No abstract

Small-scale testing under 1 g conditions as well as in the centrifuge presupposes that a model and prototype have comparative behavior. The chief condition for agreement between model and prototype is that the initial soil states of both must be at equal proximity to the steady state line. Then, when stresses are normalized to the initial mean stress, the model will in all aspects behave similarly to the prototype. Scaling rules are presented that indicate the relations between stress, strain, and displacement for the model and the prototype in terms of geometric scale and stress scale. An obvious limit of scales is imposed by that the soil in the model can be no looser than the maximum void ratio. Similarly, it must not be denser than a value that corresponds to a prototype soil at the minimum void ratio. Three main areas of application of the approach in engineering practice are identified: design of representative 1 g small-scale model tests; reanalysis of data from conventional small-scale tests; and improving the versatility of centrifuge facilities in recognition of the fact that the centrifuge test does not need to be performed at equal levels of stress, when designed according to the new approach. Key words : physical modeling, sand, scaling relations, steady state, centrifuge testing.

In Part I of this report the results are given from 43 months of measurements of forces and bending moments on two instrumented precast piles driven through 40 m (130 ft) of soft clay and 15 m (50 ft) into underlying silt and sand. The force in the piles increased due to negative skin friction. After the first 5 months a force of nearly 40 tons was observed at the bottom of the clay layer. During this time the reconsolidation of the clay after the driving took place. The force due to the reconsolidation effect amounted to about 30 tons, while the rest was due mainly to negative skin friction caused by a small regional settlement. The latter force increased linearly with time by about 15 tons per year. Seventeen months after the driving the pile heads were loaded with 44 tons and one year later another 36 tons were added. The load on the pile head eliminated the negative skin friction, which however started to return with the continued regional settlements.In Part II of the report general design formulae for piles considering negative skin friction are given. The formulae should be used to check that the permanent and transient working loads, which have been chosen according to ordinary design rules, are not too large when negative skin friction develops.When settlements due to negative skin friction are not acceptable, the negative friction can be reduced by applying a thin coat of bitumen to the piles. References are made to investigations concerning reduction of skin friction, and practical difficulties are pointed out.

The variation of the coefficient of earth pressure in normally consolidated and overconsolidated soil and the effect of soil compaction on the change of the horizontal effective stress are discussed based on cone penetration test (CPT) data. A method is outlined for estimating the increase in the effective earth pressure based on sleeve friction measurements. Soil compaction increases not only soil density, but also horizontal effective stress. Since the cone stress is influenced by the vertical and horizontal effective stress, particularly at shallow depths, the cone stress needs to be adjusted for effective mean stress. A relation is presented for determining the soil compressibility from the adjusted cone stress. A case history is presented where a 10 m thick sand fill was compacted using vibratory compaction. Cone penetration tests indicated a significant increase in cone stress and sleeve friction and a decrease in compressibility (increase in modulus number) due to compaction. The friction ratio was unchanged. It was concluded that the earth pressure about doubled corresponding to an increase in the overconsolidation ratio of at least 5. The results of settlement calculations based on the Janbu method demonstrate the importance of considering the preconsolidation effect in the analyses.Résumé : La variation du coefficient de pression des terres dans un sol normalement et surconsolidé et l'effet du compactage de sol sur le changement de la contrainte horizontale sont discutés sur la base des données de CPT. On décrit une méthode pour estimer l'augmentation de la pression effective des terres basée sur les mesures du frottement du manchon. Le compactage augmente non seulement la densité du sol, mais aussi la contrainte effective horizontale. Puisque la contrainte sur le cône est influencée par les contraintes effectives verticale et horizontale, particulièrement aux faibles profondeurs, la résistance du cône doit être ajustée en fonction de la contrainte moyenne effective. On présente une relation pour déterminer la compressibilité du sol à partir de la résistance ajustée du cône. Une histoire de cas est présentée dans laquelle un remblai de sable de 10 m d'épaisseur a été soumis à un compactage par vibration. Les essais de pénétration au cône ont montré une augmentation significative de la résistance du cône et du frottement sur le manchon et une diminution de la compressibilité (augmentation du nombre modulaire) dues au compactage. Le rapport de frottement est inchangé. On a conclu que la pression des terres a environ doublé, ce qui correspond à une augmentation du rapport de surconsolidation d'au moins 5. Les résultats des calculs de tassement basés sur la méthode de Janbu démontrent l'importance de prendre en compte l'effet de la préconsolidation dans les analyses.

Several full-scale, long-term tests on instrumented piles performed since the 1960s and through the 1990s are presented. The results of the tests show that a large drag load will develop in piles installed in soft and loose soils. The test cases are from Norway, Sweden, Japan, Canada, Australia, United States, and Singapore and involve driven steel piles and precast concrete piles. The test results show that the transfer of load from the soil to the pile through negative skin friction, and from the pile back to the soil through positive shaft resistance, is governed by effective stress and that already a very small movement will result in mobilization of ultimate values of shaft shear. The pile toe resistance, on the other hand, is determined by downdrag of the pile and the resulting pile toe penetration. Reconsolidation after the pile installation with associated dissipation of pore pressures will result in significant drag load. An equilibrium of force in the pile will develop, where the sustained loads on the pile head and the drag load are equal to the positive shaft resistance plus the pile toe resistance. The location of the force equilibrium, the neutral plane, is also where the pile and the soil move equally. The drag load is of importance mostly for very long piles (longer than 100 pile diameters) for which the pile structural strength could be exceeded. Downdrag, i.e., settlement of the piled foundation, is a very important issue, however, particularly for low-capacity short piles. Soil settlement at the neutral plane will result in a downdrag of the pile. The magnitude of the downdrag will determine the magnitude of the pile toe penetration into the soil, which will determine the pile toe resistance and affect the location of the neutral plane. Nature's iteration of force and soil settlement will decide the final location of the neutral plane.Key words: piles, negative skin friction, drag load, downdrag, neutral plane, pile settlement.