The trench shield must reach at least 18 inches above the vertical side's top. The sharpest maximum authorized slope for combined slope/bench systems in TYPE C soil and a trench depth of less than 20 feet is 34 degrees from the horizontal and is allowed only if a trench support or shield is installed. For trenches 20 feet or deeper, the slope may be no greater than 28 degrees. For trenches between 18 and 20 feet deep, the slope may be as steep as 42 degrees. Slopes greater than 46 degrees are illegal in most areas. The reason for this is that children playing in the trenches can get their legs trapped under the weight of the vehicle driving over them. The guideline is to always keep kids' safety in mind when digging holes or trenches. If you require further information on how to dig safe trenches, then we recommend that you look into our other articles here on Treehouse Science.