14.3 Slope Staking from Typical Sections

14.3 Slope Staking from Typical Sections

Quick Answer: Compute the catch from the typical and an OG shot, do not drive the Department stake on the catch, and set an RPSS generally 10 to 20 ft outside it. In the field, occupy a trial offset, compare ground elevation to the design slope plane, and slide toward the hinge if ground is above the plane on a fill (or the reverse). Worked fill: hinge 18.00 ft RT at 100.00 ft, 2:1, trial 28.00 ft RT at ground 96.50 ft → next trial 25.00 ft RT is the catch when that shot is also 96.50 ft.

This is still Domain V items A and B in the field: the procedure (A) applied to hinge, catch, and grade breaks (B). Horizontal- and vertical-curve stationing of the alignment itself is Chapter 15; here the instrument is already on the cross-section at a known station.

Office: Build the Catch from the Template

At each slope-stake station you need: PG (or the hinge elevation from the typical), hinge offset, slope ratio, and OG. Level-ground catch is Section 14.2. Real ground almost never sits on that horizontal plane, so the first office catch is a trial, not a final hub.

Caltrans 12.5-2: slope stakes control earthwork slopes. They mark the designed intersection of the constructed slope and original ground — the catch point. Stakes are not set at the catch points. Elevations of design catch positions should still be checked in the field. Instead, reference points (RPSS) are set at a constant offset from the catch, generally 10 to 20 ft, out of the work. Color is yellow. Longitudinal spacing is 100 ft, tightened to 50 ft for intervisibility, curves with radius < 1,000 ft, or cuts/fills > 10 ft. Intermediate slope stakes appear only in very heavy grading (cuts/fills > 30 ft) or for benches, as the Resident Engineer and Surveys decide.

RPSS tolerances: within 1.0 ft in station; 0.1 ft offset and elevation when cut/fill < 10 ft; 0.2 ft offset and elevation when cut/fill ≥ 10 ft. Marks are to 0.1 ft.

If the vertical difference between the design catch and ground is over 0.5 ft, locate the catch on the ground, set the RPSS from that ground catch, and report consistent differences to the Resident Engineer (and check right-of-way / easement conflict). Automated machine guidance does not erase slope stakes at conforms, beginnings and ends of curves, and transitions used for inspection.

Field Procedure

1. Occupy CL or a convenient offset line at the station (normal or radial).
2. Measure out to a trial catch from the template (hinge offset + depth × ratio, or last iteration).
3. Take an OG shot at that offset.
4. Compute the design slope elevation at the same offset: from the hinge, elev = hinge elev ∓ (horizontal from hinge) / n for an n:1 slope (minus on fill, plus on cut).
5. Error = OG − slope-plane elev. On a fill, if OG is above the plane you have overshot the catch (the slope is already underground) — move toward the hinge by |error| × n. If OG is below the plane, move away from the hinge by |error| × n.
6. Repeat until OG and the slope plane agree within the 0.1 ft slope-stake world (tighter if the job says so).
7. Set the RPSS the agreed 10 to 20 ft outside the catch (away from CL), not in the excavation or the fill prism.
8. Mark RPSS, alignment, station, offset, hub elev, then C/F and distance to CP, then point-to-point inward: catch → (bench) → hinge, each trio cut/fill, distance, slope ratio.

Point-to-point trap. Only the first C/F and distance after the hub identity is from the RPSS to the catch. Every later trio is from the previous slope point. A card that shows C 16.9, 16.9 ft, 1:1 to a bench, then F 1.0, 10.0 ft, 10:1 to a lip, then C 30.0, 30.0 ft, 1:1 to the hinge is measuring along the template, not repeating the RPSS offset.

Worked: Level-Ground Iteration (Exam Arithmetic)

Given: hinge 18.00 ft RT, hinge elev 100.00 ft, 2:1 fill. First trial at 28.00 ft RT (10.00 ft from the hinge, a 5.00 ft assumed fill).

Design slope elev at 28.00 = 100.00 − 10.00 / 2 = 95.00 ft.
OG shot = 96.50 ft. Ground is 1.50 ft above the plane.
Correction toward the hinge = 1.50 × 2 = 3.00 ft.
Second trial = 25.00 ft RT.

Slope elev at 25.00 = 100.00 − 7.00 / 2 = 96.50 ft.
If OG at 25.00 is 96.50 ft, you are on the catch.

Catch = 25.00 ft RT, elev 96.50 ft. Hinge-to-catch run = 7.00 ft. Fill on the slope = 3.50 ft.

Set RPSS 10 ft outside: 35.00 ft RT. If the RP hub is 97.10 ft, the card reads C 0.60 and 10 ft toward the stationing line to CP, then F 3.50 on 2:1 for 7.00 ft to HP.

If the second OG had been 96.20 ft instead of 96.50, ground would sit 0.30 ft below the plane: move 0.60 ft away from the hinge to 25.60 ft RT, where slope elev = 100.00 − 7.60 / 2 = 96.20 ft.

Worked: Ground Not at the Design Template (Sloping OG)

Same hinge 18.00 ft RT at 100.00 ft, 2:1 fill, but OG at the hinge is 96.00 ft and falls 8% to the right.

Level-ground product uses 4.00 ft of fill × 2 = 8.00 ft, catch 26.00 ft RT. That is only a trial.

Closed form, x = distance from hinge:
Slope: 100.00 − 0.50x
OG: 96.00 − 0.08x
100.00 − 0.50x = 96.00 − 0.08x → 4.00 = 0.42x → x = 9.52 ft.
Catch = 18.00 + 9.52 = 27.52 ft RT, elev 95.24 ft.

The 0.5 ft Caltrans check: design catch at the level-ground 26.00 ft offset is 0.64 ft above OG — over the 0.5 ft trigger — so you must walk it to 27.52, not leave the office 26.00 ft catch in the ground.

Exam traps: (1) driving the hub on the catch in Caltrans practice; (2) applying the |error| × n correction the wrong direction; (3) reading every distance on the lath from the RPSS; (4) treating 2:1 as 2% ; (5) keeping a 26.00 ft catch when OG falls away from the hinge.