How deep should a trench be without shoring?

How much do memorial plaques cost? How many miles does the Indian ocean stretch from the southernmost point to Africa to the sothernmost point of Austraila? Is there sunlight in the Arctic Ocean? What does one long blast followed by one short blast signal to a river lock attendant? Do people work in the leaning tower of pisa? Where is the Bloomingdale Museum in Bloomingdale Michigan located? Has the story about an archaeologist finding many chariots in the Red Sea been published yet? What are people who study forests? What is the size of the great sand dunes of Colorado? How many animals live in one kapok tree? What are three ways man can effect an estuary? How are ocean floor technologies used? Why people engage in tourism? Why do people believe that aliens take people in the Bermuda triangle? Why is the forest service part of the USDA? Where can a list of parks in a certain area be located? What is magellan's strait? How many people go to the western wall each year? Where is RFK buried? WHAT IS THE NAME OF THE RIVER THAT FORMS PART OF ALABAMA'S EASTERN BORDER!!!!!!!!!!!!!!!!!!!!!!?