Rock Stress Measurements for Unlined Pressure Tunnels: A True Triaxial Laboratory Experiment to Investigate the Ability of a Simplified Hydraulic Jacking Test to Assess Fracture Normal Stress

Rock Mechanics and Rock Engineering ◽

10.1007/s00603-021-02452-9 ◽

2021 ◽

Author(s):

Henki Ødegaard ◽

Bjørn Nilsen

Keyword(s):

Normal Stress ◽

Test Method ◽

Sudden Increase ◽

Normal Stresses ◽

True Triaxial ◽

Step Rate ◽

Stress Measurements ◽

Fracture Closure ◽

Rate Test ◽

Hydraulic Jacking

AbstractTo avoid hydraulic failure of unlined pressure tunnels, knowledge of minimum principal stress is needed. Such knowledge is only obtainable from in situ measurements, which are often time-consuming and relatively costly, effectively limiting the number of measurements typically performed. In an effort to enable more stress measurements, the authors propose a simplified and cost-effective stress measuring method; the Rapid Step-Rate Test (RSRT), which is based on existing hydraulic testing methods. To investigate the ability of this test to measure fracture normal stresses in field-like conditions, a true triaxial laboratory test rig has been developed. Hydraulic jacking experiments performed on four granite specimens, each containing a fracture, have been performed. Interpretation of pressure-, flow- and acoustic emission (AE) data has been used to interpret fracture behaviour and to assess fracture normal stresses. Our experimental data suggest that the proposed test method, to a satisfactory degree of reliability, can measure the magnitude of fracture normal stress. In addition, a clear correlation has been found between fracture closure and sudden increase in AE rate, suggesting that AE monitoring during testing can serve as a useful addition to the test. The rapid step-rate test is also considered relevant for field-scale measurements, with only minor adaptions. Our findings suggest that the RSRT can represent a way to get closer to the ideal of performing more testing along the entire length of pressure tunnel, and not only at key locations, which requires interpolation of stress data with varying degree of validity.

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In-Situ Stress Measurements at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) Site

Energies ◽

10.3390/en13215842 ◽

2020 ◽

Vol 13 (21) ◽

pp. 5842

Author(s):

Pengju Xing ◽

John McLennan ◽

Joseph Moore

Keyword(s):

Geothermal Energy ◽

Stress Gradient ◽

In Situ Stress ◽

Perspective View ◽

Step Rate ◽

Stress Measurements ◽

Closure Stress ◽

Fracture Closure ◽

In Situ Stress Measurements

A scientific injection campaign was conducted at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) site in 2017 and 2019. The testing included pump-in/shut-in, pump-in/flowback, and step rate tests. Various methods have been employed to interpret the in-situ stress from the test dataset. This study focuses on methods to interpret the minimum in-situ stress from step rate, pump-in/extended shut-in tests data obtained during the stimulation of two zones in Well 58-32. This well was drilled in low-permeability granitoid. A temperature of 199 °C was recorded at the well’s total depth of 2297 m relative to the rotary Kelly bushing (RKB). The lower zone (Zone 1) consisted of 46 m of the openhole at the toe of the well. Fractures in the upper zone (Zone 2) were stimulated between 2123–2126 m measured depths (MD) behind the casing. The closure stress gradient variation depended on the depth and the injection chronology. The closure stress was found to increase with the pumping rate/volume. This stress variation could indicate that poroelastic effects (“back stress”) and the presence of adjacent natural fractures may play an important role in the interpretation of fracture closure stress. Further, progressively increasing local total stresses may, consequently, have practical applications when moderate volumes of fluid are injected in a naturally fractured or high-temperature reservoir. The alternative techniques that use pump-in/flowback tests and temperature signatures provide a valuable perspective view of the in-situ stress measurements.

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Evaluation of the Constant Strain Rate Test Method for Testing Stress Corrosion Cracking in Aluminum Alloys

CORROSION ◽

10.5006/0010-9312-34.1.32 ◽

1978 ◽

Vol 34 (1) ◽

pp. 32-36 ◽

Cited By ~ 10

Author(s):

HENK. F. de JONG

Keyword(s):

Aluminum Alloys ◽

Strain Rate ◽

Stress Corrosion ◽

Stress Corrosion Cracking ◽

Constant Strain Rate ◽

Corrosion Cracking ◽

Test Method ◽

Rate Test

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Shear strength and dilative characteristics of an unsaturated compacted completely decomposed granite soil

Canadian Geotechnical Journal ◽

10.1139/t10-015 ◽

2010 ◽

Vol 47 (10) ◽

pp. 1112-1126 ◽

Cited By ~ 42

Author(s):

Md. Akhtar Hossain ◽

Jian-Hua Yin

Keyword(s):

