Negative Pore Water Pressure Research Articles

UNDRAINED SHEAR STRENGTH PARAMETERS OF SAPROCK FROM A WEATHERING PROFILE OVER PORPHYRITIC BIOTITE GRANITE AT KM 31 OF THE KUALA LUMPUR - KARAK HIGHWAY, PENINSULAR MALAYSIA

Three broad zones can be differentiated at the weathering profile; an upper, 9.4 m thick, pedological soil (zone I), an intermediate, 31.7 m thick, saprock (zone II) and the bottom bedrock (zone III). The saprock (zone II) comprises gravelly silty sands that distinctly preserve the minerals, textures and structures of the original granite and can be separated into sub-zones II A, II B, II C, and II D, based on differences in preservation of relict structures and content of litho-relicts (core-boulders). To characterize the undrained strength of saprock, samples were collected from sub-zones II A, II B, II C and II D and their physical and soil index properties determined before unconsolidated undrained triaxial tests were carried out on remolded samples. Three to four individual samples from each sub-zone were compressed under confining pressures of 138 kPa, 207 kPa, 276 kPa and/or 345 kPa. Plots of pf = [(σ1 + σ3)/2] versus qf = [(σ1 - σ3)/2] were then used to calculate apparent cohesions of 41.9 kPa, 100.3 kPa, 76.1 kPa and 73.9 kPa, and friction angles of 32.2o, 28.1o, 26.6o and 27.8o, for the samples from sub-zones II A, II B, II C, and II D, respectively. Regression analyses show apparent cohesions to decrease with increasing clay contents, and degrees of saturation; features indicating the influence of negative pore water (or suction) pressures. Regression analyses also show apparent friction angle to increase with increasing sand contents; a feature attributed to greater inter-locking and resistance to displacement of these particles. It is concluded that the undrained shear strength parameters of saprock are characterized by an average apparent cohesion of 54.6 kPa, and friction angle of 30.5o; the parameters influenced by the degree of saturation as well as clay and sand contents.

Analysis of pore water pressures in an earth dam under operating conditions (case of Taksebt Dam, Algeria)

PurposeThe purpose of this paper is the analysis of the dissipation of pore water pressures in the core of an earth dam, under the effect of water level fluctuations in the reservoir under operating conditions, taking into account the partial emptying and filling.Design/methodology/approachThe Taksebt Dam, Tizi-Ouzou, Algeria was chosen as a case study, using a two-dimensional transient finite element numerical model. The GeoStudio calculation software is used through the SEEP/W. The latter takes into account the flow in the saturated and unsaturated zone, the formulation of SEEP/W allows the analysis of the dissipation of pore water pressures in the dyke. Starting from the maximum level of the reservoir, at least one cycle of partial emptying-filling was modelled over an eight-year operating period from 2011 to 2019. The input variables were the water level variation curve, material properties and boundary conditions.FindingsIt can be concluded that the numerical results obtained from the simulation model on the different points studied, namely, the pore water pressures are satisfactory as long as they are close to those recorded in the field by the pore pressure cells with an average error not exceeding 10% except for some measurements where the error is 20%. When the water level in the reservoir varies, the pore water pressures vary and their behaviour follows these fluctuations. Some points in the dam are affected by negative pore water pressures. No abnormal situations have been detected pore water pressures.Originality/valueThe numerical results of the simulation are analysed and validated against actual pore pressure cell measurements under operating conditions.

Shear Strength Characteristics of a Thermally Cured Sand-Bentonite Mixture

An experimental program was conducted to investigate the effects of curing time and curing temperature on shear behavior of a sand-bentonite mixture. The specimens were cured at temperatures of 40°C, 60°C and 80°C for 1, 3 and 5 days under 100kPa, 500kPa and 1000kPa confinements. The results of consolidated undrained triaxial shear tests showed that an increase in temperature from 40°C to 80°C at 1, 3 and 5 days of curing increased the shear strength by 25%, 24% and 23%, respectively. Also, the increase in curing time from 1 to 3 and from 1 to 5 days at 80°C increased the shear strength of samples 12% and 24%, respectively. The failure of pre-cured samples occurred in lower strains as a result of more induced brittleness. Moreover, the secant modulus as well as the size of yield loci and critical state line’s slope increased by pre-curing. The application of thermal cycles resulted in increasing shear strength and experiencing a negative pore water pressure which shows a transition towards the quasi-structured behavior. The results of scanning electron microscopy (SEM) studies confirmed the increase in void ratio during thermal curing.

