Pore Pressure Reduction Research Articles

Analyses of the October 2013 fatal slope failures on the metamorphic terrains of Obudu tourist area, South-Southeast Nigeria

The October 2013 fatal landslides at the international tourist zone Southeast Nigeria were analyzed. The failures were on a predominantly metamorphic terrain and only on slopes adjacent to the main road. They occurred as slides, not debris flow, but produced a wide range of casualties. The research found that all the landslides occurred on slope portions composed of schist rather than gneiss or granite. Slip surfaces developed within the regolith, and the shear zone was characterized by the presence of silty materials supported by clayey matrix. Field observations indicated that the failures generally developed as localized translational slides within the semi-consolidated, cohesive soil units (with high plasticity and low strength) within the upper to middle weathered zone of the schist. The increase in pore pressure arising from elevated water table during rainfall created instability by weakening the shear strength along the failure plane. However, differences in permeability favored the formation of perched water table which eventually triggered sliding. The initial displacement apparently destroyed cohesion, and the slide became more rapid. The mechanism of the slides was identified, and laboratory evidence showed that even at high level of saturation, there was a reduction in pore pressure resulting from dilation. Subsequent infiltration was probably inadequate to equilibrate the reduction. Therefore, the post-dilation phase marked by strain softening could not lead to a transformation to debris flow. High pore pressure that approached the normal stress was possible, but it could only develop at large displacement.

The compressibility and shear characteristics of soils associated with landslides in geologically different localities—case examples from Nigeria

Field and Shuttle Radar Topographic Mission data were used to create Digital Elevation Model and geologic maps to study the roles of geology, discontinuities, elevation, steep slopes, deep valleys, and drainage patterns on the frequent occurrence of long-travel landslides along the Nigeria–Cameroon border. The undrained shear behaviors of sandy specimens from two localities with different mode and magnitude of landslides were analyzed at different stress levels and related directly to their compressibility and permeability characteristics. The weathered granitic-sandy specimens from Kwande debris avalanche showed significant change in bulk permeability and sample height, and exhibited the greatest evidence of grain crushing. This behavior correlates well with their shear response which manifested in considerable increase in excess pore pressure and drastic reduction of shear resistance. Due to grain crushing, finer grains that lowered permeability were formed at the shear zone. It is likely that the decrease in permeability facilitated the generation of high excess pore pressures leading to significant reduction of shear resistance to low values at steady state. Contrastingly, the reduction in sample height and permeability of Iva valley sands was not as significant, which also correlates with the generation of low excess pore pressure and higher steady state strengths; conditions that may be responsible for the predominance of short-travel slides in the area. The behavior of silica sands, however, which suffered higher grain crushing and reduction in sample height and permeability, appeared closer to that of the samples from the avalanche site.

Non-linear behaviour for naturally fractured carbonates and frac-stimulated gas-shales

Gas-shales and naturally fractured reservoirs usually produce from several kilometers depth, with fracing-stimulation and eventual water-drive respectively. Due to porosity, the matrix is generally weaker than is typical for basement rocks. The potential pore pressure reduction of tens of MPa during the early life of the fields, may therefore be a significant proportion of the strength of the matrix. Inevitable non-linear rock strength behaviour for the matrix should not then be ignored. It is therefore unrealistic to utilize a linear Mohr-Coulomb strength criterion as so frequently seen. The joints or natural fractures in the shales and carbonates, which are so important for production, will have producing fracture sets with different roughness and aperture, and few of them are planar enough to follow the frequently used linear Mohr-Coulomb behaviour. Non-linearity especially applies to the favourable shear strength-dilation-permeability coupling which is relevant for both NFR and gas-shales, and to the less desirable stress-closure-permeability coupling of a stress-sensitive reservoir. Non-linear constitutive modelling, partly based on the joint- or fracture-roughness coefficient (JRC) used widely in rock mechanics, also applies to the conversion from hydraulically interpreted theoretical smooth-wall apertures (e) to the larger and non-planar-non-smooth-wall physical apertures (E) through which the oil or gas actually flows to the wells. Simple index tests which can also be applied on joints or fractures recovered in occasional and inevitably expensive core, and which can also be estimated when mapping fractured pavement analogues, have been available in rock mechanics for several decades. They were already incorporated in coupled distinct element (jointed) non-linear modelling routines in 1985. However, their implementation in petroleum industry geomechanics seems to be very rare judging by numerous workshops attended in the last seven to eight years on both sides of the Atlantic. Application of non-linear (non Mohr-Coulomb) rock mechanics, using recovered core from Ekofisk in 1986-1987 in order to model fracture shear-dilation coupling with simplified E and e tracking during compaction, may be the earliest example.

