Chapter
15 describes the use of the "Bidpod" for level monitoring. For
the monitoring markers, it advocates the use of a brass screw in a plastic wallplug
mounted as near ground level as possible. The screw is aligned with its
slot horizontal, and the hook on the tape measure is then located into the brass
screw [Figure 15.1, p241].
A simple modification provides a considerable improvement to this method. The normal end hook on the tape measure is removed, and replaced by a small aluminium plate (see attached Figure 1). An egg-shaped eye is drilled and filed out of the plate. The larger diameter of the eye enables the plate to be slipped over the head of the marker screw. The smaller diameter is marginally larger than the shank of the screw, so that it locates accurately onto the screw, under the spring-loaded tension of the tape.
This adaptation is far easier to use, as it is not necessary for the slot of the screw to be accurately aligned, and the plate is held firmly in position on the screw. This also makes it more accurate. The screw should be inserted just far enough to allow the plate to be slipped on, locating directly behind the rounded head. This ensures that there is no error if the shank of the screw has been inserted at a slight angle.
If the "Bidpod" is being used for measurements off the floor or a deep datum, the eye can be held in place by a short bolt tapped into the shaft near its base. The exact position of the hole for the bolt is immaterial, provided the same staff is always used.
See "Bidpod" Levelling Staff for further details of the modified "Bidpod", and how to order.
Figure 1: Modified end to a flexible tape, held under spring tension over
marker screw
The "Bidpod" is intended to measure from ground level upwards to the line of collimation. It is essential to measure in this way if there is any horizontal cracking. However, if there is no horizontal cracking, it can be assumed that brickwork at high level will be following any foundation movements just as accurately as at low level. Under these circumstances, a simple steel rule can be suspended from the marker screw, and measurements taken off this rule in the usual way. As with the modified plate on the "Bidpod", it is preferable if the hole from which it is suspended is drilled and filed to an egg-shape, with the larger diameter slipping over the head of the screw, and the smaller diameter matching the shank. 1m steel rules with engraved markings at millimetre intervals are available from good quality tool shops; these are preferable to the painted anodised rules available from DIY Superstores. The Fisher rule (attached figure 2) has a matt surface which is easy to use, the narrower steel rule by Stanley is more reflective and can be difficult to view.
A suspended rule of this sort is particularly useful where there are many bushes or other obstacles around a building, or where very oblique lines of site are required. Under poor light conditions the rule can be illuminated using a small Durabeam torch to which a magnet has been attached with double-sided tape [similar to Fig 12.12, p191].
It should be noted that, if the two different types of staff are used with some markers above and others below the line of collimation, there would be an increased risk of errors induced from thermal movement of the staff [p241].
Figure 2. Steel rule suspended from marker screw
See "Bidpod" Levelling Staff for
further details of the "Bidpod" and steel rules, and how to order.