DWR_3312967 - Laserfiche WebLink

24 For accessible existing conduits, any open or leaking joints or holes should be patched to minimize grout leakage. An engineered bulkhead should be installed at the downstream end of the existing conduit to resist the loadings from the grout or concrete. An air return (vent) pipe or a series of pipes should be installed at the crown of the conduit and extend from the upstream end of the conduit to the bulkhead. Grout pipes should be installed at the crown of the conduit. Grouting equipment should be capable of continuously pumping grout at required pressures. Abandoning an inaccessible existing conduit is much more problematic due to the lack of access into the interior of the conduit. Stopping the flow of water into the existing conduit may be difficult, if there is an opening through the conduit. Abandonment may be possible by drilling into the conduit from the surface of the embankment dam at several locations and pumping a thick sand and grout mix (sometimes referred to as compaction grout, limited mobility grout, or LMG) to form a bulkhead. This technique was successfully used to stop leakage in a deteriorating conduit through a 65-foot-high embankment dam in southern Maryland. In this case, the approximate location of the conduit was first established by use of several geophysical methods (magnetometer, resistivity, and self-potential). An experienced driller was able to detect when the drill bit entered the existing conduit, advanced it to the middle of the conduit, and then pumped the grout to form the bulkhead. Grout was tremied into the existing conduit through additional holes drilled from the surface of the embankment dam. Filter Diaphragm or Collar If abandonment is selected, a filter diaphragm or collar should be part of a design to intercept any flow that could potentially occur through defects in the grouted conduit or along the interface between the existing conduit and earthfill. For guidance on the design of filter diaphragms and collars, refer to Chapter 6 of the FEMA Technical Manual, Conduit through Embankment Dams. Design of the filter diaphragm or collar would need to be modified from standard designs and located further downstream to limit excavation. Grouting • Grouting plan—A grouting plan detailing the contractor’s proposed grout mix equipment, setup, procedures, sequencing, plan for handling waste, method for communication, and method for sealing and bulkheading upstream and downstream should be submitted for review by the Designer prior to initiation of grouting operations. • Grout and concrete mixes—Use a grout mix with water (ASTM C 94) to cement (ASTM C 150) ratio of approximately 0.7:1 to 0.5:1 by weight. A grout fluidifier (ASTM C 937) may be needed to promote flowabilty, reduce water requirements, reduce bleeding, reduce segregation, increase strength, and eliminate grout shrinkage during setting of the grout mix. Trial mixes should be mixed at the job site prior to grouting to confirm the expected performance of the mix. For concrete backfill, the aggregate size should be selected based on the specific application but should not exceed 3/4 – inch. A 28-day compressive strength of 3,000 lb/in2 is generally acceptable. • Procedure—Install bulkhead (if applicable) prior to sealing. Sealing the conduit with grout or concrete is typically completed in two stages: 1) backfill grout or concrete (pump conduit full of concrete under low pressure [not to exceed 5 psi]) and 2) contact grouting (pumping grout along the inside crown of the pipe under higher pressure [not to exceed 25 psi]) to fill voids left by stage 1. Stage 2 is not typical for low-head dams. Especially where deterioration could allow the pressures to impact the embankment around the conduit. Assuming only Stage 1 is required; the pressure at the crown of the conduit as measured at the vent pipe should not exceed 5 psi. Grouting is stopped when the air return pipe in the crown flows full with grout. Cap the grout and air return pipes. Remove the bulkhead upon completion of grouting operations. For grouting or backfilling of long existing conduits, the use of sections is recommended. Long grout or backfill placements could result in expansion and/or contraction of the grout that could induce cracking of the existing conduit (concrete). The use of sections is also conducive to ensuring an acceptable seal of the conduit. Figures 6 and 7 show grouting operations