Widespread Surface Subsidence Induced in Alpine Hard Rocks by the Construction of the 57 Kilometres-long Gotthard Base Tunnel (Switzerland) Observed with Satellite SAR Interferometry

Abstract Subsidence associated with the construction of the 57 kilometres-long Gotthard Base Tunnel in Switzerland on the surface of the metamorphic rock masses has been measured with SAR interferometry. Significant movements with values ranging from 2 to 12 mm/yr were detected with ENVISAT ASAR data between 2003 and 2010 above the tunnel on villages and sparsely vegetated alpine slopes, in areas where no displacement was recorded with ERS-1/2 SAR data between 1992 and 2000. The mechanism contributing to the large-scale surface consolidation is believed to be associated with pore-pressure reduction in the rockmass arising from tunnel drainage.

Effects of dissociation on the shear strength and deformation behavior of methane hydrate-bearing sediments

A series of tests were conducted in order to investigate the shear strength and deformation behavior of methane hydrate-bearing sediments during dissociation using the thermal recovery method or depressurization method. An innovative temperature-controlled high pressure triaxial apparatus which can reproduce the in situ conditions of hydrate reservoirs was used. The results indicate that: (1) the failure strength of isotropically consolidated methane hydrate-bearing sediments which dissociated completely using the thermal recovery method is less than that of pure Toyoura sand. However, the initial stiffness and volumetric strain are higher than that of pure Toyoura sand. (2) The thermal recovery method will cause the failure of methane hydrate-bearing sediments when the axial load is higher than the strength of methane hydrate-bearing sediments after dissociation. (3) The depressurization method will not cause collapse of methane hydrate-bearing sediments during depressurization. However, water pressure recovery will lead to failure when the axial load is larger than the strength of the methane hydrate-bearing sediments after dissociation. (4) The depressurization rate shows little effect on the ultimate deformation of methane hydrate-bearing sediments, while the initial deformation rate increases with increasing depressurization rate. (5) The larger the reduction of pore pressure, the larger axial strain and volumetric strain.

Excess Pore Pressures Around Underground Structures Following Earthquake Induced Liquefaction

Underground structures located in liquefiable soil deposits are susceptible to floatation following an earthquake event due to their lower unit weight relative to the surrounding saturated soil. This inherent buoyancy may cause lightweight structures to float when the soil liquefies. Centrifuge tests have been carried out to study the excess pore pressure generation and dissipation in liquefiable soils. In these tests, near full liquefaction conditions were attained within a few cycles of the earthquake loading. In the case of high hydraulic conductivity sands, significant dissipation could take place even during the earthquake loading which inhibits full liquefaction from occurring. In the case of excess pore pressure generation and dissipation around a floating structure, the cyclic response of the structure may lead to the reduction in excess pore pressure near the face of the structure as compared to the far field. This reduction in excess pore pressure is due to shear-induced dilation and suction pressures arising from extensile stresses at the soil-structure interface. Given the lower excess pore pressure around the structure; the soil around the structure retains a portion of this shear strength which in turn can discourage significant uplift of the underground structure.

