Pore Pressure Dissipation Tests Research Articles

Porous stones in permeability measurement: drawbacks and solution

Porous stones are commonly used in geotechnical laboratory testing to provide solid support at the two ends of the tested specimen. Although porous stones can have several advantages, they can induce a hydraulic impedance that might alter the flowrate results. Numerous suggestions and recommendations were issued to avoid some of the problems encountered when using porous stones, such as clogging and stones' low permeability coefficient ( Kp). However, it is proven in this technical note that the existence of a soil–porous stones interaction prevents the elimination of porous stones' influence, leading to unreliable permeability results. A practical solution based on manufacturing unique porous stones (MS) is suggested in an attempt to eliminate such an influence. The porous stones' influence is highlighted by running excess pore pressure dissipation tests under triaxial conditions with calibrated beads. The results show a soil–porous stones interaction and a flowrate increase up to 700% due to MS. This interaction is addressed through permeability measurements of three soil types in a flexible-wall permeameter using standard porous stones (SS) and the newly manufactured stones (MS). At least one order of magnitude of difference in the soil permeability measurement and a decrease in the dissipation test time are observed with MS. This technical note also validates the limitation of the superposition approach for correcting the soil's permeability ( Ksoil). It is shown that porous stones can induce a signature directly related to the soil type and applied pressure.

The Application of Piezocone Penetration Test (CPTU) in Water Area Hydrogeological Investigation

Piezocone penetration test device imported from the Netherlands was used to conduct pore pressure dissipation tests at 4 measuring points in 2 holes in the water area of Sutong GIL utility tunnel project. After dissipation, stable pore pressure was used to analyse groundwater pressure and further obtain the height of groundwater head. This method provides a successful solution for in-situ stratification testing of groundwater parameters in water area engineering, and provides reliable hydrologic parameters such as groundwater pressure for structure design and construction

Evaluating the sealing potential of young and thin mass-transport deposits: Lake Villarrica, Chile

AbstractSubaqueous mass-transport deposits (MTDs) can be important elements in hydrocarbon systems, forming potential reservoirs or seals. Most research has targeted outcrops or moderately to deeply buried MTDs and, therefore, the petrophysical properties of near-seafloor MTDs, and their influence in the trapping and release of shallow fluids, is poorly studied. Here, we investigate shallow MTDs in Lake Villarrica (Chile) by combining sub-bottom profiles, free-fall penetrometer data, pore pressure dissipation tests and geotechnical properties of sediment cores. Low undrained shear strength under a surficial MTD indicates underconsolidation caused by sudden loading and rapid sealing. Larger, buried MTDs show acoustic signatures of free gas at their base, indicating effective sealing. This is supported by degassing core gaps just below MTDs and by excess pore pressure ratios c. 30–70% within MTDs. Acoustic windows below rafted blocks suggest local fluid escape. MTDs exhibit elevated undrained shear strength and reduced porosity compared to surrounding sediments, but are comparable to upslope source sequences. This suggests that MTD sealing capacity in Villarrica relates to the apparently overconsolidated nature of the slope sequence, leaving a minor role for shear densification. This study shows that shallow MTDs can form a relatively rapid seal for fluid migration, locally degraded by rafted blocks.

The influence of overpressure and focused fluid flow on subaquatic slope stability in a formerly glaciated basin: Lake Villarrica (South-Central Chile)

