geotehnica subiecte

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geotehnica subiecte

Text of geotehnica subiecte

1)SOIL STRUCTURE - Is define as the property of soil which provides the integrity of the sistem and which is responsible for response to externally applied and internally induced sets of forces and fluids.The structure of a coarse grained soil is governed by gravitational forces:-single grained structure-in honeycomb structure-in floccular structure

2)Soil texture may be defined as the visual appearance of a soil based on a qualitative composition of soil grain sizes in a gives soil mass.Texture of the cohesionless soils: -homogeneous -non-homogeneousTexture of the cohesive soil : -homogeneous -leyered -in lens form

3)Grain size distribution,: Grain size of a soil refers to the diameters of the soil particles making up the soil mass.The grain-size distribution -coarse-grained soil sire analysis -fine-grained soil can be obtained by means hydrometer analysisTo obtain the grain size distribution of a soil in laboratory we use the grain-size analysis. Sieve Analysis:realizated to dry,well pulverized soi1) the uniformity coefficient (Cu)2) the coefficient of gradation (Cz)

4)Hydrometer analysis : -is conducted on the principle of sedimentation of soil particles in water. In this test, one uses 50 grams of dry, pulverized soil. A deflocculating agent is always added to the soil. The most common deflocculating agent used for hydrometer analysis is 125 cc of 4% solution of sodium hexametaphosphate. The soil is allowed to soak for at least 16 hours in the deflocculating agent. After the soaking period, distilled water is added, and the soil-deflocculating agent mixture is agitated. The sample is then transferred to a 1000-ml glass cylinder. More distilled water is added to the cylinder to fill it up to the 1000-ml mark, and then the mixture is again agitated. The largest diameter of the soil particles still in suspension at time t can be determined by Stokes Law.

5) Uniformity coefficient ;Soil separate size limits

6) Importance of mineral composition of clay The clay minerals are commonly found in soils belong to the larger mineral family termed PHYLLOSILICATES, which also contains other layer of silicates.The clay minerals usually occur in small particle size.The two basic units in the clay mineral structures: SILICA TETRAHEDRON, with a tetrahedral silicon ion coordinated with four oxygen atoms ALUMINIUM or MAGNESIUM OCTAHEDRON, wherein aluminum or a magnesium ion is octahedrally coordinated with six oxygen atoms or hydroxyls.

Clay minerals:Kaolinete - structure of alternating silicon and octahedruHalloysite 2 forms: 2HO 4HO

Illite -consists of three layers silicate-gibbsite-silicate resulting pyraphylliteMontmorillonite consists of 3 layers of silicate-octahedral-silicate

7)Type of water in soil: - after state aggregation: -fluid water (liquid) -solid water (ice) -gaseous water (vaporous water)

- forces which act on the water molecules chemical bound water -physical bound water-electro molecular -free water: capillary(surface tension), gravitational ( hydrocolloidal forces )

8) Indices for soil characterization

In nature, soil is three-phase systems : solid soil particles, water and airTo determine the physical properties of soils we have to know three simplest characteristics: specific weight , s, of solid particles of the soil; unit weight, , of the soil of natural structure; Natural moisture content, w, of the soil.

Where: Ws = the weight of the solid particles Vs= volume of soil solids

Where: W =total weight of the soil specimenV= total volume of soil

Porosity - void volume V-specimen volume

Void ratio

saturation level [%]

moisture content : 100 [%]

dry unit weight

saturation unit weight 9) Loose and dense states of cohesionless soilsIn granular soils, can be measured relative density Dr, or density index ID, by the formula (ex):

= void ratio of the soil in the loosest state = void ratio in the densest state ; e = in situ void ratio. The relative density can also be expressed in terms of dry unit weight in situ dry unit weightdry unit weight in the densest state that is, when the void ratio is dry unit weight in the loosest state that is, when the void ratio is

10) Plasticity of soil, Atterberg limitsPlasticity may be defined as the ability of the material to change, shape continuously under the influence of a constant applied stress and to retain the new shape on the removal of the stressThe plastic limit is the water content at which a soil element will start to crumble when rolled into a pencil shape of 3 mm diameterPlasticity index

consistency index

Atterberg limits, is defined by the liquidity index LI

activity index

The figure represents the relation between the volume and weight of a soil sample at its different stages of plasticity, beginning from the dry state and ending at the liquid state.It shows clearly the different ranges of plasticity.

11) CAPILLARITY SOIL AND CAPILLARY EFFECTSThe capillary action is attributed to intermolecular forces existing between the water molecules. The capillarity rise in a tube depends on the atmospheric pressure.It is a well-known fact that when capillary tube is placed in water, the water level in the tube rises. This is caused by the surface tension effect.If the adhesion forces between a liquid and any other material are larger than the intermolecular attraction of the liquid, the surface of the dissimilar material will be wetted by liquid.Since surface tension is a material property of liquids and depends on intermolecular attraction it will be temperature dependent.So for equilibrium and .

If the equilibrium of the water column in the capillary tube is now considered, the downward acting force is the weight of the water, and the upward acting force is the vertical component of the reaction of the meniscus along the circumference. Surface tension the capillary height

the capillary meniscus

for a glass tube

The pore pressure in the capillary tube above the actual outside level is negative and its value is obtained by considering that for any elevation z must be=>for z = 0 to and z = 0, u=0 The capillary pressure The pore water pressure at any point in the zone of capillary rise in soil can be approximated as:

Where: - degree of saturation of soil ; - distance measured above the ground water table.

In this respect if the head of the free surface is considered versus the degree of saturation, it is possible to define the following limits: the highest elevation from the free surface ; the height at which the maximum degree of saturation exists, the highest elevation at which a continuous channel of water exists in the case of under drainage,

12)Permeability of soils, Darys low (1856)The flow on the unit surface is a function of height of the two pointsWhere : v=velocity [cm/sec] k=coefficient of permeability of soil [mm/sec] i=hydraulic gradient

Where: h=piezometric head difference between the sections at AA and BB L=distance between the sections AA and BBHanzen (1930) proposal for k:

(mm/sec ) Where: A an constant that varies between 10 and 15 D10 effective soil, (mm)

13)Compression and consolidation- phenomenon definitionCompressibility describes the volumetric response behavior of a soil mass and behavior characteristics under compression.Consolidation the change in volume of a soil mass with time due to remove a small amount of waterRheologic behavior the volume change performance is identified as a stress-strain-time phenomenon

For consolidation -oedometerThe result of oedometer test is plotted in compression - settlement curve.

Deformation of soil: -stiffness structure, elastic structureTest of deformation: lab compressibility, in site test with plate

14)Establishment of compressibility in situ with plate.