3 Transportation Research Record: Journal of the Transportation Research Board, No. 2363, Transportation Research Board of the National Academies, Washington, D.C., 2013, pp. 3–11. DOI: 10.3141/2363-01 C. B. Farnsworth, School of Technology, Brigham Young University, 230 Snell Building, Provo, UT 84602. S. F. Bartlett and E. C. Lawton, College of Civil and Environmental Engineering, University of Utah, Suite 2000, 110 Central Campus Drive, Salt Lake City, UT 84112. Corresponding author: C. B. Farnsworth, [email protected]. without ground treatment, the low-permeability, thick clayey soils found in the underlying Lake Bonneville sediments were expected to produce lengthy primary settlement durations greater than 2 years (1). These lengthy EOP settlement durations could not be accommo- dated in the planned construction schedule without the use of ground treatment. The installation of PV drains and extensive field monitoring of settlement progression allowed for the successful completion of the project within the allotted time (2). The use of PV drains decreased the time associated with primary settlement to about 3 to 6 months, which depends on the drain spacing. The Asaoka method for predict- ing settlement (3) was used as the primary tool for forecasting the EOP consolidation date and correspondingly allowing surcharge fill to be removed and paving operations to commence (4). Settlement projections were made solely from the surface by using settlement plates extending through the fill; thus, the projections were based on the composite settlement of the foundation soils. As monitoring progressed, the design–build team noted problems with their Asaoka projections. Typically, as the original EOP projec- tion date neared, an updated projection showed that additional settle- ment time was required. Geotechnical designers suspected that this phenomenon resulted from multiple layers consolidating at differ- ent rates, with some of the deeper, thicker layers consolidating more slowly. This “delayed” consolidation and its associated increase in construction time seriously affected the project schedule. The design- ers concluded that the Asaoka method was not valid for the sub- surface conditions found along parts of the I-15 alignment because of the heterogeneity in drainage properties of the multilayered profile. This paper summarizes subsequent research efforts to analyze the effects of differing consolidation rates within a subsurface profile on the projection of EOP consolidation settlement and to develop a more reliable projection method for such conditions (5). A critical step in the estimation of settlement behavior is a com- plete geotechnical characterization of the subsurface soils, including identification of the subsurface stratification, the thickness of criti- cal layers, and compressibility and drainage properties of these lay- ers. The quality and quantity of the subsurface investigation greatly affects the settlement projections, and for time-critical projects, it is imperative that sufficient subsurface evaluations be performed to reduce uncertainties. For foundation soils that are to be treated with PV drains, this estimation also includes having an adequate knowl- edge of the horizontal drainage properties of the various soil layers. Current methods for obtaining this information include backcalcu- lation from field performance data, the cone penetration test (CPT) with piezometer probe (CPTU) pore pressure dissipation or other in situ permeability tests, and laboratory Rowe cell testing (5). Of Estimation of Time Rate of Settlement for Multilayered Clays Undergoing Radial Drainage Clifton B. Farnsworth, Steven F. Bartlett, and Evert C. Lawton This paper demonstrates how the finite difference technique can be used to estimate the time rate of settlement for soft, compressible clayey soils treated with prefabricated vertical drains at sites where primary consoli- dation settlement is occurring in a multilayered system at varying rates. Semiempirical methods based on surface settlement monitoring have typically been used to estimate the progression of primary consolidation settlement. However, interpretation of such methods can be problematic for multilayered soil profiles. For such sites, it is crucial to obtain a reason- able characterization of the foundation soils’ horizontal drainage proper- ties and include these estimates in the time rate of settlement projections. Field monitoring of subsurface instrumentation is extremely valuable in providing additional information about the consolidation behavior of dif- ferent layers. When subsurface field measurements are coupled with the proposed numerical method, far more reliable projections are obtained. This paper focuses on how to integrate field and laboratory data with projections of time rate of settlement obtained from semiempirical and finite difference methods to predict more accurately the time rate of consolidation behavior of multilayered foundation soils. Construction of large embankments or other heavy structures atop soft, thick compressible foundation soils requires considerable time to complete end-of-primary (EOP) consolidation settlement. In urban environments, rapid construction techniques are often used to reduce construction time, thus minimizing disruption to the public and generally decreasing the cost of a project. In soft, low-permeability soils, prefabricated vertical (PV) drains are typi- cally used to decrease settlement duration. These drains allow dis- sipation of excess pore pressures primarily in the horizontal direction by shortening the drainage path and therefore markedly decreasing the time to reach EOP consolidation. Even if vertical drains are used, the time required to complete EOP consolidation settlement can still be considerable, making this a critical-path activity of many soft-ground construction projects. Thus, an accurate projection of the settlement duration is vital for project planning and construction. For example, during reconstruc- tion of I-15 through Salt Lake City, Utah (between 1998 and 2002),
Estimation of Time Rate of Settlement for Multilayered Clays Undergoing Radial Drainage
Jun 28, 2023
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