《基础工程》课程PPT教学课件（英文讲稿）Chapter 07 Mat Foundation

Chapter 7 Mat Foundation

Chapter 7 Mat Foundation

7.1 Introduction What's mat foundation-A mat foundation is a large concrete slab used to interface one column,or more than one column in several lines,with the base soil. Application of mat foundations-Mats are used to support on-grade storage tanks,several pieces of industrial equipment,silo clusters,chimneys,and various tower structures

7.1 Introduction ❖ What’s mat foundation-A mat foundation is a large concrete slab used to interface one column, or more than one column in several lines, with the base soil. ❖ Application of mat foundations-Mats are used to support on-grade storage tanks, several pieces of industrial equipment, silo clusters, chimneys, and various tower structures

7.1 Introduction Suitable conditions of mat foundation -Where the base soil has a low beating capacity and/or the column loads are so large that more than 50 percent of the area is covered by conventional spread footings. -For deep basements. -where the settlements may be a problem. Advantages of mat foundation -there is a stress compensation from excavated soil so that the net increase in pressure is controlled. -provide a water barrier when basements at or below the GWT

7.1 Introduction ❖ Suitable conditions of mat foundation ❖ -Where the base soil has a low beating capacity and/or the column loads are so large that more than 50 percent of the area is covered by conventional spread footings. ❖ -For deep basements. ❖ -where the settlements may be a problem. ❖ Advantages of mat foundation ❖ - there is a stress compensation from excavated soil so that the net increase in pressure is controlled. ❖ -provide a water barrier when basements at or below the GWT

7.2 Types of Mat Foundations -flat plate -plate thickened under columns -waffleslab (beam and slab) -plate with pedestals -basement walls as part of mat

7.2 Types of Mat Foundations ❖ -flat plate ❖ -plate thickened under columns ❖ -waffleslab (beam and slab) ❖ -plate with pedestals ❖ -basement walls as part of mat

7.3 Bearing Capacity of Mat Foundations Requirement of design -to limit settlements to a tolerable amount. -stable against a deep shear failure,which may result in either a rotational failure,or a vertical(or punching)failure. Determination of bearing capacity -the beating-capacity equations. -penetration tests(e.g.,SPT,CPT). -laboratory unconfined(or confined triaxial)compression tests. -pressuremeter -borehole shear

7.3 Bearing Capacity of Mat Foundations ❖ Requirement of design ❖ -to limit settlements to a tolerable amount. ❖ -stable against a deep shear failure, which may result in either a rotational failure, or a vertical (or punching) failure. ❖ Determination of bearing capacity ❖ -the beating-capacity equations. ❖ -penetration tests (e. g. , SPT, CPT ). ❖ -laboratory unconfined (or confined triaxial) compression tests. ❖ -pressuremeter ❖ -borehole shear

7.4 Mat Settlements The settlement tends to be controlled via the following: -use of a larger foundation to produce lower soil contact pressures. -displaced volume of soil (flotation effect). -bridging effects attributable to mat rigidity and contribution of superstructure rigidity to the mat. -allowing somewhat larger settlements,say,50 instead of 25mm

7.4 Mat Settlements ❖ The settlement tends to be controlled via the following: ❖ -use of a larger foundation to produce lower soil contact pressures. ❖ -displaced volume of soil (flotation effect). ❖ -bridging effects attributable to mat rigidity and contribution of superstructure rigidity to the mat. ❖ -allowing somewhat larger settlements, say, 50 instead of 25mm

7.4 Mat Settlements The flotation effect should enable most mat settlements to be limited to 50 to 80mm. Mat continuity results in a somewhat lower assumed amount of differential settlement relative to the total expected settlement versus a spread footing. Computer methods that incorporate frame-foundation interaction can allow one to estimate both total and differential settlements. Mat differential settlements are related to both the total estimated foundation settlement /H and the structure rigidity factor K(K,=El/EB3)

7.4 Mat Settlements ❖ The flotation effect should enable most mat settlements to be limited to 50 to 80mm. ❖ Mat continuity results in a somewhat lower assumed amount of differential settlement relative to the total expected settlement versus a spread footing. ❖ Computer methods that incorporate frame-foundation interaction can allow one to estimate both total and differential settlements. ❖ Mat differential settlements are related to both the total estimated foundation settlement ⊿H and the structure rigidity factor Kr .(Kr=EIb /EsB3 )

7.4 Mat Settlements Analysises of settlements will have to be performed where the net increase in pressure exceeds the existing in situ pressure po. It is difficult to compute settlements when heave (the base elevation rises)has occurred. Heave becomes a major problem for excavations of 10 to 20 m in clay. In practice this recovery of heave does not occur with the same rapidity as the heave if we reapply a mat pressure qo

7.4 Mat Settlements ❖ Analysises of settlements will have to be performed where the net increase in pressure exceeds the existing in situ pressure p0 ’. ❖ It is difficult to compute settlements when heave (the base elevation rises) has occurred. ❖ Heave becomes a major problem for excavations of 10 to 20 m in clay. ❖ In practice this recovery of heave does not occur with the same rapidity as the heave if we reapply a mat pressure q0

7.6 Design of Mat Foundations There are several methods to design a mat (or plate) foundation: -an approximate method conventional rigid method): The mat is divided into strips loaded by a line of columns and resisted by soil pressure.The strip is then analyzed as a combined footing.This method can be used where the mat is very rigid and the column pattern is fairly uniform in both spacing and loads.This method is not recommended at present because of the substantial amount of appoximations. -approximate flexible method -discrete element methods

7.6 Design of Mat Foundations ❖ There are several methods to design a mat (or plate) foundation: ❖ -an approximate method ( conventional rigid method): ❖ The mat is divided into strips loaded by a line of columns and resisted by soil pressure. The strip is then analyzed as a combined footing. This method can be used where the mat is very rigid and the column pattern is fairly uniform in both spacing and loads. This method is not recommended at present because of the substantial amount of appoximations. ❖ - approximate flexible method ❖ -discrete element methods

7.6.1 Approximate Flexible Method The approximate flexible method requires the following steps: -compute the plate rigidity -compute the radius of effective stiffness -compute the radial and tangential moments,the shear,the deflection using the following equations where load P acts:

7.6.1 Approximate Flexible Method ❖ The approximate flexible method requires the following steps: ❖ -compute the plate rigidity ❖ -compute the radius of effective stiffness ❖ -compute the radial and tangential moments, the shear, the deflection using the following equations where load P acts: