Lecture11-Mat-Foundations

Lecture11-Mat-Foundations - Foundation Engineering...

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Unformatted text preview: Foundation Engineering Foundation Engineering Lecture #11 Lecture #11 The Design of Mat Foundations- Compensated mats- Rigid mats- Semi-rigid mats L. Prieto-Portar 2009 A mat foundation is primarily a shallow foundation. In essence, it is an expanded continuous footing and is usually analyzed in the same way. Mat foundations are sometimes referred to as raft foundations (especially in the UK) . Mat foundations are selected when: 1. The area covered by the individual footings exceeds 50% of the structural plan area. This is usually the case for buildings higher than 10-stories, and/or on relatively weak soils where q < 3 ksf & 150 kPa ; 2. The building requires a deep basement and below the phreatic surface ( WT ). Basements may be required to build several levels of parking, or to install mechanical systems, access to a subway station, etc; 3. The Engineer wishes to minimize the differential settlement in variable soils, or if pockets of extremely weak soils are known to be present; 4. The Engineer wishes to take full advantage of the soils increasing bearing capacity with depth by excavating basements, and thereby seek a fully or a partially compensated foundation. Problem Soils That May Necessitate the Use of Mat Foundations. 1. Compressible soils occur in highly organic soils including some glacial deposits and certain flood plain areas. Highly plastic clays in some glacial deposits and in coastal plains and offshore areas there can be significant amounts of compressible soils. Problems involved are excessive settlements, low bearing capacity, and low shear strength. 2. Collapsing soils such as the settlement of loose sands and silts. Densification occurs by the movement of grains to reduce the volume. Typically includes shallow subsidence. May occur in sandy coastal plain area, sandy glacial deposits, and alluvial deposits of intermountain regions of the western United States. 3. Expansive soils , containing swelling clays, mainly from the Montmorillite/Smectite group, which increase in volume when absorbing water and shrink when loosing it. Climate is closely related to the severity of the problem. Semi-arid and semi-humid areas with swelling clays are the most severe because the soil moisture active zone has the greatest thickness under such conditions. Foundation supports should be placed below the active soil zone. Expansive soils are most prevalent on the Atlantic and Gulf coastal plain and in some areas of the central and western United States. One of the largest mat foundations cast in Florida is this 9,900 yd 3 performed on Saturday, 11 May 2002, at Sunny Isles Beach, Miami. The mat supports the 51-story Acqualina Ocean Residences and Resort that belongs to the Trump Group. The pour took 14 hours (starting at 0300 hours to 1700 hours, non-stop). (Acqualina continued)....
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Lecture11-Mat-Foundations - Foundation Engineering...

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