High Vertical Stress Research Articles

Laboratory investigation of shale rock to identify fracture propagation in vertical direction to bedding

Affected by beddings and natural fractures, fracture geometry in the vertical plane is complex in shale formation, which differs from a simple fracture in homogeneous sandstone reservoirs. However, the propagation mechanism of a hydraulic fracture in the vertical plane has not been well understood. In this paper, a true tri-axial pressure machine was deployed for shale horizontal well fracturing simulation experiments of shale outcrops. The effects of multiple factors on hydraulic fracture vertical propagation were studied. The results revealed that hydraulic fracture initiation and propagation displayed four basic patterns in the vertical plane of laminated shale formation. A hydraulic fracture would cross the beddings under the high vertical stress difference between a vertical stress and horizontal minimum stress of 12 MPa, while a hydraulic fracture propagates along the beddings under a low vertical stress difference of 3 MPa. Four kinds of fracture geometry, including a single main fracture, a nonplanar fracture, a complex fracture, and a complex fracture network, were observed due to the combined effects of flow rate and viscosity. Due to the influence of binding strength (or cementing strength) on the fracture communication effects between a hydraulic fracture and the beddings, the opening region of the beddings takes the shape of an ellipse.

Hydraulic Fracture Growth in a Layered Formation based on Fracturing Experiments and Discrete Element Modeling

Hydraulic fracture (HF) height containment tends to occur in layered formations, and it significantly influences the entire HF geometry or the stimulated reservoir volume. This study aims to explore the influence of preexisting bedding planes (BPs) on the HF height growth in layered formations. Laboratory fracturing experiments were performed to confirm the occurrence of HF height containment in natural shale that contains multiple weak and high-permeability BPs under triaxial stresses. Numerical simulations were then conducted to further illustrate the manner in which vertical stress, BP permeability, BP density(or spacing), pump rate, and fluid viscosity control HF height growth using a 3D discrete element method-based fracturing model. In this model, the rock matrix was considered transversely isotropic and multiple BPs can be explicitly represented. Experimental and numerical results show that the vertically growing HF tends to be limited by multi-high-permeability BPs, even under higher vertical stress. When the vertically growing HF intersects with the multi-high-permeability BPs, the injection pressure will be sharply reduced. If a low pumping rate or a low-viscosity fluid is used, the excess fracturing fluid leak-off into the BPs obviously decreases the rate of pressure build up, which will then limit the growth of HF. Otherwise, a higher pumping rate and/or a higher viscosity will reduce the leak-off time and fluid volume, but increase the injection pressure to drive the HF to grow and to penetrate through the BPs.

A new methodology for estimating field horizontal stress from microseismic focal mechanisms

The full stress state in a formation is characterized by the directions and magnitudes of the three principal stresses. It is common practice to take the vertical stress, ϭv, and the minimum and maximum horizontal stresses, ϭhmin and ϭHmax, as the principal stresses in unconventional reservoirs. Considering the relatively high depth of most unconventional reservoirs, which results in a very high vertical stress magnitude, this assumption holds true in most cases unless a geological feature such as a fault or fold changes the stresses locally. The magnitude of vertical stress can easily be determined by integrating the density of overburden rocks from density logs. A good estimate of ϭhmin magnitude can be obtained from well test results such as a mini-frac test or a diagnostic fracture injection test. The magnitude of ϭHmax, however, remains the most challenging stress component to determine.

IN-SITU STRESS MEASUREMENT BY OVERCORING AND HYDRAULIC FRACTURING OF PAHANG-SELANGOR RAW WATER TRANSFER PROJECT

Estimation of in-situ stress orientation and magnitude is necessary for assessing the excavation risks for Pahang-Selangor Raw Water Transfer (PSRWT) tunnel project. However, the in-situ stress state of the rock generally differs according to area and depth. Therefore, the in-situ stress measurements test in the tunnel are determined in three (3) locations, which are at Adit 2, TBM 2, and Adit 3, in which the overburdens are 227, 1130, and 570 m, respectively. The stress relief method of overcoring technique and hydraulic fracturing by high stiffness system were applied for this project. The results demonstrate that the existence of high vertical stress was estimated in particular in the TBM 2. The maximum principal stress is determined nearly along the vertical direction. Meanwhile, the stress in the horizontal plane is relatively small, and the horizontal to vertical stress ratio is less than one (1). The direction of the horizontal stress obtained is N8E, N14W, and N41E. Results indicate that this method is suitable for estimating in-situ stresses in deep tunnels. The above data and their interpretations enhance the stress database for Peninsular Malaysia

