Geotechnical HOPE: Topics

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Submerged Soil

Admin.Desk Monday, November 03, 2025 Soil Mechanics , Topics

In many soil mechanics problems, it is necessary to determine the net intergranular weight, or effective weight, of a soil when it is below the groundwater table. (In this context, intergranular refers to the weight or force that acts at the point, or on the surfaces, where soil particles are in contact.) Effective soil weight is used to determine effective stress in a soil deposit, a value that influences factors such as soil shear strength, soil compressibility and settlement, and slope stability—topics discussed in later chapters. For this “underwater” condition, the soil solids are buoyed up by the pressure of the surrounding body of water, and the submerged soil weight becomes less than for the same soil above water. The effective soil weight then becomes the unit weight of the soil material when it is weighed under water. The water in the voids has zero weight (when submerged, all voids can be assumed to be filled with water), and the weight of the soil solids is reduced by the weight of the volume of water they displace. Therefore, a submerged soil weight (W_sub) equals the soil weight above water minus the weight of water displaced, or:

Since unit weight is total weight divided by total volume:

Similarly, in terms of density:

For a given soil (the subject soil) the effective unit weight when submerged (or the submerged effective density) will be the same regardless of depth below the water surface.

For the condition where the soil is 100 percent saturated and the wet unit weight is known, the equations for submerged soil unit weight reflect that the weight (or mass) of both the soil particles and the voids water are for the buoyant condition, to become:

The equation indicates that an accurate determination of the submerged unit weight requires that the specific gravity of the soil solids and the void ratio be known. Unfortunately, in terms of time and expense, some testing or physical analysis is required to determine the specific gravity, which is then used to calculate the void ratio. Also, soil properties such as particle size distribution, void ratios, and soil weights vary somewhat over even relatively limited distances (including areas identified as “uniform deposits”). Commonly for foundation studies at building sites, representative unit weights are selected on the basis of values determined from tested soil samples (samples obtained from borings, test pits, etc.) and assumed for areas between locations of known conditions (borings, etc.). Because of the various practical aspects, the effort to make highly accurate determinations of submerged soil weights is seldom undertaken when studies and designs are done. Instead satisfactory estimates, which can be made from knowing a wet weight, are frequently utilized. For many soils, and fortunately for ease of computation, the submerged unit weight is on the order of half the wet soil unit weight above the water table (related to the relatively limited range of specific gravity values for particles and typical void ratios), see the Eqs. below; a notable exception to this condition is soils containing significant decomposed vegetation or organic material. For many practical applications, the effects from adopting the simplification are negligible (but where accuracy is required, the Eqs. above for γ_sub or ρ_subshould be used).

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The Geological Time Scale

Admin.Desk Wednesday, October 29, 2025 Geology , Topics

The major subdivisions of geological time is into eras, periods, and epochs. These separations are based on fossil evidence and on extinctions. For example, the Mesozoic era ended with an abrupt termination of the dinosaurs, now usually attributed to a severe climate change following the impact of a meteor. Breaks in deposition of sediments create erosional surface ‘‘unconformities.’’

The ‘‘Paleozoic era’’ literally means ‘‘ancient-life-time’’ and was dominated by invertebrates including many varieties of now-extinct shellfish. The ‘‘Mesozoic era,’’ or ‘‘middle-life-time,’’ was a time of dinosaurs and the earliest mammals. The ‘‘Cenozoic era,’’ or ‘‘recent-life-time,’’ is dominated by mammals.

Unconformities are very common and subdivide a rock column into ‘‘formations’’ of consistent geological age. ‘‘Formation’’ is not a time designation but a rock designation, and a single formation may incorporate a variety of rock layers. Formations for the most part are identified from fossils, which is very important in exploration drilling for oil.

Radioactive dating depends on constant rates of radioactive decay of certain isotopes, and indicates that the majority of geological time was without life except for single-celled plants, or algae. These rocks generally have been buried under younger rocks and constitute part of the ‘‘basement complex.’’ They may be lumped together in age as ‘‘pre-Cambrian,’’ which means that they originated prior to the earliest, Cambrian, period of the Paleozoic era. Granitic shield areas often are referred to as ‘‘pre-Cambrian shield.’’

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Major Soil Types

Admin.Desk Saturday, October 18, 2025 Soil Mechanics , Topics

The major engineering categories of soil are gravel, sand, silt, and clay. There is not unanimous agreement on the exact division between each of these major soil types, but gravel and sand are universally considered coarse-grained soil, for the individual particles are large enough to be distinguished without magnification. Silts and clays are considered fine-grained soil because of their small particles—too small, for the most part, to be seen unaided.

The most commonly used divisions for classifying soils for engineering and construction purposes are shown in the table. On a comparative basis, the division sizes between gravel and sand (4.76 mm or 2.00 mm) and between sand and silt–clay (0.074 mm or 0.05 mm) are actually quite close. As a result, lack of agreement on these division sizes normally does not cause serious problems.