Geomechanical modeling for CO2 storage in Nisku aquifer in Wabamun Lake area in Canada

During CO2 injection, increasing pore pressure and temperature reduction create geomechanical deformation of both the reservoir and surrounding rocks. One of the most important concerns with respect to the long term CO2 storage is that stress changes caused by injection could lead to formation fracturing or reactivation of fracture networks and fault movements which could potentially provide pathways for CO2 leakage through previously impermeable rocks.Nisku geological formation located in Wabamun lake area was chosen as storage target based on storage capacity, expected ease of injectivity, low leakage risks and lack of interference with current petroleum production in the area. In order to determine whether the injection-induced stress changes affect the viability of this target formation to act as an effective storage unit, the geomechanical assessment model of the injection process has been developed, which couples the thermal flow model and geomechanical model covering all layers from the basement to the surface, and incorporates the modeling of dynamic injection-induced fracture. This work established the methodology for investigating the increase of well injectivity by allowing fracturing, and assessing the effect of the injection temperature.The pressure and stress variations were modeled during and after CO2 injection utilizing a coupled flow and geomechanical software. Since injected CO2's temperature may be considerably lower than the formation temperature, thermal effects were incorporated into the models. In order to increase the well injectivity, the case of allowing fracture initiation and propagation in Nisku was considered. The modeling shows that total minimum in situ stress is not constant and can be dramatically reduced by injection of cold CO2, thus leading to spontaneous fracturing. The cases of isothermal and thermal injection (without containment) show that operating above the fracture pressure in Nisku will increase the well injectivity (from 1Mt/year to at least 2Mt/year) but creates potentially long fractures and also the possibility of fracturing the caprock.Decreasing temperature due to CO2 injection will result in stress reduction and smaller surface deformations compared to the isothermal model. Thermal effects of cold CO2 injection will also reduce the fracture (injection) pressure and enhance the horizontal fracture propagation through Nisku. However, to make recommendations on a design that does not jeopardize isolation capacity of the containment requires site specific geomechanical and in situ stress data for all overlaying strata, in order to assess the risk of vertical propagation more accurately.This study predicts that CO2 injection in the Nisku zone is not likely to cause any significant surface heave and is not likely to have any environmental impact associated with surface deformations.In summary, the geomechanical factors put significant constraints on CO2 storage in Nisku in terms of injection pressures that may be limited by fracture propagation, injection rates that can be achieved without compromising the seal and excessive fracture propagation, and the CO2 placement affected by a fracture. The demonstration of the importance of thermal effects on CO2 storage is the most important contribution of this work.

An evaluation of pore pressure diffusion into a shale overburden and sideburden induced by production-related changes in reservoir fluid pressure

It is commonplace in the simulation of reservoir fluid flow induced by hydrocarbon production to regard shales as barriers to flow. Whilst this appears correct for fluid exchange, this is not the case for the fluid pressure component of this process. Indeed, the authors observe that pore pressure reduction due to reservoir depletion can propagate significant distances into the shale overburden or sideburden over the production time scale. Shales may deplete their pore pressures by more than 10% of that experienced in the reservoir sand for distances of tens of metres to kilometres into the shale, depending on the production history, duration and the specific shale properties. An important factor controlling these results is heterogeneity of the shale sediments, and the pressure diffusion process can be considerably enhanced by the presence of silt laminations and streaks. These results suggest a possible risk to drillers when advancing towards the top of a depleting reservoir or when drilling a well alongside an already depleted reservoir. Our analyses conclude that pore pressure diffusion should be considered as a factor in geomechanical and fluid flow reservoir modelling, and in mud weight determination during infill drilling.

Effect of fibre type and geometry on maximum pore pressures in fibre-reinforced high strength concrete at elevated temperatures

This paper presents results of an experimental and statistical study which investigates the effect of fibre type and geometry on the amount of maximum pore pressures measured at different depths in fibre-reinforced high strength concrete (HSC) exposed to elevated temperatures. Polypropylene, polyvinyl alcohol and steel fibres of varying lengths and diameters were used. Pore pressure measurements showed that addition of organic fibres regardless of the type significantly contributes to pore pressure reduction in heated HSC. Polypropylene fibres were more effective in mitigating maximum pore pressure development compared to polyvinyl alcohol fibres while steel fibres had a slightly low effect. Longer organic fibres of length 12mm with smaller diameters of 18μm showed better performance than shorter ones of length 6mm with larger diameters of 28 and 40μm. Based on experimental observations and using statistical analysis, a relationship to predict maximum pore pressures in heated concrete was developed.

