FLOOR LEVEL SURVEYS
The behavior of concrete floor slabs, decks and other flatwork that are cast directly on the ground surface is one of the best indicators of the nature of any underlying ground movement. Deflection within such surfaces can be detected by means of level surveys performed by methods that are essentially non-destructive and, therefore, very desirable as a first means of assessing ground movement that could be affecting a structure.
DHLA utilizes two types of instruments to perform level surveys. The simplest instrument is commonly called a water manometer, but is more correctly referred to as a water level device. The water level device operates on the basis that water seeks the same level in a closed system that is open to atmospheric pressure. The level device consists of a reservoir of water connected to a graduated tube by means of a flexible plastic hose. The tops of both the graduated tube and reservoir are open to the atmosphere. A level survey is conducted by first “zeroing” the level of water in the graduated tube at a datum location on the surface to be surveyed. The graduated tube is affixed to a metal rod with a sharp point attached to its base to allow readings to be taken through carpeting. As the rod is moved about the surface being checked it will move up or down in response to irregularities in the surface while the water level in the tube remains at the same level as the water in the reservoir. The water level in the tube is read relative to the zero level reading at numerous locations throughout the surface, with the readings being plotted on a scale base map of the surface. The plotted relative elevation points are then digitized and plotted after which contours of equal relative elevation on the surface can be generated using a contouring software program. Anomalies in the surveyed surface can then be viewed and analyzed with respect to the types of earth movement that can be affecting it. A number of such level surveys, performed over an appropriate period of time, can also detect the amount of change in movement on the surface between the surveys, with the results being used to determine the rate of any progressive movement.
The primary drawback with using the manometer system described above is that if there are significant elevation differences, on the order of over eight inches or so, the volume of the tube, hose and reservoir system changes significantly enough that it can affect the readings. This can necessitate time-consuming moves or recalibration of the entire system where several levels of surfaces are involved. To overcome this, DHLA has acquired and has begun employing a Compulevel® device, previously manufactured by the Stanley Tool Company, but which is now produced as the Ziplevel® by the Technidea Corporation. These devices use hydraulic pressure in a system that is closed to the atmosphere to register elevation differences of up to 40 vertical feet electronically. The actual survey process is performed in the same manner as the water level device. With its versatility and accuracy, DHLA’s personnel also makes use of the Compulevel® for such tasks as determining the relative elevations of boreholes or test pits and the tops of observation well casings.
An argument that can be made against using floor level surveys as a means of determining total slab deflection, especially in the case of small elevation differences, is the fact that the only positive way to tell if the slab has moved is by comparison of two level surveys, one of which has been performed shortly after the slab was cast. Otherwise, the assumption must be made, and usually is, that the slab was originally perfectly level. In other words, there is no way of knowing if a floor level survey is merely an indication of the way the slab was originally finished or if it is indicative of actual movement, unless comparison surveys can be made. It is DHLA’s policy to consider the minor built-in irregularities that are acceptable in most floor slabs when analyzing level surveys, and to use state-of-the-art techniques to properly analyze the actual anomalies that are indicative of significant slab movement. With this in mind, we would encourage developers and builders to have level surveys performed on newly cast floor slabs or other flatwork as a comparison for protection against future frivolous lawsuits in which minor slab irregularities are used to suggest earth movement.