Labour Based Road Construction Methods - Assignment Example

Labour Based Road Сonstruction Methods Abstract A project involving both up gradation and construction of 30 miles of a highway is to be carried out. There are a number of key civil engineering activities that need to be done for the completion of this project. The activities that would go into the execution of this project form the basis of this assignment. Introduction The actual laying of a highway is preceded by a number of engineering activities. This is based on the following (Johannessen Bjorn ,1997 ) (i) The depth to which foundation can be laid is first assessed and soil studies are adequately done prior to fixing this depth. (ii) The bearing capacity of this foundation needs to be calculated. (iii) The settlement that could happen once the work is over and vehicle movement starts is calculated depending on a number of known external factors. Reinforcements are thus made accordingly. (iv) The foundation must be so designed to take minimum loads and should not fracture before a certain calculated period of time. (v) An expansive soil can cause severe lateral movement of the subsurface below the top layer. The foundation should be strong enough to resist this expansion and contraction. (vi) Areas that are seismic prone should be designed adequately to overcome seismic moments. Resources Before the actual construction of the road begins there are three criteria’s that need to be considered which are the Technical feasibility, Economic justification and Environmental impact. The 30 mile roadway that is to be constructed will traverse a length of the country that might contain soft soils, hard terrains, sloped terrains, mid-streams or rivers. When initially planning such a project it should be assessed whether building materials like gravel, sand and water are available at distances close to the work site. Facilities for future maintenance are also deciding criteria. The construction of the road should be economically justified in terms of the savings by travellers and freight costs. The reduction in number of accidents and also the total economic development of the region due to the construction of this road also need to be considered. Environment Impact (Johannessen Bjorn , 1997) The environmental impact of any major construction work is inevitable. The major considerations with regard to the environment are (i) Preservation of wildlife and our natural habitat. (ii) Whether the construction would cause damage to any nearby archaeological site (iii) Pollution of atmosphere (iv) Prevention of groundwater contamination (v) Proper disposal of waste generated Materials The materials that are required for the construction and lying of roads include asphaltic concrete. Bitumen and Road Oyl act as resins for bonding activities. Gravel and bricks for laying and compacting of foundation are also required. (USGS , 2006) Principles of Underpinning activities Underpinning is the method of supporting structures while providing a new foundation below an existing foundation. Underpinning is achieved by the Pit method or the Pile method. The Pit method involves creating holes in existing walls and inserting needle beams with bearing plates through these holes resting on crib supports. The excavation is done to the new level and filled with concrete. (Singh Bharat and Gupta DL, 1999) The Strip Method involves saw cutting the cracked concrete part to a distance of 1 feet on both sides of the crack. The new reinforcement is then laid and filled with concrete. Works to be carried out Of the major works to be carried out, five of the relevant works have been discussed here. Retaining Walls Retaining walls can be divided into (i) Gravity walls which resists overturning by virtue of its entire mass. Walls of this type are economical for heights up to 2 to 3m. The base width should be 0.40 to 0.65 times the overall height. The flat face should be given a slight backward slope of 1 in 24 to account for the forward push that would inevitably follow. This combined effect provides the best counter for earth pressure. Vertical joints should be located in concrete walls at distance of 20m. ( Day Robert W, 2006) (ii) Cantilever walls which depend on the resistance to withstand bending moment of the cantilevered slab about its base. These are suited for heights between 4.5m to 6m. The width used is between 0.4 to 0.65 times overall height. The wall thickness for single layer reinforcement is 150mm and 230mm for front and back reinforcement. Expansion joints are to be provided to account for the thermal expansion and the resulting movement. Spacing’s between 20m to 30 m are considered for British conditions. (iii) Counter fort walls prevent overturning of the walls by the reactive force exerted by the earth located behind the wall. These are very effectively tied to the base slab and these counter forts are spaced at one-third the height of the wall. Pressures and stresses are thus transmitted to the pile foundations (iv) Buttressed walls transmit stress to the soil with the help of buttresses extending from the front of the wall. These are used for walls higher than 6m. The buttresses transmit the load to the top of soil. Hence the wall is ably supported except for the 1m which stands cantilevered from the base slab. (Singh Bharat and Gupta DL, 1999) (v) Tied-back diaphragm walls are anchored at one or more points thus preventing overturning of the wall. After the stage I excavation, a portion of the wall is anchored. The remaining portions of the wall are