Characteristics of Structureal Engeneering - Case Study Example

Introduction: The report includes an introduction to the brownfield sites and a detailed mechanism of remediation of a brownfield site. An appropriate system of foundation for the required building is selected with the retaining wall and ground floor system. The characteristics of the super structure are discussed followed by a proposal of environment friendly cladding. A brief mention of some other environment friendly techniques is also given at the end. Brownfield Site: A brownfield site is defined as previously developed land that has the potential for being redeveloped. These are the sites which had been mostly used for industrial and commercial purposes in the past and are mostly contaminated. These are often polluted by hazardous wastes and pollutants of low level. The brownfield sites have become very popular recently because of the lack of availability of green lands (lands which have not been built-upon in past). These sites are very popular in urban areas where demand for residential and commercial buildings is very high. In such areas the old buildings are destroyed and new buildings take their place. The government in UK is taking considerable steps to encourage the use of brownfield sites and these sites have become very popular in recent past. Brownfield sites are being developed on priority basis. Brownfield sites can be redeveloped in many ways, not only by constructing residential or commercial buildings but also as recreational and open spaces. The first step in the redevelopment of a brownfield site is its assessment through certain specified experimental procedures. These experimental procedures involve analysis of the soil, analysis of surface and ground water through testing for hazardous compounds. After the conduction of these tests test reports are obtained which dictate the extent of pollution and the nature of remedial measures. Certain specific laws are present which govern the redevelopment of brownfield sites, these laws are subjected to strict environmental regulations and these can be prohibitive for the developers. A brownfield site developer should not only know about the construction techniques but also should be highly equipped to cope with environmental challenges which are to be faced during the redevelopment of a brownfield site. After the correct assessment of the potential risks involved in the redevelopment of the brownfield site, the next step is the remediation. Remediation means the removal of all the assessed contaminants to an extent that will bring the contamination level to a very low which is considered safe for human health. Remediation of a brownfield site is necessary prior to redevelopment. The remediation process is usually very expensive and a cost benefit analysis has to be done before arriving at a final conclusion that whether it is useful to redevelop it or not. In the present case the analysis has been done and the company wants to establish its office and the showroom at the brownfield site. Several techniques have been developed for the remediation of the brownfield sites. A combination of the following techniques would be used in the present scenario: Bioremediation: It allows natural processes to clean up harmful chemicals in the environment. Microscopic biological organisms or microbes which dwell in the groundwater and on the surface feeds on certain harmful chemicals which are most commonly found on the brownfield sites. The microbes decompose these chemicals into harmless chemicals, water and gases like carbon dioxide. Th ese micro biological organisms need optimum conditions for their proper activity. These optimum conditions include the right temperature, nutrients which act as fertilizers for these microbes and a certain amount of oxygen should also be present in the soil. In these optimum conditions the activity of the microbes is greatly enhanced, they consume chemicals quickly and multiply which in turn increases the rate of consumption of chemicals. Failure to provide the optimum conditions does not only make the microbes ineffective but can also make them start producing chemicals which are rendered harmful for the environment. Therefore it is very much necessary to improve the conditions to optimum for proper remediation. This can be done by pumping nutrients and air underground. In the present condition the soil needs to be dug out first and then microbes along with nutrients would be added below the soil and then the soil will be backfilled. This will ensure that the microbes are present beneath the soil and they have the proper supply of the nutrients. After this air will be pumped into the soil which will enhance the activity of the microbes. Some of the chemicals start evaporating when the mixing of the microbes with the soil is started; this can cause the pollution of air and is threatening for the environment. To avoid air pollution due to evaporation of chemicals a Formica sheet temporary roof will be fixed above the site. The roof will be slanting with collectors at the ends on the underside. As the chemicals will evaporate they will condense on the Formica sheet and will be collected in the collectors and properly disposed off. This will also slow down the evaporation process by saturating the air above the site giving the microbes more time to decompose the chemicals. At brownfield sites, the groundwater is also polluted in addition to the surface soil. Microbes can help clean polluted groundwater. Some of the groundwater is pumped into specified tanks. Here the water is mixed with nutrients, air and microbes and then the water is pumped back