In the development of coal seams with the use of flexible slabs and various kinds of shields is often necessary to correctly determine the lateral pressure of the tab on the vertical or inclined walls. Methods of determining the value of vertical pressure are mainly covered in the technical literature. Much less investigated is the lateral pressure, which can be considered as a consequence of spacing between pieces of rock under vertical load.Studies show that the state of the bulk material has a great influence on the magnitude of the lateral pressure. When the bulk material transitions from a stationary to a mobile state, the degree of its loosening increases and the value of lateral pressure increases significantly. Meanwhile, the known calculation formulas are built on the basis of schemes that provide a stationary equilibrium state of the bulk solids, so their use does not provide the correct solution in all cases.