摘要:

A review of underground openings, excavated in varying rock masstypes and conditions, indicates that the initiation of brittlefailure occurs when the damage index, Di, expressed as theratio of the maximum tangential boundary stress to the laboratoryunconfined compressive strength exceeds approx0.4. When thedamage index exceeds this value, the depth of brittle failure around a tunnel can be estimated by using a strengthenvelope based solely on cohesion, which in terms of theHoek-Brown parameters implies thatm= 0. It is proposed that inthe brittle failure process peak cohesion and friction are notmobilized together, and that around underground openings thebrittle failure process is dominated by a loss of the intrinsiccohesion of the rock mass such that the frictional strengthcomponent can be ignored for estimating the depth of brittlefailure, an essential component in designing support for theopening. Case histories were analyzed using the Hoek-Brownfailure criterion, with traditional frictional parameters, and withthe proposed brittle rock mass parameters:m= 0 ands= 0.11. Theanalyses show that use of a rock mass failure criteria withfrictional parameters (m> 0) significantly underpredicts thedepth of brittle failure while use of the brittle parametersprovides good agreement with field observations. Analyses usingthe brittle parameters also show that in intermediate stressenvironments, where stress-induced brittle failure is localized, atunnel with a flat roof is more stable than a tunnel with anarched roof. This is consistent with field observations. Hence,the Hoek-Brown brittle parameters can be used to estimate thedepth of brittle failure around tunnels, the support demand-loadscaused by stress-induced failure, and the optimum geometry of theopening.Key words: spalling, depth of failure, rock mass strength, brittle failure criterion, cohesion loss, Hoek-Brown brittle parameters

ISSN:0008-3674    

DOI:10.1139/t98-072

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

An excavation in soft clay for the construction of a deep basement frequently is accompanied by large ground movements that may damage piles preinstalled at the base of the excavation. In a recent project involving the construction of a 10 m wide, 3.7 m deep drain, the construction method adopted entailed excavating the site soils and then driving precast concrete piles. The excavation was supported by steel sheet piles braced by one level of struts. Large settlements and horizontal movements of the ground were observed during pile driving. These movements exceeded those occurring during the excavation phase. Concrete aprons outside a one-story building adjacent to the excavation were badly damaged during excavation and pile driving. However, the building supported on steel piles was undamaged.Key words: deep excavation, sheet piles, pile driving, ground movement, basement construction.

ISSN:0008-3674    

DOI:10.1139/t98-071

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The reactivity coefficient, defined as lambda = rhowGsSatheta (rhowis the mass density of water,Gsis the specific gravity,Sais the specific surface, and theta is the double-layer thickness), has been proposed as a parameter to assess the sensitivity of a clay-fluid system to changes in physical and chemical properties of the pore fluid. In this study, the liquid limit of a clay-fluid mixture,wL, is shown to be related to the reactivity coefficient. Double-layer theory is applied to determine the void ratio at the liquid limit, which is assumed to occur at a stress close to 10 kPa. Liquid limit data were measured for a bentonite clay at different pore-fluid concentrations; other data were collected from the literature. Both measured and collected data followed the proposed simple expression, suggesting that there is a unique relationship betweenwLand lambda, at least for the three-layer sheet minerals (or for clays withwL> 50). The proposed expression can be used as a simple approach to determine the sensitivity of clay-fluid systems. A parametric study based on the lambda-wLrelationship is presented to study the effects of temperature and pore-fluid concentration on the liquid limit.Key words: clay, specific surface, pore fluid, ion concentration, double layer.

ISSN:0008-3674    

DOI:10.1139/t98-073

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

This note examines the effect of unloading duration on unconfined compression test results. Artificial clayey specimens were prepared using the slurry consolidation technique. Extracted specimens were loaded vertically underK0conditions, and the load was kept constant until the end of primary consolidation. Specimens were unloaded and unconfined compression tests were carried out at different times after unloading. It is observed that the longer the unloading duration, the lower the measured unconfined strength. This behavior is attributed to the presence of negative excess pore pressure that dissipates with time, reducing the strength. Using the measured coefficient of consolidation, the degree of excess pore pressure dissipation and therefore the average mean effective stress near the failure zone can be calculated at the time of failure. Mohr circles are drawn tangential to the total shear envelope, using the calculated mean effective stresses. Reasonable agreement between predicted and measured unconfined compressive strengths has been observed, suggesting that consolidation theory can be adopted to assess the effect of unloading duration on unconfined compressive strength. Finally, engineering applications using a similar concept are briefly discussed.Key words: clays, unloading, consolidation, unconfined compression tests, triaxial tests.

ISSN:0008-3674    

DOI:10.1139/t98-074

出版商:NRC Research Press    

年代:1999

数据来源: NRC

ISSN:0008-3674    

DOI:10.1139/t98-083

出版商:NRC Research Press    

年代:1999

数据来源: NRC

ISSN:0008-3674    

DOI:10.1139/t98-084

出版商:NRC Research Press    

年代:1999

数据来源: NRC

ISSN:0008-3674    

DOI:10.1139/t98-080

出版商:NRC Research Press    

年代:1999

数据来源: NRC

ISSN:0008-3674    

DOI:10.1139/t98-079

出版商:NRC Research Press    

年代:1999

数据来源: NRC

ISSN:0008-3674    

DOI:10.1139/t98-087

出版商:NRC Research Press    

年代:1999

数据来源: NRC

ISSN:0008-3674    

DOI:10.1139/t98-082

出版商:NRC Research Press    

年代:1999

数据来源: NRC