Pore fluid pressure is a key parameter for subaquatic slope stability and has been put forward to explain the development of many submarine landslides, especially when occurring on very gentle slopes or in areas with high sedimentation rate. Due to the sparse availability of in-situ pore pressure data, a profound understanding and quantification of excess pore pressure development and its influence on subaquatic landslide processes is still missing. In this study, we use glacigenic Lake Villarrica (Chile) as a model basin, in which we document in-situ excess pore pressure, focused fluid escape features and subaquatic slope failures at relatively shallow subsurface depth (<10m). A subaquatic slope was characterized in great detail by a dense network of high-resolution seismic-reflection profiles, multibeam bathymetry and sediment cores. In-situ undrained shear strength and formation pore fluid pressures were documented using free-fall piezocone penetrometer (CPTU) and subsequent pore pressure dissipation tests. We show four independent lines of evidence for overpressure of variable magnitude within the sedimentary slopes: i) pockmarks and other fluid escape features on seismic profiles and bathymetric maps, ii) estimation of ambient in-situ pore pressure via CPTU dissipation tests, iii) underconsolidation and downward decrease in in-situ undrained shear strength within units of uniform lithology, and iv) hydrofractured glacio-lacustrine sediments. Locally, overpressure reaches about 80–90% of the hydrostatic vertical effective stress. We hypothesize that glacier-proximal sediments are strongly underconsolidated and overpressured due to rapid deposition and/or subglacial meltwater pumping. Postglacial consolidation of a thick glacier-proximal unit leads to the upward expulsion of excess pore water. A permeability barrier is formed by the overlying glacio-lacustrine clays and fine silts, focusing these migrating fluids to topographic highs. There, very high overpressure ratios facilitate the development of hydro-fractures, repeated fluid escape and pockmarks on the lake bottom. Overpressure may decrease slope stability in Lake Villarrica in different manners: by focused fluid escape weakening sediments, by generating underconsolidated sequences of low undrained shear strength, and by facilitating static failure. Based on high-quality in-situ data, this study confirms that high overpressure ratios in shallow sediments may be a typical feature at formerly-glaciated marine and lacustrine environments and highlights the major role of pore fluid overpressure for preconditioning subaquatic slopes to failure.

The use of neural networks for the prediction of cone penetration resistance of silty sands

In this study, an artificial neural network (ANN) model was developed to predict the cone penetration resistance of silty sands. To achieve this, the data sets reported by Ecemis and Karaman, including the results of three high-quality field tests, namely piezocone penetration test, pore pressure dissipation tests, and direct push permeability tests performed at 20 different locations on the northern coast of the Izmir Gulf in Turkey, have been used in the development of the ANN model. The ANN model consisted of three input parameters (relative density, fines content, and horizontal coefficient of consolidation) and a single output parameter (normalized cone penetration resistance). The results obtained from the ANN model were compared with those obtained from the field tests. It is found that the ANN model is efficient in determining the cone penetration resistance of silty sands and yields cone penetration resistance values that are very close to those obtained from the field tests. Additionally, several performance indices such as the determination coefficient, variance account for, mean absolute error, root mean square error, and scaled percent error were computed to examine the performance of the ANN model developed. The performance level attained in the ANN model shows that the ANN model developed in this study can be employed for predicting cone penetration of silty sands quite efficiently.

Characterization of thick varved-clayey-silt deposits along the Delaware River by field and laboratory tests

The existence of thick compressible varved clayey silt deposits along the Delaware River presents one major obstacle for design and construction of structures or industrial facilities in this region. To understand the characteristics of this thick weak soil layer and then develop reliable soil parameters for future earthwork design and construction, a comprehensive soil testing program was carried out. The program consisted of both in situ tests [e.g., standard penetration test borings, piezocone penetration test (CPT) soundings, Marchetti flat dilatometer test soundings, seismic shear wave test soundings, CPT pore pressure dissipation tests, and field vane shear tests] and laboratory tests (e.g., soil index tests, consolidation tests, triaxial tests, and chemical tests). Additionally, a field surcharge test program with and without installation of wick drains was carried out to investigate the compressibility of the thick clayey silt deposit and the efficiency of wick drains to help accelerate consolidation. The field and laboratory test results showed that the varved clayey silt deposit was normally consolidated to slightly overconsolidated and moderately to highly compressible. Its physical properties were intermediate between those of Savannah muddy clay and Shanghai soft clay, while much closer to that of Shanghai soft clay. Due to its lamination structure in the horizontal direction, the horizontal consolidation coefficient of the varved clayey silts was significantly greater than that in the vertical direction. The measured undrained shear strength of the varved clay silt almost increased linearly with depth. The installation of wick drains in the thick clayey silt deposits accelerated primary consolidation, but showed no effect on the secondary consolidation under constant loading. The findings obtained from this study will be beneficial for future design and construction of earthworks along the Delaware River.

