The loads involved with modern buildings usually dictate the use of steel or concrete but many older buildings are still supported by timber piles. If any changes in the level of groundwater occur in the vicinity of such buildings, rotting of the wood may result and cause trouble. Settlement of the Boston Public Library was found to be due to this cause. If unusually heavy and concentrated loads have to be transferred from building, to rock, then the maximum practicable size of pile may not be adequate and larger columns in the form of caissons must be used. Correspondingly, if preliminary investigations have indicated the presence of boulders or any other condition that will make pile driving difficult or impossible, caissons must be considered unless an altogether different type of foundation design is possible.
Ingenious machines are now available for excavating the circular holes for caissons.
With good soil conditions and lined holes, the shafts can be inspected down to bedrock before Concrete is placed in position. Belling out the bottom of the caisson to obtain extra bearing area on bedrock is common practice if soil conditions are suitable. With such information, it is possible to estimate costs accurately, so that they can then be compared with the cost of the alternatives now available for founding structures directly upon soil. Buildings Founded on Soil Soils are solid materials not dissimilar to rock except that, they are not so strong and may consist, as does sand, of small fragments of rock that have to be confined if they are to carry load. Accordingly, soils will be susceptible to stress and strain in exactly the same way as rock or any other solid material.