into the groundwater where the microbes treat ht remaining water. The microbes automatically die when the chemicals are completely consumed as the microbes do not get food. Bioremediation is very safe because it relies on microbes that naturally occur in soil. These microbes are helpful and pose no threat to people at the site or in the community. There is no dangerous chemical use in bioremediation. The nutrients for bioremediation microbes are the commonly used fertilizers. During bioremediation tests are conducted on both the soil and the groundwater to ensure proper working of the bioremediation. Keeping in view the allocated site the time required for bioremediation should not be very large but time required largely depends on the types of harmful chemicals encountered at the site. It is a very useful technique as it is a natural making least harmful for the environment. It is quick as it does not require complex equipment and procedures. The groundwater can be cleaned without moving it to some other place. Chemical Oxidation Remediation: Some of the areas at the site which have high concentrations of chemicals and would require a longer time for remediation by microbes will employ chemical oxidation remediation as it is a relatively quick process where harmful chemicals have high concentrations. In-situ chemical oxidation can be accomplished by introducing chemical oxidants into the groundwater and soil at the site. Figure 3 Chemical Oxidation Remediation [http://www.regenesis.com/images/products/chemical-oxidation.jpg] This is done through vertical injection wells. Another approach is to pump the groundwater and treat it above the surface. This water is pumped back after completion of treatment by mixing with the chemical oxidants. The remediation and redevelopment of brownfield site is very important for the sustainable construction strategy and this building will serve as another step forward in environmentally sustainable construction. Site Investigation After remediation of the brownfield site the next step is to conduct the site investigation for getting information about the geological properties of the soil. For brownfield site the site investigation for previous building can also be consulted but new site investigation is also very necessary. Site investigation describes the process of carrying out investigations on land to determine the geological properties of the soil and to collect suitable data for the purpose of risk assessment. The investigation is normally carried out in several stages. These stages range from desk study and simple visual inspection to full intrusive investigation using trial pits and boreholes etc and the sampling and analysis of materials. A wide range of techniques and methods for gathering site data are available however, it is essential that any site investigation is designed based upon the available conceptual site model and will produce sufficient quantity and quality of data which is required for risk assessment. Site investigation at the site will be done by using washing type boreholes. In this method a light bit is used and by chopping, twisting and jetting action of the bit as circulating drilling fluid removes cuttings from the holes. It is difficult to obtain undisturbed samples in this method. Changes indicate rate of progress. The site investigation will give the type of strata of the soil and the variation in bearing capacity of soil at different places as it is a brownfield site. Type of Strata and Bearing Capacity: The site is located in London (assumption) so the geology of the site (London) as described by Gall Zeidler Consultants (2010) is made ground, terrace gravels and alluvium (gravel, sand, silt and clay), London Clay and various deposits of the Lambeth Group (sand, silt, clay). So the upper stratum of the site is mainly clay with strong load bearing strata lying below so a suitable foundation will be one from the deep foundations. Micropiles: As the site is a brownfield site the bearing capacity of the soil is variable. There are many approaches to solve this problem the one that should be used here is by micropiles. Micropiles are often used to improve the bearing capacity of the foundation against applied loading. In many cases steel pipes of 50 to 200 mm diameters are used as micropiles. The strengthened ground acts as coherent mass and the behavior of such soil is remarkable as it is capable of sustaining very high compressive loads. In the present case the portions on the site which have comparatively less bearing capacity will be improved using micropiles to match their bearing capacity with other portions on the site. According to Lizzi (1982) and Plumelle (1984) micropiles create an in situ coherent composite reinforced soil system and the engineering behavior of micropile-reinforced soil is highly dependent on the group and network effects that influence the bearing capacity and the strength of the soil. Micropiles restrict displacements to tolerable levels and thus stabilize the soil. In the present case galvanized steel pipes of 100 mm diameter and 10 m in length should be driven at an angle of sixty degrees should be driven in the soil to achieve the desired bearing capacity for a 10 storey building utilizing the friction between the pile and the soil. Foundation system for the building Having improved the brownfield site to constructible levels with complete remediation against harmful chemicals and having improved the bearing capacity of the soil along with acquiring the knowledge about the geology of the site through site investigation the next step in construction is the selection of foundation system for the building. On