Soil Mechanics

Soil consists of different phases of solid, liquid, and gas and its characteristics depend on the interacting behavior of these phases, and on the stress applied. The solid phase includes clay, non-clay minerals, and organic matter. These elements are categorized by their size as clay, sand, and gravel. The liquid phase is composed of water that contains organic compounds available from chemical spills, wastes, and ground water, while the gas phase is normally air. The size, form, chemical properties, compressibility, and load carrying capability of the soil particles are determined by soil mineralogy, which is a science related with the chemistry, structure, and physical properties of minerals.

The structure of a soil depends upon the arrangement of particles, particle groups, pore spaces, and the composition.

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These basic characteristics determine the type of structure to be built and what external support measures, if any, has to be taken to make the structure last long and bear the effects of earthquake, water seepage, and other external factors. Consolidation of soils is also an important factor that needs to be studied to make strong and durable structures. Consolidation is a procedure according to which the volume of soils is reduced, by the application of a stress due to which the soil particles are packed together firmly, thereby decreasing the volume.

With the removal of the stress, the soil will bounce back and recover some of the volume lost during the process of consolidation. While studying consolidation, the crucial factors to be analyzed are the rate of consolidation and the amount of consolidation. Another important factor is permeability of the soil. The following cases of active earth pressure on cohesionless backfill will now be considered: Dry or moist backfill Submerged backfill Partly submerged backfill Backfill with uniform surcharge Backfill with sloping surcharge.

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Notes , Soil Mechanics. Several methods are available to estimate the increase in pressure at any depth z from the applied load. To Determine the Specific Gravity of Soil Take at least 25g of soil which has been passed through sieve 4 and place it in an oven at fixed temperature of C0for 24hours to dry it completely. Clean and dry the pycnometer thoroughly and find its mass M1. Find the mass M2 of pycnometer by placing dried soil in it. Add sufficient quantity of water to fill the pycnometer up to the given mark and then.

Shear Strength of Pervious and Impervious Soils Soils derive their strength from contact between particles capable of transmitting normal as well as shear forces. The contact between soil particles is mainly due to friction and the corresponding stress between the soil grains is called the effective or inter-granular stress s'. Thus, the shear strength of a soil is mainly governed by the effective stress. Book , Soil Mechanics. Total Stress and Effective Stress Analysis in Soil Plastic saturated soils silts and clays usually have lower shear strength than non-plastic cohesion less soil and are more susceptible to bearing capacity failure.

Soil Mechanics in Engineering Practice Lectures

For saturated plastic soils, the bearing capacity often has to be calculated for different condition. Total Stress Analysis Short term condition that uses the un-drained shear strength of the plastic soil. Material Properties , Notes , Soil Mechanics. Engineering Properties of Silt and Clay Silt and Clay are considered to be smaller family members of soil group, Even small amounts of fines can have significant effects on the engineering properties of soils.

Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids usually air and water and particles usually clay , silt , sand , and gravel but soil may also contain organic solids and other matter. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils.

Principles of soil mechanics are also used in related disciplines such as engineering geology, geophysical engineering , coastal engineering , agricultural engineering , hydrology and soil physics. This article describes the genesis and composition of soil, the distinction between pore water pressure and inter-granular effective stress , capillary action of fluids in the soil pore spaces, soil classification , seepage and permeability , time dependent change of volume due to squeezing water out of tiny pore spaces, also known as consolidation , shear strength and stiffness of soils.

The shear strength of soils is primarily derived from friction between the particles and interlocking, which are very sensitive to the effective stress. The primary mechanism of soil creation is the weathering of rock. All rock types igneous rock , metamorphic rock and sedimentary rock may be broken down into small particles to create soil. Weathering mechanisms are physical weathering, chemical weathering, and biological weathering [1] [2] [3] Human activities such as excavation, blasting, and waste disposal, may also create soil.

Over geologic time, deeply buried soils may be altered by pressure and temperature to become metamorphic or sedimentary rock, and if melted and solidified again, they would complete the geologic cycle by becoming igneous rock.

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Physical weathering includes temperature effects, freeze and thaw of water in cracks, rain, wind, impact and other mechanisms. Chemical weathering includes dissolution of matter composing a rock and precipitation in the form of another mineral. Clay minerals, for example can be formed by weathering of feldspar , which is the most common mineral present in igneous rock.

The most common mineral constituent of silt and sand is quartz , also called silica , which has the chemical name silicon dioxide. The reason that feldspar is most common in rocks but silica is more prevalent in soils is that feldspar is much more soluble than silica. Silt , Sand , and Gravel are basically little pieces of broken rocks. According to the Unified Soil Classification System , silt particle sizes are in the range of 0. Gravel particles are broken pieces of rock in the size range 4.

Particles larger than gravel are called cobbles and boulders. Soil deposits are affected by the mechanism of transport and deposition to their location. Soils that are not transported are called residual soils —they exist at the same location as the rock from which they were generated. Decomposed granite is a common example of a residual soil. The common mechanisms of transport are the actions of gravity, ice, water, and wind. Wind blown soils include dune sands and loess. Water carries particles of different size depending on the speed of the water, thus soils transported by water are graded according to their size.

