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Mooring Anchors: Deadweight

A Subaqueous Soil Interpretation

Mooring Anchors: Deadweight 2

Table of ContentsIntroduction. .....................................................................................................................................3Purpose ...........................................................................................................................................4Subaqueous Soil Survey .................................................................................................................4Use and Explanation of Soil Interpretations ....................................................................................5Web Soil Survey (WSS) ..................................................................................................................5Evaluation Criteria ...........................................................................................................................5Soil Rating Classes .........................................................................................................................6References ......................................................................................................................................6Nondiscrimination Statement. ..........................................................................................................7

Issued April 2018

Mooring Anchors: Deadweight 3

IntroductionA mooring refers to a structure or anchor used to hold a boat in a certain area, with a float or buoy attached (fig. 1). The safety of a boat on a mooring depends on a number of elements, including the type of bottom materials or soil.

There are two primary classes of anchors—temporary and permanent. A permanent anchor, often called a mooring, is rarely moved. The anchor needs to hold the vessel in all weathers, including the most severe storms, and only needs to be lifted on rare occasions. The two common types of permanent anchors are mushroom and deadweight. The deadweight anchor is the focus of this soil interpretation.

Deadweight anchors work on the principle of being heavy. Whether the anchor is a block of stone, concrete, or iron, its holding power is weight (fig. 2). Deadweight anchors provide the greatest reliability. They are the best choice for rocky, gravelly, or sandy soils that have surface layers with low n value or hard bottom types of material. If a deadweight anchor is dragged, it will resist with constant force. By contrast, once a mushroom anchor breaks free, it will not reset and will simply skip along the bottom.

Granite is a common choice of material for deadweight anchors in the Northeast as it is readily available and inexpensive. Although it can be difficult to handle, once in position it will not move. South of northern New England, the anchors are more commonly made of concrete or iron (INAMAR, 2000).

Figure 1.—A boat mooring area in Little Narragansett Bay. In the last decade, the boating population has exploded and moorings are increasing in number.

Figure 2.—Diagram showing use of a deadweight anchor. Deadweight anchors work best in hard bottom materials, such as gravel and coarse sands that have surface layers with a low n value.

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Deadweight anchors are used where mushroom anchors are unsuitable. An advantage of a deadweight anchor over a mushroom is that if it does become dragged, it continues to provide its original holding force. A disadvantage is that a deadweight anchor needs to be around ten times the weight of a similar-sized mushroom anchor (INAMAR, 2000).

PurposeA significant number of mooring failures during storms have been vessels driven ashore with their moorings. It was important to find out why these vessels dragged their moorings ashore fully intact. For example, the type of anchor used may have been unsuited to the soil type. The safety of a boat on a mooring depends on four elements of the mooring system: (1) the strength and design of the desk hardware (cleats, etc.); (2) the strength of the mooring lines (rodes); (3) the integrity of the chains or shackles leading to the anchor system; and (4) the resistance of the anchoring system to being pulled out of the soil. Depending upon what type of anchor is used and what type of soil material it is set in, the holding capacity of the mooring can be compromised. The type of soil material is very important for the holding power of anchors.

Subaqueous Soil SurveyTraditional soil mapping is conducted by a field soil scientist trained to understand the interaction of soil-forming processes and soil-landscape relationships. It involves mostly field work. The soil scientist traverses the landscape and digs many holes to observe the soil condition and classify the soil. Subaqueous soil mapping is performed in much the same way, except the soil is under water (fig. 3). Instead of using topographic maps to provide landscape position, subaqueous soil mapping uses bathymetric maps to identify landscapes and landforms. Augers and special tools, such as peat corers and vibracores, are used to obtain soil samples (fig. 4).

Soil samples are described to depths up to 2 meters. Based on soil descriptions and landform maps, representative soils are sampled for laboratory analysis using a vibracore. If the soils are very soft and fluid (high n value soils) or high in organic material, a peat sampler or push tubes are used. Field observations and laboratory analyses indicate significant differences in such characteristics as particle-size class, organic carbon content, pH, and fluidity (n value). The boundaries of these soils are drawn on aerial photos and digitized for use with multiple types of data.

The National Cooperative Soil Survey (NCSS) is a nationwide partnership of Federal, regional, State, and local agencies and private entities and institutions. This partnership works to cooperatively investigate, inventory, document, classify, and interpret soils of the United States and disseminate and publish soil information. With national standards, taxonomy, and publication platforms, the NCSS provides a standard for collecting, developing, and distributing information on coastal soils.

Figure 3.—Diagram of coastal and subaqueous soils, their parent materials, and landscapes.

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Use and Explanation of Soil Interpretations Soil survey interpretations are predictions of soil behavior for specified land uses and management practices. They are based on soil properties or characteristics that directly influence the specified use of the soil. The interpretations allow users of soil surveys to plan reasonable alternatives for the use and management of soils.