Stability of Railway Embankment in Saturated and Unsaturated Conditions

Slope failure is a vital issue in civil engineering infrastructures. Besides, climate change implications made this problem more critical. Therefore, in this study, a railway embankment’s slope stability has been studied in saturated and unsaturated conditions. The soil sample was collected from Universiti Teknologi Malaysia, Johor Bahru. The necessary and required data obtained through lab experiments. The obtained data from lab experiments then used in Seep/W and Slope/W to model the railway embankment numerically. The results obtained from this study indicate that rainfall infiltration plays a vital role in the safety factor value. The obtained results also depicted that both rainfall intensity and duration play an important role in slope stability. The factor of safety showed a declining pattern with increasing rainfall and duration of rainfall. The safety factor decrement occurred since the matric suction was reduced (negative pore water pressure) by rainfall infiltration. Besides, the safety factor decrease was more acute when train load was applied in a rainy period. Thus, it is suggested that in a long period of rainy time, the railway authorities should stop the train operation, or the embankment stability should be increased using some stabilizations methods in the planning and designing stage.

New Osmotic Tensiometer Development

Abstract Climatic variations induce negative pore-water pressure or soil suction oscillations in unsaturated soil with daily and seasonal cycles. The dramatic suction changes of unsaturated soil in shallow depths can lead to engineering failures. However, suction measurements using conventional water-based tensiometers are limited to 98-kPa suction that is due to water cavitation. Therefore, they are not able to capture variations of high suction values near the ground surface. A newly proposed osmotic tensiometer was developed using a cross-linked polymer-water solution to allow the high suction measurement range and provide a reliable and consistent suction response under long-term field monitoring. The unique swelling behavior of the polymer-water solution extends the suction measurement capacity to suction values above 1.5 MPa. Laboratory verification and calibration were carried out with centrifuge (covers the lower range of suction measurement from 0.66 to 250 kPa) and WP4C potentiometer (covers the higher range of suction measurement beyond 250 kPa). The proposed osmotic tensiometer can measure rapid suction changes during drying and wetting paths. Long-term laboratory monitoring using the submerged osmotic tensiometer in a deionized water reservoir demonstrated reliable and consistent suction measurements. Osmotic pressure decay was observed after a long testing period because of the leakage of polymer particles through the high air-entry ceramic disc. A series of parametric studies was performed to explore the optimal tensiometer configuration to minimize the pressure decay. Polymer adsorption measurements from ultraviolet-visible spectrometry were used to quantify the polymer leakage. The newly proposed osmotic tensiometer is expected to capture field suction changes that are due to rainfall precipitation (wetting) and evaporation (drying) and provide long-term, accurate, and consistent suction measurements for geotechnical engineering applications.

Suction measurement in freezing soils using pore pressure transducers

Frost heave is major problem for infrastructures build in cold regions. Frost heave occurs due to suction (negative pore water pressure) generated due to the freezing process close to the frost line, i.e., at the frozen fringe. To understand and predict these negative pore water pressures is a key factor to accurately calculate the segregation heave, i.e. heave related to the formation of ice lenses. Segregation heave is the major part of the total heave and also the most challenging to predict. Many attempts have been presented in literature where the generated suction during freezing is related to temperatures, temperature gradients, grain size of the freezing soil etc. Very few laboratory tests have been presented in which the actual suction is measured during the ice lens formation process and compared with theoretical estimations. One reason is that these measurements are challenging. This paper presents results from laboratory measurements of generated suction during freezing. Laboratory tests were conducted on a silty soil sample and suction was measured at the frozen fringe using small pore pressure transducers (PPT’s). The samples were subjected to one-dimensional freezing from top to bottom in an open water system at a constant temperature gradient. Temperatures were measured at various points along the height of the soil sample while suction was measured at middle of the sample. Test results have shown that PPTs do not show pressure change in long-term static pressure test under sub-freezing temperature. For suction measurement at the frozen fringe, pore pressure readings should be measured at various points along the sample height.

