Distribution Of Excess Pore Water Pressure Research Articles

Simple approach to assessing excess pore water pressure induced by shield tunneling in saturated-unsaturated clay soil

Excess pore water pressure (EPWP) induced by shield tunneling has a significant influence on the stability of the tunnel face, post-construction settlement, and the mechanical behavior of the tunnel lining. However, the three-dimensional and unsaturated property of the soil field is seldom considered in current research. Considering the non-uniform radial convergence model, a modified three-dimensional displacement solution induced by shield tunneling was established first. Then based on unsaturated EPWP elastic theory, a reliable and efficient method is developed to expedite the evaluation of EPWP distribution in three-dimensional saturated and unsaturated clay soil. The validity of the method is confirmed through comparison with field test and numerical outcomes. The analysis examples demonstrate that negative and positive EPWP are generated above the tunnel crown and beneath the tunnel invert, respectively. In the vertical direction, the negative EPWP exhibits a decreasing trend ahead of the heading face and an increasing trend behind it. Along the longitudinal direction, the influence zone of EPWP extends to 1D ahead of and 6D behind the heading face. With the decrease of soil saturation, the EPWP values tend to diminish. The maximum EPWP values observed in saturated conditions can be 16.13 times higher than those under unsaturated conditions.

Model tests on penetration and seepage behaviors of a scaled suction caisson in sand

This study presents a new type of the offshore mooring foundation named as a scaled suction caisson (SSC), of which concept is inspired from the kinetic function of the scaled skin of snakes. When the SSC penetrates into the soil, the penetration resistance effectively decreases due to decreasing the friction force between the soil and sidewall. However, the bearing capacity increases due to increasing the friction force between the soil and sidewall when the SSC is subjected to pull load. A series of the suction-assisted installation model tests are carried out to investigate the penetration and seepage behaviors of the scaled suction caisson in sand. The result indicates that penetration resistance of the SSC increases with decreasing the aspect ratio H1/D1 of the scale structure. For example, when the aspect ratio H1/D1 equals 10, the penetration resistance decreases by 3.1% compared with the traditional suction caisson. And as the aspect ratio H1/D1 decreases from 10 to 5, the penetration resistance increases by 9.9%. In spite of these cases, it shows that reasonable selection of H1/D1 is helpful or not an obstacle to penetration. Furthermore, it is found that the height of the soil plug effectively decreases by the intermittent suction-assisted installation. The formula of calculating the penetration resistance of the SSC during suction-assisted installation is presented in terms of the excess pore water pressure distribution inside and outside the sidewall. The calculation results agree well with the model test results, indicating that the proposed formula of calculating the penetration resistance is reliable.

The role of creep deformation in pit lake slope stability

Large-scale open-pit mining activities have profound impacts on the surrounding landscape and environment. At the cessation of open-pit mining, the rehabilitation of large void spaces can be achieved by pit-lake filling, where the water body provides a confining pressure on surrounding mine surfaces, reducing both the likelihood of slope failure and the need for ongoing slope maintenance. Although pit-lakes present a range of long-term benefits, the geotechnical performance of mines containing soft soils that are susceptible to creep under increasing loads due to pit-lake filling is seldom considered. From a geotechnical standpoint, creep induced failure is commonly associated with slow, downslope movements, prior to critical slope failure events. In this research, time-dependent slope stability analyses based on creep-sensitive materials are presented for an open-cut mine undergoing pit-lake filling. Numerical simulation provides a mechanism for the assessment of materials exhibiting soft soil creep constitutive behaviour under various loading conditions due to pit-lake filling. The response of mine surfaces is investigated for various filling regimes, highlighting location-dependent deformation rates, pore pressures and slope Factors of Safety for a large Australian open-pit brown coal mine. Results are presented for two separate creep-sensitive materials, identifying the ability to achieve final, stable landforms for a range of long-term pit-lake conditions.Article HighlightsTime-dependent creep deformation behaviour is investigated for a large Victorian open-pit brown coal mine undergoing pit-lake rehabilitation.The soft soil creep model is implemented for a large open-pit rehabilitation model, to assess long-lasting creep movements of a specific mine slope.Mine void filling rates are simulated for a range of rehabilitation scenarios over a 5 to 40 year period, identifying the excess pore water pressure distributions in addition to vertical and horizontal deformations rates.The long-term behaviour of 8 cross-section profiles is presented, identifying the effect of pit-lake filling for silt and clay interseam materials.

