Pipe-soil interactions during backfill placement

This dissertation documents a study of installation practices for buried pipe. Current practice was reviewed via the literature and surveys of users, manufacturers and others actively involved in the use of buried pipe. Typical backfill materials were characterized through standard and variable effort compaction tests, CBR tests, penetration tests, and one-dimensional compression tests. Standard classification systems were compared and standard groups of backfill materials were evaluated. The soil properties that were used to develop the AASHTO SIDD designs are proposed for use as standard properties for all types of pipe. The constrained modulus, M$\sb{\rm s}$, is proposed as the standard measure of soil stiffness replacing the empirical modulus of soil reaction, E$\sp\prime$. Laboratory soil box tests and full scale field tests were conducted to investigate soil behavior during installation. Variables included pipe type and size, in situ soil condition, trench width, backfill type, compactive effort, haunching effort, and bedding condition. The tests show that all of the backfill related test variables have a significant effect on pipe behavior. Tests with controlled low strength material show that this is an excellent type of backfill. Computer modeling demonstrated that finite element analysis can effectively model installation effects as well as effects of fill over the pipe. The elastic solution for behavior of a buried pipe, developed by Burns and Richard shows promise as a basis for a simplified design method. Recommendations for future practice include the use of soft bedding under the bottom of the pipe, and uncompacted backfill directly over the top of the pipe. Selection of trench width must consider the ability to place and compact backfill in the haunch zone and at the sides of the pipe. Hand tampers, sized differently for different types of backfill, were shown to be useful for providing haunching effort. It was shown that relatively small changes in backfill density can have significant effects on backfill stiffness. The project shows that pipe performance is controlled by installation practices. Proper implementation of a design process that realistically assess how a project will be built, and construction that understands and implements the design assumptions is imperative for successful long-term pipe performance