Tunnel Behaviour Caused by Basement Excavation in Clay

Many researchers have investigated the effect of basement excavation on tunnel deformation. However, the influence of consolidation on the interaction of basement-tunnel-soil is rarely considered or systematically studied in clay. In this study, three-dimensional coupled-consolidation finite element analyses were conducted to investigate the effect of consolidation on the tunnel response to excavation. An advanced nonlinear constitutive model was adopted, and numerical parametric investigations were conducted to study the effect of the excavation depth, tunnel stiffness, soil permeability coefficient, and consolidation time on the tunnel response. The results revealed that the basement excavation led to stress release, which caused tunnel heave. Owing to the dissipation of excess negative pore water pressure, the tunnel heave further increased to become approximately twice as large compared with that observed when the foundation pit excavation had just been completed. As the consolidation time increased, the longitudinal tunnel heave and tunnel diameter change caused by the foundation pit excavation gradually increased, but the growth rate was slower down. When the consolidation time changed from 50 days to 150 days, the maximum tunnel heave at the crown and the maximum tunnel diameter change increased by 1.18 and 1.48 times, respectively. The soil’s permeability coefficient did not have a significant effect on the tunnel heave at the crown nor on the tunnel diameter change. The results obtained by this study are expected to be useful as an engineering reference for the analysis of soil structure problems in clay.

Bioengineering Solution to Prevent Rainfall-Induced Slope Failures in Tropical Soil

This paper presents test results of comprehensive laboratory and field-testing program efforts for the development of bioengineering solutions such as growing vegetation for protection of slopes from erosion and landslides in a tropical environmental setting. Saturated shear strength of soil was determined using direct shear tests and unsaturated soil properties, such as soil water retention curve (SWRC), were obtained using a computer-controlled hydraulic property analyzer (HYROP) system as well as a WP4C instrument. Climate data were obtained via field instrumentation and appropriate vegetation data were assumed to perform a finite element method-based transient seepage analysis and coupled slope stability analysis to test the potential of tropical hillslope to fail with and without vegetation over a period of one month. Results show that the factor of safety (FOS) for test slope considering case (a) the rainfall and bare ground, case (b) no rainfall with vegetation, and case (c) rainfall with vegetation were found to be 1.630, 1.763, and 1.650, respectively. Although FOS is marginally improved during storm events due to consideration of vegetation as compared to bare slope, this improvement in FOS is much pronounced during antecedent rainfall (i.e., long duration and small intensity) up to the first 26 days of analysis before the storm event (i.e., high intensity and short duration rainfall), which occurs on 27th day and can be instrumental in preventing slope failures. Similarly, the negative pore water pressure (i.e., matric suction) in the top layer is reduced for case (a) from −260 kPa to −40 kPa, increased for case (b) from −260 kPa to −320 kPa, and decreased for case (c) from −260 kPa to −60 kPa. The practical application of these findings is more applicable to the engineered slopes with vegetation during the dry season when the slope is more stable due to high FOS which, however, will need careful watering just to keep them healthy but prevent complete loss of developed matric suction resulting from root water uptake (RWU). In addition, the small improvement in FOS due to matric suction induced from RWU could play a key role in keeping the slope just stable during extreme storm events especially, when FOS of the bare slope is close to 1. To the best knowledge of the authors this is the first documented geotechnical study, using the tropical soil of Guam, which considers the hydro-mechanical effect of RWU-induced matric suction in slope stability analysis in a tropical setting.

A hydro-mechanical coupled solution for electro-osmotic consolidation in unsaturated soils considering the decrease in effective voltage with time

For electro-osmotic consolidation in unsaturated soils, coupled deformation, inconsistent changes in hydraulic conductivity and electro-osmotic permeability with matric suction, and a decrease in effective voltage with time occur simultaneously. To consider all of these processes, an analytical solution was proposed. The time-stepping method was adopted in the calculation of the solution to depict the nonlinear change in shrinkage deformation with matric suction. The accuracy of the proposed analytical solution was validated by comparison with results from laboratory tests and from a previously presented solution without considering coupled deformation. Then, a parametric study was conducted. The results indicate that the dissipation of the pore water pressure is slow when considering coupled deformation, but a large volume of water is discharged in the end, especially for soils with a large volumetric shrinkage index Cm. A larger final negative pore water pressure and discharged water can be predicted by considering the inconsistent changes in hydraulic conductivity and electro-osmotic permeability. Finally, with the consideration of the decrease in effective voltage over time, both the maximum negative pore water pressure and final drainage decrease, which is consistent with field observations.