Effect of several patterns of floating stone columns on the bearing capacity and porewater pressure in saturated soft soil

One of the common geotechnical problems is the construction on soft soil and the improvement of its geotechnical properties to meet the design requirements. A stone column is one of the well-known techniques used to improve the geotechnical properties of soft soils. Sometimes thick layers of soft soil imposed the designer to use floating stone columns for improvement of such soil; in this case, the designer will be lost the end bearing of the stone column. In this study, the effects of several patterns of floating stone columns distribution under footing on the bearing capacity of soil and the distribution of excess porewater pressure are investigated. The soft soil used in this study has a very low undrained shear strength (cu) of 5.5 kPa and improved by several patterns of stone columns (single, two linear, triangular, square, and quadrilateral). The stone column has a length of 180 mm and a diameter of 30 mm. The material of the stone column is poorly graded sand has an angle of internal friction (48.5°) at a relative density of 65%. The results indicated a significant increase in the ultimate bearing capacity of soft soil when treated with floating stone columns despite the small ratio of area replacement and reducing the excess porewater pressure and settlement. Also, the ultimate bearing capacity of soil calculated from experimental work is compared with the corresponding values obtained from the proposed equations in the previous studies to evaluate the validity of using such equations.

Refined 3D modelling of spatial-temporal distribution of excess pore water pressure induced by large diameter slurry shield tunneling

Tunneling induced excess pore water pressure (EPWP) is crucial to face stability, time-dependent ground deformation and structural performance of tunnel lining. This paper numerically investigates the short-term spatial–temporal distribution of EPWP induced by large diameter slurry shield tunneling in saturated soils. A refined 3D finite element model is developed to consider the construction process including tunnel advance, segment installation, tail void grouting and grout material hardening. The commonly neglected shield conicity is fully considered in the refined modelling. The proposed fluid–solid coupled numerical model is well validated by the field monitoring data of Shanghai Yangtze River Tunnel after considering shield conicity. Temporal distribution of EPWP is equivalent to a certain spatial distribution in the longitudinal direction. Longitudinal distribution of EPWP is separated into five parts (Δua ~ Δue) induced by different construction process. The fives of EPWP are largely influenced by shield conicity induced overcut, soil compressibility, grouting pressure and tunnel diameter. The distribution of EPWP ahead of tunnel face is mainly determined by slurry support pressure and is hardly influenced by other factors.

Method for estimating initial excess pore pressure during pile jacking into saturated fine-grained soil

This study presents a novel method for estimating the initial excess pore water pressure (EPWP) over depth during pile jacking into saturated fine-grained soil. Three-dimensional (3D) numerical models considering the pore water effect are presented to simulate the pile installation. Seven coefficients are defined to describe the variation of the EPWP with depth. Numerical studies are conducted to investigate the effects of soil and construction parameters on the normalized variables relevant to the coefficients of the EPWP distribution curve. Subsequently, a formula is proposed to describe the vertical distribution of EPWP at different pile jacking depths. The proposed method is then validated against field data.

Alternative Analytical Solution of Consolidation at Constant Rate of Deformation

In this paper, we present an alternative solution of consolidation at constant rate of deformation (CRD). Governing equation of CRD consolidation is formulated considering excess pore water pressure as primary variable. Formulation of the governing equation is done for soil as linear as well as nonlinear materials. An analytical solution to the governing equation is derived which consists of transient state and steady state components. The solution derived herein yields profile of excess pore water pressure distribution with depth at any time during the test. The present paper analyzes the excess pore water pressure distribution profile during CRD consolidation and factors affecting it in detail. Analysis of the obtained solution indicates the existence of moving boundary condition within the test specimen at early stage of the test where transient state condition prevails. It also indicates that the pore water pressure ratio affects significantly the excess pore water distribution profile, in turn, average value of pore water pressure and average effective stress in the specimen. An expression for consolidation parameters for steady state condition are derived using the present solution. Further, evolution of excess pore water pressure at the base of CRD consolidation test specimen during the test is predicted for normally consolidated as well as for over consolidated soils at different rates of deformation.

