Unit 2. Planning, Surveying and Excavation
THE CONSTRUCTION PLANNER
Construction project planning is receiving growing attention as the limitations of formal deterministic planning are becoming more widely recognised. There is growing concern over the failure of construction planning to achieve its goals in spite of the considerable resources allocated to it. Deficient planning techniques are commonly blamed for this state.
There has been considerable debate over the last decade or so on the effectiveness of construction project planning. However, there is remarkably little research into what construction project planners actually do.
Laufer and Tucker provide a critique of construction planning.
They protest that:
· the planning and evaluation of planning processes are non-existent;
· there is over-emphasis on critical path methods;
· planners lack construction experience;
· planners have poor information gathering methods;
· planning is control-oriented instead of action-oriented;
· Plans have been poorly presented with overly-complex information.
In a subsequent paper Laufer and his colleagues look at the definition and allocation of planning work. They found that there was no clear system at work and planning was done in a multiplicity of ways. They call for the process to be properly co-ordinated by a single individual who will 'own' the planning process and improve communication. Winch and Kelsey believe the specialist planner has the time to do the work but incomplete practical knowledge. The line manager has the practical knowledge but does not have the quality time to carry out the task. The specialist planner has better strategic decision-making skills than the short-term decision-making focus of the line manager. They then go on to say “Line managers see the delegation of key decision-making to another as a threat to their position”.
These problems are confirmed in Laufer wider project management study and can result in:
· The planner preparing a plan which has incomplete information and in-adequate decision making authority.
· The line manager treating such plans as merely an irrelevant forecast prepared by another.
Allen & Smallwood believe Construction planners and the role they perform are little understood both within the construction industry and the wider environment where construction has a daily impact.
Planning occupies a central position in the functions of the manager. His responsibilities may vary with organisational philosophy and contingency but planning invariably remains an essential ingredient of his duties. Much Research and Development effort has been made during the last three decades but progress with techniques has not removed the dissatisfaction with the application and results of construction planning.
Effective construction planning demands that it is carried out by competent and experienced personnel. While the estimator must remain in charge or pricing the work, most contracts, today, are too complex for the estimator to handle the technical content of pre-tender appraisal. It is here that the construction planner has a major function.
WHAT IS TRIANGULATION SURVEYING?
Triangulation is the tracing and measurement of a series or network of triangles in order to determine the distances and relative positions of points spread over an area, especially by measuring the length of one side of each triangle and deducing its angles and the length of the other two sides by observation from this baseline. This was first introduced by a Dutch man named Sneli.
Triangulation is preferred for hills and undulating areas, since it is easy to establish stations at reasonable distances apart, with intervisibility. In plane and crowded areas it is not suitable as the intervisibility of stations is affected. The difficulty is overcome by building towers which is quite expensive. The main disadvantage of triangulation is the accumulation of error in the lengths and direction of lines, since both of them, for successive lines, depend upon the computations for those of the preceding line, which necessitates the check bases.
In triangulation, entire area to be surveyed is covered with a framework of triangles. For the triangle, the length of the first line, which is measured precisely, is known as Base line. The other two Computed sides are used as new baselines for two other triangles interconnected with the first triangle. By extending this process, a chain or network of triangles can be spread over the entire area.
The field work of a triangulation is carried out in the following well defined operations:
· Reconnaissance.
· Station preparation.
· Base line measurement.
· Measurement of angles.
Besides field work, triangulation consists of the specifications, the design of stations and signals, and the reduction and adjustment of the observations.
Applications of Triangulation Surveying:
· Establishing accurately located control points for plane and geodetic surveys of large areas.
· Establishing accurately located control points in connection with aerial surveying.
· Accurate location of engineering projects such as Centre lines, terminal points and shafts for long tunnels, and Centre lines and abutments for long span bridges.
Triangulation Systems:
It is a system consisting of triangulation stations connected by a chain of triangles. The complete figure is called Triangulation System. The most common type of figures used in a triangulation system are:
· Triangles.
· Quadrilaterals.
· Polygons.
Geometric conditions to be fulfilled by above figures in triangulation system are:
· The sum of interior angles should be (2n-4)x90o, where n = no. of sides of the figure.
· If all the angles are measured at a station, their sum should be 360o.
· The length of sides calculated through more than one routes should agree.
It is impossible to fulfil all the geometric conditions, owing to the errors, until the field measurements have been adjusted.
DEEP EXCAVATION
Deep excavation, unlike a shallow one, often requires protecting the sides of cut using suitable support. Besides, the problem of ground water cannot be avoided. There are methods to overcome this, such as:
Dumpling method. This is used where there are buildings or streets in the proximity. The method is to construct a series of retaining walls in trench, section by section, around the site perimeter, leaving a centre Called “dumpling”. When the perimeter walls are in place, excavation may start at the centre of the dumpling, until exposing a section of the wall. Then the wall may be side supported by struts, shoring or soil anchor etc., again section by section in short length, until the excavation is all completed. This method does not require much heavy mechanical equipment and thus the cost of work is relatively lower. It can excavate up to a maximum depth of about 3m. Sometimes in very poor soil or in waterlogged ground, interlocking steel sheet pile may be driven to confine the area to be excavated. Excavation can be done after that. Excavation may reach the maximum of about 15m with the sheet piles application. However, the cost of work will be increased.
Diaphragm walling. This method is necessary for constructing a R.C. retaining wall along the area of work. Because the wall is designed to reach very great depth, mechanical excavating method is used. The typical stages of work are: a) construct a guide wall; b) excavation for the diaphragm wall; c) excavation support using bentonite slurry; d) inert reinforcement and concreting.
A guide wall is two parallel concrete beams running as a guide to the clamshell which is used for the excavation of the diaphragm wall.
Excavation for the diaphragm wall is done using a clamshell or grab suspended by cables to a crane.
Excavation for the diaphragm wall produces a vertical strip in soil which can collapse easily. Bentonite slurry is used to protect the sides of soi1. Bentonite is a naturally occurring clay which, when added to water, forms an impervious cake-like slurry with very large viscosity. The slurry will produce a great lateral pressure sufficient enough to retain the vertical soil.
Reinforcement is inserted in form of a steel cage, but may require extending to the required length. Concreting is done using a tremie.
Diaphragm walling cannot be constructed continually for a very long section due to tremendous soil pressure. Two stop end tubes will be placed at the ends of the excavated trench before concreting. The tubes are withdrawn at the same time of concreting so that a semi-circular end section is formed. Wall sections of this type are built alternatively leaving an intermediate section in between. The interior sections are built similarly but without the end tube. At the end a continual diaphragm wall is constructed with the sections tightly joined by the semi-circular groove.