Civil Engineering Construction - Assignment Example

Chintan Ghuntla HNC Year 25 March 2006 Civil Engineering Construction - Foundations/Retaining Walls/Superstructures Foundations The lowest artificially prepared part of a structure which is in direct contact with the ground and which transmits the load of the structure to the ground is known as a foundation or substructure. It is often misunderstood that the foundation is provided to support the load of the structure. In fact, it is a device to transmit the load of the structure to the soil below. Foundation is provided for the following main purposes- To distribute the weight of the structure over a large area so as to avoid overloading of the soil beneath. To load the sub-stratum evenly and thus prevent unequal settlement. To provide a level surface for building operations. To take the structure deep into the ground and thus increase its stability, preventing overturning. Depending on the type of structure and the soil conditions prevalent, different types of foundations such as Strip Footing, Spread or Isolated Footing, Combined Footing, Strap or Cantilever Footing, Raft Foundation, Pile Foundation, Drilled Piers, Caisson, Well Foundation may be provided. Case Study- Building Additions at New York Court of Appeals (Albany, New York) The project consisted of additions and improvements to the New York State Court of Appeals located at the corners of Pine and Eagle Streets in the city of Albany, New York. The main portion of the existing structure was constructed in the 1840's with a large addition to the east in 1917. The existing 1840's portion of the structure was found to be supported upon dry laid stone with an estimated width of 6 feet, while the 1917 addition was found to be supported on a cast in place concrete foundation with an estimated width of 10 feet. It was estimated by the structural engineers that the bearing pressure on these existing foundations was on the order of about 4000 to 6000 lb/sq. ft. The project consisted of four storey addition to the north and east of the existing building, as well as two level parking garage set into the site grades to the east. The four storey additions were supported on micro-piles drilled and socketed into the bedrock at depths of nearly 100 feet below grade, while the parking deck was supported on spread foundations. Development of the site in an urban area and adjacent to an existing building presented some peculiar challenges for excavation. The project required excavations to locally extend up to about 10 feet below the existing footing for an elevator addition and about 12 feet below grade in close proximity to Pine Street. Underpinning is preferred when placing a new foundation below the existing foundation or during strengthening of existing foundation. Due to the nature of this site and being in an urban area, the project had strict requirements for lateral earth support and underpinning to protect the existing structure and adjacent roadways. Jet grout walls and underpinning techniques were employed to complete the work. Alternatively, pit method or pile method could have been used. But as the structure was very old with two different types of foundation (stone and concrete slab), jet grout walls was found out to be most preferable in this case. Cantilever jet grout walls with heights up to 12 feet were used for lateral earth support along Pine Street, with individual elements extending about 12 feet below the excavation bottom. Jet grout underpinning elements, which were required for temporary and permanent support of the existing structure, extended up to depths of 30 feet below grade adjacent to and beneath the existing foundations. Subsurface investigations consisting of test borings and test pits were carried out for planned construction and consequently, recommendations for foundation and underpinning design were given. Foundation design analyses included establishing allowable axial and lateral capacities for micro piles as well as parameters for lateral earth support and underpinning design. 2) Retaining Walls A retaining wall is constructed to retain the artificial filling. The earth pressure increases as the depth of retaining wall increases from the top. Hence a retaining wall section is gradually increased from top to the bottom to counter the acting earth pressure. The back of a retaining wall is generally stepped while the face may be vertical, inclined or curved. The retaining walls of rectangular cross-sections are usually economical up to a height of two meters or so. Retaining walls are commonly required in construction of hill roads, masonry dams, abutments and wing walls of bridges and so on. Depending upon the site conditions, type of material to be retained (type of soil) and the height of the wall to be constructed, retaining walls may be built in dry stone masonry, stone masonry, brick masonry, plain cement concrete and reinforced cement concrete. A satisfactory retaining wall must meet the following requirements to ensure its stability- The wall should be structurally capable of resisting the pressure applied to it. The section of the wall should be so proportioned that it will not overturn by lateral pressure. The wall should be safe from consideration of sliding, that is, the wall should not be pushed out by lateral pressure. The weight of the wall together with the force resulting from the earth pressure acting on it should not stress its foundation to a value greater than the safe bearing capacity of the soil on which it is founded. Particular care should be taken for accumulation of water behind the wall. Sufficient drainage for this purpose should be provided. Very long masonry walls should be provided with expansion joints located at 6 to 9 meters apart. Gravity Retaining Walls- A gravity retaining wall is one in which the earth pressure exerted by the