Lesson Note On Establishing a site master benchmark

Establishing a site master benchmark 

One of your fist tasks as a site engineer on a new site is to establish height control. 

It is usual to establish a site Master Bench Mark (MBM) from nearby Ordnance Bench marks and the following lists the usual procedure 

a) The Master Bench Mark must be constructed in apart of the site clear of any construction activity and must be of very stable construction. 

A typical MBM comprises a steel pin firmly driven into the ground and surrounded by concrete. Provide protection by surrounding it with a fence and the reduced level of the bench mark should be written on that fence as well being recorded in the field book. 

b) The reduced level of the MBM is found by flying leveling from a nearby Ordnance Bench Mark (OBM) using the MBM as a change point (intermediate sights are not permitted) and then closing the circuit onto a second bench mark, if such exists. This not only provides the closing error but also checks the OBM for any possible movement.

 c) The leveling operation must be carried out a minimum of three times, ensuring that a good closing error is obtained each time. The range of results must not exceed 5 mm, if it does then further leveling must be carried out until such a range is achieved. The mean value can then be adopted as the MBM level. 

d) Having established the MBM, any number of temporary bench marks (TBM) can be located and leveled where and as required, working to the same order of accuracy as above. 

The ideal situation regarding TBMs is to locate them in such positions that a level can be set up anywhere on the site and be within sighting distance from it, thus avoiding the need to fly level to the area of operation.

Maintenance of Levelling Equipment

Maintenance of  Levelling Equipment A) Levels (i) Store in a dry, secure place (ii) If the instrument gets wet, all surplus water should be removed with a dry cloth, using a clean tissue for the lens. Store the instrument in a warm secure place with the lid removed, to dry. (iii) Do not oil moving parts, clean them with a dry cloth. B) Tripods(i) Clean them at the end of each working day. (ii) Store them in a closed position carefully, and not thrown in the corner of a site hut. Remember that the purpose of a tripod is to provide a stable working base for the level. It will not do this if mishandled. (iii) Ensure that tripods feet do not become loose. (iv) The clamps on tripods legs tend to become less effective after long usage. Do not use nails, paper, etc., in order to tighten the legs, move the clamps further along the leg. (v) Bolts securing the tripod top to the legs work loose regularly – keep them tight. C) Levelling staves i) Clean each day after use and store correctly. ii) Wooden staves swell if wet. Take it apart and wipe off surplus water. Allow to dry naturally before reassembly.

Lesson Npte On Introduction to Drainage and Dewatering

Introduction to Drainage and Dewatering

What is drainage?

As the name itself suggests, drainage is the process in which the free water is removed with the help of gravity in order to maintain stable soil conditions.

Drainage can be classified into two categories:

1. Surface drainage
2. Sub-surface drainage

What is Surface drainage?

The water that runs on the surface of the ground is collected and or diverted into a water body.

Sub-surface drainage

It is the collection and disposal of ground water. It is also called as dewatering.

What is dewatering?

The process in which the water is removed from a foundation pit;

• when it is situated below the ground water table or
• when it is surrounded by a coffer dam.
• 

Why is dewatering done?

Only understanding the definition of dewatering is not enough. It is essential to know the reason as to why it is done.

Some times in some areas, the water table is high. This causes trouble and discomfort during construction. The water is pumped out so as to keep the excavation dry. Excavation has to be kept dry so that concreting can be done.

Sometimes temporary dewatering may suffice. Temporary dewatering is draining out the water at the time of construction and it is then followed by restoration to its original water level as soon as the construction is complete.

What is permanent dewatering and why it is done?

Permanent dewatering is removing the subsurface gravitational water throughout the life of the structure. This is done to protect the structure from moisture and dampness.

Advantages of Subsurface drainage

It helps in improving the properties of the soil

Helps in the stabilization of soils

Let us consider the case of “Fine grained soil”

The water removed from fine grained soil is not much but the improvement in the properties of the soil is significant.

Helps in reducing hydrostatic pressure acting at the base of the structure

Methodology to be adopted for Sub-surface drainage:

The methodology adopted at a site would depend on various factors. They are as follows:

• Characteristics of the soil
• Position of the water table
• Time period.

Lesson Note On Definitions of GIS

Definitions of GIS

Geographic Information System (GIS) has been defined in various ways by different authorities. A typical GIS can be understood by looking at its various definitions. In this section, we present different definitions (or descriptions) of GIS that have been offered by people.
Burrough (1986) defines GIS as a "set of tools for collecting, storing, retrieving at will, transforming and displaying spatial data from the real world for a particular set of purposes".

In his own definition, Arnoff (1989) defines GIS as "a computer based system that provides four sets of capabilities to handle geo-referenced data: data input, data management (data storage and retrieval), manipulation and analysis, and data output."
“A geographic information system (GIS) is a computer-based tool for mapping and analysing things that exist and events that happen on earth. GIS technology integrates common database operations such as query and statistical analysis with the unique visualisation and geographic analysis benefits offered by maps” (ESRI, 1990).

". . . The purpose of a traditional GIS is first and foremost spatial analysis. Therefore, capabilities may have limited data capture and cartographic output. Capabilities of analyses typically support decision making for specific projects and/or limited geographic areas. The map data-base characteristics (accuracy, continuity, completeness, etc.) are typically appropriate for small-scale map output. Vector and raster data interfaces may be available. However, topology is usually the sole underlying data structure for spatial analyses" (Huxhold, 1991 p.27).

"A geographic information system is a facility for preparing, presenting, and interpreting facts that pertain to the surface of the earth. This is a broad definition . . . a considerably narrower definition, however, is more often employed. In common parlance, a geographic information system or GIS is a configuration of computer hardware and software specifically designed for the acquisition, maintenance, and use of cartographic data" (Tomlin, 1990 p xi).

"A geographic information system (GIS) is an information system that is designed to work with data referenced by spatial or geographic coordinates. In other words, a GIS is both a database system with specific capabilities for spatially-reference data, as well as a set of operations for working with data . . . In a sense, a GIS may be thought of as a higher-order map" (Star and Estes, 1990, pp. 2-3).
A GIS is "an organised collection of computer hardware, software, geographic data, and personnel designed to efficiently capture, store, update, manipulate, analyse, and display all forms of geographically referenced information" (ESRI, 1990, pp. 1-2).

“A Geographic Information System (GIS) is a collection of computer hardware, software and geographic data used to analyse and display geographically referenced information” (URL1).
“A GIS is a computer system capable of capturing, storing, analysing, and displaying geographically referenced information; that is, data identified according to location. (Some GIS) practitioners also define a GIS as including the procedures, operating personnel, and spatial data that go into the system” (URL2).

“In the strictest sense, a GIS is a computer system capable of assembling, storing, manipulating, and displaying geographically referenced information, i.e. data identified according to their locations. GIS practitioners also regard the total GIS as including operating personnel and the data that go into the system” (URL2).
“GIS is an integrated system of computer hardware, software, and trained personnel linking topographic, demographic, utility, facility, image and other resource data that is geographically referenced” (URL3).

A list of additional definitions of GIS can be found in Longley et al (2001). By way of summary, GIS can be considered to be a computer-based system comprising hardware, software, geographically-referenced data, personnel and procedures put together for the input, storage, retrieval, analysis, manipulation, query, update and output of geographical data.