Flashcards
Shallow foundations
Cards in this deck (10)
- 1What are strip footings?One that is long compared with its width. Typically used to support masonry walls. Modern strip footings are constructed by excavating a trench, placing a reinforcement cage and pouring concrete.
- 2What are pad footings?A pad footing is used to support isolated loads, such as from columns or from support of raised slab or flooring. They are often recommended forms of foundation for sandy sites or those that may be subjected to wind or water erosion. they are generally constructed at least 300mm below the ground surface to prevent undermining. In cold climates they are placed below the frost penetration depth to prevent the effects of frost heave. Constructed in a similar way to strip footings.
- 3What are raft/mat footings?Raft or mat foundations can be used in cases where pad footings would need to be very large to support the loading, or in areas of swelling soils. They are often reinforced concrete slabs and for tall buildings can be several metres thick. At a smaller scale rafts may be stiffened at the loading point by making the slab locally thicker. Ribs or beam-and-slab construction are used to mkae the raft slab stiffer.
- 4What is meant my Ultimate Limit State design?To avoid excessive deformation under working loads resulting in undesirable cracking or impaired durability.
- 5What is meant by Serviceability Limit State design?Avoidance of excessive deformation which tends to lead to damage to structures which are often relatively stiff and brittle. Consider the average value and the variation in settlement across the loaded area or structure.
- 6How might differential settlement occur?1. Lateral variation in strata/layers of soil or rock
2. Variations in iste conditions (same soil, different stress history laterally)
3. Differences in construction time/period of adjacent parts of the structure
4. Variation in applied load laterally
5. Flexible foundations - 7What methods can we use to design for SLS?Divide the failure load found by a F.O.S to obtain the working loading. ADVS- Stress/strain behaviour of the soil is not required for calculation
DIS- Previous knowledge and experience of the soil and the particular circumstances is required. May lead to over/under design. - 8What methods can we use to design for SLS?Use the stress/strain response of the soil at wokring loads to determine the expected elastic deformations and hence settlements directly. ADVS- Elastic methods of analysis give a reasonable approximation to soil deformation. DIS-full stress/strain relationship for soil are complicated, not actually elastic so a good understanding of theoretical behaviour is required.
- 9What is the secant modulus?Found by drawing a straight line from a datum point to a point on the non-linear stress-strain curve. It is defined as a ratio of change in stress to strain as measured from the datum point.
- 10What is the tangent modulus?Found by drawing a tangent to the stress-strain curve at a particular point. It is defined as the rate of chnage of stress with strain.
Information
- Description
- Geotechnical Engineering Flashcards on Shallow foundations, created by Billie Santos on 04/03/2017.
- Created by
- Billie Santos
- Views
- 1
- Rating