Drainage Performance of Different Sizes Tire Chips used Alone and Mixed with Natural Aggregates as Leachate Drainage Layer Material

Results of compressibility and hydraulic conductivity testing performed under different levels of average vertical stress were used to determine the appropriate tire chips size (in particular aspect ratio) and tire chip—gravel random state mix combination. Proper sidewall treatment was provided to reduce side wall friction up to minimum level and constant head conditions were maintained with appropriate design flow rate. Results showed that tire chips (length 25 mm, aspect ratio 1) mixed with 40 mm gravel in the random state in mixing ratio of 3:1 (ds:dg) was observed to undergo minimum strain, maximum hydraulic conductivity and hence free drainage even under high vertical stress levels. By selecting tire chips of appropriate aspect ratio and optimum mix combination for the construction of drainage layer of MSW landfill leachate collection system lower strain response, higher hydraulic conductivity values and relatively free drainage even under anticipated higher vertical stress levels may be achieved.

Countermeasure Method for Stope Instability in Crown Pillar Area of Cut and Fill Underground Mine

Maintaining stability as well as optimizing recovery of crown pillar, a pillar separating surface area with the uppermost stope in overhand cut and fill underground mining method, is important. Failures in stope may lead to crown pillar failures and cause surface subsidence. Increasing crown pillar thickness will increase crown pillar stability yet reduce mining recovery because part of crown pillar is formed by ore body. Preventing stope failure is the key to maintain stability and optimize recovery of crown pillar. Therefore, it is important to study countermeasure method for stope failure especially in crown pillar area. An attempt has been made to investigate the effectiveness of various countermeasures for stope failure in crown pillar area by means of parametric study. The result shows active type support system is effective for supporting stope in high vertical stress condition while the passive one needs to be installed if the stope is opened in high horizontal stress condition. In general, more supporting capacity from both type support systems is needed if the stope is opened in more severe geological condition. Another countermeasures, sill pillar and surface pile, are introduced for stope instability in crown pillar and non-crown pillar area. Sill pillar is an abandoned slice of unstable stope based on stability analysis. Sill pillar is very effective to stabilize stope both in crown pillar and non-crown pillar area, especially for stope in high horizontal stress condition. Sill pillar application in model with stress ratio 2 can optimize 20 meter thickness of crown pillar into 5 meter. Another proposed countermeasure is surface pile. Surface pile can be installed from the surface to improve stability of crown pillar and stope. The most effective use of surface pile is found in simulation of model with stress ratio 0.75 where surface pile can optimize 15 meter thickness of crown pillar into 5 meter.

In situ identification of high vertical stress areas in an underground coal mine panel using seismic refraction tomography

Because high stress anomalies can induce dynamic failures in the underground panels of coal mines, the identification of high stress areas is critically important for mining safety. We have identified areas of high vertical stress of an in-situ longwall panel with seismic refraction tomography, and interpreted our results using rock physics models constrained by core measurements. Since P- and S-velocities in rocks are stress sensitive, mapping of them can provide useful information to identify areas of potential high stress. We have inverted P- and S-velocities of main roof of an in situ panel with refraction tomography, and converted them into vertical stress estimates using a calibration function from core measurements. Those estimates are classified into three groups (high, medium, and low), respectively. Identified zones of high vertical stress show qualitative agreement with monitored in-panel microseismic data. Zones derived from P-waves give a more plausible assessment than S-waves. We also discuss the link between high vertical stress areas and in-panel faults. Because of the impact of the regional stress field and previously mined out voids, the existing fault zone affects the redistribution of vertical stress in the underground panel.