Stability Assessment of Zoned Earth Dam under Water Particles Fluidity Effect: Hemren Dam as Case Study

Recently the numerical modeling using finite element method is take into account as a very effective tool to investigate the desired behavior of structures in geotechnical engineering. Earth dams are a water retention structures that are normally wide constructed around the world due to its significant features. These structures may be failed due to exposure to an earthquake and this will result in disaster. The main objective of this study is to assess the slope stability and the seismic response of an earthen dam. Since the matter of seismic response is still have a considerable lack of information for earth dams as a unique structure. Hemren zoned earth dam that is located in Diyala governorate, northeast of Iraq that considered as an active seismic zone has been considered as case study. Numerical modeling has been done in this study using Geo studio software. Factor of safety was calculated with different water levels in order to evaluate the dam safety with different operation water level. The excited earthquake is Elcentro while three values of peak ground acceleration were used which are 0.2, 0.25 and 0.3 g and the duration time is scaled to 10 seconds. In addition, three key points (at the core, the shell and the foundation) that represent the dam construction material are used to evaluate the dynamic response within the dam body. The results revealed that the factor of safety is increased when the water level is increase, but in the increasing in the magnitude of factor of safety with water depths of (10 and 15) m was more than the other depth. The zone of the dam core shows a negative pore water pressure value. That leads to an increasing in effective stress at the core of the dam.

Case study of a driven pile foundation in diatomaceous soil. II: Pile installation, dynamic analysis, and pore pressure generation

Abstract This paper presents the results from a case study highlighting the difficulties of pile driving in diatomaceous soils. In the companion (first) paper to this article, results of an extensive laboratory and in situ testing program were presented while the results from pile driving and further analysis of field observations were presented herein. Unexpected high pile rebound (HPR) was observed during driving of a closed-end pipe pile, with refusal occurring at a depth of less than 5 m. Subsequent open-ended piles were thus driven. Piezometer and case pile wave analysis program (CAPWAP) data were collected during driving of both closed- and open-end piles. Piezometer data indicated that negative pore water pressures (PWPs) were generated while the closed-ended pile exhibited high rebound. Results from in situ tests indicated change in material stiffness and strong dilative tendencies near the depth of refusal. A hypothesis for observed behavior was proposed that considers the soil beneath the pile as a medium with an effectively infinite bulk modulus.

Study on Electroosmosis Consolidation of Punctiform Electrode Unit

In the electroosmosis method, when the distance between the opposite electrode and the same electrode is equal, the two‐dimensional effect of electroosmotic consolidation is significant, and the use of one‐dimensional model will overestimate the potential gradient, making the calculated pore pressure value too large. Aiming at this problem, according to the electrode arrangement rule and the minimum composition, a punctiform electrode unit model is proposed, and electroosmotic experiments are carried out on the symmetric and asymmetric unit models. The two‐dimensional electroosmotic consolidation governing equation of the punctiform electrode unit is established. The electric potential field of the electrode unit and the finite element form of the electroosmotic consolidation equation are given by the Galerkin method. The PyEcFem finite‐element numerical library is developed using Python programming to calculate. The research results show the following: (1) The two‐dimensional effect of the potential field distribution of the punctiform electrode unit is significant. The reduction of spacing of the same nature electrode in the symmetrical unit can make the potential distribution close to a uniform electric field. The asymmetry prevents the electric potential field distribution from being reduced to a one‐dimensional model. (2) The number of anodes will affect the electroosmosis effect of the soil. The more the anodes, the better the electroosmosis reinforcement effect of the soil, and the distribution of negative excess pore water pressure will be more uniform. (3) In the early stage of electroosmosis, the more the drainage boundaries, the faster the generation of negative pore pressure, but in the middle and late stages of electroosmosis, the potential value becomes the decisive factor, and the amplitude of negative pore water pressure in asymmetric units is higher than that in symmetric units. The potential distribution will not affect the degree of consolidation but will affect the extreme pore water pressure.

Estimation of unsaturated flow parameters and hysteresis curve from field instrumentation

Abstract: The negative pore water pressure or soil suction has significant effect on the performance of geotechnical infrastructures (e.g., slope, pavement, embankment etc.). The unsaturated behavior of soil is not static, rather offers variation in response to climatic loading. The objective of the study was to evaluate field-based techniques of SWCC construction in terms of capturing these variation as compared to laboratory methods and predictive models. The field assessment could allow the quantification of hysteresis effect on the SWCC. Instrumentation data from one Texas, USA highway was used in this study. Soil Water Characteristic Curves (SWCCs) were regenerated utilizing co-located moisture and suction data from the field. Laboratory and field measured SWCCs from the instrumented site were fitted by van Genuchten model. Previously developed predicted models were also utilized to evaluate the SWCC parameters. Based on the evapotranspiration and rainfall amounts, distinct drying and wetting cycles were recorded. Though hourly data was collected in this study, average daily values were used for the analysis. Unsaturated flow parameters (α, n, m) were determined from both laboratory testing and field moisture-suction data along with the predictive models. Clear differences were observed between the values obtained from predictive models and field generated SWCC. The outcome from this study revealed that field reconstructed SWCCs can be used to simulate higher precision in numerical modeling in numerous geotechnical applications.

