excavation machine employing a wheel fitted with rim buckets, or with a boom or ladder on which an endless chain of buckets or scrapers revolves. The machine is self-propelled on rubber tires or crawlers (continuous metal treads driven by wheels). As the machine moves forward, it rotates the ladder or wheel so that the buckets dig at their forward edge. They dump onto a conveyor belt or a chute that piles the cuttings on either side. Trenching machines can be equipped to cut hard ground and even soft rock, but they encounter difficulty with boulders.