Movements of footings and retaining walls

The objectives of this dissertation are: (1) to examine the relationship between the accuracy and reliability of methods of estimating settlements of footings on sand and gravel, (2) to develop a procedure for estimating horizontal movements and rotations of footings without the need of determining soil modulus values, and (3) to develop a simple procedure for calculating movements of retaining walls due to the weight of backfill.

The accuracy and reliability of twelve methods of estimating settlements of footings on sand and gravels were examined by comparing calculated settlements with the measured values. Eleven of the methods are based on Standard Penetration Test Results, while Schmertmann’s method is based on Cone Penetration Test Results. The study showed that methods which are more accurate tend to underestimate settlements about half of the time; while those which are more reliable (in the sense that they infrequently underestimate settlements) tend to be less accurate.

The study also indicated that these methods of estimating settlements of footings on sands and gravels involve approximately the same relationship between accuracy and reliability, regardless of the approach that they use to calculate settlement. The results demonstrate that there is a tradeoff between accuracy and reliability. Any of the methods can be adjusted to achieve approximately the same combination of accuracy and reliability as other method.

A simple procedure is presented to relate horizontal movements and rotations of footings to settlements. The procedure does not require the determination of soil modulus, and its accuracy and reliability can be assessed qualitatively by association with the method used to calculate the settlement.

A simple procedure based on elastic theory was also developed to estimate movements of abutments and retaining walls due to the weight of backfill placed behind them. To avoid the inherent difficulty in determining the soil modulus, a procedure for relating these movements to the settlement of the wall was also developed. The new procedure was applied to a case history, and the calculated movements agree quite well with those calculated using the finite element method, and with field observations.