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Engineering Properties and Geologic Setting of Old Bay Clay Deposits, Downtown San Francisco, California


The Sangamon-aged, marine clay, locally known as the “Old Bay Clay” or the “Yerba Buena Mud” has an engineering significance due to its prevalence in the San Francisco Bay Area subsurface profile. Throughout the Bay Area, large-scale construction projects have been recently completed, begun construction, or are in planning that depend on understanding the small- to large-strain properties of deposits that were previously less affected by construction activities, deposits that include the Old Bay Clay. The construction of a transit center in Downtown San Francisco, California provided a direct opportunity to obtain unique data from a large, urban excavation 1500 feet long, 200 feet wide, and 65 feet deep. The excavation occurred in a densely constructed urban environment over a subsurface soil profile up to 240 feet thick, including a layer of Old Bay Clay approximately 80 feet thick. High quality samples were obtained at various depths and locations at the project site within the Old Bay Clay deposit, as well as in clay layers of other units in the subsurface. This work includes a multi-year study to document the laboratory properties of these clays.

The Old Bay Clay was characterized by contextualizing the deposit within the San Francisco Bay Area geologic setting, prior engineering geologic characterization, and with knowledge gained from subsurface exploration and sampling. The project site lies approximately on the southern limb of a Franciscan formation bedrock valley that dips to the northeast. Above the bedrock is the Alameda formation, which contains some interlayers of estuarine or marine clay. The Old Bay Clay overlies the Alameda and is remarkably consistent at the project site. Old Bay Clay is described as a Dark Greenish Gray, stiff to hard, fat clay, with water content from 33 to 44%, total unit weight from 105 to 117 pcf, 95 to 100% fines content, Liquid Limit from 60 to 68, and Plasticity Index from 37 to 44. The Old Bay Clay is a single transgression that was deposited during the last interglacial from a previous Bay that was deeper than the current San Francisco Bay. A microfossil study was performed that identified foraminifers and diatoms within Old Bay Clay samples that suggest the project site materials came from the lower units deposited in the Sangamon San Francisco Bay. Above the upper erosional contact of the Old Bay Clay are late Pleistocene to Historic materials that contribute to the understanding of the Bay Area subsurface profile. The history of the development of the Bay Area subsurface profile is documented from the late 19th Century through present works, including the locations and significance of the engineering and geologic studies.

The deep, thick layer of the Old Bay Clay and deep clays within the Alameda formation prompted detailed study of compression parameters and stress history for this project. Ten Incremental Loading Consolidation (IL) tests and 21 Constant Rate of Strain Consolidation (CRS) tests were performed with all IL tests paired with one or more CRS tests. A particular focus was on recompression and secondary compression properties of the materials, so many cycles of unloading and reloading were performed for each test at various stress states. The Old Bay Clay soils were found to be moderately to slightly overconsolidated. For the Old Bay Clay, preconsolidation pressure ranged from 6.3 to 9.0 kg/cm2 and OCR ranged from 1.6 to 2.4. In Old Bay Clay, compression index was found to be from 0.51 to 1.06, recompression index from 0.01 to 0.07, secondary compression index from 0.019 to 0.34, and coefficient of consolidation from 0.4 to 4.4 m2/yr. These compression parameters were compiled and compared with Young Bay Mud and Alameda clays. A sample quality assessment was made of the Old Bay Clay specimens, which was Very Good to Excellent (Lunne et al., 2006), which was important to justify the value of the other small strain testing performed.

Testing these materials also led to a focus on how consolidation testing methods and results unique for deep, stiff clays is different than for routine tests. The importance of a large soaking stress to prevent swelling was documented. Six methods of preconsolidation pressure were assessed and three methods routinely used in order to characterize dependence of methodology on estimates of preconsolidation pressure. Compression index was shown to vary depending on certain characteristics of compression curves. Recompression index was compiled at various stress states to show the dependence on unloading magnitude and stress level. Secondary compression was compiled for every unload and reload increment to show dependence on stress history. The coefficient of consolidation was compared between IL and CRS testing for understanding of instantaneous coefficient of consolidation estimates in CRS testing.

Monotonic and dynamic testing was performed on project samples with several different test types in order to explore small strain stiffness properties of the Old Bay Clay. Ten Anisotropic-Consolidated Undrained Triaxial (CKoUTX) tests were performed that involved a suite of tests, including bender elements, cyclic loading, and/or small to large strain compression and extension tests. A group of five CKoUTX tests were performed under special “lateral unloading” conditions to understand shear behavior and measure stiffness values at various stress states. Using the UC Berkeley high-performance triaxial cell in combination with a graphical user interface version of Georobot, small strain properties were meaningfully measured on high quality specimens fully characterized with index testing. Specimens were consolidated past their in-situ preconsolidation pressure and then anisotropically consolidated to field values of OCR and Ko. Values of Gmax, from the several testing types, ranged from 57.2 to 133 MPa (1190 to 2780 ksf) with a mean value of approximately 80 MPa (1680 ksf). Results of small strain properties, including Gmax Vs, and modulus reduction curves, appear consistent with results of other research on the Old Bay Clay as well as for similar clay materials.

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