Design and Construction of Access Features for Toe Drains

Many existing embankment dam toe drains do not include adequate access features and, unfortunately, some new systems are still being designed without these features. Modern best practice for the design of toe drains in embankment dams includes access for inspection and maintenance in order to assure the drain system and dam are functioning as intended. Inspection is typically achieved with video cameras, and maintenance can be performed by cleaning with jetting tools. In addition to inspection and maintenance, access to drain systems is also used to measure flow and trap sediment in the interest of monitoring performance over time. Access to drain systems can be separated into two categories: inspection wells and clean outs. Inspection wells provide entry by personnel, contain flow measurement instrumentation, and allow inspection at the junction of several drains. Cleanouts provide indirect access to the drain system by remote video and jetting equipment and are much less expensive than inspection wells. This paper presents specific design elements for inspection wells (IW), including precast concrete products for the risers, considerations for the base slab, and cylinder size. Interior design elements include doors, ladders, landings, lighting, and ventilation. Instrumentation design includes selection of weir or flume, energy dissipation, and sediment traps. Outside of the well, design of a barrier (underground dam) to force all water through the well is described. Two types of cleanouts are described. The first is the “end cleanout” or “end access,” which consists of transition of the drain pipe to the ground surface using 22.5° fittings. The second, lateral cleanout, is located along drain lines utilizing a “Y” fitting and transition to the ground surface. Example designs and case histories of several types of inspection wells and cleanouts are presented.