Culverts and Bridges
As in other types of layout for construction, the stakeout of culverts and bridges generally includes providing line and grade. The procedures and precision required will vary with the magnitude and complexity of the job.
DITCHES AND CULVERTS.—For minor open drains or outfall ditches a few feet deep, a single line of stakes will serve for both alignment and grade. By running profile levels, you can determine the elevations of the tops of the stakes. As a guide to the construction workers, mark the cut on each stake to show the depth of drain below each station. For drains that are very deep, you must cross-section the line and set slope stakes. The grade for a ditch is measured along the flow line; that is, along the bottom of the ditch.
When pipe culverts without wing walls and aprons are staked, only the alignment and invert grade are required; however, when head walls, wing walls, and aprons are used to intercept drainage water, to retain earthwork, and to prevent erosion, grade stakes, as well as horizontal alignment stakes, will be required. Large bridge culverts and box culverts require stakes and hubs for batter board alignment similar to those required for a building layout.
Figure 10-11 illustrates the stakeout of a box culvert that crosses below an airfield taxiway. The angle at which the culvert crosses below the taxiway may be written on the plans, or it may be taken from the plans.
Assume that this angle is 84°30´, as shown. To runthe center line of the culvert, setup the transit at A and turn the 84°30´ angle from the center line of the taxiway.
Place reference stakes at B, C, D, and E along theculvert center line far enough beyond the limits of the culvert to make sure they are not disturbed by the construction work. In this case, points B and D are set arbitrarily at 5 feet (measured at right angles) from the location of the outside face of the culvert headwalls.
Figure 10-11.—Stakeout of a box culvert.
To facilitate the stakeout, set a stake at point h.From h the locations of points j and k may be measured and staked. The distance used is one half of the length of the headwall as that length is shown on the design plans. Set stakes at points F and G directly opposite and on lines at right angles to the ends of the headwalls. Set stakes similarly at L and M. Set grade
Figure 10-12.—Ditch inlet and pipe culvert
stakes near B and D for the invert or flow line of theculvert.
The stakes set in this way are sufficient to locate the forms for the headwalls and for the barrel of the culvert. Figure 10-12 shows one of a number of types of pipe inlets and culverts. The type shown is suitable for picking up side-surface drains adjacent to a landing strip or roadway embankment. Stakes for both horizontal alignment and elevations are required. Figure 10-13 shows the stakeout of a pipe culvert, wing wall, and apron.
BRIDGE SUBSTRUCTURES.—As you know from chapter 2 of this TRAMAN, the substructure of a fixed bridge consists of the end and intermediate supports and their foundations. Bridge substructures are divided into two main types of supports: end supports called abutments and intermediate supports called bents or piers.
Abutments.—The ground support at each end of a bridge is called an abutment. Construction plans will show the details of the abutments. Check the layout after excavation and before pouring the concrete. You must check abutment elevations, and when concrete is used, establish lines for setting forms. Abutments must be staked by following the construction plans,
Figure 10-13.—Stakeout of a pipe culvert, wing wall, and apron.
and abutment stakes should be tied to the horizontalcontrol system to meet accuracy requirements. The following is a typical procedure for survey-ing an abutment that is to be at right angles to the center line of the bridge. In figure 10-14, the foundation of a concrete abutment, ABDC, is shown in the plan. AB is the face of the abutment foundation. Establish two convenient points, H and J, near the abutment CD, on the bridge center line. Set a stake at E (station 41 + 37.50)—the station designated on the plan for the abutment face.
Set up the transit at E, train on H, match the zeros,and turn 90° angles to locate A and B at the correct distance from E. Reference the line AB by setting stakes at F and G at the indicated distances from A and B. Set temporary stakes at C and D to mark the other corners of the foundation.
Sometimes the alignment of a bridge is not at rightangles to the center line of the stream or road it crosses. When this occurs, the abutment is askew (other than a right angle) to the center line of the stream or road. Then slight modifications are necessary to stake out an askew abutment.
Figure 10-15 shows the plan for an askewnear-side abutment of a railroad bridge over a highway. The outside line of the foundation is ABCD. The neat line of the face of the abutment is MN. Set stakes to define the direction of MN and ends AD and BC. The stakes P, S, U, R, V, and T are offset from the abutment so they will not be disturbed by foundation excavating. The general procedure is as follows:
Figure 10-14.—Staking a right-angle abutment.
Figure 10-15.—Staking an askew abutment.
1. Take the dimensions for setting necessary stakesfrom the abutment plans. Set the temporary point O at the station location indicated.
2. With the instrument at O, sight along the centerline of the railroad, turn the skew angle (71°45´), set the permanent stakes P and R, and set points M and N.
3.With the instrument at M, sight R, turn 90°, and set permanent stakes S and T.
4. With the instrument at N, sight P, turn 90°, andset permanent stakes U and V.
The face of the abutment is defined by P and R.
Stakes S, T, U, and V define the face of the end forms. When construction begins, set stakes at A, B, C, and D by measuring from the offset stakes. (These stakes are knocked out as the excavation progresses.) Concrete for the foundation is poured into the excavation; if forms are needed for the foundation, measure the distances from the reference offset stakes. Set the elevations of the top and bottom of the foundation from bench marks outside the excavation area.
When the foundation has been poured to grade and has had a day to set, mark temporary points on the top at M and N by measuring 10 feet plus the distance AM and BN from the offset stakes S and U. Check the forms by measuring the equal diagonals MC and ND. Mark points denoting elevation directly on the forms and give the data to the petty officer in charge of the construction project.
After the bridge seat is poured, mark point O. After the rear wall has been poured, mark points defining the girder center lines: a, b, c, d, e, and f. These points will be used for the accurate location of the bearing plates that will support the girders.