Horizontal Coefficient Of Consolidation Research Articles

Factors influencing nonstandard piezocone dissipation curves and interpreting the coefficient of consolidation

Piezocone dissipation curves can be categorized as standard (monotonic) or nonstandard (nonmonotonic) curves. However, most methods to evaluate the horizontal coefficient of consolidation (ch) from piezocone dissipation tests are only applicable to standard dissipation curves. The factors that influence nonstandard piezocone dissipation curves were numerically investigated, and a method to interpret the ch from nonstandard dissipation curves was proposed. Nonstandard dissipation curves appear if the maximum excess pore pressure is located some distance away from the pore pressure sensor (sensor located on the cone shoulder or penetration in overconsolidated soils) when the penetration is halted to carry out the dissipation test. Some factors result in more marked nonstandard dissipation phenomena, e.g., a larger overconsolidation ratio (OCR) and a smaller horizontal permeability (kh). Assuming that the ch calculated based on the time for 50 % dissipation of the maximum excess pore pressure (t50) from the standard dissipation curve by the existing method is the “real” value, an equation for correcting t50 from nonstandard dissipation curves was proposed based on the initial value of excess pore pressure (ui), the maximum value of excess pore pressure (umax) and the time for reaching the maximum excess pore pressure (tumax). The ch values interpreted by the corrected t50 (t50c) were larger than those interpreted by the method of shifting log t and were more representative of the actual ch values in the field.

Research on experimental evaluation horizontal coefficient of consolidation Ch on CPTu dissipation test to determine permeability parameters in the field to improvement of soil soft.

In the solution of soft soil improvement by preloading method combined with prefabricated, the horizontal consolidation coefficient Ch is a very important parameter to help predict the settlement speed of soft soil. But when designing the coefficient of horizontal consolidation, it is often assumed that the coefficient of vertical consolidation is quite large. This can lead to settlement over time that will fluctuate in a rather large range of values. In this paper, research and evaluation of the horizontal consolidation coefficient Ch by dissipation test on piezocone penetration device with pore water pressure measurement has been mentioned to determine the permeability parameters in-situ for the soft soil improvement at the projects.

Assessment of the coefficient of consolidation with Queensland data

Established relationships between the coefficient of consolidation (cv) and index tests are used during both preliminary design and as a cross check during detailed design. The laboratory oedometer test provides compressibility parameters and a lower bound of cv, while the coefficients of consolidation are preferred from the field dissipation tests. However, cv is dependent on the method used to determine its value, stress level, and over-consolidation ratio. In practice, the coefficient of consolidation values obtained from dissipation tests are used to predict settlement time, while oedometer tests are useful in obtaining the parameter required to predict the magnitude of settlement. However, dissipation tests measure the horizontal coefficient of consolidation (ch) which needs to be related back to the vertical value. These standard approaches are discussed using test data from Queensland sites. Inconsistencies in correlations are used to show that design should consider the wide variability in interpretations that can occur, and correlations of cv with index tests should not be used in detailed design. Additionally, the cv values obtained from oedometer testing is a poor predictor of time for consolidation. This could also be due to the size of samples being not large enough for the soil structure. Monitoring data from construction sites are used to assess a “moderately” conservative design value from dissipation and lab tests.

Influence of type of drainage boundary on coefficient of horizontal consolidation

Vertical drains are used widely to accelerate the consolidation of soft clay deposits when preloading is used as a ground improvement technique. One of the essential input parameters required in Barron's theory is the coefficient of horizontal consolidation, ch. The values of ch can be determined by the radial consolidation test, using either a central sand drain or a porous plastic peripheral drain. This paper presents the laboratory tests carried out to understand the reason for the difference in values of ch determined from inward and outward radial flow consolidations tests. A 150 mm wide instrumented consolidation cell was used to carry out the inward or outward radial consolidation tests. The total stress measurements during consolidation showed non-uniform stress distribution in clay with higher effective stress values close to the drainage boundary. This stiffening of the clay close to the drain retards the consolidation rate resulting in reduced values of ch. As a result, the ch values determined by radially outward consolidation tests with a larger drainage boundary area are lower to those obtained by inward radial flow tests. The pore water pressure measurements showed significantly higher undissipated pore water pressure away from the drainage boundary for the outward flow tests.

