Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 7—Potential Failure Modes Associated with Conduitsconnection between a conduit and valve allowed water under pressure to erode thedownstream soils by combination of internal erosion and backward erosion piping,and an embankment dam failure occurred. The failure occurred when lead caulkingbetween the outlet conduit and gate valve deteriorated and allowed water underpressure to erode the embankment dam. The failure of Lake Tansi Dam (Heckeland Sowers, 1995) was also attributed to this type of failure mode.In some cases, multiple failure modes may be involved at a single site. Tin Cup Damdeveloped sinkholes associated with Failure Mode No. 1, as described in theprevious section, when the masonry outlet conduit collapsed. To address theseproblems, an HDPE pipe was sliplined in the masonry outlet tunnel and the annulusgrouted. Additional sinkholes and distress symptoms related to Failure Mode No. 2occurred when the downstream control gate that was installed as part of the firstrepair allowed pressurized flow in the conduit. More extensive repairs were requiredto address these second series of distress symptoms, including relocating the controlgate back to the upstream side of the embankment dam and placing a downstreambuttress fill (Luehring, Bezanson, and Grant, 1999, p. 7). Failure Mode No. 3, whichis discussed in a following section, was probably also active at this site (see FailureMode No. 3 for further discussion of Tin Cup Dam).7.2.1 Design measures to prevent failureDesign measures that eliminate or reduce the possibility of a conduit deterioratingand developing a defect that would allow this failure mode include (1) using conduitmaterials that are resistant to deterioration, (2) ensuring watertight joints for pressureflow conduits, and (3) designing conduits to resist cracking from applied loads andfoundation movements. Chapters 1 through 6 discuss many of these designmeasures in more detail.Two general methods might be used to address this type of failure mode once itoccurs. They are (1) barrier cutoffs, and (2) filter diaphragms and collars. A barriercutoff consists of a grouted zone surrounding the conduit or sliplining of theconduit. The grout can be chemical or cementitious grout, depending on the sizeand shape of the suspected voids in the soil and the nature of the soils. For guidanceon grouting around conduits, see section 14.1. Rarely would grouting be consideredadequate without also installing an inverted filter over the area where seepage isoccurring. An inverted filter is a series of layered filters placed on a soil surface thatis discharging seepage. This filter is designed to filter any soil particles beingdischarged with the seepage and to provide capacity for releasing the collected water.The layers usually consist of a layer of finer sand placed on the ground surface wherethe seepage is discharging, which is covered by a layer of coarser gravel that is filtercompatible with the fine sand. A third layer of small cobbles may overlay the gravelfilter. In some cases, a fourth layer of rip rap size rock may be used to armor thefilters beneath and protect them from damage. When multiple layers of filters are167