Heat and moisture migration in a geomembrane–GCL composite liner subjected to high temperatures and low vertical stresses

This paper presents the results of an experimental and numerical modelling of heat and moisture migration conducted on a composite liner comprised of a geomembrane (GMB) and a geosynthetic clay liner (GCL), over a compacted subgrade and subjected to prolonged elevated temperatures at low overburden stresses typical of brine storage ponds or solar evaporation ponds. Results are presented for a GMB sitting on a fully hydrated GCL. Heating the top of the composite liner caused a measurable increase in subgrade temperature to at least to 250 mm below the GCL. However, the presence of an air gap, simulating the presence of a wrinkle in the geomembrane, at the interface between the GMB and the GCL reduced the impact of the high temperatures on the subgrade temperature profile with depth. The change in temperature profile was accompanied by moisture migration from the GCL to the subgrade material. However no desiccation cracks were observed in the GCL and the bentonite was still in a gel form at the end of the time period investigated. Numerical modelling using finite element method (FEM) was performed to simulate the results obtained experimentally. It was found to predict accurately the temperature changes that have occurred in the subgrade material and moisture changes that occurred in both the GCL and subgrade materials.

A finite element model for pre‐stressed or post‐tensioned concrete wind turbine towers

AbstractWind turbine support towers have been traditionally formed of structural steel tubular sections, being fabricated in large sections under factory conditions before being transported to site for erection. Given the trend towards developing turbines with hub heights in excess of 90 m, it is now necessary to develop towers of concrete and other materials that can provide improved dynamic properties and ease transportation difficulties over the structural steel solutions. Concrete towers of this height require pre‐stressing to overcome high vertical stresses induced in bending, which would otherwise lead to cracking in the concrete, with a resulting reduction in the tower's natural frequency. In order to properly understand the behaviour of concrete towers, it is necessary to take account of both material and geometric non‐linearity. Material non‐linearity of concrete is well understood, and geometric non‐linearity arises because of the imposition of an initial stress into the concrete by way of the application of pre‐stress. In this paper, a finite element model is proposed, which will describe the concrete as a continuum of four‐noded, two‐dimensional Reisser–Mindlin shell elements. The pre‐stressing tendons are to be represented by one‐dimensional bar elements, with an imposed pre‐stress. For the numerical examples considered in the paper, tendons are modelled to be post‐tensioned and debonded. The effect of varying the design parameter of magnitude of pre‐stress and the time dependence of pre‐stress force has been investigated using the model described. The impact that concrete compressive strength had on overall tower stiffness was also investigated. Copyright © 2014 John Wiley & Sons, Ltd.

Bimodal pore size distribution of a high-plasticity compacted clay

The microstructure of compacted clay plays an important role in its hydraulic and mechanical behaviour. However, little experimental evidence is available for high-plasticity clay compacted on the wet side of the standard Proctor optimum water content. The results of an oedometric and microstructural investigation (mercury intrusion porosimetry) on reconstituted and compacted high-plasticity clay are presented. The microstructural results indicate that a bimodal pore size density function is not limited to clays compacted on the dry side of the standard Proctor optimum. The compacted clay prepared on the wet side can display a bimodal pore size distribution dependent on the level of compaction, current void ratio and degree of saturation. The bimodal distribution of the compacted clay can be lost by compression to high vertical stress accompanied by an increase in the degree of saturation. In this case the compression path ultimately approaches the normal compression line of the reconstituted saturated material. The results demonstrate that the observed behaviour of the compacted clay can be interpreted with reference to the reconstituted clay.

Cutting Roof Roadway Regional Stress Characteristics Numerical Analysis of Thin Coal Seam Group

The stress state characteristic of the cutting roof roadway region was simulated and analyzed by FLAC3D. The results showed that the low side of return airway and gas roadway had higher vertical stress while the horizontal stress concentration was obviously in floor and low side of the return airway, the stress of cutting roof roadway was obviously, thus the level of rock burst hazard increased. range of stress concentration are 5~10m when using cutting roof roadway, thus distance between two cutting roof roadway are no less than 20m, to avoid stress concentration regions by two adjacent cutting roof roadway superposition.