Mitigation of lateral slope movement and soil improvement using the vacuum-PVD scheme

This paper interprets the results from field monitoring which was carried out during vacuum-PVD improvement in a site located near an actively moving slope. Interestingly, the monitoring results showed, among other things, mitigation in the outward lateral movements during and after the preloading process indicating relative stability in the slope and the efficiency of vacuum to mitigate lateral movements during the preloading period. Analyses were made on other field parameters such as pore pressure and settlement, as well as back-calculation of flow parameters to be considered during vacuum preloading design, such as permeability ratio (kh/ks) and horizontal consolidation coefficient (Ch) due to vacuum-PVD, were carried out. Post improvement, appropriate geotechnical properties were obtained from laboratory tests of clay specimens from borehole samples and undrained shear strengths were measured from unconfined compression and field vane shear test. The obtained properties indicated improvement in soft soil properties with a reduction in water content and an increase in maximum past pressure, OCR and undrained shear strengths. The prediction made for final shear strength using past literature, where applied additional incremental stress was reduced with depth, matched well with the shear strengths recorded from field testing.

Interpretation of pore pressure dissipation of CPTu in intermediate soil considering partial drainage effect

The degree of drainage that occurs during structure-soil interaction is a crucial factor that affects the stress on the structure in the intermediate soil. The consolidation coefficient is the most important index to evaluate partial drainage conditions, but the pore pressure dissipation curve used for consolidation coefficient inversion is also affected by drainage conditions. This paper adopts an effective stress numerical analysis work which combines finite-element penetration analysis and critical-state soil mechanics considering the partial drainage effect through changing the penetration rate. The significant advances of this work are the evaluation of the partial drainage effect on pore pressure test data and the influence mechanism of partial drainage on consolidation coefficient test results of intermediate soil. A modified consolidation coefficient inversion method can describe the pore-pressure response curve for dissipation tests performed in soils where pore pressure has dissipated partially. The modified theoretical relationship of vertical and horizontal consolidation coefficient and the theoretical relationship of ch under partial drainage and undrained condition are given. The consolidation coefficient inversion method of intermediate soil with high permeability is proposed, and the inversion results are consistent with the actual value.

Study of Anisotropy Characteristics of Bogor Volcanic Soil

Anisotropy in soil results from the deposition process which describes the characteristics of the soil grains or is caused by stress or from the consequences of stresses caused during deposition and subsequent erosion. All soils behave in general anisotropy and some exhibit undrained shear strength. This study conducted 2 tests, namely the first field testing with original soil samples in the form of CPTu and dilatometer. The CPTu test's objective is to determine the vertical soil parameters, while the dilatometer is to determine the horizontal soil parameters. This study indicates that the indication of anisotropy in all shear strength tests is evident in the results of the CPTu test and the Dilatometer test. TX - UU and consolidation show that the horizontal shear strength (Suh) is greater than the vertical slope shear strength (Suv). In this case, the ratio obtained for shear strength is Suh = 1.3 Suv. And from the results of the consolidation test in the laboratory, it was found that the horizontal compression index parameter (Cc horizontal) was greater than the vertical (Cc vertical) and the horizontal coefficient of consolidation (Ch) is greater than the vertical coefficient of consolidation (Cv).

Reclamation and Ground Improvement of Soft Marine Clay for Development of Offshore Terminal 4, JNPT, Navi Mumbai

A container yard of around 100 Ha area was developed as part of the fourth terminal for Jawaharlal Nehru Port Trust (JNPT) in Navi Mumbai by means of reclamation in sea over soft, compressible marine clay. Ground improvement of the subsoil was warranted in order that the finished reclamation satisfies the stringent serviceability criteria. Extensive offshore geotechnical investigation campaign comprised conventional borehole sampling, laboratory tests and various in situ field tests such as CPTu, field vane shear tests, etc. Investigations revealed clay thicknesses of 4 m–22 m, with top layer showing undrained shear strengths less than 7 kPa and increasing with depth, underlain by weathered basalt. The ground was improved using prefabricated vertical drains (PVDs) with preloading. Radial consolidation and cone dissipation tests were carried out to establish the coefficient of horizontal consolidation (ch). The marine reclamation is confined on all sides by a perimeter bund with revetment, Stability analyses carried out for this bund showed that it was necessary to have overfill beyond the reclamation cope line along with tension geotextiles so as to achieve the prescribed factors of safety against slope failure. After removal of the preloading, the overfill was cut back to the final geometry and revetment was provided along the perimeter. Extensive geotechnical instrumentation and monitoring were conducted using multilevel magnetic extensometers, settlement gauges, inclinometer and piezometers to monitor the behaviour and performance of ground improvement. Field ch and smear coefficients were back-calculated. Around 200 confirmatory boreholes (CBH) were carried out after completion of ground improvement to ascertain the post-improvement and validate the design parameters, thus eliminating the risk of post-construction settlement.

