An experimental investigation of soil-structure interaction in a cohesive soil. Volume 2
Jester, Guy Earlscort.
Technical reportAbstract: This study was an experimental investigation of the behavior of an idealized structure buried at various depths in a compacted cohesive soil (buckshot clay, water content = 26%). Eight static and 20 dynamic plane-wave loadings up to 310 psi were conducted. The cylindrical test devices (5 inches high and 6 inches in diameter) were oriented vertically and their stiffness relative to the soil was varied. In addition a device whose top could be extended and contracted hydraulically was buried at various depths and the relation between load and deformation changes was studied at static overpressures of 37.5 and 75 psi. At low static and dynamically applied surface pressures (Ps = 37.5 psi) and a depth of burial of one structure diameter (H/B = 1), the amount of active arching depended upon the stiffness of the structure relative to that of the soil. Under these conditions, it was possible to relieve practically all the overpressure on the test structure just by decreasing its stiffness. At H/B = 1 , the structure behaved as if it were fully buried under dynamic and static pressures less than 40 psi. As the surface pressure was increased, the amount of arching at H/B > 1 became more dependent upon the shear strength of the soil. When the scaled depth of burial was increased to H/B = 3 at surface pressures in the 150- to 250-psi range, the differential pressure, as calculated by subtracting the average pressure acting on the top of the device from the surface pressure at the same time interval, increased but it did not increase as much as the load on the structure. At P s = 150 psi under dynamic conditions the differential pressure was 32 psi or 2.5 times the shear strength of the soil as determined by unconfined compression tests (qu/2) as compared to 25 psi or 1.4 times the shear strength of the soil at H/B = 1. When the surface pressure was increased to 240 psi under dynamic conditions at H/B = 3, the differential pressure was only 35 psi. Under static conditions, the differential pressure was 37 psi at P8 = 150 psi and 54 psi at Ps = 175 psi. When the static surface pressure was increased to 240 psi, the differential pressure only increased to 58 psi or 5, 2 times the shear strength of the soil. Once the strength of the soil at a particular depth had been fully developed, increasing the surface pressure had very little effect on the amount of arching. There was a transition zone between those surface pressures at which the amount of arching was determined by relative structure flexibility and the pressure at which it was more dependent upon soil strength. The pressures which limited the transition zone depended upon depth of burial and the time in which the load was applied. Within the transition zone, the role played by the relative stiffness changed gradually. Based on the very limited amount of data developed in this test program (P8 < 65 psi and H/B = 1), passive arching does not appear to be sensitive to structure stiffness. Once the relative structure stiffness exceeded a value of approximately 4, there was no increase in the amount of arching with an increase in the structure stiffness. The maximum scaled differential pressure (never exceeded a value of 1.1. Regardless of the stiffness of the structure or the state of arching considered, static arching curves produced by lowering or raising the top of the structure by internal means could not be used to estimate the amount of arching that a similar spring test device would induce under static or dynamic external loads. In addition it was found that static arching data produced with the spring device could not be used to predict the design loads on a comparable structure at dynamically applied surface pressures in excess of 40 to 70 psi, depending on the depth of burial.
Weapons Effects Laboratory (U.S.)Engineer Research and Development Center (U.S.)
Clays; Cohesive soils; Interactions; Shear properties; Soil arching; Soil strength; Soil-structure interaction; Structures; Subsurfaces; Structures; Subsurface structures
Technical report (U.S. Army Engineer Waterways Experiment Station) ; N-70-7 v.2.
Approved for public release; distribution is unlimited.
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