3.4 The CONSTRUCTION_ELEMENT entity

The combination of construction_elements makes up what has been built at the site of a groundwater_feature. This spans from a simple situation where natural groundwater_features such as a spring may have been slightly modified to increase access to groundwater (eg. a length of casing inserted) to fully constructed features such as production bores with the full suite of casing, well screens, bentonite seals, gravel pack, pump and concrete pad.

Construction_elements can be placed into six basic types (Figure 3.3):

Table 3.4 Attributes of the construction_element entity

As a bore can be made up of different components of casing, screens etc, a groundwater_feature can have many construction_elements. Each construction_element is uniquely defined by the combination of feature_identifier and construct_number.

The position of each construction_element is defined by an interval (interval_from, interval_to) with a datum_plane acting as an origin, such as casing from 0 to 50m depth relative to ground surface. The convention that intervals above the datum_plane are recorded as negative is followed (refer Convention 4.2). Hence, the interval_from for casing commencing 1m above the ground surface is recorded as -1 using a ground surface datum_plane. Details of the datum_plane can be referenced using the unique combination of feature_identifier and datum_number.

The method (eg. drilling technique for voids) and the material of the construction_element (eg. steel, PVC, wood) can also be recorded. The configuration describes how the construction_element spatially relates to the reference line of the groundwater_feature. In a simple bore, components tend to be in a linear configuration, stacked one on top of another. Casing may be telescoped in other bores, being positioned through previously installed larger diameter casing. In multilevel piezometers, many small diameter casings or tubes may be set in parallel but at different depths. Who constructed the element (make, person, license, organisation) and when it was constructed (start_date, finish_date) can be recorded. A construction_element may replace or be formed from another construction_element. For example, part of a length of casing may be slotted to provide the inlet for groundwater. In the case of an uncased or open hole, the void-type construction_elements are the precursors of the defined inlet. The precursor_number is used to define these relationships.

Construction_elements can have different cross_section shapes. For bore casing and well screens this shape is circular, but with manually excavated wells for example, the cross_section of the lining may be square or rectangular. The minimum_dimension, maximum_dimension, thickness and dimension_type are used to define the cross section size of a construction_element. The dimension_type sets whether internal, external or nominal dimensions are being defined for the element. Hence, either the inside diameter or the outside diameter of a casing length can be defined. Figure 3.4 shows how these attributes are used. For circular cross sections, the minimum_dimension and the maximum_dimension have identical values, regardless of whether inside diameter or outside diameter is used (as dictated by the dimension_type). Hence, the minimum_dimension is not necessarily the inside diameter and the maximum_dimension is not necessarily the outside diameter. If external dimensions are being recorded and the thickness is known, then the internal diameter of the casing can be easily derived, and vice-versa. The minimum_dimension and maximum_dimension attributes are more meaningful for rectangular or elliptical profiles, as these would have different values.

A construction_element can also have a functional status. A well screen may become blocked or corroded and be physically removed and replaced. This information is described in the separate status entity.

In addition to the generic attributes of a construction_element, some particular components have specific properties. Each of these specific properties is stored as a construction_element_property, uniquely defined by feature_identifier, construct_number and property. The material that is used to fill the void or annulus has specific properties such as fill volume, grain size grading, and grain size distribution (maximum grain size, minimum grain size, modal grain size, and uniformity coefficient). Slotted casing can be described in terms of slot aperture, slot density, slot length and slot orientation. Fixtures such as pumps can be described in terms of depth to intake, power source and power rating. Casing can have different end seals, holding methods and joining methods, while different fluids can be used during drilling of holes (voids). Hence, additional properties of a construction element can be described, either numerically as a value or a text string in the description attribute.

Table 3.5 Attributes of the construction_element_property entity.

Figure 3.3 Different types of construction_elements found in groundwater_feature