Nairobi, Feb. 18 -- The sun is rising in Kenya's Kajiado county, just outside Nairobi, and a truck is rumbling over dusty ground towards a riverbank. Young men guide the driver to a parking spot and then spring into action, each with a scoop, filling the truck from a heap of the most desirable building sand for which the area is famous.

The driver passes the time with a snack and a mug of tea poured from a flask by a mobile vendor. He pays each of the young men around $10 for their labour and the landowner $40-$50 for the sand. The driver then starts out on his journey to deliver sand to hardware stores, building sites or informal selling points in Nairobi and its suburbs, paying county taxes andpolice bribesalong the way.

Sand mining s...