a brownfield site there is a rare possibility of constructing on the foundations of the previous building if deemed suitable. But in the present scenario the foundations of the previous building which are raft footing (assumption) will be removed prior to the start of the construction of foundation system of the building. The foundation system selected for the building is Pile Foundation. Pile Foundation: As the subsoil at the site consist of a 13-15m thick very soft clay layer overlying medium to stiff clay layer. A sand layer occurs at depth of 20m to 35m below the stiff clay layer. A second layer occurs intermittently between sand layers, according to the typical geology of London where the site is located. A dense layer of sand is below 50 m where the toe of the deep bored piles would lie. The piles for the retail outlet will be bored to a depth of 25m where the first layer of sand lies. As pile foundations are used to carry and transfer load of the structure to the bearing ground located at a certain depth below the ground therefore firstly we need to identify this depth. The main components of the foundation are the pile cap and the piles. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity. Concrete piles should be used here considering the geotechnical conditions and the extent of rise of the building. Pile foundations are particularly useful in case of brownfield sites as the behavior of the surface soil and the soil at shallow depths is unpredictable because of the previously done construction therefore instead of spending large sum of money on soil improvement techniques pile foundations are a very good option for using as the foundation system for the building particularly in areas where the soil with high bearing capacity is not very deep as is the case here. As the ground contains both sand and clay so a combination of friction and cohesion piles would be used. These are actually an extension of end bearing piles. The pile is driven far enough into the lower material to develop adequate frictional resistance. The piles should have enlarged bearing areas by forcing a bulb of concrete into the soft stratum immediately above the firm layer to give an enlarged base. This can also be done by forming a large cone or bell at the bottom with a special reaming tool. Bored piles which are provided with a bell have a high tensile strength and can be used as tension piles as well. The mechanism to be followed for piling is driving with casting in place. For this Franki Pile approach should be used. In this method a steel tube is erected vertically over the place where the pile is to be driven and about a meter depth of gravel is placed at the end of the tube. Hammer is dropped having a weight of 1500 to 4000 kg compacting the aggregate into a solid plug which then penetrates the soil and takes the steel tube down with it. After achieving the required depth the tube is raised slightly and the aggregate is broken out. Dry concrete is then added and hammered until a bulb is formed. Then reinforcement is placed in position with the addition of more dry concrete. Here the piles should be permanently cased (casing should be left in the ground). In a brownfield site even after remediation there is a danger of contamination of soil levels which are neither very deep nor are near the surface. Such depths cannot be reached easily during the remediation process therefore in case of a brownfield site, which is the case here the casing for the piles should be permanent i.e. it should be left in the soil after the construction of piles is complete. The selection of cast-in-place piles has the following advantages: 1. These are relatively inexpensive 2. These have low noise levels 3. Pile lengths are readily adjustable 4. An enlarged base can be formed which can increase the relative density of a granular founding stratum leading to much higher end bearing capacity Some of the potential problem areas which should be kept in mind while using the piling foundation system are: 1. Heaving neighboring ground surface which can be avoided by carefully observing the nearby ground surfaces during the piling operation. 2. Displacement of nearby retaining walls, this can also be avoided by carefully executing the boring and driving process. Basement Parking The basement will be used as the parking for the building. The retaining walls will be cantilevered. Cantilevered retaining walls are made from an internal stem of RCC cast in place. The cantilever retaining walls will be of inverted T shape and will convert the horizontal earth pressure to vertically downward acting load. The walls will be buttressed on the front. The advantage of using cantilevered retaining walls is less use of material as compared to the gravity retaining wall. Ground Floor System: The ground floors will be composite floors consisting of profiled and formed steel decking with a concrete topping. These floors are very light in weight but have high strength. These floors do not need form work. The use of lightweight concrete results in the reduction of dead load. The decking will distribute the shrinkage strains thus preventing serious cracking and will stabilize the beams against lateral buckling. The cells in the decking help in locating the services which have been provided underground and are otherwise sometimes very difficult to locate. Concrete Frame Structure: A proposed structure for the building is concrete frame structure and it should be moment resisting frame. The column orientations and locations are such that there are no moment-resisting frames of more than one bay in either direction of the building in the top most storey. The first story should be tall as it has to incorporate the clear span showroom/factory outlet of the company. Storey height for this building would range from 3.5 to 4.5m in the first storey and 2.8 to 3.0 in the upper storeys. The columns would be blade columns with an aspect ratio of approximately 3. Column plan dimensions should range between 150 mm cross 500 mm to 250 mm cross 800 mm. The longitudinal rebar ratio would range from 1% to 2%. 