Jamieson, Fausset and Brown on 2 Samuel 17:18

18. and came to a man's house in Bahurim, which had a well in his court--The court was that of the house, and the well an empty cistern. All the houses of the better class are furnished with such reservoirs. Nothing could more easily happen than that one of these wells, in consequence of a deficiency of water, should become dry and it would then answer as a place of retreat, such as David's friends found in the man's house at Bahurim. The spreading of a covering over the well's mouth for the drying of corn is a common practice.

2Sa 17:23-29. Ahithophel Hangs Himself.