IV. ANALYSIS: INFORMATION NEEDED FOR ANALYSIS
IV-13
USING OBSERVATIONS (Continued)
pervious foundation. Observations indicate that split-well piezometer pressures are higher than
anticipated by design calculations, and small pin boils are observed in the relief well collector
ditch. The problem here is that the relief wells are too widely spaced, and the remedy is to
install intermediate wells.
In other cases, use observations to furnish input data for the various methods of analyzing
seepage. All the methods require reasonable assumptions for boundary conditions, and
material properties and observations are usually the best source of information for establishing
or changing boundaries, or determining the flow paths through a complexly stratified
foundation.
One example is locating the phreatic surface and surface of seepage for an embankment dam.
In design, the ratio of vertical to horizontal permeability may be misjudged. After several years
of operation, visual and piezometer observations can locate the surfaces with considerable
accuracy. Or, a finite element computer model can be calibrated and adjusted by using actual
pressures obtained from piezometers at the appropriate locations in the element grid.
After the seepage problem is clarified and a remedial action proposed, information is sometimes
needed for design and construction. Consideration of potential future needs during your
original investigation will minimize cost and time delay in implementing remedial action.