Dewatering, Shoring & Excavation Support

Every church project touches the ground, and the ground in Ontario is full of water. Whether you are digging a 1.2 m trench for a storm sewer or excavating 6 m for a full basement under a new sanctuary, you will encounter groundwater, unstable soil, or both. This module covers the systems used to control water and support excavation walls so that crews can work safely and foundations are built on solid, dry bearing surfaces.

This article assumes you already understand basic excavation procedures covered in the Site Preparation & Earthwork module. Here we go deeper — literally and figuratively — into the specialized techniques that separate a competent superintendent from someone who is just moving dirt.

1. Dewatering Fundamentals

Why We Dewater

Groundwater in an open excavation causes three critical problems:

1. You cannot place concrete in standing water. Water dilutes the cement paste, destroys the water-to-cement ratio, and creates planes of weakness. Even 25 mm of water on a footing subgrade is unacceptable. The Ontario Building Code (OBC) and CSA A23.1 both require that concrete be placed “in the dry” unless tremie or other underwater placement methods are specifically engineered.
2. Saturated soils lose bearing capacity. A silty clay that provides 150 kPa allowable bearing when dry may lose 30–50% of its capacity when saturated. The geotechnical engineer’s bearing capacity value assumes proper dewatering. If you pour footings on a wet, softened subgrade, you are building on a lie.
3. Water destabilizes excavation walls. Seepage forces push soil particles outward, causing sloughing, piping, and potentially catastrophic wall failures. Hydrostatic pressure behind a shoring wall can double the lateral earth pressure the wall must resist.

Ontario Groundwater Conditions

Southern Ontario’s geology is dominated by glacial deposits, and conditions vary enormously within short distances:

• Clay plains (e.g., much of Waterloo Region, Hamilton, London): Low permeability glacial till and lacustrine clays. The water table may be shallow (1–3 m), but flow rates are low. Dewatering is often manageable with sump pumping, but the clay itself is challenging — it swells when exposed to water and shrinks when dried, and it can be extremely soft when disturbed.
• Sandy glacial deposits (e.g., Brantford, Cambridge outwash plains, Oak Ridges Moraine): High permeability sands and gravels with a water table that can be within 1–2 m of the surface. These soils transmit water readily. Sump pumping alone often cannot keep up, and wellpoints or deep wells are required.
• Bedrock areas (e.g., Niagara Escarpment, Canadian Shield fringe): Fractured limestone or shale can transmit large volumes of water through joints and fractures. Flow is unpredictable — you may drill 10 boreholes and find water in only two, but those two may flow at 50 L/min each.

2. Sump Pumping

Sump pumping is the simplest dewatering method. You dig a sump pit at the low point of the excavation, let water flow to it by gravity, and pump it out. It does not lower the water table — it only removes water that has already entered the excavation.

When Sump Pumping Is Adequate

• Minor seepage from perched water tables or surface infiltration
• Shallow excavations (less than 2–3 m) in low-permeability soils (clays, silty clays)
• Temporary conditions — e.g., rain event cleanup
• As a backup/supplement to a wellpoint or deep well system

Setup Procedure

1. Excavate a sump pit at the lowest point of the excavation, typically 1 m × 1 m × 1 m deep (deeper than the lowest footing elevation by at least 300 mm).
2. Line the pit with clear stone (19–50 mm) wrapped in non-woven geotextile to act as a filter and prevent soil from clogging the pump.
3. Install a submersible trash pump (typically 50–100 mm / 2–4” discharge) with a float switch for automatic operation.
4. Run the discharge hose to the approved discharge point. Ensure the hose is secure and will not fall back into the excavation.
5. Cut shallow drainage ditches (150–200 mm deep) in the excavation floor sloping toward the sump at a minimum 1% grade.

Pump Sizing Basics

A pump must overcome both flow rate and total dynamic head (TDH). TDH = static lift (vertical distance from water surface to discharge point) + friction losses in the hose. For a typical 5 m deep excavation discharging at grade:

• Static lift: ~5 m
• Friction loss in 30 m of 75 mm hose at 500 L/min: ~2 m
• TDH: ~7 m
• Select a pump that delivers your required flow rate at 7 m TDH — check the pump curve, not just the nameplate maximum

The Piping Problem

3. Wellpoint Systems

Wellpoint dewatering is the workhorse system for moderate-depth excavations in sandy and gravelly soils. Unlike sump pumping, wellpoints lower the water table before you excavate, so you dig into dry ground.