Numerical analysis for widening embankments over soft soils treated by PVD and DJM columns

The appropriate selection of treatment methods for a soft soil subgrade is critical to highway widening projects. In this paper, plane strain conditions are considered in the analysis of prefabricated vertical drains (PVD) and dry jet mixing (DJM) columns. PLAXIS is used to develop the finite element models for the simulation of PVD and DJM treated embankments over soft soils for highway widening projects. The model results include the settlements during and after embankment construction and the lateral displacements of the subgrade. This paper also investigates the working mechanisms when the two subgrade treatment methods are adopted for the widening of embankments over soft soils by analysing the excess pore water pressure distribution in the soft soils. In-situ embankment tests are used to validate the proposed numerical model. The results show that the conventional PVD method causes a soil arching effect in the old embankment and excessive ground disturbance during construction despite the fact that this method can reduce the post-construction soil settlement and the lateral displacement during construction. In comparison, the DJM treatment can greatly reduce the settlement during construction, the settlement and differential settlement after construction, and the lateral displacements. Finally, several techniques are summarised to improve the conventional PVD method, providing practical value for widening highway embankments over soft soils.

Evaluation of train-induced settlement for metro tunnel in saturated clay based on an elastoplastic constitutive model

In this study, a two-dimensional (2D) soil–water coupling dynamic finite element (FE) analysis is conducted to investigate the effect of repeated train vibrations on the long-term settlement of a metro tunnel in saturated clay. Particular attention is paid to the leakage problem of the metro tunnel by assuming different permeability conditions, namely fully permeable, fully impermeable, and partially permeable, on the periphery of the tunnel for simplicity. The train vibration load is first evaluated using a rail–fastener–tunnel–subgrade model and averaged over a characteristic length for 2D numerical analysis. Cyclic Mobility model is used to simulate the mechanical behaviors of saturated soft clay in the FE analysis. Excess pore water pressure (EPWP) and associated tunnel settlement in trial operation and normal operation are calculated using the FE code DBLEAVES for different permeability conditions. It is found that a very low EPWP is generated in the trial operation, which then increases rapidly to peak values at the early days of normal operation. Afterward, the EPWP diminishes gradually as the train vibration continues. The permeability of the tunnel lining plays a significant role in the distribution of EPWP around the tunnel but produces a minor influence on the development of tunnel settlement. The train-induced tunnel settlement is mainly caused by the static settlement resulting from the EPWP dissipation during train interval, while the dynamic settlement arising from dynamic consolidation in each train vibration only accounts for a small portion. According to the 2D dynamic FE analysis, the final train-induced settlement of the metro tunnel in saturated clay is estimated to reach 160 mm while the peak EPWP value can reach 26.55 kPa. The settlement discrepancies between the numerical method and empirical method are discussed in detail.

INNOVATIVE EXPLICIT FINITE DIFFERENCE SOLUTION TO TIME RATE OF SETTLEMENT IN CLAY

The time rate of settlement process in clay involves changes in excess pore water pressure (EPWP) with time. Exact solutions have been published for constant initial EPWP and other simple distributions. The finite differences methods are generally used in solving complex initial EPWP distributions. Such methods suffer from roundoff errors at each time increment and truncation errors proportional to the step size used. The explicit finite difference method produces stable solutions when proper time and depth increments are used. An innovative explicit finite difference model involving eigenvalues and eigenvectors is proposed that will permit arbitrary initial EPWP distributions and reduce roundoff errors. This method is numerically stable and convergent. Unlike traditional methods, the proposed solution will also eliminate the need to calculate the EPWP vector traditionally required at each time increment. Instead, the EPWP can be computed directly for any number of time increments.

INNOVATIVE IMPLICIT FINITE DIFFERENCE SOLUTION TO TIME RATE OF SETTLEMENT IN CLAY

The primary consolidation process in clay involves changes in excess pore water pressure (EPWP) with time. The changes in the EPWP are related to permeability, void ratio, and boundary conditions. Closed-form solutions for simple initial EPWP have been published. Such solutions include simplifying assumptions and limited to few impractical initial EPWP distributions. Generally, the initial EPWP encountered in practice is complex, in which case, numerical methods are used. The finite differences and the finite element methods are the primary tools employed in the analysis of settlement behavior of fine-grained soils. Unfortunately, these methods suffer from three main problems: (1) they require evaluation of EPWP vectors at each time increment which results in a large number of calculations; (2) roundoff errors at each time increment; and (3) the solution for a given initial EPWP distribution is achieved at a predetermined integer time increment. An innovative explicit finite difference model is proposed that will permit any initial EPWP distribution, substantially reduces the number of calculations and roundoff errors.

