Can anyone provide any guidance in determining minimum cover over a highway cross culvert in the 750mm to 1050mm (30in to 42in) size range for RC pipe? The general practice in this area has been to try and place the top of the pipe below the frost line which is about 1.5m (5ft) below grade to avoid any potential frost heaving. As you can imagine this can be very restrictive in shallow draining areas. Is this approach too conservative when compared to the suggested 0.3m (1ft) minimums for live/dead loads?