Frost pressures in the tie-back system at the National Arts Centre excavation

The effect of frost on a prestressed tie-back system of excavation bracing is described. Measurements of the horizontal movement of the wall as well as the changes in the prestressing force in the tendons, were made during the winter of 1965-66. These indicated that although considerable frost pressures did exist, the tendon loads were lower than had been anticipated owing to plastic flow creep in the bracing system. Suggestions for future installations were made.Les auteurs d\ue9crivent les effets du gel sur un blindage assujetti par un syst\ue8me pr\ue9contraint d'ancrage. Les d\ue9 placements horizontaux de la paroi des fouilles ainsi que les modifications affectant la tension impos\ue9e aux c\ue2bles d' ancrage ont fait l'objet de mesures au cours de l'hiver 1965-1966. Les r\ue9sultats indiquent qu'en d\ue9pit des pressions consid\ue9rables engendr\ue9es par le gel, les contraintes subies par les c\ue2bles d'ancrage ont \ue9t\ue9 moindres que pr\ue9vues en raison du fluage ayant affect\ue9 le blindage. Les auteurs donnent quelques indications pour la r\ue9alisation d'autres installations.Peer reviewed: NoNRC publication: Ye

The performance of tied-back sheet piling in clay

In Ottawa in 1969 a tied-back sheet pile wall was installed to provide temporary support in one side of a 12 m deep excavation through Champlain Sea deposits to shale bedrock. The wall was designed to permit as little yield as possible in order to safeguard the vital operation of an adjacent transformer building. To assess the performance of this structure, measurements of vertical movements of the surface adjacent to the wall, horizontal displacements of the wall, tendon loads and ground-water pressures were made as the excavation progressed. A series of triaxial tests was carried out in the laboratory to determine the form and magnitude of soil deformations under stress changes approximating those derived from the field measurements. Reasonable correlation is obtained when the results of these tests are used to estimate soil displacements in the field situation. The measured tendon loads are compared with those that would be expected using current design methods.En 1969, dans un chantier d'Ottawa, on a \ue9rig\ue9 un mur en palplanches hauban\ue9es afin de fournir un appui temporaire sur l'un des c\uf4tes d'une excavation d'une profondeur de 12 m\ue8tres, traversant des d\ue9p\uf4ts de la Mer de Champlain et atteignant un tuf schisteux. Le mur fut concu de facon \ue0 minimiser le fl\ue9chissement afin de prot\ue9ger un important poste de transformateurs adjacent. Afin d'\ue9valuer le comportement de ce mur, on a mesur\ue9, au cours de l'excavation, les mouvements verticaux de la surface adjacente au mur, les d\ue9placements horizontaux du mur, les charges sur les haubans et les pressions de la nappe aquif\ue8re. Une s\ue9 rie d'essais triaxiaux a effectu\ue9e en laboratoire afin de d\ue9 terminer la forme et l'ampleur des d\ue9formations du sol sous l'effet des changements de contrainte semblables \ue0 ceux qu' on a mesur\ue9s sur place. On obient une correspondance convenable lorsqu'on utilse les r\ue9sultats de ces essais pour \ue9valuer les d\ue9placements du sol sur place. Enfin, les auteurs comparent les charges sur les haubans d\ue9termin\ue9es sur place \ue0 celles que fourniraient les m\ue9thodes courantes de calcul.Peer reviewed: NoNRC publication: Ye

Measured contact pressures below raft supporting a stiff building

Contact pressures between supporting clay and the foundation raft of a very stiff, 15-storey building indicate that pressures near the edge of the raft were up to three times those near the center. This condition persisted over a 2- year period. The difference in pressure is discussed in the light of the stiffness of the building. The raft design method suggested by a A.C.I. Committee in 1966 is compared with the measured performance and found to be reasonable for the raft itself but questionable for the foundation walls above the raft.Les contraintes au contact entre l'argile portante et le radier de fondation d'un \ue9difice tr\ue8s rigide de 15 \ue9tages indiquent que les contraintes pr\ue8s du bord du radier sont jusqu'\ue0 trois fois plus grandes que pr\ue8s du centre. Cette condition a persist\ue9 pendant une p\ue9riode de 2 ans. L'auteur examine cette diff\ue9rence de contrainte \ue0 la lumi\ue8re de la rigidit\ue9 de l'\ue9difice. En comparant avec le fonctionnement mesur\ue9 la m\ue9thode de conception sugg\ue9r\ue9e en 1966 par un comit\ue9 de l'American Concrete Institute, on constate que celle-ci est convenable pour le radier m\ueame mais douteuse pour les murs de fondation au-dessus du radier.Peer reviewed: NoNRC publication: Ye

Strain-time-strength relationships in a marine clay

The main purpose of the testing program was to determine if the strength of a particular clay deposit decreases with time when subjected to a constant shear stress. Strains were examined to ascertain if a critical strain existed. Distinguishing features of clay examined (Leda clay), are its high sensitivity, high liquidity index and tendency toward decreasing rather than increasing shear strength with depth. The consolidated-undrained triaxial compression test was used. A constant consolidated pressure was selected for all tests, and a back pressure of 1 tsf was applied, with a confirming pressure of the sample value. Three series of tests were carried out; first at controlled rates of strain 20%, 2% and 0.2% per hour; next at constant stress, with time to failure being measured; and lastly uniform increments of stress were applied for uniform increments of time, to determine Housel's "yield stress". The loss of shear strength with time up to 25% seems to be a real time effect. Safety factors should be selected to allow for this. The presence of a plastic element of strain ( irreversible strain not dependant on time) and the decreasing modulus of deformation with increasing deviator stress makes a simple application of a rheological model difficult. Normal stress-strain plots of either constant stress rate or strain rate must be curved. The controlled strain rate test may or may not give information equivalent to the constant stress test. Finally some of the test data indicated that the strain at failure might be constant with respect to time for a given clay.Le principal objet du programme d'essai consistait \ue0 d\ue9 terminer si la r\ue9sistance d'un gisement d'argile donn\ue9 pouvait dimineuer avec le temps lorsque soumise \ue0 une contrainte de cisaillement constante. Certaines des donn\ue9es d'essai ont indiqu\ue9 que la d\ue9formation \ue0 la rupture pouvait \uea tre constante dans le temps pour une argile donn\ue9e.Peer reviewed: NoNRC publication: Ye