COMPUTATIONAL PROGRAM FOR EVALUATING PRESSUREMETER TEST RESULTS OF CLAY SOIL

Pressuremeters are often used in applications to test soils onsite; major developments for these devices are related to methods of manufacturing and analytical solutions to enable operation with different types of soils. In this study, pressuremeter tests were conducted in situ to obtain information on the horizontal at-rest pressure, elastic modulus, undrained shear strength, limit pressure, and horizontal coefficient of consolidation. Owing to the increasing importance of pressuremeter test results and their applicability in determining the engineering properties of soils, different methods and theories have been used to analyze and interpret pressuremeter parameters; many of the assumptions for these parameters reflect the uncertainties of soil properties: some assumptions are that the soil is elastic perfectly plastic and that the tests ensure plain strain conditions; furthermore, horizontal at-rest and limit pressures are critical to the design of foundations. A computational program was thus developed to explore the results of conventional and theoretical limit pressures using different analysis methods. This program was intended to simplify the methods used to determine the conventional and theoretical limit pressures without obtaining the curves necessary for determining the various parameters used in the conventional analysis; moreover, the program could unify the metrics used in the different analysis methods. Using the proposed program, the results of the conventional and theoretical limit pressures were shown to have distinct differences when using different analysis methods.

Research on Horizontal Coefficient of Consolidation of Vietnam’s Soft Soil

The horizontal coefficient of consolidation is the most important parameter for designing the improvement of soil soft by prefabricated vertical drains (PVDs) combined with surcharge and vacuum preloading. This paper presents the experimental study on the horizontal coefficient of consolidation (ch) of some soft soils distributed in Vietnam. Thechvalue was determined by the laboratory test and CPTu dissipation test. The laboratory tests included the Rowe consolidation cell test and constant rate of strain consolidation with radial drainage test. Two types of consolidation laboratory tests were performed. The experimental results indicated that thechvalue is always larger than the vertical coefficient of consolidation of soil (cv). The ratio ofch/cvdepends on the consolidated pressure, type of soil, and the anisotropy of soil. The ratio ofch/cvis different in different types of soft soil in Vietnam. In the normally consolidated state, thech/cvratio ranges from 1.35 to 10.59. It was necessary to choose thechvalue at the consolidated stress level for calculating the PVD spacing.

Modified Time Factor to Estimate the Duration of Pile Setup

Pile setup is defined as the increase in pile capacity with time. This increase in pile capacity starts immediately after end of driving (EOD). Majority of the setup conducted during the consolidation period followed by a small amount during the post-consolidation period. However, there is no available methodology to estimate the duration of setup during the consolidation period, and to establish a theoretical upper time limit for pile setup capacity beyond which pile setup capacity can be neglected. This paper examines or focuses to develop a methodology to estimate the time duration of pile setup. Three instrumented test piles (TPs) were driven mainly in clayey soil in Louisiana besides regular bridge construction projects. The piles were instrumented with sister bar strain gages to measure the load transfer along the piles’ length, and four sets of pressure cells and piezometers were installed on each pile face to measure the total stress, effective stress and excess pore water pressure (PWP) with time. Dynamic and Static load tests were performed for a duration of 6 months after EOD and hence study setup. Piezocone dissipation tests were performed at each TP locations at different depths to measure the horizontal coefficient of consolidation (ch) besides the regular laboratory and in situ cone penetration test (CPT) to characterize the subsurface soil condition. The piezometers that were installed on the pile face showed that the dissipation continued from 2.5 h to 90 days after EOD depending on the soil properties. The equation that was proposed by Teh and Houlsby (Geotechnique 41(1):17–34, 1991) for dissipation test of piezocone was adopted to calibrate the time factor for pile. A new modified time factor for pile (Tpile*) was proposed in this study to estimate the duration of pile setup.

