, high-speed video recordings were synchronised with the pressure measurements for a detailed investigation of the coupling between the run-up and run-down flow processes and the measured pressure variations. Outward directed pressure gradients were found which exerted a lift force up to ≈60% of the submerged weight of the core material. These maximum outward directed pressure gradients were linked to the maximum run-down event and were in general situated at, or slightly below, the maximum run-down level. Detailed velocity and turbulence measurements showed that the large outward directed pressure gradients in general coincide, both in time and space, with the maximum bed-shear stresses on the armour layer based on the Reynolds-stresses. The bed-shear stresses were found to result in a Shields parameter in the same order of magnitude as the critical value for movement of the armour stones.

Dynamic Response of a Model Levee on Sherman Island Peat: A Curated Data Set

A model levee resting atop soft compressible peaty organic soil in the Sacramento/San Joaquin Delta was shaken by forced vibration to study the seismic deformation potential of the underlying peat and measure dynamic levee–peat interaction. Forced vibration testing occurred over a frequency range of 0 Hz to 5 Hz and produced force amplitudes applied to the embankment crest that induced elastic to nonlinear levee-foundation responses. Available data include acceleration records from sensors mounted on the model levee and on the ground surface near the model levee, and acceleration and pore pressure measurements from sensors embedded in the underlying peat. A remote data acquisition system measured settlements and pore pressures over a span of more than a year, encompassing time before and after the dynamic testing. Small pore pressures were generated in the peat during testing although embankment settlements from cyclic loading were small.

On the integrated use of geophysics for quick-clay mapping: The Hvittingfoss case study, Norway

Abstract Quick-clay landslides are a known hazard in formerly glaciated coastal areas; hence, large efforts are devoted to map the distribution of quick clays. In this paper, we focus on one particular Norwegian site (Hvittingfoss, 80 km south-west of Oslo), which was remediated against potential landsliding in 2008. A set of geophysical methods including Electrical Resistivity Tomography, P-wave seismic refraction tomography, S-wave seismic reflection profiling, and Ground Penetrating Radar were jointly analysed and complemented with laboratory data and in situ geotechnical measurements (i.e., seismic and resistivity Cone Penetration Testing with pore pressure measurement) in order to improve our geological understanding of the site and to establish a suitable, integrated and multi-disciplinary approach to better map the special extent of the quick-clay zone. The integration of the different geophysical methods and geotechnical measurements allow a more precise imaging and characterisation, both spatially and vertically, of the sedimentary sequences and of the underlying bedrock. The resulting geological model is then populated with the quantitative parameters derived from the geophysical measurements. Considering the inherent complexity of quick-clay mapping, the collected data illustrate the benefit of an integrated approach, and emphasise the need for high resolution, proper imaging, calibration and ultimately joint inversion of the different data.

Predrill prediction of subsalt pore pressure from seismic impedance

A target-oriented, predrill, pore-pressure prediction method using seismically estimated acoustic impedance is applied to recently acquired and processed dual-coil data in the Green Canyon and Walker Ridge areas of the Gulf of Mexico. Three deepwater subsalt wells are used in the study. One was used as a calibration well, and the other two were used as blind wells for comparing prediction results with the measured pore-pressure and mud-weight data. All three wells are in the Green Canyon area, penetrating thick salt bodies. A new seismic-inversion method is used for inversion of seismic impedance. The pore-pressure method uses a direct transform of the inverse of acoustic impedance, with two adjustable parameters. The optimization of the parameters is done through an iterative process to match the pore-pressure gradient obtained from the well acoustic impedance with those of pore-pressure measurements and mud weights within a tolerable range of those data for the calibration well. The optimized parameters are then used to transform the seismic acoustic-impedance volume to pore-pressure gradient volume. The predicted pore pressure at the blind wells from the well impedance and seismic impedance match reasonably with well data.

