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CT2 Study Notes
Construction Technology 2 (Substructure) (300721)
Western Sydney University
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CT2 STUDY NOTES 2012 MODULE 1-6 Module 1: The strength of foundations Foundation Soils are fairly incompressible. Footings and retaining walls mainly fail when the soil particles are shifted sideways and upwards. Buildings can be designed to float on poor foundation soil. The soil shears when it moves sideways. One layer of soil moves to the side in relation to the main bulk of the soil. This behaviour occurs ground coupled construction systems, including footing beams, slabs and retaining walls. Shearing Strips A building cannot fail by compression of the soil, therefore the shear capacity of the soil determines the load-carrying capacity of the footing. The ability to resist shear is important for soil. Prediction of Shear Strength Two mechanisms, soil resist loads by cohesion and friction between the soil particles. Cohesion is important in clayey soils It results from, chemical bonding amongst soil particles and between the soil particles and water in the pores. The type of chemical bonding depends on the moisture content, electrolyte concentration and the compaction of the soil. Flocculent Clay Structure Capillary forces within the soil pores. Menisci develop within the soil voids that are capable of binding the soil together. Capillary forces are optimised at a particular moisture content. The meniscus is an example of surface tension Apparent cohesion in soil Meniscus will spread the forces like an arch, moisture content is to high then the pore is overwhelmed and cannot develop and if it's to low it cannot develop as well. Friction is caused by the physical interlock of soil particles, frictional resistance is more important in sandy or granular soils, larger sized particles soils usually possess greater frictional properties. Clay particles are smooth and are platelets Dispersed clay structure FF are important in sandy soils, sand particles are rougher then sand :S, less chance of slippage between particles also known as cohesion less soils. Theories of shear strength There are two types of shear resistance have been developed in two theories. The Coulomb Theory Failure occurs when the supported weight exceeds the shear capacity of soil. The weight on the soil has reached a limit that is related to the bonding within the soil. Work done with an excavated depth greater than 1 metres is classified as high risk construction work and a MSDS is required. The Mohr Theory Progressive failure occurs so that the soil adopts an angle of repose. Which is when the frictional resistance of the soil is just balancing the weight of the soil particles. When other particles are added it will just roll down the surface. You can increase the angle of repose by using coarser soil and gravel and patting the soil down increases the interlocking of the particles. Module 2: Earth Pressure Retaining wall types: Gravity: Geosynthetic, Gabions, Crib-Walling and Soil-Nailed. Cantilever Wall Made from an internal stem of steel reinforced, cast in place concrete or mortared masonry, converts horizontal pressure from behind the wall to vertical pressures on the ground below. Some situations they are strengthened with buttresses on the front, or include a counter fort on the back resisting higher soil loads. Sheet Pile Retaining Wall Used in soft soils and tight spaces, made out of steel, vinyl or wood planks which are driven into the ground. Sheet piling is usually driven 1/3 above ground, 2/3 below ground, design may be altered to soil conditions and environment. Tied back to soil to the soil to be retained by use of a ground anchor also referred to a dead man place a distance behind the face of the wall into the soil. Anchored Retaining Wall Construction types mentioned but also include additional strength using cables or other stays anchored into rock or soil behind the face of the retaining wall. Ground anchors i. steel ropes are usually installed in the soil or the material to be retained with direction drilling plant, complex but effective with high soil loads. Module 3: Failure Modes of Retaining Walls Internal Failure When the structural members bend that constitutes the retaining wall lateral earth pressure can cause the retaining wall to distort, introducing compressive and tensional stresses to the wall, the wall face in contact with the retained earth is forced to stretch and the exposed wall face contracts. Thickness of base is 1/3 of the height, when the earth pressure is greater a cantilever retaining wall may be used. Even more resistance reinforcement can be used. The heel is pushed upward as the rest of the retaining wall rotates under earth pressure applying tensional forces to the top side of the heel and simultaneously, the toe is bent downwards applying tensional forces to the underside side toe. This results in the reinforcement being placed on the tension side of the stem, it is for dry shrinkage and temp movements. Cantilever Retaining Wall Buttress and Counter fort lessen bending stresses by using stiffening fins. They can be constructed both in front and behind the wall and reduce the flexing and bending of the stem and require less steel reinforcement. Anchored Retaining Wall its different, installing walls by installing a rock/ ground anchor or dead man thus providing additional support to the top of the retaining wall. This tying back turns the cantilever retaining wall into a simple beam structure, but with the intro of anchors will reduce the flexibility of the entire wall. Indeterminate Beam Action Negative curvature (over strong points in the soil) reinforcement needed in the top of the beam, Positive curvature (over weak points in the soil) reinforcement needed in the bottom of the beam. Module 6: Settlement Monolithic Action If the entire structure moves vertically or rotates as a plane rigid body, this does not always cause structural distress. Differential Action Differential settlement may cause serious damage to individual parts of the building, it leads to progressive failure and the ultimate collapse of the structural system. Heave A structure can be disrupted by ground heaves, as well as settlement. Ground Heave occurs on the following situations: Moisture ingress causes expansion of clayey soils, dishing and doming of a slab structure may result. Frost heave is caused by the expansion of groundwater when it freezes. Ground jacking is caused by the inappropriate use of ground injection grouting.
CT2 Study Notes
Course: Construction Technology 2 (Substructure) (300721)
University: Western Sydney University