How a Wellpoint System Works

1. Install wellpoints by jetting. A wellpoint is a slotted screen (typically 50 mm diameter, 600–1000 mm long) attached to the bottom of a riser pipe (typically 38–50 mm diameter). To install it, you connect a high-pressure water jet to the riser pipe and force it into the ground. The jetting water washes the soil away as the wellpoint descends. Once at the target depth, you pull the jet pipe and backfill around the riser with coarse sand to create a filter pack.
2. Connect riser pipes to the header. A horizontal header pipe (typically 150–200 mm diameter) runs along the perimeter of the excavation. Each wellpoint riser connects to the header via a swing connection with an individual valve so you can shut off any single wellpoint.
3. Connect the header to the pump. A vacuum-assisted self-priming centrifugal pump (or multiple pumps for long runs) connects to the header. The pump creates a vacuum in the header, which draws water up through each wellpoint, along the header, and to discharge.
4. Run the system 24/7 until the excavation work is complete and backfilled above the water table. The water table typically draws down to its target level within 24–72 hours of startup, depending on soil permeability.

Wellpoint Spacing by Soil Type

The Suction Limitation

Wellpoint pumps work by suction, and atmospheric pressure limits suction lift to a theoretical maximum of 10.3 m (one atmosphere). In practice, accounting for friction losses, vacuum inefficiency, and the need for pump NPSH (net positive suction head), the practical drawdown limit for a single stage of wellpoints is 4.5–6 m.

Multi-Stage Wellpoints

When you need more than 5–6 m of drawdown, you install wellpoints in stages. The first stage lowers the water table by ~5 m. You then excavate to that level and install a second ring of wellpoints on a bench at the new excavation level. The second stage draws the water down another 5 m. This is labour-intensive and takes up excavation space, but it works. For church basements in sandy soils where the water table is high, two-stage wellpoints are common.

4. Deep Wells

Deep wells are used when wellpoints cannot achieve sufficient drawdown — typically for excavations deeper than 6 m below the water table, in high-permeability soils where flow rates exceed wellpoint capacity, or in layered soils where you need to depressurize a confined aquifer beneath the excavation.

Installation Procedure

1. Drill the borehole. Use a truck-mounted drill rig to bore a hole typically 250–400 mm in diameter to the required depth (usually 3–6 m below the base of excavation). Drilling method depends on soil: hollow-stem auger in cohesive soils, mud rotary in sands below the water table.
2. Install well screen and casing. Lower a PVC or steel well screen (slotted section, typically 150–200 mm diameter) into the borehole. The screen section spans the water-bearing zone. Solid casing extends from the top of the screen to the surface.
3. Place the filter pack. Pour graded gravel or sand (filter pack) into the annular space between the well screen and the borehole wall. The filter pack prevents soil particles from entering the well while allowing water to flow freely. Filter pack grain size is selected based on the soil gradation — typically 4–6 times the D₅₀ of the native soil.
4. Seal the top. Place a bentonite seal above the filter pack to prevent surface water from short-circuiting down the annulus.
5. Install the submersible pump. Lower a submersible pump (typically 2–10 HP depending on flow requirements) into the well, suspended on the discharge pipe. Connect electrical supply and discharge piping.
6. Develop the well. Before connecting to the system, surge and pump each well to remove drilling fines from the filter pack and near-well zone. The well is developed when discharge water runs clear and the sand content is below 5 mg/L.

Deep Well Spacing and Layout

Deep wells are typically spaced 10–30 m apart, depending on soil permeability and required drawdown. A hydrogeologist designs the system using analytical or numerical models to predict the cone of depression from each well and confirm that the combined drawdown achieves the target level across the entire excavation.