Silt and clay may settle out in a lake, and gravel and sand collect at the bottom of a river bed. Wind blown soil deposits aeolian soils also tend to be sorted according to their grain size. Erosion at the base of glaciers is powerful enough to pick up large rocks and boulders as well as soil; soils dropped by melting ice can be a well graded mixture of widely varying particle sizes.

Gravity on its own may also carry particles down from the top of a mountain to make a pile of soil and boulders at the base; soil deposits transported by gravity are called colluvium. The mechanism of transport also has a major effect on the particle shape. For example, low velocity grinding in a river bed will produce rounded particles. Freshly fractured colluvium particles often have a very angular shape. Silts, sands and gravels are classified by their size, and hence they may consist of a variety of minerals.

Owing to the stability of quartz compared to other rock minerals, quartz is the most common constituent of sand and silt. Mica, and feldspar are other common minerals present in sands and silts. The common clay minerals are montmorillonite or smectite , illite , and kaolinite or kaolin. The specific surface area SSA is defined as the ratio of the surface area of particles to the mass of the particles. Clay minerals typically have specific surface areas in the range of 10 to 1, square meters per gram of solid. As home builders and highway engineers know all too well, soils containing certain high-activity clays make very unstable material on which to build because they swell when wet and shrink when dry.

This shrink-and-swell action can easily crack foundations and cause retaining walls to collapse. These clays also become extremely sticky and difficult to work with when they are wet. In contrast, low-activity clays, formed under different conditions, can be very stable and easy to work with. The minerals of soils are predominantly formed by atoms of oxygen, silicon, hydrogen, and aluminum, organized in various crystalline forms.

These elements along with calcium, sodium, potassium, magnesium, and carbon constitute over 99 per cent of the solid mass of soils. Soils consist of a mixture of particles of different size, shape and mineralogy.

The Basics of Soil Mechanics in Civil Engineering

Because the size of the particles obviously has a significant effect on the soil behavior, the grain size and grain size distribution are used to classify soils. The grain size distribution describes the relative proportions of particles of various sizes. The grain size is often visualized in a cumulative distribution graph which, for example, plots the percentage of particles finer than a given size as a function of size. Sands and gravels that possess a wide range of particle sizes with a smooth distribution of particle sizes are called well graded soils. If the soil particles in a sample are predominantly in a relatively narrow range of sizes, the sample is uniformly graded.

If a soil sample has distinct gaps in the gradation curve, e. Uniformly graded and gap graded soils are both considered to be poorly graded. There are many methods for measuring particle-size distribution. The two traditional methods are sieve analysis and hydrometer analysis. The size distribution of gravel and sand particles are typically measured using sieve analysis. A known volume of dried soil, with clods broken down to individual particles, is put into the top of a stack of sieves arranged from coarse to fine. The stack of sieves is shaken for a standard period of time so that the particles are sorted into size bins.

This method works reasonably well for particles in the sand and gravel size range. Fine particles tend to stick to each other, and hence the sieving process is not an effective method. If there are a lot of fines silt and clay present in the soil it may be necessary to run water through the sieves to wash the coarse particles and clods through. A variety of sieve sizes are available. The boundary between sand and silt is arbitrary.

According to the Unified Soil Classification System , a 4 sieve 4 openings per inch having 4. According to the British standard, 0. The classification of fine-grained soils, i. If it is important to determine the grain size distribution of fine-grained soils, the hydrometer test may be performed.

In the hydrometer tests, the soil particles are mixed with water and shaken to produce a dilute suspension in a glass cylinder, and then the cylinder is left to sit.

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A hydrometer is used to measure the density of the suspension as a function of time. Clay particles may take several hours to settle past the depth of measurement of the hydrometer. Sand particles may take less than a second. Stoke's law provides the theoretical basis to calculate the relationship between sedimentation velocity and particle size. ASTM provides the detailed procedures for performing the Hydrometer test. Clay particles can be sufficiently small that they never settle because they are kept in suspension by Brownian motion , in which case they may be classified as colloids.

There are a variety of parameters used to describe the relative proportions of air, water and solid in a soil. This section defines these parameters and some of their interrelationships. Note that the weights, W, can be obtained by multiplying the mass, M, by the acceleration due to gravity, g; e. It is easily measured by weighing a sample of the soil, drying it out in an oven and re-weighing.

Standard procedures are described by ASTM. Geotechnical engineers classify the soil particle types by performing tests on disturbed dried, passed through sieves, and remolded samples of the soil. This provides information about the characteristics of the soil grains themselves. It should be noted that classification of the types of grains present in a soil does not account for important effects of the structure or fabric of the soil, terms that describe compactness of the particles and patterns in the arrangement of particles in a load carrying framework as well as the pore size and pore fluid distributions.

Engineering geologists also classify soils based on their genesis and depositional history. In the USCS, gravels given the symbol G and sands given the symbol S are classified according to their grain size distribution. The Liquid Limit is the water content at which the soil behavior transitions from a plastic solid to a liquid. The Plastic Limit is the water content at which the soil behavior transitions from that of a plastic solid to a brittle solid. The Shrinkage Limit corresponds to a water content below which the soil will not shrink as it dries. As the transitions from one state to another are gradual, the tests have adopted arbitrary definitions to determine the boundaries of the states.

The liquid limit is determined by measuring the water content for which a groove closes after 25 blows in a standard test.

The undrained shear strength of remolded soil at the liquid limit is approximately 2 kPa.