Shear Strength ◽

Normal Stress ◽

Matric Suction ◽

Water Retention Curve ◽

Direct Shear ◽

Normal Stresses ◽

Soil Water Retention Curve ◽

Strength Data ◽

Decomposed Granite ◽

Completely Decomposed Granite

Shear strength and dilative characteristics of a re-compacted completely decomposed granite (CDG) soil are studied by performing a series of single-stage consolidated drained direct shear tests under different matric suctions and net normal stresses. The axis-translation technique is applied to control the pore-water and pore-air pressures. A soil-water retention curve (SWRC) is obtained for the CDG soil from the equilibrium water content corresponding to each applied matric suction value for zero net normal stress using a modified direct shear apparatus. Shear strength increases with matric suction and net normal stress, and the failure envelope is observed to be linear. The apparent angle of internal friction and cohesion intercept increase with matric suction. A greater dilation angle is found at higher suctions with lower net normal stresses, while lower or zero dilation angles are observed under higher net normal stresses with lower suctions, also at a saturated condition. Experimental shear strength data are compared with the analytical shear strength results obtained from a previously modified model considering the SWRC, effective shear strength parameters, and analytical dilation angles. The experimental shear strength data are slightly higher than the analytical results under higher net normal stresses in a higher suction range.

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Nondestructive residual stress measurements in railroad wheels using the low-field magnetoacoustic test method

NDT & E International ◽

10.1016/0963-8695(91)90744-n ◽

1991 ◽

Vol 24 (1) ◽

pp. 44

Author(s):

M. Namkung ◽

D. Utrata

Keyword(s):

Residual Stress ◽

Test Method ◽

Stress Measurements ◽

Low Field ◽

Railroad Wheels

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Investigation of the Stress - Induced Gel - Sol Transition in Polymers Using Parallel Superposition and Normal Stress Measurements

Progress and Trends in Rheology V ◽

10.1007/978-3-642-51062-5_226 ◽

1998 ◽

pp. 467-468

Author(s):

B. A. de L. Costello

Keyword(s):

Normal Stress ◽

Stress Measurements

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A temperature- and stress-controlled failure criterion for ice-filled permafrost rock joints

The Cryosphere ◽

10.5194/tc-12-3333-2018 ◽

2018 ◽

Vol 12 (10) ◽

pp. 3333-3353 ◽

Cited By ~ 6

Author(s):

Philipp Mamot ◽

Samuel Weber ◽

Tanja Schröder ◽

Michael Krautblatter

Keyword(s):

Normal Stress ◽

Failure Criterion ◽

Slope Failure ◽

Rock Slope ◽

Shear Resistance ◽

Stress Conditions ◽

Rock Joints ◽

Normal Stresses ◽

Permafrost Rock ◽

Limestone Sample

Abstract. Instability and failure of high mountain rock slopes have significantly increased since the 1990s coincident with climatic warming and are expected to rise further. Most of the observed failures in permafrost-affected rock walls are likely triggered by the mechanical destabilisation of warming bedrock permafrost including ice-filled joints. The failure of ice-filled rock joints has only been observed in a small number of experiments, often using concrete as a rock analogue. Here, we present a systematic study of the brittle shear failure of ice and rock–ice interfaces, simulating the accelerating phase of rock slope failure. For this, we performed 141 shearing experiments with rock–ice–rock “sandwich”' samples at constant strain rates (10−3 s−1) provoking ice fracturing, under normal stress conditions ranging from 100 to 800 kPa, representing 4–30 m of rock overburden, and at temperatures from −10 to −0.5 ∘C, typical for recent observed rock slope failures in alpine permafrost. To create close to natural but reproducible conditions, limestone sample surfaces were ground to international rock mechanical standard roughness. Acoustic emission (AE) was successfully applied to describe the fracturing behaviour, anticipating rock–ice failure as all failures are predated by an AE hit increase with peaks immediately prior to failure. We demonstrate that both the warming and unloading (i.e. reduced overburden) of ice-filled rock joints lead to a significant drop in shear resistance. With a temperature increase from −10 to −0.5 ∘C, the shear stress at failure reduces by 64 %–78 % for normal stresses of 100–400 kPa. At a given temperature, the shear resistance of rock–ice interfaces decreases with decreasing normal stress. This can lead to a self-enforced rock slope failure propagation: as soon as a first slab has detached, further slabs become unstable through progressive thermal propagation and possibly even faster by unloading. Here, we introduce a new Mohr–Coulomb failure criterion for ice-filled rock joints that is valid for joint surfaces, which we assume similar for all rock types, and which applies to temperatures from −8 to −0.5 ∘C and normal stresses from 100 to 400 kPa. It contains temperature-dependent friction and cohesion, which decrease by 12 % ∘C−1 and 10 % ∘C−1 respectively due to warming and it applies to temperature and stress conditions of more than 90 % of the recently documented accelerating failure phases in permafrost rock walls.

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