Advantages Over Wellpoints

• Each well is independent — failure of one well does not compromise the entire system
• Can achieve drawdown of 20 m or more with no multi-staging
• Works in all soil types, including layered soils and confined aquifers
• Wells can be installed inside or outside the excavation footprint
• Lower maintenance — no vacuum system to monitor, no header pipe connections to check

Sand Pumping

The biggest risk with deep wells is sand pumping — if the filter pack is poorly designed or the well is not properly developed, the pump will draw sand from the formation. Sand damages the pump impeller (reducing efficiency and eventually destroying the pump), creates voids in the ground around the well (potentially causing settlement of adjacent structures), and clogs discharge piping. If a well starts producing sandy water, shut it down immediately and redevelop or replace it.

5. Eductor (Ejector) Wells

Eductors fill the gap between wellpoints and deep wells for fine-grained soils where neither system works well on its own.

How Eductors Work

An eductor is a jet pump installed at the bottom of a well. High-pressure supply water (typically at 700–1000 kPa) is pumped down a supply pipe to a nozzle-and-venturi assembly at the wellpoint. As the supply water passes through the nozzle and enters the venturi, it creates a low-pressure zone (Bernoulli’s principle) that draws groundwater into the venturi. The combined flow (supply water + groundwater) returns to the surface through a separate return pipe.

When to Use Eductors

• Fine-grained soils (silts, fine sands) where wellpoints cannot generate enough vacuum to pull water through the low-permeability soil, and where deep wells would need to be so closely spaced that they become impractical
• Deep drawdown requirements in fine soils — eductors can achieve 25–30 m of drawdown in a single stage
• Where you need both dewatering and pore pressure reduction — eductors are excellent at dissipating excess pore water pressure in silty soils, which improves slope stability and bearing capacity

Limitations

• Energy-intensive: You are pumping large volumes of supply water to recover relatively small volumes of groundwater. The ratio of supply water to recovered groundwater can be 5:1 or higher.
• Complex piping: Each eductor requires a supply line and a return line, plus the well itself. A system with 30 eductors means 60 pipes connected to the supply and return headers.
• High-pressure pump required: The supply pump must deliver 700–1000 kPa at high flow rates. This is a specialized piece of equipment.

6. Dewatering Method Selection Guide

7. Ontario Water Taking Regulations

Every dewatering operation removes groundwater from the environment, and Ontario regulates this activity under the Ontario Water Resources Act (OWRA). As a superintendent, you need to understand the permitting framework because starting dewatering without the correct permit is an offence that can result in fines up to $100,000 per day for individuals and $1,000,000 per day for corporations.

Permit to Take Water (PTTW)

A PTTW is required for any water taking exceeding 50,000 litres per day (about 50 m³/day). For context, a single 50 mm wellpoint pump running at moderate capacity can easily produce 200,000–500,000 L/day. Almost every wellpoint or deep well dewatering system will require a PTTW.

EASR (Environmental Activity and Sector Registry)

For lower-risk water takings between 50,000 and 400,000 L/day from certain sources (not from a municipal drain or within a wellhead protection area), you may qualify for an EASR instead of a full PTTW. An EASR is a self-registration process through the MECP (Ministry of the Environment, Conservation and Parks) that is faster and less expensive than a PTTW.

• EASR processing time: Immediate upon registration (online portal)
• PTTW processing time: 90–120 days typical; can be expedited in some cases but do not count on it
• PTTW validity: Usually issued for the duration of the construction project (1–5 years)

Discharge Requirements

The water you pump out has to go somewhere, and “somewhere” is regulated:

• Discharge to ground surface / natural drainage: Permitted under most PTTWs, but you must settle sediment first. Typically, you pump through a dewatering bag (geotextile filter bag), settling pond, or sediment tank before allowing the water to enter any watercourse or drainage ditch.
• Discharge to municipal storm sewer: Requires permission from the municipality. Many municipalities have by-laws requiring sediment removal to below 25 mg/L TSS (total suspended solids) before discharge to storm sewers.
• Discharge to municipal sanitary sewer: Requires a sewer use agreement. The municipality may charge a fee based on volume. This is sometimes the only option in urban areas.
• Discharge to a watercourse (creek, river, wetland): Most restrictive. Must comply with the federal Fisheries Act (no deposit of deleterious substances) and may require a permit from the local Conservation Authority. Turbidity must be controlled to near-background levels.

8. Shoring Systems — Soldier Pile and Lagging

Soldier pile and lagging is the most common shoring system for commercial construction in Ontario. It is versatile, relatively economical, and well-understood by local contractors. If you are building a church with a basement, there is a good chance you will encounter this system.

