state the condition needed for construction of well
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Answer:
Introduction
Most of what we know about Marlborough’s groundwater resources comes from drilling records, observations and related testing and without it the groundwater resources would be a complete mystery.
The technology of drilling a well has been around for thousands of years with many cultures independently developing similar drilling techniques in diferent parts of the world. By the mid 11th century the ancient Chinese were drilling wells to depths of several hundreds of feet to fnd salt. They used the same percussion principle as modern cable tool rigs operating in Marlborough today (Kurlansky - 2003).
The types of wells used around the world vary depending on local conditions and the end use of the water. Most local wells apart from monitoring piezometers or infltration galleries consist of a sealed upper section near the surface with an opening at the lower end (Fig. 6.1).
The closed section is called the casing and is normally made of mild steel welded together in sections, or PVC. This lining acts as a conduit for groundwater to be pumped to the surface, to house the pump and to prevent the well from collapsing. The intake at the base is to let groundwater enter the well and is called the screen.
Drilling a well is a very skilled business. One of the most important aspects involves keeping the casing true or vertical so that once a pump is ftted, it won’t get stuck. This is not as simple as it might appear, especially if there are boulders or gravels pushing the drill-bit of-course.
In Marlborough we have had the beneft of some extremely skilled and ingenious craftsmen over the years (Fig. 6.2).
Figure 6.1: Conventional water well
Locating a site and making a well
The frst step in making a well is to choose a site. Locating an aquifer is straightforward in an area like Rapaura where groundwater is generally available everywhere. Elsewhere water availability is much more uncertain and to improve the chances a well owner will do some research frst. There are also many practical considerations such as power supply, location of crops or irrigation systems and maintaining a reasonable separation distance from other wells.
The next decision to be made involves selecting the diameter of the well casing. Well diameter has little to do with yield as fuid dynamics tells us that doubling the casing diameter will only result in about a 10 percent increase in fow.
Figure 6.2: Cable tool resting on the ground to the right of the driller, Colin Simpson during construction of Renwick municipal wellfeld 2008
Generallythecasingdiameterischosentoaccommodate the size of pump required and allow it to operate efciently. This means minimising friction losses within the annulus, which is the space between the casing and pump, or delivery pipe to the surface. The larger the casing diameter the more expensive the well becomes. This refects the higher price of wider steel pipe and the fact that the drilling process is slower. Once the site is selected there are a number of considerations including how deep to drill and what construction method to use. In more explored areas of the Wairau Plain the depth to the aquifer is known but in other areas the location of water bearing layers is unpredictable.
Many shallow or exploratory wells are drilled using the percussion method to provide the best samples for describing the sub-surface geology, or because it is relatively cheap. This approach is commonly used by the MDC where detailed information is needed for making a decision on where to place the screen and the most efcient slot size in public water supply or test wells.
Answer:
availability of soft soil
ground water table should be high