anchored in a series of stages. This is so designed that the resistance provided by the soil in front of the wall prevents any forward movement. Foundations A foundation is that part of a structure which is in direct contact with the ground and transfers the load to the structure below. The design of foundations requires the designer to have a thorough knowledge of the mechanics of the soil and to have a judgement regarding the amount of settlement expected. Foundations are classified as Shallow foundation and Deep foundation. If the depth is equal to or less than its width it is called a shallow foundation and if the depth is equal to and greater than its width it is called a deep foundation. Functions of a Foundation (i) It distributes the load over a wider area thus preventing overloading of the subsoil. (ii) It prevents unequal settlement. (iii) It prevents overturning and tilting of structures. (iv) It prevents lateral force from the adjoining soil and provides a level and firm base for construction. Different Types of Foundation 1. Spread or Open Footing- This is suited in places where hard soil is available within 1.5m to 3.5m depth. These can be classified as wall footings and reinforced concrete footings. Wall footings consist of several courses of bricks with the lowest course twice the thickness of the wall above. The depth of each course is about 10cm. Reinforced concrete footings on the other hand is used where loads to be handled are heavy and bearing capacity of soil is very low.( Unified Facilities Criteria, 2005) 2. Raft Foundation- If the loads are heavy and soils have low bearing capacity then a raft or a mat foundation is provided covering more than half the required area. Differential settlement is thus avoided as load is transmitted uniformly. The design of raft foundation varies with the type of soil below the structure. A raft foundation over sand will have a different design to a foundation over clay as a raft on clay may fail by shear as opposed to sand. The recommended factor of safety against failure is three. 3. Well foundation-This method consists of excavating up to half a meter and then placing a well curb in place. Suitable coffer dams are erected to prevent water ingress in areas where the highway crosses midstream. After laying the curb, excavation is begun inside. When it reaches about 1m steining is done using brick masonry up to 1m above the bed level. After this the bottom is dredged to maintain a uniform level. A thick layer of concrete is poured and the bottom is plugged. After the plugging procedure all the water is pumped out from inside the well and the top sealed by concrete. ( Day Robert W, 2006) 4. Caisson Founadtion-This is used in sandy soils and is constructed by installed by installed a box made of timber masonary or concrete and can be used where the required excavation is minimal. The procedure involves sinking the box in position and filling it with masonry. These are generally used for roads traversing over bridges and piers. (Singh Bharat and Gupta DL, 1999) Slope Stabilization When there is a difference in elevation, gravitational forces try to cause a movement within the point at a higher level. If the slope is unstable and the shear resistance is less than the gravitational resistance it can result in a landslide. Slopes may become unstable due to the following reasons. (i) Loss of cohesion with in the soil particles (ii) Reduced weight of soil due to absorption of water thus forming a clayey deposit. (iii) Temperature changes within the environment causing alternate expansion and shrinkage. (iv) Natural calamities like earthquakes and tsunamis. There are several methods that can be used to assess the stability of slopes. These include (a)Swedish Slip Circle Method (b) Bishops Method (c) Janbu’s method (d) Prices method (e) Spencer’s method. The Swedish slip circle method is discussed here. The basic principle used in assessing factor of safety, F in slopes is by equating the disturbing moment that has the tendency of causing slips with the restoring moment that is acting to counter this moment. The mass of soil of weight wg cause a bending moment of wg x a about point a, which is the centre of the arc. The restoring moment is provided by the shearing force of the arc soil profile. ( Day Robert W, 2006) The factor of safety can be increased by constructing buttresses at the toe of the slope or installation of piles or pier walls. Suitable means of anchoring the soil to the layers below are also envisaged. ( Saudi Aramco Desk Top Standards, 2005) Improving the drainage facilities also decrease the force acting on these slopes. Inserting a shear key at certain points along the slope also help in increasing the shear strength of these soils as the shear key, which is a wide trench is filled with soil of higher shear strength. This acts as a deterrent to the loose soil in sliding down. Groundwater control Groundwater is water that is present beneath the ground surface in soils pores and in fissures within rocks. The control of water available underground when designing substructures forms an important part of Civil Engineering Design. Ingress of water causes delay in work, flooding and also erosion of certain work areas. This water is located within the soil up to the water table. Controlling ground water consists of permanent exclusion or temporary exclusion of groundwater. (Unified Facilities Criteria, 2005) Temporary exclusions are done by using pumping out water from sumps or well points from depths limited to 7.5m. Providing lateral