In situ determination of hydraulic conductivity in Yangtze Delta deposits using a modified piezocone model

In order to more precisely determine the in situ hydraulic conductivity of soils as an essential parameter in geotechnical engineering, this article presents a new method based on piezocone tests. In light of results obtained from a series of classical numerical simulations of piezocone dissipation tests and in situ tests, the modified direction and value assumptions of excess pore water pressure distribution are fundamental: (1) the flow surface of pore water is assumed to be cylindrical in shape at larger scales, and (2) the initial state of induced excess pore pressure is assumed to satisfy a negative exponential distribution in dissipating. After detailing the existing approaches, a comparison of data in the Yangtze Delta region between them and the proposed method based on graphical and statistical analysis has been accomplished; the comparison revealed the accuracy and validity of the proposed method, with five indices utilized, including a new relative error index. The reasonable assumptions, logical derivation and mathematical analysis together indicate the academic value and application potential of the proposed method.

Deformation behaviour of geotechnical materials with gas bubbles and time dependent compressible organic matter

Geotechnical materials may contain organic matters and gas bubbles during the process of their formation. The existence of compressible gas bubbles and organic matters may affect the excess pore water pressure distribution during compression and the deformation behaviour of the materials. Immediate settlement and creep have been observed in the early stage of consolidation curves of brown coal and peat samples, which can not be explained with Terzaghi's consolidation theory where soils are considered as fully saturated and soil particles are incompressible. A numerical model has been developed to model the consolidation behaviour of brown coal and peat considering the inclusion of gas bubbles and the time dependent compressible organic matters. In the model, the materials are assumed to contain incompressible minerals, compressible organic matters, gas bubbles and water. Consolidation of the materials is considered as a coupling process of water extrusion and volume variation of gas bubbles and organic matters. The model has been validated using the experimental results from one dimensional consolidation curves of brown coal and historical test results of peat. The gas content and the compressibility of organic matters can be obtained using the proposed model. The model can be used to analyze compression behaviour of geotechnical materials containing compressible particles and gas bubbles.

Closure to “Tall Oedometer Testing: Method to Account for Wall Friction” by Julie Lovisa and Nagaratnam Sivakugan

The authors greatly appreciate the interest shown by Dr. Kang in his discussion on the paper describing an analytical solution for consolidation resulting from arched initial excess pore-water pressure distributions in a tall oedometer. The work carried out by the discusser using a large-sized floating-type oedometer appears to support the assumptions made by the writers—namely, that the vertical normal stresses were uniformly distributed at any depth and that the adhesion between the oedometer walls and the soil was equal to the cohesion of the soil.

Radial consolidation of soil layer under truncated cone fill

A given quantity of fill may be placed over a given circular area in the shape of a cone, a truncated cone, with nearly uniform thickness. Depending on the shape of the fill, different non-uniform initial excess pore water pressure distributions can exist in the underlying saturated soil layer. This paper investigates the influence of the shape of the fill on the time-dependant consolidation behaviour of the underlying circular soil layer, which is draining only along a peripheral drain. Plots of the average degree of consolidation against time factor and degree of consolidation isochrones are developed for different axisymmetric embankment geometries. The results show that the way the load is spread within the circular area has significant influence on the rate of pore water pressure dissipation throughout the soil layer. The exact degree of consolidation at a point within the clay is computed to illustrate the variation in the degree of consolidation with the radial distance.

