A CROSS-ARM SUPPORT TREE

The purpose of this article is to describe a device which helps to measure fill settlements when the cross-arm settlement gage is kept ahead of the fill. The gage uses horizontal steel beams as reference points in the body of the fill. In a typical installation the cross beams are placed vertically apart and joined by a column of tubes. The tubes are of two sizes so that the smaller slides easily into the larger and the cross-beams are bolted to the smaller tubes at midlength. The sensing device that is lowered into the column of tubes consists of a steel cylinder with spring-loaded pawls projecting from its side. The settlement gage can be installed in two ways. In suitable fill when boring and other excavating machinery is available, it may be kept below fill level during construction and only exposed when cross-arms are added and measurements taken. This technique allows the upper end of the column of tubes to be buried until the fill is high enough for a further pair of tubes and attached cross-arm to be added, but it involves excavation and very accurate boring to relocate the tube end. An alternative approach, necessary with rock fill and other materials that are difficult to excavate, is to keep the tubes above the fill level. The fill can be satisfactorily compacted around the tube by paver rammer, small vibratory roller or other equipment. This second technique, which is described, was used to install settlement gages in the center of the impervious core at the Gitaru Dam in Kenya.