The corrugated metal conduit barrel depends on the backfill or embankment to maintain its proper shape and stability. When the backfill does not provide the required support, the conduit will deflect, settle, or distort. Shape changes in the conduit therefore provide a direct indication of the adequacy and stability of the supporting soil envelope. By periodic observation and measurement of the conduit's shape, it is possible to verify the adequacy of the backfill. The design or theoretical cross- section of the conduit should be the standard against which field measurements and visual observations are compared. If the design cross section is unknown, a comparison can be made between the unloaded conduit ends and the loaded sections beneath the roadway or deep fills. This can often provide an indication of Conduit deflection or settlement. Symmetrical shape and uniform curvature around the perimeter are generally the critical factors. If the curvature around the conduit becomes too flat, and/or the soil continues to yield under load, the conduit wall may not be able to carry the ring thrust without either buckling inward or deflecting excessively to the point of reverse curvature. Either of these events leads to partial or total failure.

Corrugated metal pipes can change shape safely within reasonable limits so long as there is adequate exterior soil pressure to balance the ring compression. Therefore, size and shape measurements taken at any one time do not provide conclusive data on backfill instability even when there is significant deviation from the design shape. Current backfill stability cannot be reliably determined unless changes in shape are measured over time. It is therefore necessary to identify current or recent shape changes to reliably check backfill stability.

Locations in sectional pipe can be referenced by using pipe joints as stations to establish the stationing of specific cross-sections. Stations should start with number 1 at the outlet and increase going upstream to the inlet. The location of points on a circular cross section can be referenced like hours on a clock. The clock should be oriented looking upstream. On structural plate corrugated metal conduits, points can be referenced to bolted circumferential and longitudinal seams.

It is extremely important to tie down exact locations of measurement points. Unless the same point is checked on each inspection, changes cannot be accurately monitored. The inspection report must, therefore, include precise descriptions of reference point locations. It is safest to use the joints, seams, and plates as the reference grid for measurement points. Exact point locations can then be easily described in the report as well as physically marked on the conduits. This guards against loss of paint or scribe marks and makes points easy to find or reestablish. All dimensions in conduits should be measured to the inside crest of corrugation. When possible, measurement points on structural plate should be located at the center of a longitudinal seam. However, some measurement points are not on a seam.

Visual observation of the shape should involve looking for flattening of the sides, peaking or flattening of the crown, or racking to one side. Form A-1 may be used to record measurements of corrugated metal conduits. Minimum measurements include the vertical distance from the crown to the invert and the horizontal distances from each side at its widest point to a vertical line from the highest point on the crown. These horizontal distances should be equal. When they differ by more than 10 inches or 5 percent of the span, whichever is less, racking has occurred and the curvature on the flatter side of the arch should be checked by recording chord and mid-ordinate measurements. Racking can occur when the rise checks with the design rise. When the rise is more than 5 percent less than the design rise, the curvature of the top arc should be checked.

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