When soil interpretations are used in connection with delineated areas on soil maps, the information pertains to the soil for which the soil area is named. The named soil is the major soil component of the soil map unit. Other soils that are too small to map but are observed within the delineated area are called minor soil components or inclusions.

This soil interpretation does not include the minor soil components or inclusions. For example, a soil delineation with the name “Rhodesfolly fine sand, 0 to 1 meter water depth” may also include small, unmappable areas of other soils, such as Napatree and Anguilla. The interpretation applies to the Rhodesfolly part of the delineated soil area and not to the included soils. More detailed studies are required if small, specific sites are to be developed or used within a given soil delineation.

Soil interpretations do not eliminate the need for onsite study and testing of specific sites for design and construction for specific uses. Interpretations should be used as a guide to planning more detailed investigations and for avoiding sites undesirable for an intended use.

No consideration was given in this interpretation to the size and shape of a soil delineation or to the pattern of the delineation with that of other soils on the landscape. For example, some soil areas identified as desirable are too small, are too irregular in shape, or occur with less desirable soils in a pattern too complex for the intended use. Although not considered in the interpretation, these factors may influence the final selection of a site.

Web Soil Survey (WSS)Web Soil Survey provides soil data and information produced by the National Cooperative Soil Survey (NCSS). It is operated by the USDA Natural Resources Conservation Service (NRCS) and provides access to the largest natural resource information system in the world. The site is updated and maintained online as the single authoritative source of soil survey information. In obtaining these data from USDA-NRCS, it is understood that the user has the right to use them for any internal purpose.

Evaluation CriteriaRatings are based on soil properties and qualities that affect the capacity of the soil to support a deadweight anchor.

Fluidity of MaterialsThe n value (Pons and Zonneveld, 1965) characterizes the relation between the percentage of water in a soil under field conditions and the soil’s percentages of inorganic clay and humus. It is helpful in predicting whether a soil can support loads.

The critical n value of 0.7 can be approximated closely in the field by a simple test of squeezing a soil sample in the hand. For example, if some of the soil flows between the fingers with difficulty, the n value is between 0.7 and less than 1.0 (slightly fluid manner of failure class); if the soil flows easily between the fingers, the n value is 1 or more (moderately fluid or very fluid manner of failure class) depending on what remains in the palm of the hand.

Figure 4.—Vibracore used to sample subaqueous soils.

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Soil Consistence Terms and Their DescriptionTerm Description

Nonfluid None of the specimen flows through the fingers after exerting full compression (Soil Survey Division Staff, 2017). The approximate equivalent n value is less than 0.7 (Pons and Zonneveld, 1965).

Slightly Fluid After exerting full compression, some of the specimen flows through the fingers, but most remains in the palm of the hand. The approximate equivalent n value is 0.7 to 1 (Pons and Zonneveld, 1965).

Moderately Fluid After exerting full compression, most of the specimen flows through the fingers; a small residue remains in the palm of the hand. The approximate equivalent n value is 1 to 2 (Pons and Zonneveld, 1965).

Very Fluid Under very gentle pressure most of the specimen flows through the fingers like a slightly viscous fluid; very little or no residue remains in the palm of the hand (Soil Survey Division Staff, 2017). The approximate equivalent n value is equal to or greater than 2 (Pons and Zonneveld, 1965).

Soil Rating ClassesSoils rated for mooring types using deadweights are not limited, somewhat limited, or very limited.

Soils are placed into interpretive rating classes per their rating indices. A rating index uses the sum of the thicknesses of horizons that each have an n value condition between 0 and 150 centimeters. The n value associated with the highest sum of horizon thicknesses is used to place the soil into one of the rating classes.

Not Limited: Soils with a high n value (very fluid) between 0 and 150 centimeters from the soil surface. The soil has surface layers with low n value or a hard bottom type of material.

Somewhat Limited: Soils with an n value of 0.7 to 2 (slightly or moderately fluid) between 0 and 150 centimeters from the soil surface.

Very Limited: Soils with a low n value (nonfluid) between 0 and 150 centimeters from the soil surface. The soil has surface layers with high n value or a soft bottom type of material. The size of the vessel moored or the weight of the anchors used may be limited.

Not Rated: Soils or miscellanous areas that do not have a subaqueous drainage class.

Limitation Ratings for Deadweight AnchorsCriteria Not Limited Somewhat Limited Very Limited

Fluidity (n value) of soil materials

Nonfluid (low n value soils)

Slightly and moderately fluid Very fluid (high n value soils)

ReferencesINAMAR. 2000. Moorings: Important recommendations for safe moorings from INAMAR. pp. 1-8.

Pons, L.J., and I.S. Zonneveld. 1965. Soil ripening and soil classification: Initial soil formation in alluvial deposits and a classification of the resulting soils. Publication 13. International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands.

Soil Science Division Staff. 2017. Soil survey manual. C. Ditzler, K. Scheffe, and H.C. Monger (eds.). USDA Handbook 18. Government Printing Office, Washington, DC.

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