Pore pressure development in hybrid fibre-reinforced high strength concrete at elevated temperatures

The present experimental work investigates the build-up of pore pressure at different depths of High Strength Concrete (HSC) and Hybrid-Fibre-Reinforced High Strength Concrete (HFRHSC) when exposed to different heating rates. First, the effect of the measurement technique on maximum pore pressures measured was evaluated. The pressure measurement technique which utilised a sintered metal and silicon oil was found to be the most effective technique for pore pressure measurement. Pore pressure measurements carried out showed that addition of polypropylene fibres is very effective in mitigation of spalling and build-up of pore pressure inside heated HSC. Addition of steel fibres plays some role in pore pressure reduction at relatively higher pressures in deeper regions of concrete during fast heating. Pore pressure development is highly influenced by the rate of heating with fast heating leading to higher pore pressures in the deeper regions of concrete compared to slow heating.

Surface subsidence and uplift above a headrace tunnel in metamorphic basement rocks of the Swiss Alps as detected by satellite SAR interferometry

Surface subsidence associated with the construction of a headrace tunnel in the Swiss Alps at more than 2000 m above sea level (a.s.l.) has been detected at two locations with satellite differential Synthetic Aperture Radar (SAR) interferometry. At the first location, a subsidence trough of about 4 cm in the satellite line-of-sight direction following the headrace tunnel axes has been measured between August 1995 and August 1996. Similar values from SAR data of ascending and descending orbits indicate displacements in the vertical direction of the movement. In the second case, a symmetric cone of depression with a maximum displacement of about 4 cm between 1995 and 1997 has been observed above the tunnel. Differences in the results from satellite SAR data of ascending and descending orbits indicate that the direction of displacement in this second case was not entirely vertical. Large-scale consolidation associated with pore-pressure reduction in the rock mass arising from tunnel drainage at about 200–400 m depth beneath the topographical surface is believed to be the contributing mechanism (Zangerl et al., 2008a, 2008b). Evidence for this process is based on pore pressure recordings in nearby deep wells. In both areas, the subsidence was followed by a small uplift of about one centimeter between 1997 and 1999, after the tunnel was cased with permeable concrete segments. This partial recovery is also visible in pore pressure records and can be related to the elastic components of rock mass deformation.

Geomechanical modeling for CO2 Storage in Wabamun Lake Area of Alberta, Canada

Abstract Based on experience gained from existing sequestration pilot projects and enhanced oil recovery practices, geologic storage is a technically viable means to significantly reduce anthropogenic emissions of CO2. During CO2 injection, increasing pore pressure and temperature reduction create geomechanical deformation of both the reservoir and surrounding rocks. One of the most important concerns with respect to the long term CO2 storage is that stress changes caused by injection could lead to formation fracturing or reactivation of fracture networks and fault movements which could potentially provide pathways for CO2 leakage through previously impermeable rocks. The cumulative CO2 emission from power plants located in Wabamun lake area in central Alberta, Canada reaches 30 Mt/year. Nisku geological formation located in Wabamun lake area was chosen as storage target based on storage capacity, expected ease of injectivity, leakage risks and interference with current petroleum production in the area. In order to determine whether the post-injection stress changes affect the viability of this target formation to act as an effective storage unit, the geomechanical assessment model of the formation has been developed, which couples the flow model and geomechanical model covering all layers from the basement to the surface. This work was a part of a comprehensive feasibility study of large scale CO2 storage potential in the Wabamun area, called the Wabamun Area CO2 Sequestration Project (WASP). The model used rock mechanical properties, reservoir characterization and flow properties of the Nisku provided by extensive work in other teams of the WASP project. The present modeling work focused on evaluation of a single well injecting 1 Mt/year for 50 years, followed by simulation of 50 years of shut-in. The pressure and stress variations were modeled during and after CO2 injection utilizing geomechanical software, GEOSIM, commercial code of Taurus Ltd. In order to increase the well injectivity, the case of allowing fracture initiation and propagation in Nisku was considered. The results show that injection above the fracture pressure will have the potential to increase the well injectivity (to at least 2 Mt/year) but also create the possibility of fracturing the caprock. The possible upward fracture propagation strongly depends on the caprock stress state and mechanical properties. However, the results of the simulation of vertical propagation have been obtained under the most unfavorable assumption of constant minimum stress gradient and are only preliminary. Since injected CO2’s temperature is considerably lower than the formation temperature, thermal effects were incorporated into the models. Decreasing temperature due to CO2 injection will result in stress reduction and smaller surface deformations compared to the isothermal model. Thermal effects of cold CO2 injection will also reduce the fracture (injection) pressure and enhance the horizontal fracture propagation through Nisku. CO2 injection in the Nisku zone is not likely to cause any significant surface heave and is not likely to have any environmental impact associated with surface deformations. Surface deformation data can be used in conjunction with seismic measurements to validate mechanical properties of Nisku and overlying layers. It also can help to plan for the location of the instrumentation and surface monitoring. Given the best estimates of cohesion and friction angle, the probability of reaching shear failure in Nisku or in the caprock is low. However, the likelihood of fracturing due to thermal effects is high. The importance of modeling of thermal effects in CO2 storage is the most important contribution. The results of this study are in agreement with our research on CO2 storage in Ohio River Valley.

