Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damssuspected voids or stopes are air- or water-filled. Placement of sources and receiversinside the conduit, when accessible, can improve the technique.The parameters recorded can provide important information about different featuresthat may damage the embankment dam’s structure, such as fractures, low densityregions, saturated zones, and high stress regions. The results may be presented ascross-section images (figures 139 and 140) of compression (P- [primary]) wavevelocity, or of seismic wave attenuation. These properties may be correlated to otherengineering parameters of interest, such as possible fractured zones, and potentialvoid areas. For an embankment dam in Maryland (Schaub, 1996, p. 3), thetomographic investigation interpretations revealed that the relative compaction ofthe earthfill around a CMP spillway conduit ranged from 65 percent to nearly 100percent. The areas with the lowest interpreted densities were found to be under,along, and above the conduit.For concrete, high compression (P-) and shear (S- [secondary]) wave velocitiesindicate competent concrete. Lower velocity values may indicate cracking,deterioration caused by ice and other weathering, alkali reaction, or defects.10.1.1 Spectral analysis of surface wavesRecently developed geophysical procedures called Spectral Analysis of SurfaceWaves (SASW) and Multichannel Analysis of Surface Waves (MASW) measure the“dispersion” of surface wave velocities to evaluate material properties. (Billingtonand Basinger, 2004, p. 4; Park et al., 2001; and Miller et al., 1999). These techniques,termed “indirect methods,” since the measurements can be made from one side of astructure, provide estimates of material properties averaged over relatively largedistances.The SASW/MASW techniques can be used on a large scale to evaluate embankmentdams, such as for locating possible voids or potential seepage zones along a conduit(Stokoe, 1999, p. 3). On a smaller scale, these techniques can be used to evaluate thequality of conduit materials, such as concrete deterioration and loss of wall thicknessdue to corrosion.Use of surface wave data is a powerful technique that allows measurement of softlayers beneath harder layers. This means that SASW/MASW may be able to detectpossible voids in the backfill adjacent to a conduit by making measurements frominside of the conduit. (In comparison, the seismic refraction technique generallycannot be used to locate softer layers under harder layers.)The basis of the methods is measurement of the “dispersion” of Rayleigh typesurface waves (USACE, 1995c, p. 3-24). Essentially, surface waves of different wave248