cavity in bread loaves

It's called 'layering.' It means that you haven't adequately kneaded the dough to develop the gluten (protein) in the flour. If you knead bread dough for time suggestions (and that's all they are) rather than for gluten development, layering is often a risk. Instead, knead bread for the windowpane test. Pinch off a walnut-size piece of dough and tease it out over your fingertips to see how thin you can get it without it shredding in places. When you can get a good, thin windowpane, your gluten development is great. Read about windowpaning here: http://www.food52.com/recipes/15517_italian_style_boule