10.5 Support of Excavation (Trenching, Cofferdams)

Introduction to Support of Excavation

Excavation and trenching are among the most hazardous construction operations. A soil cave-in occurs when the shear stress in the soil wedge exceeds the soil's shear strength, causing the walls of the excavation to collapse. Under OSHA 1926 Subpart P, excavations deeper than $5\text{ feet}$ require a protective system (sloping, shoring, or shielding) unless cut entirely in stable rock. For excavations deeper than $20\text{ feet}$, the protective system must be designed by a registered professional engineer. In addition, a safe means of egress (ladders, steps, or ramps) is required for trenches $4\text{ feet}$ or deeper, with a maximum lateral travel distance of $25\text{ feet}$ for workers.

Soil Classification and Sloping Systems

Under OSHA regulations, soil is classified into three types based on unconfined compressive strength ($q_u$):

• Type A: Cohesive soils (clays, silty clays) with $q_u > 1.5\text{ tsf}$ (tons per square foot). No soil can be classified as Type A if it is fissured, subject to vibration, or has water seeping.
• Type B: Cohesive soils with $0.5 < q_u \le 1.5\text{ tsf}$, or granular cohesionless soils (silt, sandy loam), or previously disturbed soils.
• Type C: Cohesive soils with $q_u \le 0.5\text{ tsf}$, granular soils (sand, gravel), submerged soils, or soils with active water seepage.

The maximum allowable slope (Horizontal to Vertical, H:V) for sloping systems is dictated by these soil types:

• Type A: $0.75:1$ ($53^\circ$)
• Type B: $1:1$ ($45^\circ$)
• Type C: $1.5:1$ ($34^\circ$)

Benching (excavating steps) is permitted only in cohesive soils (Type A and B) and is strictly prohibited in granular Type C soils.

Earth Retention Systems

In urban areas or deep excavations where sloping is impossible due to property limits, structural support systems are required. These include sheet pile walls, soldier piles with lagging, and cofferdams.

Sheet Pile and Soldier Pile Walls

Sheet piles are interlocking steel sections driven into the subgrade. Cantilever sheet piles rely on embedment depth for stability and are limited to shallow cuts. Anchored sheet piles use ground anchors (tiebacks) or internal struts to resist lateral loads. Soldier pile walls use vertical steel H-piles driven at intervals, with horizontal timber lagging inserted between them as excavation proceeds.

Unlike permanent retaining walls designed using Rankine active pressure, braced excavations with struts do not rotate about their base. Therefore, they do not mobilize a linear triangular active pressure distribution. Instead, engineers use Peck's Apparent Earth Pressure Diagrams to design struts, walers, and lagging. For sand, the maximum apparent lateral pressure is constant at $0.65 K_a \gamma H$. For clays, the pressure envelope is trapezoidal, varying based on the stability number $N_b = \gamma H / c$, where $\gamma$ is unit weight, $H$ is depth, and $c$ is cohesion.

Cofferdams and Groundwater Seepage

A cofferdam is a temporary watertight enclosure driven into water-bearing ground or open water and dewatered to allow construction in a dry environment. A major failure mode for cofferdams is piping (also known as a boiling or quicksand condition). Dewatering the inside of the cofferdam creates a hydraulic head difference, forcing water to flow under the sheet piles and seep upward through the excavation floor. This upward flow exerts a drag force on the soil particles.

If the upward seepage pressure equals the buoyant unit weight of the soil, the effective stress drops to zero ($\sigma' = 0$). The soil loses all shear strength, behaving like a liquid. The critical hydraulic gradient ($i_c$) at which this occurs is: $i_c = \frac{\gamma'}{\gamma_w} = \frac{G_s - 1}{1 + e}$ where:

• $\gamma'$ is the buoyant unit weight of soil ($\text{pcf}$).
• $\gamma_w$ is the unit weight of water ($62.4\text{ pcf}$).
• $G_s$ is the specific gravity of soil solids (typically $2.65\text{ to }2.70$).
• $e$ is the void ratio of the soil.

The actual hydraulic gradient is $i = \Delta h / L$, where $\Delta h$ is the head difference and $L$ is the length of the flow path. The Factor of Safety ($\text{FS}$) against piping is: $\text{FS} = \frac{i_c}{i}$ A minimum Factor of Safety of $1.5\text{ to }2.0$ is required.

Worked Engineering Example: Piping Safety Check

Problem Scenario: A cofferdam is excavated in a fine sand deposit. The sheet piles are driven to an embedment depth that provides a groundwater flow path length of $L = 15\text{ feet}$ beneath the bottom of the excavation. The groundwater table outside the cofferdam is $9\text{ feet}$ above the excavation floor, which is pumped dry ($\Delta h = 9\text{ feet}$). Laboratory testing indicates the sand has a specific gravity of $G_s = 2.68$ and an in-place void ratio of $e = 0.60$. Calculate the critical hydraulic gradient ($i_c$), the actual hydraulic gradient ($i$), and determine the Factor of Safety against piping.

Step 1: Calculate the critical hydraulic gradient ($i_c$). $i_c = \frac{G_s - 1}{1 + e} = \frac{2.68 - 1}{1 + 0.60} = 1.05$

Step 2: Calculate the actual hydraulic gradient ($i$). $i = \frac{\Delta h}{L} = \frac{9\text{ ft}}{15\text{ ft}} = 0.60$

Step 3: Calculate the Factor of Safety against piping. $\text{FS} = \frac{i_c}{i} = \frac{1.05}{0.60} = 1.75$

Conclusion: The Factor of Safety is $1.75$. Since $1.75 \ge 1.50$, the excavation is safe against piping under these conditions.

Critical Field Traps in Excavation Support

• Incorrect Soil Classification: Classifying a soil as Type A based solely on visual inspection when it is actually a fissured clay or near heavy traffic vibrations. These factors automatically downgrade the soil to Type B or C.
• Spoils Pile Placement: OSHA requires that the excavated soil (spoils pile) and any construction equipment be placed at least $2\text{ feet}$ away from the edge of the excavation. Placing loads too close creates a surcharge that increases lateral earth pressure and triggers shallow slope failure.
• Water Accumulation: Water seeping into a trench must be actively pumped out. Water accumulation increases lateral pressure (hydrostatic force) and reduces the soil's effective stress and cohesion, accelerating cave-ins.
• Misusing Trench Boxes: A trench box does not prevent trench wall movement. If a large void exists between the trench box and the soil wall, a cave-in can impact the box dynamically, crushing it or knocking it over. Backfilling the space between the box and the soil wall is critical.