Swash zone moisture dynamics and unsaturated infiltration in two sandy beach aquifers

The intertidal zone of coastal aquifers is a dynamic region of mixing between saline surface water and fresh groundwater. Groundwater circulation in this zone is affected by complex forcing mechanisms that operate on a range of timescales and can regulate chemical fluxes to marine environments. We evaluated wave swash-induced infiltration and associated flow dynamics in the unsaturated region of two sandy beach aquifers with differing wave conditions and beach morphologies. Moisture and pressure sensors were used to measure fluctuations in water content, water table elevation, and hydraulic gradients at high frequencies (5 Hz) in the swash zone at mean lower low water, mean sea level, and mean higher high water. Water content in the unsaturated region of the swash zone responded to wave overtopping and swash infiltration, with a rapid rise in water content followed by a slower decline. Swash-induced unsaturated infiltration rates, calculated from water content response, were lowest near low tide and increased up the beachface with inflow highest near high tide at both sites, consistent with an increase in water table depth up the beach. Unsaturated infiltration was 1.6 m3/m per tidal cycle at the wave-dominated beach and 0.4 m3/m per tidal cycle at the tide-dominated beach. Saturated pore pressure measurements show that a water table mound formed as a consequence of swash infiltration that migrated up the beach during rising tide, leading to divergent seaward/landward groundwater flow. The results demonstrate the significant and spatially variable effects of wave swash on moisture dynamics in the unsaturated zone of beach aquifers, and show that these effects depend on wave conditions and beach characteristics. Results have implications for understanding transport and reaction of solutes in this biogeochemically active zone.

Stress History Estimation Method of Underconsolidated Soil by Partial Piezocone Dissipation Tests

The paper discusses a new method to put forward to determine the initial pore pressure by extrapolating the last segment of measured pore pressure versus the inverse square root of time scale through incomplete pore pressure dissipation test. For underconsolidated soil, the estimated initial pore pressure is greater than the hydrostatic pore pressure. With the calculation of the initial pore pressure, the status of the consolidation of underconsolidated soil can be evaluated by calculating the apparent degree of consolidation which is defined as excess pore pressure generated by piezocone penetration divided by the difference between the total pore pressure measured by piezocone and in situ hydrostatic pore pressure. The apparent degree of consolidation is less than one as the soil is underconsolidated. The Northern Expressway Connection project of Chongqi Bridge is introduced as an example of practical application. In this case, the studied area is slightly underconsolidated, which is consistent with the results of the laboratory oedometer tests. Finally, compared with overconsolidation ratio (OCR) values from the oedometer tests, a new formula to estimate the OCR of underconsolidated soil using the apparent degree of consolidation was presented. It indicates that the OCR of underconsolidated soil can be proposed directly from partial piezocone dissipation tests.

In situ dynamic piezocone penetrometer tests in natural clayey soils — a reappraisal of strain-rate corrections

Cone penetration testing with pore pressure measurement (CPTU) is a cost- and time-efficient way of collecting in situ geotechnical parameters of near-surface marine soils for cable and pipeline tracks, offshore foundations, and geohazard identification. The measured dynamic CPTU parameters (cone penetration resistance, sleeve friction, pore pressure) are higher than the measured static CPTU parameters. This mismatch is caused by the different penetration rates used for dynamic and static CPTU tests (dynamic tests have up to 500 times higher penetration rates than the static tests; i.e., with the commonly used 2 cm/s penetration rate). This study presents comprehensive Calypso piston and gravity core as well as dynamic and static CPTU datasets acquired in the landslide-prone Sørfjorden area (Finneidfjord, northern Norway). The fjord-marine sediments at the study site are characterized as normally consolidated to slightly over consolidated clay-dominated soils with embedded layers of sandy silt to sand. The dynamic CPTU results were corrected to match the nearby static CPTU (i.e., distance less than 10 m) using strain-rate factors (SF) derived from three known correction methods. Based on a statistical test and visual comparison of dynamic and static CPTU profiles, the modified inverse sin-hyperbolic correction method is found to be best suited for the strain-rate correction of dynamic CPTU tests, and results in SF less than 1.35 for the corrected cone penetration resistance and up to 2.4 for the sleeve friction. Our data illustrate a positive correlation between penetration rate and penetration depth, which is governed by the consolidation state of the clays. The good agreement between SF-corrected dynamic CPTU data from 34 deployments (acquired within less than 36 h shiptime) with data obtained from static CPTU and laboratory experiments on sediment cores further demonstrates that the MARUM dynamic CPTU device is a powerful tool for characterizing the properties of surficial seafloor sediments in shallow-water environments.