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Bay mud consists of thick deposits of soft, unconsolidated clay, which is saturated with water; these soil layers are situated at the bottom of certain estuary, which are normally in that have experienced cyclical glacial cycles.
Example locations are Cape Cod Bay, Chongming Dongtan Reserve in Shanghai, China, Banc d'Arguinpreserve in Mauritania, The Bristol Channel in the United Kingdom, Mandø Island in the Wadden Sea in Denmark, Florida Bay, San Francisco Bay, Bay of Fundy, Casco Bay, Penobscot Bay, and Morro Bay.
Bay mud manifests low shear strength, high compressibility and low permeability, making it hazardous to build upon in active regions like the San Francisco Bay Area. Typical bulk density of bay mud is approximately 1.3 grams per cubic centimetre.
Bay muds often have a high organic matter content, consisting of decayed organisms at lower depths, but may also contain living creatures when they occur at the upper soil layer and become exposed by low tides; then, they are called , an important ecological zone for and many types of marine organisms. Great attention was not given to the incidence of deeper bay muds until the 1960s and 1970s when development encroachment on certain North American bays intensified, requiring geotechnical design of foundations.Farzad Naeim, The Seismic Design Handbook, Kluwer Academic Publishers, (2003)
Bay mud has its own official geological abbreviation:Floyd Fusselman, Environmental Concerns: Learn the Acronyms, Trafford Publishing, Victoria, B.C. (2002) the designation for Quaternary older bay mud is Qobm and the acronym for Quaternary younger bay mud is Qybm. An alluvial layer is often found overlying the older bay mud.
In relation to , it is often necessary to dredge bay bottoms and barge the excavated material to an alternate location. In this case, chemical analyses are usually performed on the bay mud to determine whether there are elevated levels of heavy metals, PCBs or other toxic substances known to accumulate in a Benthic zone environment. It is not uncommon to dredge the same channel repeatedly (over a span of ten to thirty years) since further settling sediments are prone to redeposit on an open estuarine valley floor.
Some depositional formation is quite recent, such as in the case of Florida Bay, where much of the bay mud has accumulated since 2000 BCE, and consists of primarily decayed organic material.R.B. Halley, E.J. Prager, R.P. Stumpf, K.K. Yates, and C.H. Holmes, sea level rise and the Future of Florida Bay in the Next Century, U.S. Geological Survey, (2001) In the case of Florida Bay these bay muds can accrete as much as 0.5 to 2.0 centimeters per annum, although the dynamic equilibrium of erosion, wave action redistribution and deposition complicate the net rate of layer growth. In the case of the Bristol Channel in the United Kingdom bay, mud formation has been occurring at least since the Eemian Stage (known as the Sangamonian Stage in North America), or about 130,000 years ago.R.Kirby and W.R.Parker, Settled mud deposits in Bridgewater Bay Bristol Channel, Report no. 107, Institute of Oceanographic Sciences, Surrey, UK (1980) In other cases such as with San Francisco Bay, deposition has been interrupted by sea-level changes, and strata of vastly different vintages are found. In the San Francisco Bay Area, these are called Young bay mud and Older bay mud by . Human activities can also affect deposition; close to half of the Young Bay Mud in San Francisco Bay was placed in the period 1855–1865, as a result of placer mining in the Sierra Nevada foothills.
A number of notable buildings have been constructed over bay muds, typically employing special mitigation designs to withstand seismic risks and settlement issues. Complicating design issues, fill (beginning about 1850 CE) is sometimes found deposited on the surface level. For example, the Dakin Building in Brisbane, California, was designed in 1985 Judges praise innovative ideas, San Francisco Examiner, Page F-6, January 12, 1992 to sit on piles 150 feet deep, anchoring to the Franciscan formation, below the bay muds and through an upper fill layer. Furthermore, the structure's entrance ramp has been set on a giant hinge to allow the surrounding land to settle, while the building absolute height remains constant. The Crowne Plaza high-rise hotel in Burlingame, CaliforniaC.M.Hogan, Kay Wilson, Ballard George, Marc Papineau et al., Environmental Impact Report for the Crowne Plaza Hotel, prepared by Earth Metrics Inc and. published by the city of Burlingame, California, circulated by the State of California Clearinghouse, (1985) was also designed to sit over bay muds, as was the Westin Hotel in Millbrae, California, and Trinity Church in Boston's Copley Square. Indeed, Boston's entire Back Bay district is named for the tidal bay that it now covers. Logan International Airport and the San Francisco International Airport are also constructed over bay mud.
Mammals such as the Harbor seal may use mudflats to haul out of estuary waters; however, larger mammals such as may become accidentally stranded at low tides. Note that normally humpback whales do not frequent estuaries containing mudflats, but at least one errant whale, publicized by the media as Humphrey the humpback whale, became stuck on a mudflat in San Francisco Bay at Sierra Point in Brisbane, California.
In Asia the Chongming Dongtan Nature Reserve in Shanghai, China, is an example of a large scale bay mud formation. The Atlantic Ocean of Africa holds the Banc d'Arguin, a World Heritage nature preserve in the country of Mauritania. Banc d'Arguin is a vast area underlain by bay mud.
In the case of dredging within the United States, a permit is almost always required from the United States Army Corps of Engineers, after submission of extensive data on the project limits, chemical properties of the bay muds to be disturbed, a dredge disposal plan and often a complete Environmental Impact Statement pursuant to the National Environmental Policy Act. Further review by the United States Coast Guard would normally be required. Within individual state jurisdictions, such as California, an Environmental Impact Report must be filed for dredging of any significance; furthermore, agency reviews by the California Coastal Commission and the Regional Water Quality Control Board would normally be mandated. All of these regulatory bodies serve an important role in deciding whether an area may be dredged or not. However, the most important body is the California Environmental Quality Act (CEQA). This guiding legislation is the reason for Environmental Impact Reports, costly mitigation measures and arduous review processes. One of CEQA's main goals is to promote interagency cooperation in the review process of a project. This is one of the main reasons why it is the overseer of all projects in California.
For buildings proposed over bay mud layers, typically the municipality involved will, in addition to the usual engineering and design review issues common to all building projects (which are more complicated because of the site conditions), require an Environmental Impact Report [1]. This process would include reviews by that city's building department, as well as applicable regional and state agencies such as those cited above for dredging projects, except that Coast Guard agencies would not typically be concerned. In developing in California, proposed development over bay mud layers would also have to go through a planning commission and a city council in order to be allowed. This process would respect the EIR, CEQA, and all the other bodies discussed above. In the case of San Francisco the project would have to get approved by the San Francisco Board of Supervisors.
The Millennium Tower, an example of a tall building built on bay mud, was completed in 2008 and subsequently experienced sinking. This has had a negative impact on the residents of this building. In response to this subsidence, San Francisco's city attorney filed a lawsuit against the developer, because the developer failed to inform the residents of the accelerated speed that the building was sinking at.
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