Step-by-Step Procedure

1. Install soldier piles. Steel H-piles (typically W200×46 to W310×97, depending on design loads) are installed at 1.8–3.0 m centres along the perimeter of the excavation. Installation methods:
   • Driven: Impact or vibratory hammer drives the pile into the ground. Fast in soft to medium soils but generates vibration and noise.
   • Drilled: A hole is augered to the required depth, the pile is lowered in, and the hole is backfilled with lean concrete or compacted granular fill. Preferred near existing structures to avoid vibration damage.
2. Excavate in lifts. Dig down in horizontal lifts of 1.0–1.5 m. After each lift, install lagging before excavating the next lift.
3. Install lagging. Timber planks (typically 75–100 mm thick rough-sawn spruce or hemlock, or 38 mm plywood for lighter loads) are slid horizontally between the flanges of adjacent H-piles. The lagging retains the soil between the piles. In cohesive soils, a small gap (25–50 mm) may be left between lagging boards to allow drainage and prevent hydrostatic pressure buildup behind the wall. In granular soils, lagging must be tight with filter fabric behind to prevent soil loss.
4. Install tiebacks (if required). For excavations deeper than about 4–5 m, or where surcharge loads from adjacent structures are significant, the wall needs lateral support beyond what cantilever alone can provide. Tieback anchors are the standard solution.

Tieback Anchors: Detailed Procedure

1. Drill the anchor hole. A drill rig mounted on the excavation floor drills through or adjacent to the soldier pile, through the lagging, and into the retained soil at a downward angle (typically 15–30° below horizontal). The hole is typically 100–150 mm diameter and extends 6–15 m behind the wall, depending on the required anchor capacity and the location of the no-load zone (the active wedge of soil behind the wall that does not contribute to anchor resistance).
2. Install the tendon. A high-strength steel tendon (either a single bar or a bundle of 7-wire strands) is inserted into the drilled hole. The tendon has a bonded zone (the portion that will be grouted into the soil to develop pullout resistance) and a free length (unbonded section that allows the anchor to stretch and develop load).
3. Grout the bonded zone. Cement grout (typically w/c ratio 0.40–0.50) is injected under pressure into the bonded zone. In granular soils, pressure grouting at 500–1500 kPa can increase anchor capacity by 50–100% compared to gravity grouting. The free length is kept ungrouted (typically by a smooth plastic sheath over the tendon) to allow the anchor to act as a spring.
4. Stress and lock off. After the grout reaches required strength (typically 24–72 hours, verified by testing grout cubes), a hydraulic jack stresses the anchor to the design load. A bearing plate and anchor head transfer the load to the soldier pile.
5. Testing:
   • Proof test (every anchor): Load to 133% of design load, hold for 10 minutes, verify total movement is within limits, lock off at design load.
   • Performance test (typically 5% of anchors, minimum 2): Incremental loading to 133% with measurements at each increment. Plots load vs. movement to verify elastic behaviour.
   • Creep test (on selected anchors, especially in clay soils): Hold at 133% for extended period (up to 60 minutes) and measure time-dependent movement. Creep rate must be below 2 mm per log cycle of time.

9. Sheet Piling

Sheet piles are interlocking steel sections driven into the ground to form a continuous wall. Their key advantage over soldier pile and lagging is water cutoff — when the interlocks are engaged, a sheet pile wall is essentially watertight (or close to it).

Common Section Types

• Z-sections: The most common type for retaining walls. The interlocks are at the outer flanges, creating a wall with a sinusoidal cross-section. Strong in bending. Examples: AZ-18, AZ-26.
• U-sections: Interlocks at the neutral axis. Used where interlock friction (sliding resistance) is important, such as cellular cofferdams. Slightly less efficient in bending than Z-sections.
• Flat sections: Used for circular cofferdams and marine structures where hoop stress is the primary load. Low bending strength.

