MODULE 3.pdf

BT 251 SOILS AND FOUNDATION SYSTEMS MODULE 3 Kofi Agyekum (PhD) [email protected] // 0246 761 879 Sept 2015  FOUNDATION SYSTEMS Foundation- is that part of the building in direct contact with t...

BT 251 SOILS AND FOUNDATION SYSTEMS MODULE 3 Kofi Agyekum (PhD) [email protected] // 0246 761 879 Sept 2015  FOUNDATION SYSTEMS Foundation- is that part of the building in direct contact with the soil and which transmits the loads of the building to the soil. A foundation means the soil or rock that ultimately supports the load and any part of the structure which serves to transmit the load into the soil. Often the term foundation describes only the structural elements but this definition is incomplete, because  the ability of the structural element to transmit the load is limited by the capability of the soil to support the load. 1  A foundation failure may destroy the superstructure as well while a failure in the superstructure might result in only localized damage and does not essentially mean failure of the foundation. - Natural or sub-foundation - Artificial (man-made) or constructed foundation 2  FUNCTIONS OF FOUNDATIONS 1. Distribution of loads 2. Stability against sliding & overturning 3. Minimize differential settlement 4. Safe against undermining 5. Provide level surface 6. Minimize distress against soil movement 3 FUNCTIONS OF FOUNDATIONS CONT’D Distribution of loads  Foundation help to distribute the loads of super-structure to a large of the soil.  Therefore, the intensity of load at its base does not exceed the safe bearing capacity of the soil.  In the case of deep foundations, the super imposed loads are transmitted either through end bearing or both by side friction & end bearing. FUNCTIONS OF FOUNDATIONS CONT’D Stability against sliding & overturning  Foundation imparts lateral stability to the super structure by anchoring it to the ground.  It increases the stability against sliding & overturning due to horizontal forces to wind, earthquake, etc. FUNCTIONS OF FOUNDATIONS CONT’D Minimize differential settlement  Foundation distribute the super-imposed loads evenly on the sub-soil, even in the case of non-uniform loads.  This can be achieved by constructing combined footing or raft foundation. FUNCTIONS OF FOUNDATIONS CONT’D Safe against undermining  Foundation provide safety against scouring or undermining by flood water or burrowing animals. PROVIDE LEVEL SURFACE  It provides level surface over which super-structure can be raised.  FUNCTIONS OF FOUNDATIONS CONT’D Minimize distress against soil movement  Distress or failure due to expansion or contraction of the sub-soil due to moisture variation in clayey & black cotton soils are minimized by the provision of special type foundations.  FOUNDATION DESIGN PRINCIPLES The main objectives of foundation design are to:-  ensure that the structural loads are transmitted to the subsoil safely, economically and without any unacceptable movement during the construction period and throughout the anticipated life of the building or structure BASIC DESIGN PRINCIPLES  Factors influencing Choice of foundation (Design & Construction) Nature of the Super-structure or building The loads that are due to the structure or building Nature, bearing capacity and shear strength of the soil (sub soil) Nature and compressive strength of the construction materials.  FOUNDATION SETTLEMENT Uniform settlement Differential/uneven/ relative settlement  TYPES OF FOUNDATION SYSTEMS Shallow foundations (good bearing strata near soil surface) There are three main types:  Strip foundations;  Pad/isolated column bases; and  Rafts  Pad Foundation  Suitable for most subsoil except loose sand, loose gravels and fill areas  Usually constructed of reinforced concrete, square in plan  Typical pad foundation types are:  Isolated or pad foundation  Steel grillage  Rectangular pad  Combined column foundation  STRIP FOUNDATION Suitable for most subsoil & light structure loadings Suitable for those encountered in low to medium rise domestic dwellings where mass concrete can be used Typical strip foundation types:  Traditional strip  Deep strip or trench fill  Reinforced concrete strip  Continuous column  RAFT FOUNDATION Used to spread the load of the structure over a large base to reduce the load per unit area being imposed on the ground Particularly useful where low bearing capacity soils are encountered & where individual column loads are heavy Typical raft foundation:  Solid slab raft  Beam & slab raft  DEEP FOUNDATIONS PILE FOUNDATION  Can be defined as a series of columns constructed or inserted into the ground to transmit the loads of a structure to a lower level of subsoil  Can be used when suitable foundation conditions are not presented at or near ground level  Classification of piles (may be classified by their basic design function or method of construction): End bearing piles Friction or floating piles Replacement piles Displacement piles  Depth of foundations Mainly influenced by the zone of seasonal changes in the soil In average soils- - No changes are expected in soil behaviour (max. stability) at 2m depth. - Very little change is expected at 1m depth In shrinkable soils because of the expected movement, the depth of foundation must be greater than 1m. Width of foundations Depends on the total load imposed on the foundation and allowable bearing capacity of the soil. Dead loads (load due to the structure / materials) Live loads (inclusions in the building) Wind and moving loads (including cranes if any) Thickness of foundation Depends on the shear stress of construction material and must be such as will resist punching shear due to the load Thickness = Total load on column Perimeter x 2 x shear stress of material (concrete) Example: A pad foundation is required for column in reinforced concrete framed building. The total estimated load to be transmitted through the column is 360KN, the allowable bearing capacity of the soil is 200KN/m2 and the shear stress of concrete is 40KN/m2Determine the size of the pad. Solution: Required area of pad is obtained by dividing the building load by the bearing capacity of the soil Required pad size = 360 = 1.8m2 200 Assuming a square pad: Pad size = Sq. root of 1.8 = 1.342m Say; 1.35m x 1.35m = 1.8225 m2  1.8 m2 Thickness = 360 = 0.83m 4(1.35) x 2 x 40 (Hint: In practice you will have to make allowance for self weight of the pad)  Thank you For any concerns, please contact [email protected] [email protected] 0246 761 879