Experimental study on vibration reduction by using soilbags

The effectiveness of soilbags in reducing mechanical vibration is investigated, and the influential factors are examined through a series of laboratory tests, including cyclic lateral shear tests, vertical excitation tests and small shaking table tests as well as a soilbag-filled trench field test. The test results illustrate that soilbags have a relatively high damping ratio and variable horizontal stiffness, which are greatly influenced by the materials inside the bags at low vertical stress but become nearly independent of the materials inside the bags at high vertical stress. We find that vertical and horizontal vibrations can be effectively reduced through the use of soilbags.

A Novel Reinforcement Pattern for Exterior Reinforced Concrete Beam-Column Joint

Abstract Current design philosophy permits moment-resisting frames subjected to cyclic loading to be induced into the inelastic range where the forces that develop in parts of the structure will exceed their design values. In this phase of inelastic range the beam-column joints are obliged to resist high horizontal and vertical shear stresses coming from the adjacent beams and columns. This occurs during a large number of inelastic cycles and while the joints need to dissipate large energy values. However, in spite of systematic research that has been carried out investigating the behavior of reinforced concrete beam-column connections in the past 20 years in the US, New Zealand, Japan, we have not succeeded in improving satisfactorily this behavior, the specific features of which are extreme decrease of strength, stiffness and energy dissipation capacity, all of which may lead to collapse of the whole building. Attempts at improving beam-column joint performance resulted in nonconventional methods of reinforcing, such as the use of fiber concrete. Paulay, Park and Phillips first introduced the concept of using inclined main reinforcement to prevent brittle failure in short reinforced concrete coupling beams. Following the work of Paulay, Park and Phillips, Minami and Wakabayashi and Tegos and Penelis applied the idea in short columns. The results showed that the columns with inclined reinforcing bars performed considerably better than those with conventional reinforcement. Beam-column joints have many similarities in geometry, state of stress and mechanical behavior with short columns and coupling beams. For these reasons, the efficiency of a novel nonconventional reinforcing pattern by the use of crossed inclined bars in column is examined as a way of improving the seismic performance of type 2 exterior beam-column joints.

Parametric study of the physical properties of hydrate‐bearing sand, silt, and clay sediments: 2. Small‐strain mechanical properties

The small‐strain mechanical properties (e.g., seismic velocities) of hydrate‐bearing sediments measured under laboratory conditions provide reference values for calibration of logging and seismic exploration results acquired in hydrate‐bearing formations. Instrumented cells were designed for measuring the compressional (P) and shear (S) velocities of sand, silts, and clay with and without hydrate and subject to vertical effective stresses of 0.01 to 2 MPa. Tetrahydrofuran (THF), which is fully miscible in water, was used as the hydrate former to permit close control over the hydrate saturation Shyd and to produce hydrate from dissolved phase, as methane hydrate forms in most natural marine settings. The results demonstrate that laboratory hydrate formation technique controls the pattern of P and S velocity changes with increasing Shyd and that the small‐strain properties of hydrate‐bearing sediments are governed by effective stress, σ′v and sediment specific surface. The S velocity increases with hydrate saturation owing to an increase in skeletal shear stiffness, particularly when hydrate saturation exceeds Shyd≈ 0.4. At very high hydrate saturations, the small strain shear stiffness is determined by the presence of hydrates and becomes insensitive to changes in effective stress. The P velocity increases with hydrate saturation due to the increases in both the shear modulus of the skeleton and the bulk modulus of pore‐filling phases during fluid‐to‐hydrate conversion. Small‐strain Poisson's ratio varies from 0.5 in soft sediments lacking hydrates to 0.25 in stiff sediments (i.e., subject to high vertical effective stress or having high Shyd). At Shyd ≥ 0.5, hydrate hinders expansion and the loss of sediment stiffness during reduction of vertical effective stress, meaning that hydrate‐rich natural sediments obtained through pressure coring should retain their in situ fabric for some time after core retrieval if the cores are maintained within the hydrate stability field.

