摘要:

An experimental study of piping erosion is presented. Various artificial granular filter and base soil combinations are tested in a permeameter under variable confining pressures to determine the critical gradient where soil erodes through the filter. Previous research has concentrated on establishing a filter to soil grain size ratio criteria, typicallyD15f/D85s< 4, which separates safe from potentially unsafe filters. These works often ignored self-filtration zone formation phenomena and rarely documented the influence of variables such as confining pressure, filter thickness, and hydraulic gradient. To adequately control all variables that may influence piping erosion, a new permeameter was designed and careful attention was paid to sample preparation. Artificial glass beads were water pluviated to permit consistent repeatable uniform samples. By monitoring head, settlement, confining pressure, amount of eroded soil, and water outflow rate, the onset of piping can be determined. It is shown that the grain-size ratio is the most important parameter affecting piping erosion. A soil-filter system withD15f/D85s< 8 will not fail, whereas aD15f/D85s> 12 will not be able to retain base soil. For 8 <D15f/D85s< 12, piping will only occur if the hydraulic gradient exceeds a critical threshold. The critical gradient is lower if the head is rapidly increased, as a filtration zone is inhibited from forming. A very thin filter has a similar effect. Stability is somewhat inversely related to the confining pressure level for small grain-size ratios.Key words: filters, seepage forces, confining stress, piping erosion.

ISSN:0008-3674    

DOI:10.1139/t99-116

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The estimation of base resistance is a key step in the design of piles embedded in moderately dense to dense sand. Calibration chamber plate load tests are sometimes used to investigate the base load - settlement relationship of nondisplacement piles in sand. In such tests, the sand specimens are carefully prepared to simulate the installation of nondisplacement piles. In this paper, calibration chamber tests are analyzed using the finite element method; experimental and numerical results are compared. The finite element models are axisymmetric and use a nonlinear, elastic-plastic constitutive model. Plate resistance values predicted using the finite element analysis are shown to be in good agreement with measured values, which validates the proposed numerical model. Questions regarding the existence of calibration chamber size effects have not, to this date, been adequately addressed. Finite element analyses of both pile base resistance and plate resistance for sands with various relative densities and stress states show that size effects are usually small for settlement levels of interest in practice. This suggests that the use of calibration chambers in pile base capacity studies is justified.Key words: calibration chambers, plate load tests, constitutive modeling, size effects, piles, sands.

ISSN:0008-3674    

DOI:10.1139/t99-061

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Recently there has been an increased interest in the study of the nonlinearity in soil response for large strains through in situ earthquake observations. In this paper, the downhole array acceleration data recorded at a reclaimed island, Kobe, during the 1995 Kobe earthquake are used to study nonlinear site effects. Particular attention is given to the liquefaction-induced nonlinear effects on the recorded ground motions. By using the spectral ratio and the spectral-smoothing technique, the characteristics of the ground motions are analyzed. It is shown that the peak frequencies in spectral ratios were shifted to lower frequencies when the strongest motions occurred. The increase in the predominant period was caused primarily by a strong attenuation of low-period waves, rather than by amplification of long-period motions. Based on the spectral analyses, the nonlinearity occurring in the shallow liquefied layer during the shaking event is identified, manifested by a significant reduction of the shear modulus. A fully coupled, inelastic, finite element analysis of the response of the array site is carried out. The stress-strain histories of soils and excess pore-water pressures at different depths are calculated. It is suggested that the stress-strain response and the build up of pore pressure are well correlated to the variation of the characteristics of ground motions during the shaking history.Key words: site response, ground motion, nonlinearity, soil liquefaction, array records, Kobe earthquake.

ISSN:0008-3674    

DOI:10.1139/t99-092

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The La Grande 1 (LG-1) hydroelectrical project, part of the La Grande Complex in Northern Quebec, required the construction of a 2444 m long dyke on the north bank of the river. The presence of sensitive marine clay, covered with deltaic sand and silt deposit, and river sand deposit, called for special design features such as downstream bank and upstream bank stabilization berms to avoid the occurrence of potentially disastrous retrogressive slides. This paper describes the geotechnical and hydrogeological conditions of the northern terrace and presents the different construction phases of the riverbank stabilization, with emphasis on the control of groundwater pressures in the lower aquifer by the use of relief wells.Key words: sensitive clay, river bank, dyke, rockfill, relief well, slope stability.

ISSN:0008-3674    

DOI:10.1139/t99-083

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Tapered piles have a substantial advantage with regard to their load-carrying capacity in the downward frictional mode. The uplift performance of tapered piles, however, has not been fully understood. This paper describes the results of an experimental investigation into the characteristics of the uplift performance of tapered piles. Three instrumented steel piles with different degrees of taper were installed in cohesionless soil and subjected to compressive and tensile load tests. The soil was contained in a steel soil chamber and pressurized using an air bladder to facilitate modelling the confining pressures pertinent to larger embedment depths. The results of this study indicated that the pile axial uplift capacity increased with an increase in the confining pressure for all piles examined in this study. The ratios of uplift to compressive load for tapered piles were less than those for straight piles of the same length and average embedded diameter. The uplift capacity of tapered piles was found to be comparable to that of straight-sided wall piles at higher confining pressure values, suggesting that the performance of actual tapered piles (with greater length) would be comparable to that of straight-sided wall piles. Also, the results indicated that residual stresses developed during the compressive loading phase and their effect were more significant on the initial uplift capacity of piles, and this effect was more pronounced for tapered piles in medium-dense sand.Key words: tapered piles, uplift, axial response, load transfer, experimental modelling.

