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Geotechnics - Quest'
Descriptive Questions
| Question | Answer |
|---|---|
| Explain the variation found in the undrained shear strength of individual samples taken from one borehole | Results vary depending on the specimen size of the sample due to tube sampling disturbance |
| Describe dilatency | Measures the change in volume of a soil when it is distorted by shearing |
| Describe 3 moduli and explain the significance of using different moduli when carrying out a geotechnical design. | Three types - Elastic, tangential and secant. Secant safest - it is used in uncertain ground conditions |
| Sketch and annotate graphs showing the relationship between volume change and shear stress for a dense sand | Show before and after - particles getting further apart (after) |
| Describe Terzaghi's principle of effective stress | When soil is subjected to stress, it is opposed by the fluid pressure of pores in the soil and all quantifiable changes in stress to a soil are a direct result of a change in effective stress - Effective Stress = Total stress - pore pressure |
| Describe effective stress and its importance | Effective Stress = Total stress - pore pressure - influences strength and the volume/shape of the soil |
| Changes in water levels above ground have zero effect on the stress below ground | Changes in water levels above ground have zero effect on the stress below ground. |
| Describe what is meant by the term "coefficient of lateral earth pressure" | Ratio of horizontal of effective stress to the vertical effective stress. Uses are deep foundations, retaining walls and tunnels. |
| Describe how as an engineer you would lower the water table on a site to enable 'dry' works | Ground freezing or shallow bored wells with an underlying aquifer could be used depending on what work is being carried out. |
| What would the effect on your calculations if the coefficient of volume compressibility was taken directly from the Oedometer testing, when calculating settlement? | Reduce value for consolidation settlement |
| Describe 4 of Terzhagi's principles of 1 dimensional consolidation | 1.) The soil is homogenous, 2.) The soil fully saturated, 3.) The solid particles and water are incompressible, 4.) Strains in the soil are relatively small |
| Discuss the importance of understanding the applied loadings before calculating consolidation settlements using Mv | The applied loadings must be known as the voids and stress change when subjected to loading affecting Mv value which in turn can affect the rate of consolidation |
| Describe boiling and its effects on the ground | When the upward seepage force is equal to the force of the soil due to its weight causing it to behave like a viscous fluid. |
| Explain the difference in seepage velocity and velocity of water in a soil | Seepage velocity is the actual rate of movement of the water while velocity of water is the speed and direction of the movement of water in the soil |
| What would the effect of a rising water be affected as the bearing capacity of soil reduces as the water table rises above the base of the footing | The ulitimate bearing capacity would be affected as the bearing capacity of soil reduces as the water table rises above the base of the footing |
| Briefly describe some examples of deep and shallow foundations and in what situation they may be used? | Shallow foundations - (pad, raft) are used when the surface of the soil is strong and stiff enough to support the imposed loads. Deep foundations (Caissons, piles) are used when there are very large design loads or poor soil at a shallow depth |
| Describe the applications and uses of raft foundations | Resist differential movement thus reducing differential settlements and spread loads over large areas, used with low bearing capacity soils |
| Describe the properties that dictate the choice of foundations | Cost of foundations, design load of the structure, properties of the soil and the position of the water table, nature of the structure (building) |
Created by:
Civilman
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