Driving Methods

• Vibratory hammer: The preferred method in granular soils. A vibratory hammer clamps onto the top of the sheet pile and vibrates it at 20–40 Hz, which liquefies a thin zone of sand around the pile and allows it to sink under the combined weight of the pile and hammer. Fast (a 12 m pile can be driven in 2–5 minutes in loose sand), relatively low impact noise, and gentle on adjacent structures. Does not work well in stiff clays or dense tills.
• Impact hammer: A diesel or hydraulic impact hammer drives the pile by repeated blows. Slower and noisier than vibratory driving, but works in hard driving conditions including stiff clay, dense till, and soils with cobbles or boulders. Generates significant vibration.
• Press-in method (silent piler): A hydraulic press grips previously installed piles for reaction and pushes the next pile in using static force. Generates virtually no vibration or noise. Ideal for urban areas adjacent to sensitive structures. Slow and expensive, but sometimes the only option when vibration limits are tight.

Vibration Monitoring and Limits

PPV = peak particle velocity, measured at the nearest point of the structure to the pile-driving source. A seismograph (vibration monitor) must be installed and recording continuously during all driving operations. A common approach is to use triggered recording with real-time SMS alerts to the superintendent when vibration exceeds 80% of the limit.

Interlock Sealing for Water Cutoff

Standard sheet pile interlocks are not perfectly watertight. Water can seep through the interlock at rates of 0.5–2.0 L/min per metre of wall per metre of head. For applications requiring near-complete water cutoff (cofferdams, contaminated site containment), interlocks can be sealed with:

• Sealant compounds: Applied to the interlock before driving. Effective for moderate seepage reduction.
• Welding: After driving, the exposed interlocks above excavation level can be welded. Provides excellent sealing but is permanent (prevents extraction).
• Swellable waterstop strips: Hydrophilic strips installed in the interlock that expand on contact with water to seal the joint.

10. Secant and Tangent Pile Walls

When you need deep excavation support in an urban environment where vibration must be minimized and water cutoff must be excellent, secant pile walls are the premium solution.

Secant Walls: How They Work

1. Construct a guide wall. A reinforced concrete guide wall (typically 300–600 mm deep) is cast at the surface to guide the drilling rig and ensure pile positions are accurate.
2. Drill and pour primary (female) piles. Using a large-diameter drilling rig (typically CFA — continuous flight auger — or cased rotary), drill the primary piles at the designed spacing. Primary piles are poured with a weak concrete mix (typically 5–10 MPa at 28 days) and are not reinforced. Primary pile diameter is typically 600–900 mm.
3. Drill and pour secondary (male) piles. Before the primary piles reach full strength (timing is critical — usually within 24–48 hours), drill the secondary piles at the midpoints between the primaries. The secondary drill bit cuts through the edges of the adjacent primary piles, creating an overlap of 100–200 mm on each side. Secondary piles are poured with structural concrete (25–35 MPa) and reinforced with a steel cage (rebar or steel W-section).
4. Excavate. Once the wall is complete, excavate inside it. The overlapping piles create a continuous, watertight barrier.

Tangent Pile Walls

Tangent walls are similar but the piles just touch each other (tangent) without overlapping. All piles are reinforced and poured with structural concrete. Tangent walls are faster and cheaper than secant walls but do not provide reliable water cutoff because achieving perfect tangent contact over the full depth of the pile is nearly impossible. Small gaps between piles will allow water and fines to pass through.

Verticality Tolerance

The success of a secant wall depends entirely on maintaining the overlap between primary and secondary piles over the full depth. If a pile drifts off vertical, the overlap is reduced or lost, creating a “window” that allows water and soil to pass through. Minimum verticality tolerance is 1:200 (1 mm of lateral deviation per 200 mm of depth), and better equipment can achieve 1:300 or better. For a 15 m deep wall at 1:200, the maximum deviation at the toe is 75 mm — which is consumed entirely by the required overlap. There is zero margin for error at these depths.

11. Soil Nail Walls

Soil nailing is a top-down construction method that reinforces the existing soil to create a stable, self-supporting excavation wall. Unlike soldier piles or sheet piles, which are installed before or during excavation and act as structural elements, soil nails turn the soil itself into the retaining structure.

