The arched wall system is sometimes adopted for docks, but as it requires a longer time to construct, and is obviously more liable to be damaged from settlement and by pressure of the earth and the failure of joints and weak places, solid walls of Portland cement concrete with a hard face are more to be desired; as in such situations a monolithic and equally resisting mass is required, weight and mass being of importance. The relative bulk of materials for a certain expenditure should be considered, as it may happen that greatly increased weight and mass may be obtained for no extra expense by the employment of a certain substance. Combinations of brickwork and masonry and concrete are being abandoned in favour of one homogeneous material throughout, and no yet known aggregate fulfils this condition for such work as Portland cement concrete, owing to the difficulty of making secure and perfect joints, although hard brick facing may be necessary to protect the face from wear by the rubbing of vessels. By a judicious adoption of material for a dock-wall in order to give it weight and mass, and sufficiently deep foundations, neither counterforts nor other special means of protection against failure may be required, and slips and movements of the earth may be prevented. In the case of a stratum of soft soil of considerable depth overlying a firm foundation, dock and quay-walls upon arches, not exceeding about 30 feet span, have been successfully erected upon wells sunk in or to the solid ground, when by reason of the cost it was impracticable to carry a solid wall to the firm ground; and no slipping, sliding forward, or subsidence has occurred. To prevent movement of the earth at the back of a wall pushing or fracturing it and being the cause of a slip, it may be well to briefly state a few points to which attention should be especially directed. In adopting two systems of design of a wall, it is advisable that the change of form be gradual and not made abruptly, or cracks may occur from unequal settlement or load upon the foundations, and also when a concrete bed is used under a wall a lime concrete layer cannot be firmly joined to one composed of more solid and unyielding material, such as Portland cement concrete. Similar walls erected upon different soils are not equally strained, as one earth may scarcely change from the effects of air and water, and another may vary daily according to the weather.