ISSN:0008-3674    

DOI:10.1139/t99-090

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The satisfactory disposal of scrap tires is a major environmental problem worldwide. This waste occupies valuable space in landfill sites, and tire stockpiles pose serious health and fire hazards. The use of scrap tires as reinforcement for construction of retaining walls and slopes is a viable method towards reduction of this waste. This paper describes the construction of a 57 m high × 17 m wide instrumented test fill, comprising both retaining wall and reinforced slope sections. Approximately 10 000 whole tires and tires with one sidewall removed, tied together with polypropylene rope, were used in both cohesionless and cohesive backfills. The testing program also included plate loading tests, field pull-out tests on tire mats, water-quality assessment in the field and laboratory, and other complementary laboratory testing. This first paper, in a series of three, demonstrates the practical feasibility of constructing reinforced earth fills using scrap tires. Results of large plate load tests and the field behaviour with particular reference to the design of the retaining wall sections are presented. The paper emphasizes the role of negative wall friction in increasing the active thrust when the retaining wall becomes more compressible than the backfill. Recommendations for the design of retaining walls using scrap tires are presented.Key words: scrap tires, earth reinforcement, retaining walls, reinforced slopes, plate load test, construction, performance

ISSN:0008-3674    

DOI:10.1139/t99-084

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The disposal of scrap tires has become a major environmental concern. The reuse of scrap tires in the reinforcement of earth structures can provide an attractive solution in reducing the number of used tires disposed in overcrowded landfills. This paper, the second in a series of three papers, discusses the behaviour of slopes reinforced with scrap tires and proposes design recommendations. A mat-reinforced slope can either fail by pull-out of the reinforcement or due to rupture of the attachment tying the tires. A large number of pull-out tests were performed on whole tires and tires with one sidewall removed embedded in sand and cohesive backfill. The pull-out resistance of tire mat reinforcement was primarily governed by the effective shear strength of the soil, and therefore it can provide an efficient means of reinforcement. However, large displacements were required to fully mobilize the ultimate pull-out capacity which must be considered in design.Key words: pull-out tests, scrap tires, reinforced slope, performance, design guidelines.

ISSN:0008-3674    

DOI:10.1139/t99-085

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A reinforced earthfill was constructed using approximately 10 000 scrap tires. Water samples were collected from a drainage system installed below the tire-reinforced earthfill and analyzed for chemical quality. Additional tests on water quality were performed in laboratory test columns in which tire chips were embedded in sand or clay to provide a conservative estimate of the potential release of toxic compounds. Field monitoring of the effluent indicated that no significant adverse effects on groundwater quality had occurred over a period of 2 years. Laboratory batch tests performed on tire chips embedded in sand provided evidence of an increase in solution of certain metal elements which in some cases exceeds their respective drinking-water standards. This increase was attributed to the exposed steel reinforcement found in the tire chips. The amount of organic compounds leached from the tire chips decreased with the number of exposure periods or pore volumes flushed through the soil.Key words: whole scrap tires, water quality, tire chips, field investigation, laboratory lysimeter tests.

ISSN:0008-3674    

DOI:10.1139/t99-086

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

This study describes the behavior of embankments stabilized with prefabricated vertical drains installed in soft clay foundations, based on several fully instrumented case histories selected from Thailand and Malaysia. A multidrain analysis is conducted based on an equivalent, plane strain model developed by the authors. The deformation of soft clay beneath an embankment (i.e., along and away from the centerline) is predicted and compared with the available field measurements. The effects of both smear and well resistance are incorporated in the analytical formulation, which is used in conjunction with the modified Cam-clay theory. It is demonstrated that the installation of vertical drains significantly increases the rate of settlement, improves the dissipation of pore-water pressure, and decreases the lateral deformation of the soft clay foundation. To obtain better predictions, especially of pore pressures and lateral displacements, the equivalent discharge capacity of the drains needs to be modeled appropriately. The inclusion of the effects of smear and well resistance in the analysis of vertical drains improves the accuracy of predictions significantly, in comparison with the field measurements.Key words: clay, consolidation, embankment, finite element method, settlement, smear zone, vertical drain.

ISSN:0008-3674    

DOI:10.1139/t99-115

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A key requirement in the assessment of landslide risk in such densely populated urban areas as Hong Kong consists of the prediction of potential runout distance or the extent of the subsequent debris flow. This paper presents a three-dimensional dynamic model of unsteady gravity-driven debris flow. The Lagrangian Galerkin finite element method is used to determine the nodal velocity and depth of soil column elements within the sliding mass, with the momentum and mass conservation mathematically closed within the soil column elements. The numerical solution also features a lumped mass matrix and a volume-weighted procedure. The method of least squares approximation plays a smoothing role which enhances stability and efficiency of the numerical solution scheme. The nodal elevation during sliding is obtained via a dynamic bilinear interpolation of the elevation function for the base of the sliding mass. Furthermore, the accuracy, robustness, and generality of this method are validated by experimental results. Its application to the Shum Wan Road landslide and the Fei Tsui Road landslide, both of which occurred during a heavy rainstorm in Hong Kong on 13 August 1995 and involved fatalities, gives reasonable results in comparison to the field observations. A variety of rheological constitutive relationships have already been coded in the present program to provide flexibility and adaptability in practical applications.Key words: debris flows, three-dimensional dynamic model, runout distance.

ISSN:0008-3674    

DOI:10.1139/t99-089

出版商:NRC Research Press    

年代:2000

数据来源: NRC