Mechanism of drying induced rebound movements in a soil slope in Sai Kung, Hong Kong

This paper reports on a study conducted during 1992–1995 on the progressive movement of a slope in Sai Kung, Hong Kong. In spite of the fact that significant downslope movement was triggered during the rainy seasons, the field data show that remarkable rebound in the uphill direction was mobilized during dry season when the groundwater level dropped. This paper focuses on such rebound deformation which is due to the drying of the soils. To investigate the mechanism of the rebound deformation, a two-dimensional finite element model of the slope was established and a series of hydraulic and mechanical analyses were conducted. It is found that the volumetric contractive behaviour caused by the increase in suction within the unsaturated soil zones and the reduction in pore pressure in the submerged soils due to the lowering of groundwater level induced the rebound deformation. Moreover, the water retention properties of the unsaturated soil zones can also strongly influence such deformation.

Seismic activity triggered by water wells in the Paraná Basin, Brazil

Triggered seismicity is commonly associated with deep water reservoirs or injection wells where water is injected at high pressure into the reservoir rock. However, earth tremors related solely to the opening of groundwater wells are extremely rare. Here we present a clear case of seismicity induced by pore‐pressure changes following the drilling of water wells that exploit a confined aquifer in the intracratonic Paraná Basin of southeastern Brazil. Since 2004, shallow seismic activity, with magnitudes up to 2.9 and intensities V MM, has been observed near deep wells (120–200 m) that were drilled in early 2003 near the town of Bebedouro. The wells were drilled for irrigation purposes, cross a sandstone layer about 60–80 m thick and extract water from a confined aquifer in fractured zones between basalt flow layers. Seismic activity, mainly event swarms, has occurred yearly since 2004, mostly during the rainy season when the wells are not pumped. During the dry season when the wells are pumped almost continuously, the activity is very low. A seismographic network, installed in March 2005, has located more than 2000 microearthquakes. The events are less than 1 km deep (mostly within the 0.5 km thick basalt layer) and cover an area roughly 1.5 km × 5 km across. The seismicity generally starts in a small area and expands to larger distances with an equivalent hydraulic diffusivity ranging from 0.06 to 0.6 m2/s. Geophysical and geothermal logging of several wells in the area showed that water from the shallow sandstone aquifer enters the well at the top and usually forms waterfalls. The waterfalls flow down the sides of the wells and feed the confined, fractured aquifer in the basalt layer at the bottom. Two seismic areas are observed: the main area surrounds several wells that are pumped continuously during the dry season, and a second area near another well (about 10 km from the first area) that is not used for irrigation and not pumped regularly. The main area displays cyclic annual activity, but the second area does not. We explain the earthquake swarms as being triggered by pore pressure diffusion in the fractured basalt layer due to additional pressure from the newly connected surface aquifer. This reaches critically prestressed areas up to a few kilometers away from the wells. During periods of continuous pumping, the reduction of pore pressure in the confined aquifer stops the seismic activity. Our study suggests that this kind of activity may be more common than previously thought and implies that many other cases of small tremors associated with the drilling of water wells may have gone unnoticed.

