Trench Shoring: Method

Trench Shoring

What the tool does

It applies the federal excavation standard, 29 CFR Part 1926 Subpart P, to one trench: it classes the soil by Appendix A, works out one protective system for the entered depth and bottom width, and lists the requirements of the standard that depend on those numbers. The appendix tables are reproduced from the Code of Federal Regulations (7–1–23 edition), a U.S. government work. The standard puts the decisions in the hands of the employer's competent person, "one who is capable of identifying existing and predictable hazards … and who has authorization to take prompt corrective measures"; the tool does the look-ups and the arithmetic for that person and for the engineer who checks a system outside the tables. States that run their own plans may be stricter than the federal rule.

Soil classification (Appendix A)

The appendix ranks deposits as stable rock, Type A, Type B and Type C, in decreasing order of stability. The tool follows its definitions in this order:

Material: cohesive soil is Type A at an unconfined compressive strength of 1.5 tsf (144 kPa) or more, Type B above 0.5 and below 1.5 tsf, Type C at 0.5 tsf (48 kPa) or less; cemented soil (caliche, hardpan) is Type A; angular gravel, silt, silt loam and sandy loam are Type B; gravel, sand and loamy sand are Type C; dry rock that is not stable is Type B.

Exclusions from Type A: no soil is Type A if it is fissured, subject to vibration from heavy traffic, pile driving or similar effects, or previously disturbed; such soil becomes Type B. Layers that dip into the excavation at 4 horizontal to 1 vertical or steeper make the material Type C. Submerged soil, soil from which water is freely seeping and submerged rock that is not stable are Type C.

The strength is a plain number, tons per square foot in USCS and kPa in SI, compared with the limits as the regulation prints them in each unit. The appendix requires at least one visual and one manual test, and reclassification when conditions change (rain, a new source of vibration). A type already determined on site can be entered directly; a system may always be designed for a less stable type.

Sloping and benching (Appendix B)

Table B-1 gives the maximum allowable slope, horizontal to vertical, for excavations 20 ft or less deep: stable rock vertical, Type A 3/4 : 1 (53°), Type B 1 : 1 (45°), Type C 1 1/2 : 1 (34°). Type A excavations 12 ft or less deep that stay open 24 hours or less may be cut at 1/2 : 1. Deeper than 20 ft, sloping and benching are designed by a registered professional engineer. With signs of distress the slope is cut back by at least 1/2 horizontal to 1 vertical; with surcharge from stored material, equipment or traffic the competent person decides the reduction.

Figure B-1 adds the configurations the tool draws: a single bench or multiple benches under the same overall slope (lowest bench 4 ft high at most; higher benches 5 ft in Type A and 4 ft in Type B; benches in Type B only in cohesive soil; none in Type C); in Type A an unsupported vertical side of 3 1/2 ft at most under a 3/4 : 1 slope in excavations up to 8 ft deep and under a 1 : 1 slope from 8 to 12 ft; a vertically sided lower portion that is supported or shielded to at least 18 in above its top, under the slope of the soil type; and six two-layer cases, in which a stronger layer over a weaker one takes the slope of the weaker.

Geometry. With bottom width W, depth H and slope s, a simple slope has the top width W + 2 s H and the cross-section W H + s H². A bench of height r under the slope leaves the wedge below the slope line in place: cross-section W H + s (H² − r²). Multiple benches are drawn with each tread equal to s times the riser below it, so the outer corners lie on the slope line. A vertical side h under a slope gives W + 2 s (H − h) and W H + s (H − h)². The volume is the cross-section times the length. The spoil is drawn as a triangular windrow of the loosened volume on one side: height √(f A / m) and base 2 m × height, for a swell factor f and side slopes of m horizontal to 1 vertical. Swell factor and spoil slope are entered values; the defaults (1.25 and 1.5) are ordinary figures, not from the standard.

Timber shoring (Appendix C)

Tables C-1.1 to C-1.3 give actual sizes of mixed oak or equivalent with a bending strength of at least 850 psi for Types A, B and C; Tables C-2.1 to C-2.3 give S4S (surfaced) sizes of Douglas fir or equivalent with at least 1500 psi. For the depth band (5 to 10, 10 to 15, 15 to 20 ft) and the trench width (up to 4, 6, 9, 12 or 15 ft) each line of a table is one acceptable arrangement: the horizontal spacing of the cross braces, the cross-brace size, its vertical spacing (4 ft in Type A, 5 ft in Types B and C), the wales and their vertical spacing, and the uprights with their largest horizontal spacing or close sheeting. The tool lists every line and counts one of them. A depth on a band limit is read in the shallower band, as Appendix D words its bands ("over 10 up to 15").

Notes of the tables used by the tool: spacings are centre to centre; wales are installed with the greater dimension horizontal; with a 4 ft vertical spacing the top cross brace is no more than 2 ft below the top of the trench, with 5 ft no more than 2.5 ft; more than 2.5 ft from the lowest cross brace to the bottom needs embedded uprights (36 in at most) or a mudsill (42 in at most); saturated or submerged conditions need tight sheeting. The brace layout the tool prints is the smallest number of equally spaced levels that satisfies these limits; it is one valid layout, not the only one.

The tables are not considered adequate when structures or stored material within a distance equal to the depth weigh more than a 2 ft soil surcharge, when the vertical load on a cross brace exceeds 240 lb on one foot at its centre, when equipment heavier than 20,000 lb is present, or when only the lower portion of a sloped trench is shored and the members were not chosen for the depth from the top. The tool tests the equipment weight and flags a spoil pile taller than 2 ft that starts within the depth of the trench from its edge; that reading of "weigh in excess of the load imposed by a two-foot soil surcharge" is the tool's and is on the safe side.