backfill is resisted by dead weight of the wall. Such a type of retaining wall may either be made up of masonry or of mass concrete. The stress developed in gravity retaining wall is very low. These walls are so proportioned that no tension is developed anywhere in the body of the retaining wall, and the resultant of the forces acting on the wall remain within the middle third of the base. A gravity retaining wall usually has a vertical or near vertical face. Gravity retaining walls are made from a large mass of stone, concrete or composite material. These walls usually have a slight setback to improve wall stability by leaning back into the retained soil. Dry-laid gravity walls are somewhat flexible and do not require a rigid footing below frost. Earlier, taller retaining walls were often gravity walls made from large masses of concrete or stone. Today, taller retaining walls are increasingly built as composite gravity walls such as: geosynthetic or steel-reinforced backfill soil with precast facing; gabions (stacked steel wire baskets filled with rocks), crib walls (cells built up log cabin style from precast concrete or timber and filled with soil) or soil-nailed walls (soil reinforced in place with steel and concrete rods). Gravity retaining walls made out of soil are usually reinforced to hold the soil together. For reinforced-soil gravity walls, the soil reinforcement is placed in horizontal layers throughout the height of the wall. Common soil reinforcement materials include steel straps and geogrid, a high-strength polymer mesh, that provide tensile strength to hold soil together. The wall face is often of precast, segmental concrete units that can tolerate some differential movement. The reinforced soil's mass, along with the facing, becomes the gravity wall. The reinforced mass must be built large enough to retain the pressures from the soil behind it. Gravity walls usually are a minimum of 50 to 60 percent as thick as the height of the wall, and sometimes even larger, if there is a slope or surcharge on the wall. As gravity retaining walls cannot take tension, they are best suited where stationary soil mass is to be retained. Cases where vibrations or movement of such soil mass is likely to occur may create tensile stresses or reversal of stresses and in such a scenario, gravity walls are not recommended. If gravity walls is to be constructed in such conditions, then the section of the wall is likely to be quite heavy. Cantilever Retaining Walls- The cantilever retaining wall resists the horizontal earth pressure as well as other vertical pressures by way of bending of various components acting as cantilevers. A common form of cantilever retaining wall is the T-wall which consists of a stem, heel slab, and a toe slab. Each of these bends as cantilevers and hence are reinforced on the tension face. Another common form of cantilever retaining walls are the L-shaped walls, which also resist the soil pressures by bending. Cantilevered walls are made from a relatively thin stem of steel-reinforced, cast-in-place concrete or mortared masonry. These walls cantilever loads like a beam to a large, structural footing; converting horizontal pressures from behind the wall to vertical pressures on the ground below. Sometimes cantilevered walls are butressed on the front, or include a counterfort on the back, to improve their stability against high loads. Buttresses are short wing walls at right angles to the main trend of the wall. These walls require rigid concrete footings below seasonal frost depth. This type of wall uses much less material than a traditional gravity wall. Most modern constructions prefer cantilever retaining walls as these are easy and quick to construct (though some care has to be taken while designing and especially during construction of such walls) and can be used almost anywhere. Due to its capacity to tackle bending and especially tensile stresses, these walls are favourites with the engineers. Active Earth Pressure: Rankin's Theory- It can be shown that under conditions of one-dimensional loading (i.e. lateral constraint), the ratio between the vertical and horizontal effective stresses can be represented by, where, Ko= Rankin earth pressure coefficient for soils at rest Typical values of Ko are Basic assumptions of Rankin's Theory are: The soil and the wall are semi-infinite. The soil is dry, homogeneous and isotropic. The soil is granular with no cohesion (c' = 0). The face of the structure that is interacting with the soil is assumed to be completely smooth, so that there is no friction between the structure and the soil. In addition there may also be no adhesion between the wall and the soil. The wall is vertical, and the surface of the soil behind the wall is horizontal - the vertical and horizontal effective stresses in the soil mass are principal stresses and no shear stresses exist on vertical and horizontal planes. The wall is assumed to rotate at its base in order to develop the active and passive states. Planer failure surfaces are assumed, instead of circular or log-spiral planes. In Rankin's original derivations it was assumed that the whole of the semi-infinite mass profile exists simultaneously and uniformly in a state of plastic equilibrium in the active and passive cases, often represented by a criss-cross pattern throughout the soil mass, at angles representative of the failure plane angles, q or (90 - q). However, in reality the movement of a finite wall cannot develop plastic equilibrium in the soil mass as a whole, but rather only in a wedge of soil the size of which is dependent on the orientation of the failure planes - the remainder of the soil mass would not reach plastic equilibrium. Water Pressure- If the retained soil is either saturated or dry, the analysis is straight forward. If, however, a water table is present at some depth behind the