Assessment of soft clay ground improvement from SCPTU results

Some in situ seismic piezocone penetration tests (SCPTU) were performed in undisturbed soils and the inter-pile soils between T-shaped, bidirectional, soil–cement deep mixing columns at lacustrine clay sites to quantitatively assess the effects of ground improvement. The difference of cone resistance and sleeve friction of soils among the columns after cement column construction was analysed. From pore pressure dissipation tests, the consolidation characteristics of the inter-pile soils were evaluated. The effect of the deep mixing column on the behaviour of the inter-pile soils is discussed. Preconsolidation pressures were estimated using some methods based solely on net cone tip resistance, excess pore pressure, effective cone resistance and a multiple regression based on cone tip resistance and shear wave velocity. Cement columns increase the lateral stress locally within the zone of influence of the column. Soils initially appear to be normally consolidated to slightly overconsolidated, and a significant increase in preconsolidation pressure was noticed in clay layers after treatment.

Excess Pore Pressure Dissipation Characteristics of Different Soil by CPTU on Pearl River Delta

Through a lot of CPTU excess pore pressure dissipation tests on Pearl River Delta in China, excess pore pressure dissipation characteristics of different soil layer on Guangzhou-Zhuhai section of Beijing-Zhuhai Expressway and Taishan section of Guangdong West Coast Expressway has been analyzed. The dissipation time of 50% dissipation degree t50 of excess pore pressure dissipation curve by CPTU can be used as the auxiliary method to determine the type of soil, and through CPTU excess pore pressure dissipation tests, the t50 value of sand soil, silt, silty clay, clay soil and silt soil has been obtained; through comparison with the t50 value of different soil, the difference of sand soil, silt and clay soil can be roughly distinguished; the obvious boundary value between the t50 value of clay soil, silty clay and silt soil is not existed, so the t50 value can not be direct to determine the type of these clay type of soil. The achieved results can provide a research foundation for CPTU application research on Pearl River Delta in China.

Cyclic Pore Pressure Generation, Dissipation and Densification in Granular Mixes

Knowledge of cyclic load induced pore pressure generation, post-liquefaction dissipation and volumetric densification characteristics of sands, silty sands, and silts are important for the analysis of performance of loose saturated granular deposits in seismic areas. This article presents results from an experimental study of these characteristics for such soils containing 0 to 100% non-plastic silt. Pore pressure generation characteristics are studied using undrained cyclic triaxial tests. Pre- and post-liquefaction compressibility and coefficient of consolidation, and post-liquefaction volumetric densification characteristics are determined from consolidation data prior to cyclic tests and pore pressure dissipation tests following undrained cyclic tests. Effects of fines content on these characteristics compared to those of clean sands are examined in the context of intergranular void ratio and intergranular contact density concepts.

Coefficient of consolidation from piezocone measurements

Pore pressure dissipation tests were carried out employing two miniature piezocones, with areas of 1 cm2 and 5 cm2, in a bed of reconstituted kaolin. By using independently determined values of the coefficient of consolidation, the results were interpreted to derive appropriate values of rigidity index for use with the theory of Teh &amp; Houlsby. In accordance with the theory, different filter positions on a piezocone (tip, face and shoulder) were not found to require different values of rigidity index. However, the importance of allowing for initial load relaxation at the commencement of the dissipation test was demonstrated, and there appeared to be a systematic effect of piezocone size.

Experiments with a Miniature Piezocone in Thinly Layered Soil

Abstract A piezocone, with a cross-sectional area of 100 mm2 and a pore pressure filter either at its apex or at its shoulder, was driven at the standard rate of 20 mm/s into samples of thinly layered soil. The samples consisted of alternating layers of normally consolidated kaolin clay about 18 mm thick and layers of fine sand or silt about 2 mm thick enclosed in a 252-mm-diameter rigid-walled cell. The performance of the piezocone was evaluated with respect to the detection of the more permeable layers during penetration and the estimation of the coefficient of consolidation from pore pressure dissipation tests in a clay layer at the end of penetration. Control tests were performed on two clay samples without any sand or silt layers. Changes in the pore pressure response enabled the more permeable layers within the layered samples to be easily detected, but their spacing was too close for individual layers to be detected using the cone resistance profile. Generally, the detection obtained from pore pressures was better with the apex filter than with the shoulder filter. The permeable layers had a measurable influence on the dissipation test results, but their influence on the derived values of the coefficient of consolidation was never large by comparison with their effect on the overall consolidation behavior of the soil.

