Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 7—Potential Failure Modes Associated with Conduits7. If the tunnel continues to develop from internal erosion and proceedsbackwards until it reaches the reservoir, and the defect in the conduit is largeenough, a breaching type of failure can occur. If the tunnel erosion does notprogress completely until it reaches the reservoir, a complete breaching failuremay not occur, but sinkholes may develop that must be repaired.The Introduction of this document includes extensive discussions of factors thatshould be evaluated to determine whether internal erosion or backward erosionpiping is the correct term to describe the mechanism of failure.This type of failure mode was in progress at Tin Cup Dam (Luehring, Bezanson, andGrant, 1999). Numerous sinkholes developed in an embankment dam, when amasonry tunnel developed defects and the soils adjacent to the conduit were erodedinto the conduit. Later, after the conduit was repaired, additional problemsdeveloped, as described under Failure Mode No. 2.This type of failure mode can also occur where conduits have misaligned joints orirregularities in their walls. Joint offsets can cause high negative pressures to developat overhangs during high velocity flow within the conduit. These offsets can createnegative pressures at the offset from a Venturi effect. The negative pressures canpull or “suck” surrounding soils into the conduit through the opening, and voids candevelop next to the conduit. Continued loss of surrounding soil could lead todevelopment of a sinkhole, which, if it were to connect with the reservoir, could leadto serious consequences and eventually a disastrous failure of the embankment dam.Theoretically, this failure mechanism would develop as follows:1. High velocity flow in a conduit with an joint offset or other irregularity in thewalls causes a negative pressure to develop downstream of the offset or defect.2. If a defect in the conduit wall or a joint that has separated occurs near the pointof high negative pressure, the soil surrounding the conduit could be pulled intothe conduit from the negative pressures, even though the conduit is flowingunder pressure.3. Continued removal of soil near the defect could result in a sinkhole, if it wereallowed to continue and could even progress to connect to the reservoir orembankment surface.This failure mechanism is less likely than the one where water under positivepressure is forced through the defect in the conduit and damages the surroundingsoil (Failure Mode No. 2).163