12 to 16 mm diameter smooth rebar are generally used. Transverse ties are smooth rebar of 6 to 10 mm diameter with 90° hooks. The spacing of transverse ties is typically 200 to 250 mm along the clear height of the column. Typical beam spans ranged between 3 and 5 m. Beam depths and widths ranged between 200 to 250 mm and 500 mm to 600 mm, respectively. Transverse ties are smooth rebar of 6 to 10 mm diameter with 90° hooks. The spacing of transverse ties is typically 200 to 250 mm along the clear length of the beam. Bent-up longitudinal rebar, often used for reasons of economy to provide shear resistance to gravity loads and to increase negative moment resistance for gravity loads at supports, do not resist shear force if the loads are reversed due to earthquake shaking. Side-face column rebar should either be spliced per corner rebar or terminated above and below the joint with 180° hooks. No transverse reinforcement for the purpose of confinement should be provided in the hinge, joint, or splice regions. Common slab system thickness from 80 mm to 120 mm. Joist system is formed by hollow clay tiles. Interior and exterior infill walls should be constructed of either hollow clay tile or lightweight gas-concrete blocks. The hollow clay tile block is more widely used than the gas-concrete block and is extremely brittle. The block infill should not be reinforced nor should it be anchored to the structural framing with masonry ties. Block infill walls are proposed to be built in contact with the structural framing and add significant stiffness and strength to the framing system. Environment Friendly Cladding System: Glass is most commonly used as a cladding material in buildings but an insulated cladding material can replace glass curtain wall and at the same time provide a more aesthetically and architecturally appropriate solution. A building employing a glass cladding system uses a lot of energy but an insulation material which is aesthetically good can be used as for cladding. The proposed cladding system is a wall EIFS (Exterior Insulation Finish System). If used in a shade of beige at a thickness of 290 mm EIFS can significantly reduce structural loads; can reduce annual cooling/heating loads by 6-8%. This can also reduce the overall cost of the cladding by 60%. EIFS does not require any frames or mounting assembly as it adheres directly to the concrete superstructure. Other major advantage is that it can be pre-fabricated which will reduce the construction time and it can be painted or textured to achieve a wide variety of aesthetic and functional specifications. The mesh can protect the building from moisture. Moreover there will be no cracks in the material as it is very flexible. The material is best suited for all types of climates as it consists of layers. During the fabrication the thickness of each layer can be selected according to the different temperature and humidity ranges. In addition to selecting the thickness of various layers of the material we can also control the number of layers according to our requirement. The good thing about EIFS is that it can achieve the aesthetics of glass and also the sustainability and the energy efficiency performance is very good as compared to glass. As the material is very light it will reduce the loads on the building main structure which will in turn save the building material reducing the overall costs of the building. While designing a building for energy efficient and environment friendly techniques like passive solar heating and passive cooling glass and steel cladding are of no use; rather these materials have negative effects on the energy efficiency of the building. On the other hand EIFS increases the efficiency of such designs. EIFS along with other insulation techniques, the passive design and thermal coupling can make a building energy and environment friendly. Other Environment Friendly Techniques: The building in addition to the proposed environmental friendly cladding and design would also incorporate the following environment friendly techniques. 1. The central lobby will include skylights for lighting up, such design can save energy. 2. Concrete slab will insure earth coupling which will help in maintaining the temperature of the building. 3. The orientation of the building will be due north to facilitate the passive solar heating and proper ventilation would be ensured to facilitate passive cooling. Conclusion: Employing the above mentioned techniques for the structure and cladding system for the building the building can be converted into an environment friendly building with fulfilling all the requirements needed by the client and the client’s environment policy. References Geltman E. G., (2000) Recycling Land: Understanding the legal landscape of brownfield development, University of Michigan Press Tomlinsion M.J. (1994) Pile Design and Construction Practice, Taylor and Francis Abramson L. W. (2002) Slope Stability and Stabilization Methods, pp 511-512, John Wiley and Sons Cowan H. J. (1982) Design of Reinforced Concrete Structures, Prentice Hall Nelson P. E., Kroll R. E., (1996) Exterior Insulation Finish Systems (EIFS): materials, properties and performance, ASTM International Read More