Temperature effects on dredged slurry performance under vacuum preloading

Under appropriate temperature conditions, vacuum preloading can effectively accelerate the rate of soil consolidation. If the temperature is low (30 °C), vacuum preloading is less effective at consolidating the soil. If temperature is extremely high, vacuum preloading is less efficient at consolidating the soil due to the consummation of excess energy consumed. In this study, a series of laboratory tests was conducted to analyse the effects that temperature has on dredged slurry consolidation via vacuum pressure using constant and variable heating modes. During these tests, heat transfer, water discharge, surface settlement, and pore-water pressure dissipation were observed in the soil samples. Based on the laboratory test measurements, each soil sample’s horizontal coefficient of consolidation, water content, and shear strength were determined. To quantify the energy consumption of the different heating modes, the ratios of energy consumption as a function of the soil’s total water discharge and mean shear strength were determined. Using these parameters, an optimal soil consolidation temperature was obtained. The results indicated that vacuum preloading was most effective in consolidating the soil under a constant temperature of 75 °C rather than variable temperatures.

Effect Rate of Strain on In Situ Horizontal Coefficient of Consolidation from Pressuremeter

The conventional methods for evaluating the values of the coefficient of consolidation of soils usually carried out by Odometer or triaxial testing in laboratory by observing time rates of sample compression or pore water pressure dissipation during the application of a constant pressure. Attaching a small pore water pressure transducer to the membrane of the pressuremeter, enabling the measurement of the horizontal coefficient of consolidation of Glasgow glacial till. The horizontal coefficient of consolidation can be deduced in situ using the pressuremeter from the” holding tests “with the direction of pore water pressure drainage sensibly radial. The conventional method for carrying pressuremeter holding tests for measuring the horizontal coefficient of consolidation is by holding the diameter constant and allow the dissipation of pore water, the author developed a new method for measuring horizontal coefficient of consolidation by holding the pressure constant. Different rates were arranged to expand the membrane. The rate of strain showed distinctive discrepancies on the deduced values of the horizontal coefficient of consolidation during pressuremeter testing for Glasgow till.

Pore pressure observation: pressure response of probe penetration and tides

Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating (FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from 1.3×10-6 m2/s to 8.1×10-6 m2/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately 2.0×10-11 Pa-1, which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment.

A Simplified Approach for Estimating Settlement of Soft Clay under Vacuum Consolidation

In general, the ground improvement by Prefabricated Vertical Drains (PVD) consolidation involves a high magnitude of settlement, which results in a change in the consolidation characteristic of the soil during the process of consolidation. Under such circumstances, the consolidation problem must be treated as a large strain problem. The large strain 3-Dimensional consolidation analysis requires a large number of parameters, making it difficult for practicing engineers to carry out such analysis for a practical application. This paper aims to provide a simplified method for estimation of settlement during 3D vacuum consolidation using a single parameter, the coefficient of horizontal Consolidation (Ch). For this, the variation in Ch during the 3D consolidation was back-calculated using Hansbo’s method from a series of large-scale 3D vacuum consolidation tests carried on reconstituted marine clay. As the varying Ch cannot be employed in available analytical models, a simplified finite element analysis is presented to employ varying Ch. The estimated settlement was further compared with settlement obtained utilizing constant Ch, by trial and error method. The paper also demonstrates a potential advantage of the proposed method that the variation in Ch can also be determined from 3D consolidation carried out previously for similar strata, as it requires only basic data, which are usually available.