wall, the effects on the change in unit weight of the soil and the pore pressure distribution behind the wall have to be taken into account. The material above the water table is analyzed in the normal way using gdry or gsat as unit weight. The weight of the top layer is then added as surcharge on the saturated layer at the location of the water table. In analyzing the saturated layer the unit weight has to be changed to g' and z is taken as zero at the level of the water table. Lastly the pore pressure distribution behind the wall is added. A rise in water table from the toe to its top can more than double the horizontal pressure on the wall. This can happen relatively suddenly and is why water is one of the main causes of retaining wall failures. It is therefore important that the water table, direction of water flow and seasonal variations of water in the ground are well understood before construction starts. Rain storms and burst water mains can also create severe conditions. To counter this effect, weep holes in sufficient numbers should be provided in the section of the retaining wall to drain off the water from the earth side. Particular care must also be taken that sand particles are not drained off through weep holes. For this purpose, graded filters should be provided near the weep holes on the earth side. 3) Superstructures All the part of the structure visible above the ground level is known as superstructure. In olden days, buildings used to be load bearing structures with the walls of the structure bearing the whole weight and providing strength to the structure. With advancement in material technology and ever increasing demands, today's structures are framed type structures in which all the load is taken by the frame of the building. The walls of such buildings are only provided for partition or to save its habitants from the external agencies like sun, wind, snow, rain etc. Such frame structures usually have reinforced cement concrete or structural steel as its framework. Further, the reinforced cement concrete may be cast in-situ or maybe pre-cast and only assembled at the site. Cast in-situ concrete- This simply means concrete cast in situation. This is most commonly used, with the frame of a building cast at the site with formwork in place. In such a case, all the materials required to cast the concrete (cement, sand, aggregate, water, admixtures etc.) are procured on the site and the concrete mix is prepared by mixing all these ingredients in the required manner at the site itself. This concrete is then transported to place where it is to be caste and placed in the formwork which is prepared before hand to cast the concrete. After the required period, the formwork is taken off and the required curing is done for such concrete, which takes up the shape of the frame of the structure. Advantages of cast in-situ concrete- All the concreting work, right from concrete manufacturing to laying it in place, compacting and curing is done right under the supervision of the site in-charge according to the requirements. Concrete members of virtually any shape and size can be cast as the casting is done on the site itself. Some of the obvious disadvantages of in-situ concrete are- All the materials required to produce concrete has to be made available on site. Proper care has to be taken and strict supervision has to be carried out at every stages of casting the concrete blocks. A high level of finishing may be little difficult to achieve in this case. Pre-cast concrete- The main difference between pre-cast concrete and cast in-situ concrete is that the former is a factory made product while the latter is made at site for work. The required members are manufactured off-site and just bought at site to be assembled. As pre-cast concrete members are manufactured in a quite controlled environment, they are usually of far higher quality. Advantages of pre-cast concrete are- Very suitable in urban area where much space is not available for construction. Concrete of superior quality is produced and a far greater technical control is achieved. The work can be completed quite quickly. Some disadvantages of pre-cast concrete are- Very careful handling is required during transport. Special equipments are required for transporting pre-cast members. If the factory is far away, then pre-cast concreting turns out to be very costly due to high transportation charges. Structural Steel- Structural steel is usually used as a framework for industrial buildings where large spans and strength is required. Structural steel when used in residential or commercial buildings is encased in concrete to achieve good appearance. Structural steel is more suitable in high rises due to its strength and durability. The main advantage of using structural steel is that the work commences very fast as it just needs assembling on site. But availability of the required section in the market may not always prove to be easy. Claddings- Various types of claddings right from aluminum, steel, timber, and some modern day plastics are used as cladding materials. The suitability of the cladding material will depend on the use of the structure. Timber cladding acts as a very good insulation materials and protects against cold, but is very poor in resisting fire. Aluminum and steel claddings give a very modern industrial look to the buildings. Thus claddings need to be selected considering its use (aesthetic appearance, other use) and thus the most appropriate cladding can be used. References Arora, K. R. Soil Mechanics and Foundation Engineering. Delhi: Standard Publishers Distributors, 2003. Punmia, B. C., Jain A. K. R. C. C. Designs. New Delhi: Laxmi Publications, 1998. Rangwala, S. C. Building Construction. Anand: Rupalee Publications, 1998. Kumar, S. Building Construction. New Delhi: Standard Publishers Distributors, 1999. Read More