Effect of increased shear stress along a plate boundary fault on the formation of an out-of-sequence thrust and a break in surface slope within an accretionary wedge, based on numerical simulations

We investigated the effect on accretionary wedge structure of increased shear stress, which describes the frictional sliding resistance along a decollement arising from an increase in material friction or reduction in pore pressure. To clarify the nature of the effect, we performed numerical simulations using two models: a Stable Friction model and an Increased Friction model. The Stable Friction model produced a low-angle, smooth, surface slope and an in-sequence thrust, whereas the Increased Friction model produced a break in surface slope (scarp) and an out-of-sequence thrust (OST) that cuts through the thrust sheet. The OST formed via the connection of segments of two adjacent thrusts, and its formation resulted in a change in the thickening mode of the wedge from thrust-sheet rotation and back-thrust activity to underplating. This contrast in thickening mode between the landward high-friction zone and seaward low-friction zone resulted in the formation of a clear break in slope, as the landward zone is steeper than the seaward zone, consistent with critical taper theory. The subduction of a basement slice or seamount can produce similar structures arising from an increase in resistance to basal shear sliding. However the distinctive structures arising in an accretionary wedge as a result of increased shear sliding resistance include a flat basal plane and absence of slope-failure sediments beneath the OST. These structural features are observed in accretionary wedges of the Nankai Trough off Muroto (Japan), the Sunda Strait, and the Barbados Ridge.

Episodic slow slip events and rate‐and‐state friction

There are several ways of generating episodic slow slip events in models of rate‐and‐state friction. Here I explore the possibility that they arise on velocity‐weakening faults whose length is “tuned” in some sense. Unlike spring‐block sliders, which have a unique critical stiffness for instability, elastically deformable faults have multiple length scales (stiffnesses) relevant to nucleation. Slow slip events occur when the fault is large enough to initiate an event but too small for that event to reach dynamic instability. The available window of lengths depends upon the effective fracture energy of the growing nucleation zone. For the “aging” evolution law this fracture energy increases rapidly with slip speed, and the range of permissible lengths increases nearly as b/(b − a), where a and b are the coefficients of the velocity and state dependence of the frictional strength. This range becomes very large for faults that are near velocity‐neutral (a ∼ b). However, existing lab data firmly favor the “slip” law as a predictor of nucleation style, and for this law the effective fracture energy increases much less rapidly with slip speed. The range of fault lengths capable of hosting slow slip events in this case seems too small to explain why they are common to so many subduction zones. Slow slip can be stabilized over an exceedingly large range of fault lengths if the fracture energy increases with slip speed much more rapidly than for the aging law. An appealing mechanism is inelastic dilation of the fault zone coupled with pore pressure reduction and diffusive recovery.

Effect of pore geometry and dissolved C O2 on ultrasonic transmission during pore pressure changes

It is common practice to use changes in fluid transmission velocity to determine whether a fluid is in gas or liquid phase during pore pressure changes, but transmission amplitude also changes when fluid phase changes during pore pressure reduction. We used two sizes of glass beads in a low-pressure cell to simulate porous rock and conducted experiments with pore spaces filled with distilled water, and with distilled water in which [Formula: see text] of [Formula: see text] was dissolved. Ultrasonic transmission tests above and below bubble point showed that transmission frequencies and amplitudes were higher for distilled water and the smaller beads. There was a greater reduction in frequencies and amplitudes when gas was liberated by scattering from the small gas bubbles associated with the small beads. The water-[Formula: see text] mixture produced higher transmission amplitudes than distilled wateralone, which is consistent with increasing fluid density in the pores. Although Henry’s law was appropriate for predicting the onset of bubbles, the ultrasonic response sensed bubble nucleation before the pressure predicted by Henry’s law was reached. We also found that transmission amplitudes and frequencies changed more quickly than transmission velocity, which changed little by comparison. Our study suggests that for time-lapse monitoring of [Formula: see text] sequestration operations, changes of transmission amplitude and frequency may provide a quantitative assessment of the amount of dissolved [Formula: see text] in connate water. Observations of ultrasonic transmission amplitude and frequency are more important in this regard than velocity observations. This knowledge can be applied where [Formula: see text] migrates or changes phase after sequestration, be it at depth, or as a result of near-surface leakage. Walk-away VSP data can provide a suitable monitoring tool for this purpose.

