Is sand backfill as good as grout backfill?
No. Sand backfill has two negatives: it settles, and it washes away. One west coast department of transportation has studied this. According to them, sand backfill settles for about a month after installation, unless it is well tamped. Unfortunately, it is very difficult to tamp effectively in a small diameter borehole. This settlement means that a good initial reading cannot be obtained for a month after installation. If you obtain the reading earlier, you are likely to detect only movement of the casing within the borehole, rather than movement of the ground. In ground that has faults sand is sometimes used because grout leaks out the fault. Over time, however, the sand will wash away. Then the casing will move within the borehole and all readings are suspect. Finally, if there are gaps in backfill (and this goes for grout as well as sand), you’ll find that it takes longer for the inclinometer probe to stabilize. Evidently, movement of the probe in the casing can set up some vibratio
