Civil Engineering Estimates

An estimate is a calculation of the quantities of various items of work, and the expenses likely to be incurred there on. The total of these probable expenses to be incurred on the work is known as estimated cost of the work. The estimated cost of a work is a close approximation of its actual cost. The agreement of the estimated cost with the actual cost will depend on accurate use of estimating methods and correct visualization of the work, as it will be done. Importance of correct estimating is obvious. Under-estimating may result in the client getting an unpleasant shock when tenders are opened and drastically modifying or abandoning the work at that stage. Over-estimating may lose the engineer or estimator his client or his job, or in any case his confidence.

Estimating is the most important of the practical aspects of construction management, and the subject deserves the closest attention of one aspiring to a career in the profession. It is a comparatively simple subject to understand; however, as it brings one up against practical work, methods and procedure, knowledge of it cannot be acquired without close application.

.Purpose of Estimating:

To give a reasonably accurate idea of the cost
An estimate is necessary to give the owner a reasonably accurate idea of the cost to help him decide whether the work can be undertaken as proposed or needs to be curtailed or abandoned, depending upon the availability of funds and prospective direct and indirect benefits. For government works proper sanction has to be obtained for allocating the required amount. Works are often let out on a lump sum basis, in which case the Estimator must be in a position to know exactly how much expenditure he is going to incur on them
1. Estimating Materials
From the estimate of a work it is possible to determine what materials and in what quantities will be required for the work so that the arrangements to procure them can be made.
2. Estimating Labor
The number and kind of workers of different categories who will have to be employed to complete the work in the specified time can be found out from the estimate.
3. Estimating Plant
An estimate will help in determining amount and kind of equipment needed to complete the work.
4. Estimating Time
The estimate of a work and the past experience enable one to estimate quite closely the length of time required to complete an item of work or the work as a whole.
Whereas the importance of knowing the probable cost needs no emphasis, estimating materials, labor, plant and time is immensely useful in planning and execution of any work.

. Types of Construction Estimates:

There are several kinds of estimating techniques; these can be grouped into two main categories

1. Approximate estimates
2. Detailed estimates

1. Approximate Estimates
An approximate estimate is an approximate or rough estimate prepared to obtain an approximate cost in a short time. For certain purposes the use of such methods is justified.

2. Detailed Estimate
A detailed estimate of the cost of a project is prepared by determining the quantities and costs of every thing that a contractor is required to provide and do for the satisfactory completion of the work. It is the best and most reliable form of estimate. A detailed estimate may be prepared in the following two ways

(a). Unit quantity method
(b). Total quantity method.

(a) Unit Quantity Method
In the unit quantity method, the work is divided into as many operations or items as are required. A unit of measurement is decided. The total quantity of work under each item is taken out in the proper unit of measurement. The total cost per unit quantity of each item is analyzed and worked out. Then the total cost for the item is found by multiplying the cost per unit quantity by the number of units. For example, while estimating the cost of a building work, the quantity of brickwork in the building would be measured in cubic meters. The total cost (which includes cost of materials. labor, plant, overheads and profit) per cubic meter of brickwork would be found and then this unit cost multiplied by the number of cubic meters of brickwork in the building would give the estimated cost of brickwork.
This method has the advantage that the unit costs on various jobs can be readily compared and that the total estimate can easily be corrected for variations in quantities.

(b) Total Quantity Method
In the total quantity method, an item of work is divided into the following five subdivisions:
(I) Materials
(II) Labor
(III) Plant
(IV) Overheads
(V) Profit.

The total quantities of each kind or class of material or labor are found and multiplied by their individual unit cost. Similarly, the cost of plant, overhead expenses and profit are determined.

Qualifications of an Estimator

A good estimator should possess the following quantifications:
1. A thorough understanding of architectural drawings.
2. A sound knowledge of building materials, construction methods and customs prevailing in the trade.
3. A fund of information collected or gained through experience in construction work, relating to materials required, hourly output of workers and plant, overhead expenses and costs of all kinds.
4. An understanding of a good method of preparing an estimate.
5. A systematic and orderly mind.
6. Ability to do careful and accurate calculations.
7.Ability to collect, classify and evaluate data that would be useful in estimating.

Good instruction or careful and thorough study of a standard book will help a beginner to become a good estimator. He must, however, try to develop all the above mentioned qualities while obtaining practical experience.