An elastoplastic solution for spherical cavity undrained expansion in overconsolidated soils

In this paper, a novel elastoplastic solution is developed for the undrained expansion of a spherical cavity in overconsolidated soils, with the aim of providing a mechanical model for the interpretation of cone penetration tests in overconsolidated soils. The well-established unified hardening (UH) model is adopted in the present solution to represent the unique mechanical behaviors of the overconsolidated soil during cavity expansion. The problem considered is formulated as a system of first order partial differential equations in terms of a Lagrangian description; these equations are solved as an initial value problem with the code developed based on the Runge-Kutta algorithm. The results, including the expansion-pressure curves, the distributions of the stress components, and the evolution of the potential strength during cavity expansion are presented to show the expansion responses in overconsolidated soils. The results reveal that the unique expansion responses in overconsolidated soil during cavity expansion, such as the development of a negative excess pore water pressure, the strain-hardening and softening behavior, the peak strength and the decay of the potential strength during cavity expansion could be well represented by the present solution.

A comparative study of high capacity tensiometer designs

The High Capacity Tensiometer (HCT) is a soil sensor that can measure large negative pore-water pressures, i.e. in excess of −1 MPa, and is a key instrument for monitoring ground-atmosphere interactions. The design of HCTs has not changed since the first prototype was proposed in the 1990s and still includes three main components, namely a high air-entry value porous filter, a small water reservoir and a pressure transducer, which are all encased inside a protective sheath. Despite many successful examples of measurement of pore-water tension in the laboratory and the field, there are essentially no commercially available HCTs for industrial applications. The rare utilization of HCTs in engineering practice has always relied on bespoke devices that have been purposely designed by research laboratories or suppliers of soil sensors. The dissemination of HCTs into engineering practice has been further hindered by the relatively poor understanding of sensor design and its impact on the reliability of measurements. To overcome such gap of knowledge, this paper explores the influence of distinct design variables (i.e. the porous filter, the pressure transducer, the water reservoir size and the protective casing) on the measuring range of HCTs. Seven prototypes were manufactured with different combinations of the above design variables showing that, if HCTs are properly saturated, the air entry value of the filter has the strongest influence on the measuring range while the effects of reservoir size, pressure transducer and protective casing are relatively modest. It is expected that the results from this work will guide future design of HCTs, thus contributing to the development of resilient sensors for geotechnical applications.

Advanced Triaxial Tests on Partially Saturated Soils Under Unconfined Conditions

A series of partially drained (pore air pressure drained and pore water pressure undrained), fully undrained (both pore air and water pressures undrained) and fully drained (both pore air water and pressures drained) triaxial tests were conducted in unconfined conditions to explore the mechanical behaviour of an unsaturated silty soil. The water contents of test specimens were kept at 10, 15, 20 and 25% with the respective degree of saturation of 24, 36, 49 and 59.5%. The void ratio was 1.10 with a dry density of 1.25 g/cm3 while maintaining the relative compaction of all specimens at 80%. During the constant water content (CW) and fully undrained (FU) tests, the peak shear strength was within an axial strain range of 0–2.75% and the volumetric strain of all specimens did not vary within this range. Under shearing infiltration (SI) and constant loading infiltration (CLI) conditions, it was revealed that a significant magnitude of matric suction still exists in the specimens at the peak/post-peak deviatoric stress which depicts that a slope can fail during a rainfall under an unsaturated condition due to negative pore water pressure (matric suction) in the soil. The results of the triaxial tests suggest that the peak shear strength increases with matric suction linearly up to a matric suction of 15 kPa and the rate of increase gradually decreases with further increase in matric suction. Moreover, the failure envelopes have revealed an existence of a critical suction value of 15 kPa corresponding to 27% water content (i.e. on the dry side of optimum water content) and the increase in shear strength due to suction becomes insignificant afterwards. It is also observed that the peak shear strength increases nonlinearly with the increase in matric suction with a good agreement between the failure envelopes for CW, FU and SI tests.

