Civil - Soil Mechanics -Stress Distribution And Settlement
Important Question And Answer: Stress Distribution And Settlement
1. What are the assumptions of Boussinesq Equtions?
1. The soil mass is an elastic medium for which the modulus of Elasticity, E is constant.
2. The soil mass in homogeneous, that is all its constituent parts or elements are similar and it has indentical properties at every point in it in indentical directions.
3. The soil mass is isotropic, that is it has identical elastic properties in all directions through any point of it.
4. The soil mass is semi?infinite, that is i surface.
2. Name the vertical stress distribution diagrams drawn using Boussinesq equation?
1. Vertical stress isobar diagram
2. Vertical pressure distribution on a horizontal plane.
3. Vertical pressure distribution on a vertical line.
3. Define isobar?
An isobar is a curved or contour connecting all points below the ground surface of equal vertical pressure.
4. Define pressure bulb?
The zone in a loaded soil mass bounded by an isobar of given vertical pressure intensity is called a pressure bulb.
7. Define Influence Value?
In Newmarks influence value a circle is drawn with radius r1 equal to 0.270z and the are is divided into 20 area units, each area unit will produce a vertical stress equal to 0.005q at a depth of z cm below the centre. The arbitrarily fixed fraction 0.005 is called influence value.
10. What are the disadvantages of settlement and the components affecting settlement?
If the settlement is excessive, meaning more than what is permissible for the structure, it may cause structural damage or malfunctioning, especially whent eh rate of such settlement is rapid. The total settlement St, of a loaded soil can be recognized as having three components: the immediate settlement Si, the settlement duet o primary consolidation Sc and the settlement due to secondary consolidation Ss or creep.
St = Si+Sc+Ss
11. Briefly explain about immediate settlement
The immediate settlement or distortion settlement occurs almost immediately after the load is imposed, as a result of disortion of the soil without any volume change. The immediate settlement is usually determined by using the elastic theory even though the deformation itself is not truly elastic.
12. Define consolidation?
According to Terzaghi : “Every process i saturated soil without replacement of the water by air is called a process of consolidation.
13. What are the factors which cause the compressibility of clays?
i. The expulsion of double layer water from between the grains.
ii. Slipping of the particles to new positions of greater density.
iii. Bending of particles as elastic sheet.
14. Define hydrodynamic lag?
The delay caused in consolidation by the slow drainage of water out of a saturated soil mass is called hydrodynamic lag.
15. Define hydrodynamic pressure?
The pressure that builds up in pore water due to load increment on the soil is termed excess pore pressure or excess hydrostatic pressure.
16. Define primary consolidation?
The reduction in volume of soil which is due principally to a squeezing out of water from the voids is termed primary consolidation.
17. Define secondary consolidation?
Even after the reduction of all excess hydrostatic pressure to zero, some compression of soil takes place at a very slow rate. This is known as secondary consolidation. During secondary compression, some of the highly viscous water between the points of contact is forced out from between the particles.
18. Write an equation for consolidation settlement of a normally consolidated soil?
19. Define coefficient of compressibility, av?
The coefficient of compressibility is defined as the decrease in voids ratio per unit increase of pressure.
20. Write any 5 assumptions of Terzaghi’s theo
1. Compression and flow are one dimensional
2. Darcy’s law is valid
3. The soil is homogeneous
4. The soil is completely saturated
5. The soil grains and water are both incompressible.
21. Define isochrone?
The distribution of excess hydrostatic pressure u at any time t is indicated by the curve, joining water levels, in the piezometric tubes; this curve is known as isochrone.
22. List down the factors affecting time factor and hence the degree of consolidation?
i. Thickness of clay layer.
ii. No. of drainage faces
iii. Coefficient of permeability, k.
iv. Cv, coefficient of consolidation
v. Magnitude of consolidating pressure
vi. The manner of its distribution across the thickness of the layer.