Composite Consolidation Coefficient Analysis of Soft Soil with Drainage Water

The consolidation coefficient is the most basic parameter to calculate the consolidation rate of soil layer, and the horizontal consolidation coefficient controls the radial water flow into the drainage well. Based on the background of the soft soil in Shantou, Guangdong Province, a series of experimental studies on the consolidation characteristics were carried out by using the modified consolidation instrument. And the concept of the composite consolidation coefficient of the drained water body was put forward. The composite consolidation coefficient reflects the consolidation characteristics of soft soil with drainage water, The test results showed that: 1) The consolidation test with drainage plate is basically consistent with the load compression curve, but its consolidation rate is fast, which is reflected by the composite consolidation coefficient. 2) In the consolidation test of water bodies with drainage, the vertical consolidation coefficient and radial consolidation coefficient are calculated by “three-point method”, and then the composite consolidation coefficient is obtained. The composite consolidation coefficient decreases with the increase of drain spacing ratio, effective drainage diameter and drainage height, which is basically consistent with the theoretical formula. 3) The vertical consolidation coefficient and radial consolidation coefficient decrease with the increase of the diameter of the sample, and the difference is obvious when the load is large. The large-size model with a diameter of 100 mm and a height of 100 mm is about 1.35 times of the vertical consolidation coefficient of the conventional consolidation test.

Interpretation of horizontal permeability from piezocone dissipation tests in soft clays

Piezocone is a common site investigation tool where a cone penetrometer is pushed into ground. In fine-grained soils, the penetrometer can be stopped to measure pore pressure decay, known as dissipation testing. Conventionally the horizontal coefficient of consolidation has to be first determined before the hydraulic conductivity can be estimated. This study uses the modified Cam-clay soil model to numerically simulate dissipation tests. A total of 90 calculations are performed to derive a new dimensionless time factor that easily and directly evaluates horizontal permeability for both standard and non-standard dissipation curves. The proposed equation is validated with field data.

Effect of surcharge loading rate and mobilized load ratio on the performance of vacuum–surcharge preloading with PVDs

The results from three laboratory model tests performed under various vacuum and surcharge loads with PVDs are reported. Different SLRs were adopted to investigate the effect on the consolidation of dredged soil. To measure the lateral displacement, a refitted inclinometer was developed and tested. In the tests, the settlement, lateral displacement, and vane shear strength were measured, and the degree of consolidation (DOC), horizontal coefficient of consolidation (Ch), and bearing capacity were calculated. The results indicate that larger SLR values promote consolidation. The largest vane shear strength, settlement, and Ch values were all obtained under the highest SLR, and the bearing capacity under this SLR was more than double that under the lowest SLR. The DOC was found to increase with the growth of the SLR. However, considering the vacuum pressure was higher in Case-III, the influence of SLR on reinforcement effect may not be so significant.

Effectiveness study of prefabricated vertical drain using vacuum preloading and surcharge preloading

Prefabricated vertical drain is a method of soil improvement in order to accelerate consolidation settlement. In line with the development of knowledge, vacuum preloading was used to replace embankment preloading to produce stress in soil. This research will compare the effectiveness of vacuum preloading against the embankment preloading through the analysis of the degree and time of consolidation. The prediction of settlement is manually calculated using 1 dimension consolidation Terzaghi’s theory method, while the finite element method by PLAXIS 2D Program and ASAOKA method from settlement on the field. The horizontal coefficient of consolidation used is based on the result of back calculation using ASAOKA’s chart. The ratio of coefficient consolidation (Ch/Cv) obtained for vacuum preloading is 5.9 and for embankment preloading is 2.21-4.25. The analysis show that the vacuum preloading method can cut the consolidation time up to 74% comparing the embankment preloading method.

Plane-strain consolidation theory with distributed drainage boundary

In practice, the full arrangement of sand blankets overlying soft clays could result in an uneconomic design for soft soil treatment using the surcharge preloading method. In view of this, a novel type of distributed drainage boundary is proposed in this investigation to improve the design. A two-dimensional plane-strain consolidation problem with distributed drainage boundary is established and solved. The sensitivity of the consolidation process to the pave rate (sand blanket area over the total area), thickness–width ratio (thickness over width of the representative element) and anisotropy coefficient (horizontal consolidation coefficient over vertical consolidation coefficient) are discussed. The results show that the negative effects of distributed drainage on the consolidation process become negligible if the pave rate and thickness–width ratio are designed adequately. Remarkably, the distributed drainage boundary becomes more efficient with the decrease in anisotropy coefficient. In addition, the increasing rate of consolidation time calculated using different parameters is presented for four average degrees of consolidation to provide a theoretical reference for engineering design.