Consolidation of hard rock tailings under positive and negative pore-water pressures: testing procedures and experimental results

This paper presents laboratory testing procedures and key results on the consolidation of tailings from hard rock mines under positive or negative pore-water pressures (PWP). Specific experimental protocols have been developed and applied to assess the behaviour of low-density tailings (slurry) using compression tests in instrumented columns. The testing results on saturated specimens with positive PWP are used to determine the primary and secondary compression (consolidation) parameters of the tailings, based on excess PWP and displacement measurements. The compression tests with controlled negative PWP were conducted using two stress paths: vertical loading with a constant (imposed) suction and with a progressively increasing suction. The results from these tests illustrate specifically, for the first time, the combined effects of the net vertical stress and suction on tailings compressibility parameters, and on the evolution of PWP. The experimental procedures and related experimental results presented here can be quite useful for the analysis of tailings consolidation in the field, where both positive and negative PWP can occur.

An Improved Unsaturated Triaxial Device to Characterize the Pore-Water Pressure and Local Volume Change Behavior of a Compacted Marl

Abstract Compacted fills in earthworks are in an unsaturated state, with suctions (negative pore-water pressures) usually ranging from several kPa to several MPa. Characterizing the variation of pore-water pressure is a major concern in the estimation of embankment stability and, at the same time, a big challenge when its value falls below –80 kPa. This technical note presents an improved triaxial system based on the standard triaxial cell, equipped with a tensiometer, a thermocouple psychrometer, and local strain gauges. The main objective of these improvements is to measure the pore-water pressure and local volume changes during isotropic loading. In this study, three compacted specimens were tested under undrained conditions. Pore-water pressure was continuously measured by means of the psychrometric probe in the range of –3,000 to –100 kPa and by the tensiometer in the range of –80 to 1,100 kPa. Volume change was measured by the local strain gauges. Analysis and development of the results from both this study and the literature lead to the conclusions that i) specimens with occluded air bubbles (e.g., at the standard Proctor optimum) behave like a saturated soil, as Bw is close to unity when pore-water pressure becomes positive, and ii) a pore-water pressure coefficient Bw close to 1 is not necessarily a sign of saturation, as soil in an unsaturated state may also have a pore-pressure coefficient close to unity, especially at the beginning of the mechanical loading.

Analytical solution for nonlinear consolidation of combined electroosmosis-vacuum-surcharge preloading

Abstract In this study, analytical solutions of the consolidation of vertical drains combined with electroosmosis-vacuum-surcharge preloading are derived considering void ratio-dependent compressibility, hydraulic permeability and permeability of electroosmosis. In addition, radial and vertical flow, changes of the hydraulic and electroosmosis permeabilities in the smear zone, and time‐dependent surcharge preloading are also considered. The solution is verified by comparing it with experimental result and numerical solution in the previous literature. Finally, a parametric study is conducted to investigate the influence of considering nonlinearity, smear zone, and non-synchronous change of hydraulic and electroosmosis permeability in void ratio. The result indicates that the average negative pore water pressure under combined electroosmosis-vacuum-surcharge preloading is larger with the consideration of nonlinear change of the soil parameters, and the consolidation rate is also faster. The presence of a smear zone decreases the consolidation rate and increases the final settlement. A slower consolidation rate is obtained for the smear zone with the large radius and the less permeable. The consolidation rate, final settlement and negative pore water pressure increase for the soils with the large value of electroosmosis permeability index. In addition, the decrease of the vertical hydraulic permeability will increase the final settlement and consolidation time.

Three-dimensional numerical model of Baghdad metro with fluctuation of ground water

When a tunnel is excavated below the ground water table, water flows into the excavated faces of the opening tunnel and leads to affecting the stability by reducing the effective stress and shear resistance by generating seepage forces towards the excavation boundaries. The seepage flow may lead to ground water drawdown and time dependent subsidence due to consolidation and rearrangement of the pores. The effect of interaction between tunnelling and ground water on the soil around boring tunnel and ground surface has become an essential part of the planning, design, and construction of tunnelling project. This paper presents the analysis and the results of a numerical solution for a series of actual tunnelling processes of the proposed Baghdad metro as a case study. The finite element package, ABAQUS 2017, was used for this study. A fully coupled three dimensional stress-pore pressure model has been adopted to realistically capture the mechanical and hydrological interaction between tunnelling and ground water. The effect of fluctuation of ground water level on the pore water pressures for soil surrounding tunnel have been computed. Four sequential simulation steps have been done for the location approximately under Kasra Square where the tunnel will be approximately passed through different soil layer profiles. It is concluded that increased pore water pressures is associated with rising ground water and vice versa. It was also concluded that the left tunnel route will be under danger during the excavation only because of presence of negative pore water pressures along it. Therefore, preliminary proceedings of safety are recommended during the excavation of the left tunnel route.