Consolidation settlements above deep tunnels in fractured crystalline rock: Part 1—Investigations above the Gotthard highway tunnel

Surface subsidence associated with tunnelling in fractured crystalline rock masses is rarely considered to be large enough to be a cause for concern. Recent high precision levelling measurements along the Gotthard Pass road in Central Switzerland, however, have revealed up to 12 cm of subsidence along sections that pass several hundred metres above the Gotthard highway tunnel. Large-scale consolidation associated with pore-pressure reduction in the rock mass arising from tunnel drainage is believed to be the contributing mechanism. Although these settlements may appear to be small compared to those associated with groundwater or oil and gas withdrawal from more compliant porous media, they are large enough to adversely affect the structural integrity of sensitive concrete structures on the surface (e.g. thin-arch concrete dams). This is a concern for the 57 km long Gotthard Base Tunnel, which is currently under construction, as it will pass through similar geological conditions as the Gotthard highway tunnel and underneath several important dams. The prediction of the expected settlements requires a more complete understanding of the processes underlying the settlements than exist at present. This paper (Part 1) and a companion paper (Part 2) are a contribution towards this end. In Part 1 we examine the question of consolidation in fractured crystalline rock, introduce the geodetic data and the geological setting of the subsidence associated with the Gotthard highway tunnel. Possible mechanisms for the consolidation are described and estimates obtained for the key hydro-mechanical parameters that govern their action, such as the normal stiffness of fault zones. These data are used to condition analytical scoping calculations of expected subsidence based upon poro-elastic theory, and to provide input for a series of numerical models used to obtain a more detailed understanding of the data, which are presented in Part 2.

Polypropylene fibres in heated concrete. Part 2: Pressure relief mechanisms and modelling criteria

Polypropylene (PP) fibres play a role in reducing pore pressures via a number of possible mechanisms which include reservoirs (e.g. air bubbles and micro-cracks) to accommodate expanding steam as well as continuous channels for moisture-vapour migration (e.g. via pressure-induced tangential space (PITS) or through spaces vacated by vaporisation of the polypropylene fibres). In the melt stage, PP would not easily transport through the concrete pore structure but would be able to do so in the vapour stage. The effectiveness of the pressure release mechanisms depends not only on the characteristics of PP material but also on the characteristics of the fibre itself in terms of its dimensions. Aspect ratios by themselves are meaningless and the important parameters are the actual fibre diameter and length. Key parameters that could influence pore pressure reduction are the number of fibres, the cumulative length and cumulative surface area of the fibres as well as fibre interconnectivity, all of which increase with reduction in fibre diameter for a given individual fibre length. An optimum individual fibre length would exist that allows both interconnectivity and good dispersion. Criteria for thermo-hydro-numerical modelling would depend upon the physical, chemical, thermal and mechanical properties and their interrelation with the surrounding matrix. Smeared properties can be assumed, although meso-level modelling at the micrometre scale simulating individual fibres and other concrete constituents would be useful, especially in modelling PITS. The assumptions made for modelling would depend upon the transformations taking place as temperature increases during a fire. These are divided into five temperature ranges of 20–100°C, 100–165°C, 165–475°C, 475–550°C and above 550°C.

Vacuum pressure distribution and pore pressure variation in ground improved by vacuum preloading

This paper presents the difference between vacuum pressure and pore pressure reduction for vacuum preloading projects. The experimental results show that the pattern of the fluid flow under vacuum pressure can be classified into three categories—a single-phase water flow, an air–water two-phase flow, and a single-phase air flow. The field test results show that the vacuum pressure reaches the highest value at the ground level and the measured gradients of the vacuum pressure in the vertical direction are approximately 11 kPa/m. It is demonstrated that (i) the treatment area of vacuum preloading cannot be sealed and does not need to be airtight, (ii) the air–water mixture is drawn out from the treatment area under vacuum pressure and the groundwater level drops owing to the presence of air in practice, and (iii) there is an air–water two-phase flow in the unsaturated zone during preloading. The study shows that (i) the vacuum pressure is only a part of the pore pressure reduction along the depth of improving soil; and (ii) the vacuum pressure induces the soil to undergo isotropic consolidation, whereas the pore pressure reduction that is greater than the atmospheric pressure induces the soil to undergo one-dimensional consolidation.