Drilling and Bore Construction - Builders In Berwick

Drilling and Bore Construction:

     The simplest way to access groundwater is to dig a well. Wells can be dug manually to reach the shallow water table within the unconfined aquifer. However, if an aquifer is deeper than a few meters, a proper borehole needs to be drilled. Choosing a site, drilling method and bore construction are complex tasks requiring hydro geological knowledge, a skilled driller and specialized equipment.

     Many issues need careful consideration before a bore can be constructed. Some of the questions that need to be answered include (modified from Water watch, 2005):

• What is the geology and geomorphology of the area?
• How many aquifers exist, and which is the most appropriate one for the
 study purpose?
• How deep is the targeted aquifer?
• What is the purpose of the bore (monitoring, production, injection etc)?
• If it is a production bore, how much water is needed and how much water
 can reasonably be extracted?
• What are the licensing requirements and conditions operating within the
 State or Territory?
• Is there a groundwater management plan for the area?
• Are there any other bores in close proximity?
• Is the proposed location far enough from potential contamination sites like irrigation, septic tanks, drainage  lines, animal feedlots, etc.?
• If it is a monitoring bore, is it in a suitable position to monitor the impacts
 of potential contamination sites?
• Is the bore sited in an area where is could be prone to damage (such as by flooding, erosion, vandalism    etc)?
• Is the bore sited to minimise any disturbance or inconvenience to the land holder?
• Is the bore sited where there is infrastructure that is underground (such as
 water pipes, electrical cables, optical fibre networks) or overhead (such as
 power lines)?

     All groundwater bores should be drilled, cased and equipped according to national construction standards defined in Minimum Construction Requirements for Water Bores in Australia (ARMCANZ, 2003). This document deals with a broad scope of issues pertaining to water bore construction from licensing to construction, development and decommissioning for shallow small-diameter and low-yielding bores, through to high-yielding, deep and large-diameter bores.

 DRILLING METHODS:

     Drilling methods are many and varied, ranging from simple digging with hand tools to high speed drilling with sophisticated equipment. Each of the drilling methods has its advantages and disadvantages. The choice of drilling method employed should be made on the basis of geological and hydrogeological conditions and the type of facility to be constructed. The most commonly used drilling methods are described briefly below.
When selecting a drilling method or sampling an existing bore, the potential effects of the drilling method should be considered. Contamination of the borehole and its surrounds needs to be avoided during drilling and construction of the bore. Water contaminants, lubricants, oil, grease, solvents, coatings and corrodible materials may affect the suitability of the bore for groundwater monitoring, especially when monitoring for contaminants.

     All drilling and sampling equipment should be thoroughly cleaned before commencing drilling. Casing, drilling fluids and any materials used in the bore also need to be free of contaminants. Casing and screens should be kept in their protective covers until required for installation.

     There are many variations in methods used to drill monitoring bores. A driller experienced in the region being investigated can provide valuable advice on the best drilling method. The most common methods are described below with an overview of some of the issues that may affect the sampling of bores drilled using the technique. The selection of a drilling method and construction materials for a monitoring bore should take into account how these may influence analytes chosen for monitoring.

     When drilling a monitoring bore, a lithological log (and preferably a stratigraphic interpretation) should be made by an experienced person able to identify the important features.

1. Auger drilling:

      Auger drilling works on the simple mechanical clearing of a hole as it is drilled. Auger drilling eliminates the need for a drilling fluid (liquid or air) and hence reduces the potential influences from an introduced fluid. However, auger drilling has a high potential for smearing material such as clay or contaminants along the hole, thus affecting groundwater flow paths or increasing contaminant concentrations.

There are two major types of auger drilling:

• solid flight augers consisting of solid helical flights where extensions are added as the hole is drilled
• hollow flight augers consisting of augers that have a hollow centre.

     Auger drilling is generally used in soils and soft rock for relatively shallow bores.
It is possible to insert the casing into the hollow centre of a hollow flight auger before it is removed from the hole. This does require a large diameter borehole, but can be particularly useful in sandy ground.

2. Rotary air drilling:

     Rotary air drilling uses a rotating drill bit combined with circulating air that clears the drill cuttings, blowing them to the surface. The major advantage of rotary air drilling is that groundwater-bearing formations tend to be easily identified when encountered. The disadvantage of rotary air drilling is the potential for oxidation, volatilisation and precipitation of substances of interest. The introduction of high pressure air may also disturb flow paths and hydrochemical profiles in some aquifers.

3. Rotary mud drilling:

     Rotary mud drilling works on the same principle as rotary air drilling except that liquid is used as a circulation medium. Mud additives are used to support an open hole in soft and unconsolidated formations. The use of liquid may influence the formation, and hence groundwater samples, in the following ways:

• drilling fluids may enter the aquifer and mix with groundwater
• clay particles or other chemical products in the drilling mud may sorb or chemically alter the groundwater    properties
• mud may restrict or block groundwater flow paths.

4. Cable tool drilling:

     Cable tool drilling involves lifting and dropping a string of drilling rods with a bit at the base that cuts the hole with each blow. The cuttings are retrieved by removing the drilling rods and collected using a bailer. Cable tool drilling is slow and can compact aquifer material around the hole.

5. Direct push technology:

     Direct push (DP) technologies are an alternative method to conventional drilling techniques for sampling groundwater and installing monitoring bores in unconsolidated materials such as clay, silt, sand, and gravel. They are appropriate for sampling in the saturated zone and to depths of around 20m. Typically, a truck mounted mechanical hammer or hydraulic rig is used to push a string of steel hollow rods or a drive casing to the desired depth with a sacrificial tip. The rod assembly is disengaged from the tip and the sampling screen exposed. By directly pushing the sampler, the soil is displaced and helps to form an annular seal above the sampling zone. Direct push technologies are generally faster to install and more economical for high density sampling. They produce little or no cuttings during installation. DP installed groundwater bores are not appropriate for high volume sampling, are not recommended when telescoped bores are required to prevent migration of contaminants below confining layers, and may not penetrate hard bands, bedrock
and some unconfined layers (US EPA, 2005; ASTM, 2005).

6. Sonic drilling:

     Sonic drilling is a relatively new technique, where a high frequency vibration is combined with rotation to advance the drill stem. The core barrel is retrieved and the sample vibrated into a plastic sleeve or core trays. The advantage of this technique is relatively continuous and undisturbed geological samples, without the use of drilling fluids or other potential contaminants.

7. Vibro coring:

     Vibro coring method is used wherever soil conditions are unsuited to gravity corers or where greater penetration of the seabed is necessary. Standard size vibro coring equipment will produce 86 mm diameter core samples to a maximum depth of 6 m. In coarse aggregates larger diameters up to 150 mm can be obtained. This method is used widely throughout the geotechnical investigation industry and ca be deployed in water depths up to 1000 m.

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