drains also help in drainage. Using a solution of magnesium chloride at temperatures of -15ºC and 25ºC creates a wall of ice which is totally impervious facilitating the personnel to work inside. Permanent Exclusion of water includes sheet piling or construction of diaphragm walls by which a cut-off wall is constructed to prevent ingress of water. Grouting is another process by which the soil or rocks are injected with setting material which reduces the porosity of the material. A grout curtain is created by boring holes around the work area and injecting cement grout into these holes. Piling Works This is provided to obtain necessary bearing capacity on loose soils. It is defined as a column driven vertically into the ground to increase bearing capacity. It protects the foundation from slippage, compacts granular soils and transfers loads. Piles are classified as displacement pipes and non displacement (replacement) piles. (i) Large or small displacement piles are driven into the ground by displacing the soil outwards. These are constructed by driving either timber, precast concrete and steel tubes into the ground. The lower end of this tube is closed by a shoe or plug which is maybe left intact or expanded to form an enlarged foot. These are normally used for marine works where this pile can be driven through water or clayey soil for large depths. These can be easily modified in length and have a continuous cross section. Disadvantages include large vibration and noise. When driving slender piles, care should be taken to avoid fracture caused by bending stresses. Small displacement piles on the other hand consists of an open ended tube or a hollow section that enters the ground with ease during driving and forms a sort of binder to the loose soil. Displacement piles are available in variants of timber piles, precast and pre-stressed concrete piles and Jointed precast concrete piles. Timber piles can be used for loadings up to 300kN. These are used with combination of pile caps in concrete. Concrete piles form the upper part of the pile avoiding contact with water. The timber pile forms the rest and is located below the water table thus avoiding decay. Precast concrete have the advantage of lot of time saving and not being limited by the vagaries of site construction. However care should be taken during transportation, lifting and driving these piles into the ground to ensure no hair line cracks form on its surface due to a combination of tensile and compressive forces. These are designed on a safe stress of 250 mPa. (Day Robert W, 2006) Jointed precast concrete piles consist of the West’s Shell pie or the Herkules pile. The West’s pile consists of cylindrical hollow shells made of 380, 405, 535 and 610mm diameters which can be threaded onto a steel mandrel. Shells can be adjusted to suit depths and after attaining the required length the steel mandrel is taken out and interior space filled with concrete. Herkules pile consists of short hexagonal precast sections with locking units to increase lengths. (ii) Replacement piles are formed by drilling a hole till a certain depth. A steel cage or tube is then inserted into this hole and allowed to sink into the hole by gravity. Concrete is then gradually poured into the casing while the cage is slowly retracted. Bored or replacement piles have the disadvantage that if water is encountered during the driving process it tends to wash out the cement from the concrete. If the casing is also withdrawn before the concrete is set then cracks in the bored pile may occur. (Day Robert W,2006) Hazards There is a lot of safety issues associated with a work site. Accidents caused to machine failure, electrical motors not properly insulated and tools being worn out all contribute to risky work areas. Bitumen and tar should be handled with care since these can cause a toxic effect on reaction with human skin. Proper uniforms with protective shoes and helmets are to be used while working. People working on scaffoldings should have proper harness tied around their waist. The area of work must have proper lighting and should be properly allocated. Unsafe areas should be identified and marked suitably. (Singh Bharat and Gupta DL, 1999) Communication Positive communication with clarity among the workers at site is essential to oversee that all activities proceeds smoothly under a clear chain of command. The Civil engineer is responsible for facilitating the clear flow of instructions to his subordinates. Discussions regarding ways to overcome obstructions and pitfalls are always welcome but what comes out of that meeting should be understood equally by all heads of department so that there is no confusion when the actual work takes place. There should also be a spokesperson to address the general public at large so that any feelings of ill will from the public regarding certain delays or environmental disturbances are mitigated. Reference Lists 1. Johannessen Bjorn, “Labour Based Road construction methods”, Technical manual, August 1997, p.p 3-5 2. Day Robert W, Foundation Engineering Handbook, 2006, p.p5.3-5.17 McGraw Hill, 3. “Geotechnical Engineering Procedures for Foundation Design of Buildings and Structures”, Unified Facilities Criteria, Department of Defence, USA, August 2005, p.11-9 4. “Concrete Foundations”, Saudi Aramco Desk Top Standards SAES-Q-005, September 2005, p.p 4-7 5. “Materials for Use in US Interstate Highways”, USGS, October 2006, p.1 6. Singh Bharat and Gupta DL, Construction and Foundation Engineering, January 1999, p.p 1117-1136 Read More