Properites of Soil - Soil Mechanics

Soil Properties

The study of the chemical and physical properties of soil helps in managing resources while working with a particular kind of soil. Not all soils have similar properties, and working with them requires understanding soil properties in detail.

Understanding the chemical and physical properties of soil means understanding the soil behavior under different temperature and pressure conditions. Soil mass is always subjected to changing temperature and pressure forces and, depending upon the chemical and physical properties of the soil mass, the necessary actions will be taken. For agricultural and construction purposes, soil properties need to be studied to increase the productivity of soil and to improve the workability of the soil mass, so knowing soil behavior inside out is very important.
Soil properties can broadly be divided into two major categories depending upon their properties achieved during soil formation process.

• The physical properties of soil
• The chemical properties of soil

Physical Properties of Soil

Here are some of the physical properties of soil:

Soil Texture

The texture of soil is based on the size distribution of the constituent particles. In simple terms, the relative percentage of clay, sand, and silt in a soil mass determines its texture. Furthermore, the soil texture determines the water retention capacity of a soil sample. Sand particles have the largest diameter, whereas clay particles have smallest diameter, among the three soil constituent particles. The American Association of State Highway Officials (AASHO) and the USDA divide soil texture into twelve classes which are shown in the corresponding figure. Depending upon the percentage of sand, silt and clay in soil, it is defined as coarse, fine, or medium aggregate soil.

Soil Structure
Soil structure refers to the arrangement of sand, silt, and clay particles within a soil mass. Air and water movement through a soil mass directly depends upon the structure of soil mass. Symmetry leads to stability, so if a soil mass has symmetrical or good structure, water and air movement through it will be smooth. However, if the structure is asymmetrical, water and air movement will not be smooth and the soil will be unstable. By knowing soil structure, the moisture content of the soil mass can be worked on, which is equally important for engineering and agricultural works.

Color of Soil
Generally speaking, soils are classified as dark and light colored soils. By looking at the color of a soil mass, a rough idea of the moisture content of the soil, drainage properties, and degree of oxidation can be obtained. A dark color reflects poor drainage, high organic content, and low annual temperatures. On the other hand, light colored soils have better drainage, high annual temperatures, and highly leached conditions. However, these are rough estimates that only help engineers and agriculturists to adopt suitable measures to study soil properties in detail.

Permeability and Porosity of Soil
The ease with which soil will allow water to pass through it is called permeability, which is a very useful value for civil engineers. Constructing a building on highly permeable soil means that water proofing techniques have to be put in place before digging a foundation or raising columns. On the other hand, the porosity of a soil mass means the pore space or void space in a soil mass. It affects the strength of a soil mass and it depends upon other physical properties of the soil like texture, structure, and presence of organic matter in soil.

Chemical Properties of Soil

Chemical properties of soil are discussed in this part.

Acidity of Soil (pH)
From an engineering as well as an agricultural point of view, determination of the pH of the soil mass is essential. For healthy plants to grow, the acidic and basic nature of soil must be known. On the other hand, for construction works, highly acidic soil will affect the bitumen stability of roads and have adverse effects on concrete strength. (Salinity of soils will also increase maintenance costs.)

Silicate Clay Presence
The presence of silicate clay materials affects the chemical properties of soil mass. Clay particles have a large surface area and are the finest materials present in a soil mass. Clay particle increase the reactivity of a soil mass and affect the stability of the soil mass by forming compounds with external materials. Determination of silicate clay presence is important to find out the reactivity of the soil mass and its compatibility with admixtures and construction materials used with concrete.
Cation exchange capacity and the presence of organic matter are two other chemical properties of soils.

Index Properties Determination

Shrinkage characteristics, liquid limit, plastic limit, and different densities of soil are called the index properties of soil mass. These properties are determined using different laboratory index test methods. Collectively these properties are known as Atterberg Limits and are valid only for fine grained soils.
The plastic limit of a soil mass, or the point at which a soil ceases to be plastic and starts to crumble, is determined using the Casagrande method (apparatus shown adjacent), which also helps in the determination of the liquid limit of soil

mass. Knowing these properties helps in calculating shear stress actions on soil mass and the behavior of the soil in changing moisture conditions. Other important index test soil properties methods are listed below.

• Pyknometer (specific gravity bottle) test to determine particle density and specific gravity of soil mass
• Volumetric shrinkage test to determine shrinkage properties of soil mass
• Particle size determination using hydrometer test
• Cone penetrometer (fall cone) test to determine liquid and plastic limits, which is often considered superior to testing with the Casagrande method