Civil Engineering: Composition of Soil

Senin, 28 Maret 2011

Composition of Soil

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Diposting oleh Politeknik Negeri Medan - Teknik Sipil 09' |

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Soil composition

Soil mineralogy

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 mineral constituents of gravel may be more similar to that of the parent rock.

The common clay minerals are montmorillonite or smectite, illite, and kaolinite or kaolin. These minerals tend to form in sheet or plate like structures, with length typically ranging between

10 - 7 m

and

4 x 10 - 6 m

and thickness typically ranging between

10 - 9 m

and

2 x 10 - 6 m

, and they have a relatively large specific surface area. 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 1,000 square meters per gram of solid . Due to the large surface area available for chemical, electrostatic, and van der Waals interaction, the mechanical behavior of clay minerals is very sensitive to the amount of pore fluid available and the type and amount of dissolved ions in the pore fluid.

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.

Grain size distribution

Soils consist of a mixture of particles of different size, shape and mineralogy. 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. The median grain size,

D 50

, is the size for which 50% of the particle mass consists of finer particles. Soil behavior, especially the hydraulic conductivity, tends to be dominated by the smaller particles, hence, the term "effective size", denoted by

D 10

, is defined as the size for which 10% of the particle mass consists of finer particles.

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 soil are called uniformly graded soils. If there are distinct gaps in the gradation curve, e.g., a mixture of gravel and fine sand, with no coarse sand, the soils may be called gap graded. 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 used in geotechnical engineering are sieve analysis and hydrometer analysis.

Sieve analysis

The size distribution of gravel and sand particles are typically measured using sieve analysis. The formal procedure is described by ASTM (number needed). A stack of sieves with accurately dimensioned holes between a mesh of wires is used to separate the particles into size bins. 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 4.75mm opening size separates sand from gravel and a #200 sieve with an 0.075 mm opening separates sand from silt and clay. According to the British standard, 0.063 mm is the boundary between sand and silt, and 2 mm is the boundary between sand and gravel.

Hydrometer analysis

The classification of fine-grained soils, i.e., soils that are finer than sand, is determined primarily by their Atterberg limits, not by their grain size. 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. 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.

Mass-volume relations

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 . The basic notation is as follows:

V a

V w

, and

V s

represent the volumes of air, water and solids in a soil mixture;

W a

W w

, and

W s

represent the weights of air, water and solids in a soil mixture;

M a

M w

, and

M s

represent the masses of air, water and solids in a soil mixture;

ρ a

ρ w

, and

ρ s

represent the densities of the constituents (air, water and solids) in a soil mixture;
Note that the weights, W, can be obtained by multiplying the mass, M, by the acceleration due to gravity, g; e.g.,

W s = M s g

Specific Gravity is the ratio of the density of one material compared to the density of pure water (

ρ w = 1 g / c m 3

).

Specific gravity of solids, $G_s = \frac{\rho_s} {\rho_w}$
Note that unit weights, conventionally denoted by the symbol

γ

may be obtained by multiplying the density instead of

ρ

by the acceleration due to gravity,

g

.
Density, Bulk Density, or Wet Density,

ρ

, are different names for the density of the mixture, i.e., the total mass of air, water, solids divided by the total volume of air water and solids (the mass of air is assumed to be zero for practical purposes):

$\rho = \frac{M_s + M_w}{V_s + V_w + V_a}= \frac{M_t}{V_t}$

Dry Density,

ρ d

, is the mass of solids divided by the total volume of air water and solids:

$\rho_d = \frac{M_s}{V_s + V_w + V_a}= \frac{M_s}{V_t}$

Buoyant Density,

ρ'

, defined as the density of the mixture minus the density of water is useful if the soil is suberged under water:

$\rho ' = \rho\ - \rho _w$

where

ρ w

is the density of water

Water Content,

w

is the ratio of mass of water to mass of solid. 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.

$w = \frac{M_w}{M_s} = \frac{W_w}{W_s}$