Particles larger than gravel are commonly referred to as cobbles or boulders. Again, no unanimous agreement exists on the range of sizes. When gravel extends up to the 200 mm (8 in.) size, anything larger would be termed a boulder. Where the 80 mm (3 in.) size, or thereabouts, is taken as the upper size for gravel, the sizes between 80 mm and 200 mm may be designated as cobbles, and anything larger than 200 mm (8 in.) as boulders. However, 150 mm or 300 mm (6 in. or 12 in.) may also be taken as the division between cobbles and boulders. As for sands and gravels, these discrepancies usually do not cause serious problems. Conventionally, when a construction project requires a particular material, it has become standard practice to indicate the soil or aggregate requirements on the basis of size instead of, or in addition to, classification.

In conclusion, particle size serves as the basis for classification of sands, gravels, cobbles, and boulders.

The classification of a fine-grained soil as either a silt or a clay is not done on the basis of particle size but, rather, is based on the plasticity or nonplasticity of the material. Clay soil is plastic over a range of water content; that is, the soil can be remolded or deformed without causing cracking, breaking, or change in volume, and will retain the remolded shape. The clays are frequently “sticky.” When dried, a clay soil possesses very high strength (resistance to crushing). A silt soil possesses little or no plasticity and, when dried, has little strength. If a small sample of moist silt is shaken easily but rapidly in the palm of the hand, water will appear on the surface of the sample but disappear when the shaking stops. This is referred to as dilatancy. When a sample of moist clay is similarly shaken, the surface will not become wetted.

The reason for the difference in behavior between clay and silt relates to the difference in mineralogical composition of the soil types and particle shape. Silt soils are very small particles of disintegrated rock, as are sands and gravels, and possess the same general shape and mineralogical composition as sands and gravels (which are nonplastic). The clay minerals, however, represent chemical changes that have resulted from decomposition and alteration of the original rock minerals. The effect is that their size and shape are significantly different from those of other types of soil particles. This is discussed further in a following section.

Naturally occurring soil deposits most generally include more than one soil type. When they are classified, all the soil types actually present should be indicated, but the major constituent soil type should dominate the description, while the soils of lesser percentage are used as modifying terms; for example, a material that is mostly sand but includes silt would be classified as a silty sand, whereas a silt–clay mixture with mostly clay would be termed a silty clay.

Although a soil may be predominantly coarse-grained, the presence of silt or clay can have a significant effect on the properties of the mixture. Where the amount of finegrained material exceeds about one-third of the total soil, the mixture behaves more like a fine-grained soil than a coarse-grained soil.

The condition also exists where small fragments of decomposed vegetation are mixed with the soil, particularly fine-grained soils. Organic material mixed with the nonorganic soil can have striking detrimental effects on the strength and compressibility properties of the material. The presence of organic material should be carefully considered. A foul odor is characteristically though not always associated with such soils, as is a blackish or dark gray color. Soils in this category are designated as organic (e.g., organic silt or organic clay) in comparison to a nonorganic designation for soil free of decomposed vegetation.

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Typical Values of Modulus of Elasticity and Poisson’s Ratio

Admin.Desk Thursday, October 16, 2025 Soil Mechanics , Topics

The tables show respectively the typical ranges of values of modulus of elasticity, E and Poisson’s ratio, μ.

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Number and Depths of Boreholes

Admin.Desk Monday, October 13, 2025 Geotechnics , Topics

It is practically impossible and economically infeasible to completely explore the whole project site. You have to make judgments on the number, location, and depths of borings to provide sufficient information for design and construction. The number and depths of borings should cover the zone of soil that would be affected by the structural loads. There is no fixed rule to follow. In most cases, the number and depths of borings are governed by experience based on the geological character of the ground, the importance of the structure, the structural loads, and the availability of equipment.

Building codes and regulatory bodies provide guidelines on the minimum number and depths of borings. The number of boreholes should be adequate to detect variations of the soils at the site. If the locations of the loads on the footprint of the structure are known (this is often not the case), you should consider drilling at least one borehole at the location of the heaviest load. As a guide, a minimum of three boreholes should be drilled for a building area of about 250 m² (2500 ft²) and about five for a building area of about 1000 m² (10,000 ft²).

Some guidelines on the minimum number of boreholes for buildings and for due diligence in subdivisions are given in the Table 1.

Guidelines for the Minimum Number of Boreholes for Buildings and Subdivisions Based on Area

Some general guidance on the depth of boreholes is provided in the following:

• In compressible soils such as clays, the borings should penetrate to at least between 1 and 3 times the width of the proposed foundation below the depth of embedment or until the stress increment due to the heaviest foundation load is less than 10%, whichever is greater.

• In very stiff clays and dense, coarse-grained soils, borings should penetrate 5 m to 6 m to prove that the thickness of the stratum is adequate.

• Borings should penetrate at least 3 m into rock.

• Borings must penetrate below any fills or very soft deposits below the proposed structure.

• The minimum depth of boreholes should be 6 m unless bedrock or very dense material is encountered. General guidelines for the minimum number or frequency of boreholes and their minimum depths for common geotechnical structures are shown in the Table 2.