A simple method to account for non-uniform initial excess pore water pressures in settlement computations

The variation in percentage consolidation with time within a clay layer subjected to a non-uniform initial excess pore water pressure distribution can be difficult to evaluate, and as a result, often a uniform initial distribution is assumed in most analyses. However, by utilizing some of the key features of consolidation in terms of excess pore water pressure dissipation, it is possible to simply adjust the uniform case to account for any number of non-uniform initial excess pore pressure distributions. By observing the decay of excess pore water pressure with time resulting from various non-uniform initial distributions, it is clear that any initial asymmetry or skewness is quickly dispersed, and the distribution of excess pore pressure with depth becomes sinusoidal (or half-sinusoidal if singly drained) shortly after consolidation has commenced. In other words, once the pore pressure decay due to a non-uniform initial distribution has become sinusoidal, it will actually decay at the same rate as the uniform case – however, it will be “ahead” or “behind” the uniform case by some constant factor. Once this factor has been determined, it is possible to simply adjust the rate of consolidation resulting from a uniform initial pore pressure distribution (the values of which are widely available in literature) to account for any number of realistic non-uniform initial excess pore pressure distributions.

Cone penetration-induced pore pressure distribution and dissipation

The excess pore water pressure distribution (u) induced by the penetration of a piezocone into clay and its dissipation behaviour have been investigated by laboratory model tests, theoretical analysis and numerical simulation. Based on the results of the tests and the analysis, a semi-theoretical method has been proposed to predict the piezocone penetration-induced pore pressure distribution in the radial direction from the shoulder of the cone. The method can consider the effect of the undrained shear strength (su), over-consolidation ratio (OCR) and rigidity index (Ir) of the soil. With a reliably predicted initial distribution of u and the measured curve of dissipation of pore water pressure at the shoulder of the cone (u2), the coefficient of consolidation of the soil in the horizontal direction (ch) can be back-fitted by analysis of the pore pressure dissipation. Comparing the back-fitted values of ch with the values directly estimated by a previously proposed method indicates that the previously proposed method can be used reliably to estimate ch values from non-standard dissipation curves (where u2 increases initially and then dissipates with time).

Finite Element Analysis for Subgrade Consolidation Settlement in Soft Soil

The two-dimensional finite element model for subgrade consolidation settlement analysis within soft soil pile is developed using ABAQUS. The numerical simulation on a highway subgrade deformation is performed to study the variation of consolidation settlement and the excess pore water pressure distribution in the central location and the part under centerline of the embankment. The results show that settlement develops gradually with the increasing period of soil consolidation. The excess pore water pressure of deep subgrade soils under embankment centerline rise due to the increased load. After each soil layer was filled, the excess pore water pressure increased in the first and was stable later along with the increase of soil depth. After the embankment soil was filled completely, excess pore pressure dissipated with time developing until the completion of consolidation.

Consolidation Behavior of a Cylindrical Soil Layer Subjected to Nonuniform Pore-Water Pressure Distribution

Nonuniform, initial, excess pore-water pressure distributions in the horizontal direction will have a different impact on the consolidation behavior of the soil layer, when compared with the uniform pore-water pressure distributions discussed in the literature. In the current study, various practical instances in which the nonuniform, axisymmetric, horizontal distributions of pore-water pressure occur are identified. The influence of these distribution patterns on the consolidation response of a cylindrical soil layer draining only at its peripheral face is analyzed analytically using a simple series solution method and by finite-element analysis using PLAXIS. From the Uavg-T curves and pore-water pressure isochrones developed, it is observed that the uniform pore-water pressure assumption overestimates the degree of consolidation achieved for various pore-water pressure distribution conditions and important information about pore-water pressure redistribution is overlooked. Incorporating appropriate initial pore-water pressure distribution will enable realistic estimates of the degree of consolidation to be made.

Calculating cv based on non-uniform initial excess pore pressure

The coefficient of consolidation (cv) is often determined by comparing the characteristics of the experimental and theoretical consolidation using empirical curve-fitting procedures which are based on the theoretical U−T curve generated by a layer subjected to a uniform initial excess pore water pressure distribution. However, in cases where settlement–time data are a result of a non-uniform initial pore pressure distribution, these curve-fitting procedures are no longer valid. In this technical note, a generalised procedure for Taylor and Casagrande's popular curve-fitting procedures is proposed, where the user is directed to select appropriate adjustment factors, depending on the type of non-uniform initial excess pore pressure distribution encountered. These factors were determined by approximating separate regions of the U−T curves using simple power and exponential functions. In non-uniform cases where the power approximation only captures a small portion of the U−T curve it may be difficult to use the corresponding modified curve-fitting procedure objectively.