HC-Lecture04-Soils-Excavation-Dewatering

HC-Lecture04-Soils-Excavation-Dewatering - Heavy...

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Heavy Construction Heavy Construction Lecture #04 Lecture #04 Soils, Excavation and Dewatering L Prieto-Portar 2008
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Foundations for Buildings. The purpose of the foundation is to safely transfer the structure’s loads into the ground. The choice of the type of foundation is therefore dependent on, - the structure’s size and shape, - the total weight, and type of loads, - the sensitivity of the structure to settlements, and - economical and technically possible with the resources at hand. Foundation Loads: - Dead loads (the weight of the permanent components of the structure) - Live loads (the weight of furniture, people, equipment, etc, that can move around) - Wind loads (increases as the square of the height) - Horizontal pressures below grade (from adjacent soils and structures) - Uplift (from the hydraulic forces due to high water tables, expansive clays) - Earthquake (both horizontal and vertical components of forces) - Snow - Ice
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Types of Settlement. No settlement. Rare (on rocks or very firm soils). Uniform settlement. May only affect grade requirements. Differential settlement. Causes great damage to beams and slabs.
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A simple example of differential settlement between two paved areas.
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Differential settlement around a column.
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The Leaning Tower of Pisa is the most famous example of a differential settlement.
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Types of Soils: Granular: good foundations and well drained. - Gravels (particle size from 4” down to ¼”); above those, cobbles and boulders; - Sands (particle size from ¼” down to 0.075 mm); Cohesive: may be problematic foundations and retain water. - Silts (particle size from 0.075 mm to 0.002 mm = 2 microns); - Clays (0.002 mm and smaller – high chemical activity = sticks to everything); Organics: very weak soils and retain water; usually removed. - Peats, muck, etc. Granular soils.
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Spread footings and columns on clay soils during good conditions.
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Cohesive soils tend to retain moisture for long periods of time, and complicate construction.
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This pedestal will keep a steel column out of the moist cohesive soils.
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Subsurface Investigation (of the soils and rocks) . Who needs to know soil characteristics? - the structural engineer, to determine how to support the building; - the architect, to site the building; - the contractor, to estimate the time and cost of the building; - the owner, because all the above affect the final price of the land. When do they need to know them? - the engineer needs this information prior to the design; - the contractor, prior to pricing and planning; - the owner, prior to deciding if the project is feasible. How is that knowledge obtained? - A visual exploration of the site and adjacent buildings; test pits are dug to visually see the soils below; deeper studies require borings and dynamic penetrometers, samples, laboratory tests and a final report. Why does a contractor care about the soil on site?
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HC-Lecture04-Soils-Excavation-Dewatering - Heavy...

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