Aluminum hydraulic shoring (Appendix D)

Tables D-1.1 and D-1.2 give vertical shores for Types A and B: the largest horizontal spacing by depth (8, 8, 7 ft in Type A; 8, 6.5, 5.5 ft in Type B), a 4 ft vertical spacing of the cylinders, and the cylinder by trench width: 2 in diameter up to 8 ft, 2 in with steel oversleeves from 8 to 12 ft, 3 in from 12 to 15 ft. Tables D-1.3 and D-1.4 give waler systems with timber sheeting for Types B and C: for wales with a section modulus of 3.5, 7.0 or 14.0 in³ at 4 ft vertically, the cylinder spacing and diameter, with 3 × 12 uprights at the printed spacing or as solid sheeting. A 2 in cylinder must have a safe working capacity of at least 18,000 lb and a 3 in cylinder 30,000 lb at full extension; vertical shore rails need a section modulus of at least 0.40 in³; shores are installed at least three in a group. The cylinder and wale layouts the tool prints use the limits drawn on the appendix's typical-installation figures (first cylinder within 18 in of the top or first wale within 2 ft, 4 ft at most above the bottom).

Appendix D has no vertical-shore table for Type C and no waler table for Type A; the tool then uses the table that exists and says so. The data do not cover equipment heavier than 20,000 lb or a vertical load above 100 lb on one foot at the centre of a cylinder.

Trench shield

A shield does not hold the soil; it protects the people inside it. It is used within its manufacturer's tabulated data (1926.652(c)(2)), so the rated depth for the soil type is an entered value. The tool checks that rating against the full depth, the excavation below the bottom of the shield (2 ft at most, 1926.652(g)(2)), and the height of the shield: at least 18 in above the top of the vertical side when the upper portion is sloped (Figure B-1), or up to the ground surface when it is not. An entered rated pressure is compared with the pressure in the heading of the timber table for the soil type.

Engineered check

Pressure. The headings of the timber tables state the lateral pressure they were made for: 25 H + 72, 45 H + 72 and 80 H + 72 lb/ft² for Types A, B and C, H in feet, the 72 lb/ft² being a 2 ft surcharge. The regulation does not say how that pressure is distributed. The report the tables are drawn from, NBS Building Science Series 127, uses a pressure that is uniform over the depth (its equation 3.1), and the tool applies the appendix value the same way. The alternative is Rankine's active pressure with entered parameters, p = K_a (σ′_v + q) − 2 c √K_a with K_a = tan²(45° − φ/2), not taken below zero, with the buoyant unit weight and the hydrostatic pressure below an entered water level.

Strip surcharge. A load q on a strip parallel to the trench, from a distance a to a + b, adds σ_h = (2 q / π)(β − sin β cos 2α) at each depth, where β is the angle the strip subtends at that depth and α the angle of its centre line from the vertical. This is the elastic (Boussinesq) stress doubled for a wall that does not yield, the form the Caltrans Trenching and Shoring Manual gives as equation 5-1-2 and tabulates in its Appendix C.

Strut loads. Each level of cross braces or cylinders carries the pressure between the mid-heights to its neighbours, the top level from the surface and the bottom level to the trench bottom; the strut load is that force per unit length times the horizontal spacing. The wale is a simple span between struts (Appendix D note 9), M = w s² / 8, and the required section modulus is M / F_b. NBS Building Science Series 127 lets wales be designed for 80 percent of the tributary load to allow for arching; the tool's default is the full load.

The check is meant for systems designed under 1926.652(c)(3) or (c)(4). The members of Appendices C and D are prescriptive: they come from the NBS recommendations, existing codes and practice, and a tabulated wale checked this way against the stated bending strength can show a stress ratio above 1. The tool therefore does not run the tables through the check. Apparent-pressure envelopes for braced cuts, base heave and staged excavation are in the Sheet Pile and Braced Excavation tool.

Checklist

The depth triggers of the standard: a stairway, ladder or ramp within 25 ft of lateral travel in trenches 4 ft or more deep (1926.651(c)(2)); atmospheric testing before entry in excavations deeper than 4 ft where an oxygen-deficient (below 19.5 percent) or hazardous atmosphere could reasonably be expected (1926.651(g)); a protective system in excavations 5 ft or more deep, and in shallower ones when the competent person sees an indication of a possible cave-in, except in stable rock (1926.652(a)(1)); design by a registered professional engineer beyond 20 ft. Also: spoil and equipment at least 2 ft from the edge (1926.651(j)(2)), precautions against accumulating water (1926.651(h)), daily inspections (1926.651(k)), and the location of underground installations before digging (1926.651(b)). The number of ladders assumes they are spaced 50 ft apart along the trench.

Checks and source notes

Every cell of the ten tables was read from a 300 dpi render of the printed page and typed twice; the examples the appendices give in words (four for timber, five for hydraulic shoring) are reproduced by the look-up. Table C-1.1 prints "6X5" for the cross brace at 10 to 15 ft, 10 ft spacing and widths up to 6 ft; Example 1 of the appendix shows the intended 6 × 6, which the tool uses. The figure for the Type A vertical side at 12 ft is labelled "2′ Max." where its caption says 12 ft. The strip-load equation reproduces the 1,600 cells of the Caltrans Appendix C tables within 0.006 lb/ft² (16 cells differ by one in the last printed digit) and the worked values of its Chapter 5 and Appendix C. Pressure integrals, strut loads, wale moments and the slope cross-sections were checked against an independent numpy and scipy calculation on forty seeded cases.

Not covered: the OSHA booklet and Technical Manual examples (cited, not checked), sloping deeper than 20 ft, surcharge reductions of slopes, shield design, timber member capacities, hydrostatic uplift and dewatering design, and adjacent structures (1926.651(i)).