Variation of vertical stress in the Carnarvon Basin, NW Shelf, Australia

Variations in vertical stress magnitudes can be attributed to lateral facies variations, diagenesis, localized uplift and disequilibrium compaction overpressure. Vertical stress magnitudes have been calculated using density and checkshot data from 47 wells in the Carnarvon Basin, on the NW Shelf of Australia. The vertical stress ranges from 20.2 MPa km − 1 to 23.8 MPa km − 1 below seabed; demonstrating a significant variation across the basin. The largest vertical stress magnitudes were calculated for the Dampier Sub-Basin, the eastern Barrow Sub-Basin and on the Peedamullah Shelf. The smallest vertical stress magnitudes were calculated in the Kangaroo Syncline, the western Barrow Sub-Basin and offshore across the Exmouth Plateau. High rock densities, thus, high vertical stress magnitudes, can be due to uplifted rock that is over-compacted relative to its current depth. In the Carnarvon Basin, areas of elevated vertical stress magnitudes correspond to areas of identified uplift. Uplift at the Bambra Anticline has previously been estimated to be approximately 900 m. Calculations within the Bambra-4 well demonstrate that a 900 m increase in burial depth of a regionally consistent lithostratigraphic unit (the Muderong Shale) equates to a 0.22 g cm − 3 increase in rock density. Thus, the difference between uplifted, over-compacted rock and normally-compacted rock (no uplift) is equivalent to a difference in the vertical stress magnitude of 2.1 MPa km − 1 at the Bambra Anticline. This value, totalling the difference between over-compacted (uplifted) and normally-compacted sedimentary rock (2.1 MPa km − 1 ), accounts for some part of the 3.6 MPa km − 1 variation in vertical stress magnitude observed across the Carnarvon Basin. Therefore, recent tectonic uplift has had a significant impact on the present-day magnitude of the vertical stress and is interpreted to be responsible for the majority of observed variation of vertical stress in the Carnarvon Basin.

Influence of Surface Drainage Conditions on Dynamic Response of Saturated Asphalt Pavement

AbstractAt first, finite element model of axisymmetric transient dynamic analysis for saturated asphalt pavement subjected to dynamic loading was built, whose loading condition, boundary condition and material parameters were also listed out. Then the distribution along the depth and time history variation of vertical displacement, vertical stress, horizontal stress, shear stress and pore pressure in surface drainage case and surface non-drainage case were compared and analyzed. The result indicates that (1) the surface non-drainage condition has less influence on the displacement field, while it has an obvious impact on the stress field; (2) for the surface non-drainage case, a higher vertical stress appears at the outsides of vehicle wheels, what is more, a higher positive pore pressure is produced, which makes asphalt pavement structure in a more adverse situation.

Discussion on mechanisms and controls on the formation of sand intrusions

D. M. D. James writes: I congratulate the authors on a very clear and comprehensive account of a widespread and complex phenomenon of increasing economic importance. However, presumably for didactic ease, they have made some simplifications that I feel are counterproductive for full physical understanding. It is not entirely clear from the paper that sand intrusions require a concatenation of source/seal properties that on the face of it are very rare. These are that the source sand bed must have a negligible grain cohesion typical of burial at very low vertical effective stress (not shallow depth, necessarily) and that the seal must have effectively negligible permeability, typical of burial at very high vertical effective stress (and often associated with high cohesion). In most basins sands that can be fluidized are interbedded with ‘leaky‘ shales of high permeability (for shales) that do not sustain high overpressure during normal compaction. However, such high overpressure is vital because the intrusion is powered by a pressure decline at source that provides the mechanical energy to strain elastically the intrusion walls and to drive the ascent of the fluidized sand against frictional resistance. This cannot happen if fracture takes place when fluid pressure reaches a value merely equal to that of the horizontal stress (as illustrated by the authors in their figs 6 and 8) as this lies on the closure gradient of any fracture, not its inception gradient. How then can the requisite overpressure for fracture initiation and propagation be attained at the shallow depths normally associated with low vertical effective stress? Of the obvious ways around this problem, the development of zero permeability diagenetic or clathrate seals seems unlikely to be a universal solution. More likely, and not mentioned in the paper, is that the source pressure builds up at such a high …

Stress distribution and soil displacement under a rubber-tracked and a wheeled tractor during ploughing, both on-land and within furrows

