Coming from the Upper Midwest to the San Francisco Bay Area, I get to deal with the Bay Mud, which resembles the soft and compressible characteristics of many of the Great Lakes clays except that its deposition is different. This technical note summarizes the findings from my humble effort in the literature review of available articles, a compilation of existing data, and correspondences with several friends and experts in the subject matter, to partially fulfil my personal curiosity and also for my work.
Introduction
The Young Bay Mud (YBM) is a marine-deposited clay commonly found along the margins of the bay, between modern shorelines and the historical limit of tidal marsh. The YBM is composed of alluvial gray silty clay, with thin layers of silt and fine to medium-grained, sands, small amounts of organic material, and shell fragments. Within developed areas and city limits, the YBM is generally buried under artificial fill; therefore, YBM is typically normally consolidated or slightly overconsolidated, i.e., OCR of between 1 and 3 (Bonaparte and Mitchell, 1979; Benoit and Clough, 1986; Koutsoftas et al., 2017). Where it is exposed at the surface, it can be desiccated and stiff, and its OCR can be higher. There are mainly two types of YBM commonly encountered in the San Francisco Bay Area (Johnson and Bartow, 2019). The first type is very soft to soft clay with organic content (CH, OH). This type of soft bay mud is generally characterized by higher plasticity, higher moisture content, lower density, and low shear strength. The second type is more notable in its granular composition: sandy/silty clay (CL, SC) and silty sand (SM). This type of mud deposit primarily consists of soft to medium sandy clay or medium dense silty sand, having lower plasticity, lower moisture content, higher density, and higher shear strength. This document provides a brief review of the engineering properties, primarily the indices and consolidation properties of the first type of YBM, as it is more geotechnically concerning and applicable to the type of work discussed herein.
Index Properties
The engineering properties of YBM vary across the Bay Area. Table 1 presents a compilation of several index properties of Bay Mud from limited existing literature and author's projects.
Liquid Limit
All data points, except the samples from the Bridge Oakland Mole and Yerba Buena Cove, have Liquid Limits (\(w_{l}\)) of greater than 50% that indicates a high-plasticity or high-compressibility soil. If plotted on the plasticity chart, many of these data points also fall slightly above the A-line, which is a characteristic of many marine clays. If there is a presence of organic matter, the data points are located in the region below the A-line, e.g., plasticity index of 45% and Liquid Limit of 90% in Benoit and Clough (1986). The Liquid Limit represents the mineralogy of a soil, and often time, and can be empirically correlated to strength and other properties of a soil (TPM, 1996). Soil with higher Liquid Limit or its natural water content (\(w_{0}\)) closer to Liquid Limit tends to behave as a vicious fluid when sheared. In other words, a soil with higher Liquid Limit is highly sensitive like those of Scandinavian marine clays.
A useful ratio to indicate sensitivity of soil is Liquidity Index (LI), defined as \((w_{0}-w_{p})/(w_{l}-w_{p})\). If LI is greater than 1, remolding transforms the soil into a thick slurry (highly sensitive); If LI is between 0 and 1, the soil behaves like plastic; if LI is negative, the soil cannot be remolded or will have brittle fracture when sheared. Bonaparte and Mitchell (1979) reported that LI for YBM is slightly above 1, which as described earlier, is a sensitive clay. Based on several other studies, Bonaparte and Mitchell (1979) summarized that typical values of YBM are: Liquid Limit at ±88% and natural water content slightly higher at about ±90%. However, because water content in most soils falls between its Plastic and Liquid Limits in its natural state, the author would like to caution the use of such “typical” values, especially the Liquid Limit, including those listed in Table 1 where the upper limits of \(w_{0}\) exceed the upper limits of Liquid Limit. Recall that the first type of YBM contains some amount of organic matter. When the soil sample is prepared in accordance with the ASTM standard, air- or oven-drying causes irreversible dehydration of organic matter, reducing the Liquid Limit (TPM, 1996; Holtz and Kovacs, 1981). Therefore, the reported “typical” Liquid Limit value of ±88%, if prepared per the ASTM standard, is likely underestimated, and it is very likely that Liquid Limit is slightly above the natural water content. Often, the laboratory Atterberg Limits results are used to correct the classifications as observed in the field (e.g., CH vs. CL), while preparing boring logs. If in doubt of laboratory results, the engineer or geologist should check with the laboratory standard procedure and refer to the descriptive color (dark brown, dark gray, or black) of the retrieved samples for indications of organic materials.
Specific Gravity and Unit Weight
Two other useful properties are specific gravity (\(G_{s}\)) and total unit weight. When evaluating consolidation test results or performing settlement analysis, specific gravity is needed to compute the void ratio. Bonaparte and Mitchell (1979) summarized that specific gravity of YBM is in the range of 2.69 to 2.73, with a mean value of 2.71. The specific gravity of a soil varies based on soil constituents, e.g., organic soil has a lower \(G_{s}\) value. Denby (1978) reported specific gravities ranging from 2.52 at a depth of 5 feet to 2.65 at 50 feet; and 2.75 below 75 feet from samples obtained in Hamilton. Therefore, it is reasonable to use a lower \(G_{s}\) value for those YBM with organic materials at shallow depths. Based on the typical \(w_{0}\) of 90% and a \(G_{s}\) of 2.7, the initial void ratio (\(e_{0}\)) is roughly 2.4 for YBM without organic content, or higher void ratio for YBM with organic content. This assumes that soil stratum is fully saturated, i.e., \(e_{0}=w_{0}G_{s}\). It has been reported that the total unit weight of YBM is fairly constant with depth. At the Hamilton Air Force Base, Benoit and Clough (1986) reported a unit weight of 100 pcf in the upper 6½ feet and 94 pcf below that. Note that near-surface desiccation can cause denser soil state and organic soil tends to lower the total unit weight to around 90 pcf. For general engineering calculations, the author uses a total unit weight of 95 pcf for YBM, unless organic content is noticeable, then a lower total unit weight such as 90 pcf is used.
Drawing from the discussions above, readers can easily conclude that YBM is a highly sensitive and highly compressible clay. The sensitivity component is more prominent in stress or strength analysis, while compressibility is more noteworthy in seepage, consolidation, or settlement analysis. In soft ground engineering, it is always preferred to muck-excavate soft and compressible soil like YBM, and backfill with sand or engineered fill if it is practical and feasible to do so (also from the environmental standpoint). If a geotechnical engineer must deal with YBM, either ground improvement or deep foundations will be needed. It is important to note that the majority studies of the YBM’s engineering properties were originated from the Hamilton Air Force Base, and by now, the readers should know that engineering properties of YBM vary across the Bay Area. As a result, the author urges reader to exercise engineering judgement and perform supplemental laboratory or in-situ testing to confirm properties in question.
See Next: Consolidation Properties
No comments:
Post a Comment