Performance of miniature piezocones in thinly layered soils

Two piezocones, with cross-sectional areas of 1 cm2 and 5 cm2 and a variety of pore pressure filters, were driven at the standard rate of 20 mm/s into models of thinly layered soil, and pore pressure dissipation tests were also performed. The models consisted of alternating layers of pre-consolidated kaolin clay and more permeable layers of silt or fine sand, enclosed in a 400 mm diameter cell with either a rigid or a flexible lateral boundary. Control tests were performed on a clay model without any sand or silt layers. The experiments demonstrate the feasibility of using miniature piezocones to detect individual permeable layers as thin as 2 mm in normally and lightly overconsolidated clay deposits, provided their vertical spacing is at least two cone diameters and a sufficiently fast data recording rate is used. However, the lateral extent of a permeable layer cannot be gauged from a single piezocone probe. Pore pressure dissipation tests in clay layers are not significantly affected by neighbouring permeable layers provided they are more than two cone diameters above or below the filter element. On the basis of the experimental results, it is suggested that miniature piezocones could be used to improve the investigation of thinly layered ground, as might be found in alluvial and estuarine areas.

Performance of miniature piezocones in thinly layered soils

Two piezocones, with cross-sectional areas of 1 cm2 and 5 cm2 and a variety of pore pressure filters, were driven at the standard rate of 20 mm/s into models of thinly layered soil, and pore pressure dissipation tests were also performed. The models consisted of alternating layers of pre-consolidated kaolin clay and more permeable layers of silt or fine sand, enclosed in a 400 mm diameter cell with either a rigid or a flexible lateral boundary. Control tests were performed on a clay model without any sand or silt layers. The experiments demonstrate the feasibility of using miniature piezocones to detect individual permeable layers as thin as 2 mm in normally and lightly overconsolidated clay deposits, provided their vertical spacing is at least two cone diameters and a sufficiently fast data recording rate is used. However, the lateral extent of a permeable layer cannot be gauged from a single piezocone probe. Pore pressure dissipation tests in clay layers are not significantly affected by neighbouring permeable layers provided they are more than two cone diameters above or below the filter element. On the basis of the experimental results, it is suggested that miniature piezocones could be used to improve the investigation of thinly layered ground, as might be found in alluvial and estuarine areas.

Determination of Hydraulic Conductivity Using Piezocone Penetration Test

An elastoplastic, finite-strain, coupled theory of mixtures in an updated Lagrangian reference frame is applied to the piezocone penetration test to estimate the hydraulic conductivity of the soil via analysis of the steady-state excess pore pressure generated during piezocone penetration. The results of this approach were compared with piezocone penetration test data. It showed that reliable hydraulic conductivities can be estimated conveniently without performing pore pressure dissipation tests. This study also shows that the change in the dimensionless excess pore pressure (excess pore pressure is normalized by the effective overburden pressure) at the cone tip is almost constant when the dimensionless hydraulic conductivity (hydraulic conductivity is normalized by the penetration speed and cone radius, hereafter called DLHC) is less than 10-7 or greater than 10-4. It is also shown that the drainage condition around the cone tip is close to a fully undrained condition when the DLHC of the soil is less than 10-7, while it is close to a fully drained condition when the DLHC of the soil is greater than 10-4.