Ploughing is a field operation considered to be associated with severe soil compaction. Two experiments were carried out in Denmark on Eutric Cambisols with a Claas Challenger 2-65 E rubber-tracked tractor with a total weight of 185 kN. In Experiment A, the vertical stress under the track during ploughing was measured by stress sensors at 0.1 m depth in order to study stress distribution. In Experiment B, the vertical soil displacement and vertical normal soil stress during ploughing were measured simultaneously at three different depths. The tracked tractor was compared to a wheeled tractor with a total weight of 97 kN. The rubber-tracked tractor and the wheeled tractor were pulling a plough with 12 and 7 bodies, respectively. The tracked tractor ploughed on-land, whereas the wheeled tractor ploughed both on-land and conventionally (two wheels running in the furrow). In Experiment A, vertical stress was found to be much higher under the rear than under the front part of the tracks. It was caused by unsuitable adjustment of the plough to the tractor for given field conditions. By lowering the point of application of the draught force induced by the plough, maximum vertical stress was reduced from 304 to 158 kPa. However, the vertical stress was concentrated under the supporting rollers and wheels as well as under the centre line of the track, so that maximum stress was 3.8 times higher than average stress. In Experiment B, the vertical stress was higher below the wheeled tractor than below the tracked tractor, which had been adjusted as in Experiment A. No significant difference in maximum vertical soil stress was found between the tracked tractor and the wheeled tractor ploughing on-land at any depth. The vertical stress at 0.3 and 0.5 m depth was significantly higher during conventional ploughing (i.e. under the in-furrow wheels) than during on-land ploughing. The higher vertical stress also resulted in larger vertical soil displacement at 0.3 m depth under the in-furrow wheels when ploughing conventionally than under the on-land ploughing tractor. At 0.5 m depth, no residual vertical soil displacement occurred because the soil strength was very high. The results clearly demonstrated that on-land ploughing may reduce the risk for subsoil compaction compared to conventional ploughing. Using tracks instead of wheels may further reduce this risk. However, this is only the case if the tractor is well balanced. Thus, each particular tillage tool should be adjusted to the tractor, also with respect to the soil type and the field conditions.

The effect of callus removal on dynamic plantar foot pressures in diabetic patients.

Clinical observation suggests that neuropathic foot ulceration frequently occurs beneath plantar callosities and in areas of high dynamic shear and vertical stress underneath the foot during walking. Seventeen diabetic patients had dynamic foot pressure measurements made before and after the removal of a total of 43 forefoot plantar callosities. Peak pressures (mean +/- SE) in the treated areas were reduced by 26% from 14.2 +/- 1.0 to 10.3 +/- 0.9 kg cm-2 (p less than 0.001), with reductions at 37 of the 43 sites and in all patients. Mean heel pressures were not significantly different (5.0 +/- 0.6 vs 4.9 +/- 0.6 kg cm-2). These results suggest that callus may act as a foreign body elevating plantar pressures and that a significant reduction in pressure is achieved by local chiropody treatment.

Preuplift Method to “Anchor” Liquid Storage Tanks

Steel tanks for liquid storage often consist of a cylindrical shell attached to a steel bottom plate which rests on the ground. In many cases neither the bottom plate nor the cylindrical shell are fixed to the ground. When such an unanchored tank is subjected to seismic ground shaking, the lateral inertia of the contained liquid contributes to the overturning moment, but its weight cannot contribute to the stabilizing moment until some uplift of the bottom plate occurs. The hold‐down force, due to the weight of liquid resting on the uplifted portion of the bottom plate, provides the necessary resistance against overturning. However, this benefit of uplift comes at the cost of very high vertical compressive stresses in the tank wall at the base in the region diametrically opposite to where the maximum uplift occurs. The benefits of uplift can be reaped without some of its detrimental effects if the tank wall is preuplifted all around the circumference by means of a ring filler. An annular portion of the base plate is also preuplifted, but is not supported by the ring filler. The weight of liquid resting on that preuplifted portion of the base plate then becomes effective in resisting the overturning moment. For a 127 mm diameter Mylar tank, it is shown by experiment and analysis that preuplift increases the lateral load capacity by a factor of up to 2.