In this section, published theoretical formulatins for pull-out capacity of soil anchors are discussed [16, 17, 67, 75, 102, 103, 106].

Even though, for the case of cohesionless soils, correlations between measured pull-out capacities and those predicted by present hypotheses do not appear to be very good [19], it is still reasonable to assume that

there is a correlation between the pull-out capacity of anchors and the shear strength of the surrounding soil. Thus, any soil classification used in conjunction with predictions of anchor pull-out capacities should reasonably reflect the shear strength of the soils. Unfortunately, methods and terminologies presently used by the mobile home industry for classifying soils are neither consistent among themselves, nor do they convey much information on the shear strength of the soil. This inconsistency is a source of considerable confusion which should be eliminated.

Since the various terminologies and definitions associated with soil classification which are used by the mobile home industry differ from those accepted by the geotechnical engineering profession, it is necessary to first discuss professionally accepted methods of soil classification. Present confusion in terminology may lead to misunderstandings between the parties concerned with the selection and installation of soil anchors, namely the soils engineer, the mobile home owner, the anchor installer and the anchor manufacturer.

5.2 DEFINITIONS OF TERMS USED IN SOIL CLASSIFICATION

ASTM [11] defines soil as "sediments or other unconsolidated accumulations of solid particles produced by the physical and chemical disintegration of rocks, and which may or may not contain organic matter." Rock is defined as "natural solid mineral matter occurring in large masses or fragments." To further subdivide definitions for soil, we use the terms cobbles, gravel, sand, and fines. Fines can be either silt or clay. According to the Unified Soil Classification System [101] which is widely used in geotechnical engineering, these specific names are used to designate the size ranges of soil particles. The gravel and the sand sizes are further subdivided as shown in table 5.1. The boundaries between one soil type and another have been arbitrarily made and they are based on U.S. standard sieve sizes given in the table. For example, a fine sand would mean that the majority of a sample of sand would have particle sizes that would be between the number 40 and 200 sieve. To further classify the fines

(silts and clays), it is necessary to use geotechnical engineering laboratory tests which are beyond the scope of this report.

Typically, silts are defined as any fine-grained materials that will pass through the No. 200 sieve and exhibit little or no strength when the particles are in an air-dry state. By strength we mean its ability to hold shape under pressure from the fingers, for example. Arbitrarily, silt size is usually defined as those particles which are smaller than the No. 200 sieve (0.74 mm) but larger than .002 mm. After the individual soil particles can no longer be seen by the naked eye, the individual soil grains are finer than the No. 200 sieve size.

Clays, too, have two arbitrary classifications. One is by size and the other is by the mineralogical composition of the soil. Clay size is arbitrarily defined as any soil particle that is smaller than .002 mm.

Clay soils may be made up of particles of weathered rock that are smaller than .002 mm but more usually the definition is used for soil particles that are made up of clay minerals. Clay minerals are plates of microscopic size with very ordered atomic structures. The engi

neering properties of clays are the result of physical and chemical forces between the clay particles. Sand and silt particles are so large compared to clay particles that their engineering behavior is not dominated by the chemical and electrical forces that dominate clay soil behavior. By far the most significant characteristic of a clay soil is its ability to exhibit plasticity. Clay plasticity is that property which allows the soil to be moved around, for example, by the hand and still retain its deformed shape. Another definition of clay plasticity is for the soil to be rolled into a very thin thread (for example, 1/8 in (3.2 mm) in diameter) and still hold together. When these (plastic) clay soils are allowed to dry out, they will exhibit considerable strength upon drying. By this we mean the dried clay lump will be difficult to crumble by the hand and only great pressure would cause disintegration. All of these comments on soil classification are adequately covered in ASTM standards [11, 13, 14, 15].

It will be shown later that the pull-out capacity of an anchor is assumed to be a function of the shear strength of the material in which the anchor is embedded. When an anchor is pulled out of the soil, it can be said that it shears against the soil; or the soil immediately above a helix anchor, for example, shears the soil surrounding the anchor. The higher the shear strength of the soil, the harder it will be to pull out the the anchor. Before one can describe those tests which are used by the mobile home industry to determine anchor capacity, it is necessary to discuss terms describing the shear strength of the soils that have now been standardized to some extent.

Many states utilize the "Standard Penetration Test" as a means of classifying soils. The Standard Penetration Test (SPT) [12] is a field test in which a standard sampler which is approximately 30 inches (0.76 m) in length, 2 inches (51 mm) in outside diameter and 1 3/8 inches (35 mm)

in inside diameter is attached to a drill rod and placed at the bottom of the properly cleaned out boring. A 140-lb (63.5-kg) weight raised 30 inches (0.76 m) above an anvil or striker plate is allowed to fall freely, imparting its energy to the drill rod to which the sampler is attached. The standard sampler is advanced in three 6-inch (0.15 m) increments and the number of blows it takes for the hammer to penetrate the second and third 6-inch increment is added and is called the "blow count" or "N" value. The SPT N value is fairly reliable in determining relative properties of granular soils (sands, silts and gravels). The Standard Penetration Test is over 40 years old and soils engineers have correlated many engineering properties of soil with the Standard Penetration Test N value. Table 5.2 presents an accepted soils engineering relationship between blow counts and descriptive terms used to describe relative density of sands. The term "relative density" is used to describe the weight per unit volume (or density) of granular soils "relative" to the maximum dry density and minimum dry density. For example, a soil whose natural density is close to the minimum dry density would be described as "loose." It is suggested that the terms used in table 5.2 be used in all future soil classification charts with respect to granular materials (sands). Words like "compact," which is a geologic term, should be avoided.

TABLE 5.2 PENETRATION RESISTANCE AND ENGINEERING PROPERTIES OF SANDS