A geotechnical investigation of marine deposits in a nearshore seabed for land reclamation

The properties of marine deposits in a nearshore seabed at a land reclamation site in Hong Kong were investigated. A variety of laboratory and in situ tests were conducted to define the geotechnical and consolidation characteristics of the marine deposits. The reliability and applicability of various laboratory and in situ testing techniques in evaluating the coefficient of consolidation were examined. Coefficients of consolidation were calculated from conventional oedometer tests, large-diameter (250 mm) Rowe cell tests, field permeability tests, and piezocone dissipation tests. Three techniques were adopted to evaluate the horizontal coefficient of consolidation ch from the results of in situ piezocone dissipation tests. Results from in situ pore pressure dissipation tests are compared with those from large-diameter Rowe cell tests performed on undisturbed samples under both vertical and horizontal drainage conditions and in situ permeability tests to provide reference values of the vertical coefficient of consolidation cv and ch. The engineering implications, particularly those related to land reclamation work in the nearshore environment, of various laboratory and field tests are discussed.

Determination of permeability of soils using the multiple piezo-element penetrometer

The current method of determining the hydraulic properties of soils using piezocone penetration test (PCPT) requires the advancement of the piezocone penetrometer to the desired depth and holding (or arresting) it for the dissipation test. In order to obtain the hydraulic properties, one analyses the pore water dissipation test results by two-dimensional or three-dimensional radial drainage consolidation. This conventional procedure is methodologically simple and presents relatively reliable values of permeability compared to other field test methods. However, it is still challenging for field engineers and needs to be improved. The piezocone penetrometer intrudes into the ground with the speed of 2 cm/s. Thus, the test mechanism is a kind of strain-controlled condition with partial drainage. Therefore, the excess pore pressure during the piezocone penetration is a function of the permeability of the soil as well as the stress–strain parameters. Thus, with the proper coupled theory of mixtures which can take into account the coupling of solid and pore water flow, one can predict the permeability of the soil from the pore pressure response during PCPT. In this study, the coupled theory of mixtures of the soil grains and the pore water is used in order to predict the permeability of the soil from the excess pore pressure generated from the multiple piezo-element PCPT ‘on the fly’. An elasto-plastic, finite strain constitutive equation in an updated Lagrangian reference frame is used in this work. Using the proposed method, a reliable value of the permeability of soil is obtained conveniently without the use of the pore pressure dissipation tests. Copyright © 1999 John Wiley & Sons, Ltd

Influence of creep on in situ pore pressure dissipation tests : Fahey, M Proc 1st International Symposium on Penetration Testing, ISOPT-1, Orlando, 20–24 March 1988V1, P497–503. Publ Rotterdam: A A Balkema, 1988

Influence of creep on in situ pore pressure dissipation tests : Fahey, M Proc 1st International Symposium on Penetration Testing, ISOPT-1, Orlando, 20–24 March 1988V1, P497–503. Publ Rotterdam: A A Balkema, 1988

In situ estimation of the coefficient of consolidation in clays

An investigation was conducted to demonstrate the applicability of cylindrical piezocone and flat piezoblade tests for providing reliable estimates of the coefficient of consolidation in clays. Coefficients of consolidation were calculated from piezocone dissipation tests for different degrees of consolidation using theoretical time factors to provide a comparison with laboratory oedometer tests. Three techniques were developed to calculate the coefficient of consolidation from piezoblade dissipation tests. Results from in situ pore pressure dissipation tests were compared with laboratory oedometer tests performed on undisturbed samples oriented in both the vertical and horizontal directions, to provide reference values of cv and ch. The results of investigations conducted at several clay sites are presented. Key words: in situ tests, piezocone, piezoblade, coefficient of consolidation, oedometer test, clays.

Effect of sample size on consolidation of a fissured clay

This paper describes an experimental investigation on the effect of sample size on consolidation characteristics of fissured London Clay. Pore pressure dissipation tests on 38, 100, and 300 mm diameter samples were undertaken in the laboratory. Constant-head in situ permeability tests were conducted in four boreholes at different depths in the clay. Conventional oedometer tests on 76 mm diameter samples recovered from the same depths at which in situ permeability tests were carried out were also undertaken. The results show that both the coefficient of compressibility mv and the coefficient of consolidation determined in the laboratory are not significantly affected by sample size. It is concluded that estimate of in situ coefficient of consolidation can best be made from mv determined in the laboratory, and from in situ permeability measurements. Key words: consolidation, compressibility, fissured clay, permeability, size effect, testing.