Construction Sequence

1. Excavate a lift. Remove soil in a horizontal lift of 1.0–1.5 m. The soil must stand temporarily without support for long enough to install the nails and shotcrete. This is why soil nailing does not work in soft clays or loose sands — the face would collapse before you can stabilize it.
2. Drill nail holes. Using a track-mounted drill rig, drill holes into the exposed face at a slight downward angle (typically 10–20° below horizontal). Hole diameter is typically 100–150 mm. Nail spacing is typically 1.2–2.0 m in both directions (a grid pattern).
3. Install nails. Insert a steel bar (typically 25–32 mm diameter, Grade 400W or 500W rebar) into each hole. The bar should extend to within 300 mm of the back of the hole.
4. Grout. Fill each hole with neat cement grout (w/c 0.40–0.50) by tremie from the back of the hole forward. The grout bonds the nail to the surrounding soil, creating pullout resistance through friction along the entire grouted length.
5. Apply shotcrete face. Spray a layer of shotcrete (typically 100–150 mm thick) over the exposed face, incorporating welded wire mesh (WWM) or steel fibre reinforcement. The shotcrete face distributes the nail forces and prevents surface ravelling between nails. Bearing plates and nuts at each nail location transfer load from the nail to the shotcrete face.
6. Repeat. Excavate the next lift and repeat the process. Each lift is connected to the one above by lapping the mesh reinforcement.

How It Works

The nails act as passive reinforcement (unlike tieback anchors, which are actively prestressed). As the retained soil mass tries to move outward, the nails resist by developing tension through friction with the grout and surrounding soil. The overall effect is to create a reinforced soil block that behaves as a gravity retaining wall — it resists overturning and sliding by its own weight.

Advantages

• Fast installation — a crew can install 15–25 nails per day
• No heavy equipment required — a compact drill rig and a shotcrete pump are the main equipment
• Works on irregular wall geometries (curves, corners) where sheet piles or soldier piles are difficult
• Less expensive than tieback walls for moderate depths in suitable soils

Limitations

• The soil must have enough cohesion to stand unsupported for the short period between excavation and shotcrete application (typically 4–8 hours)
• Not suitable in soft or loose soils, organic soils, or soils with a high water table (the face will not stand)
• Nails extend into the retained soil — you need easements or rights if the nails extend beyond your property line
• Permanent soil nail walls require corrosion protection (epoxy-coated or encapsulated nails)

12. Shoring System Selection Guide

13. Excavation Monitoring

Shoring systems are designed with safety factors, but the real world does not always match the design assumptions. Monitoring is how you verify that the wall is behaving as predicted — and catch problems before they become failures.

Monitoring Instruments

• Inclinometers: A casing (typically 70 mm ABS pipe with internal guide grooves) is installed in a borehole behind or within the shoring wall. A torpedo-shaped probe is lowered into the casing, measuring lateral tilt at 0.5 m intervals. By summing the tilt measurements, you get a profile of lateral wall deflection from top to bottom. Readings are typically taken weekly or after each excavation lift. Alert threshold for most walls: 25 mm cumulative deflection or 10 mm movement between readings.
• Survey monitoring points: Prisms or targets are installed on the shoring wall face, on adjacent buildings, and on the ground surface behind the wall. A total station (robotic for automated systems) reads positions at regular intervals. Detects both lateral and vertical movement. On major projects, automated monitoring with hourly readings and wireless data transmission is standard.
• Crack monitors: Simple devices bonded across existing cracks on adjacent buildings. A plastic overlapping plate with a grid shows any differential movement across the crack. Read manually (weekly) or electronically (continuous). Install on every visible crack in adjacent buildings within the zone of influence (typically 1–2 times the excavation depth behind the wall).
• Vibration monitors (seismographs): Measure ground vibration (peak particle velocity) in three axes. Mounted on adjacent structures at the foundation level. Required during all pile driving, sheet pile driving, and any vibratory compaction near existing structures. Must be calibrated within the past 12 months.
• Load cells on tieback anchors: Measure the load in the anchor over time. Load loss can indicate creep in the anchor bond zone or relaxation of the prestress. A load drop of more than 20% from lock-off load requires investigation and possibly re-stressing.
• Piezometers: Measure pore water pressure in the ground behind the wall. Important for confirming that the dewatering system is performing as designed. A rise in pore pressure that was not predicted by the dewatering design can indicate a failing well or an unexpected source of recharge.

Trigger Levels and Action Plans

Every monitoring program must define three response levels:

1. Green (normal): Measurements within expected range. Continue monitoring at standard frequency.
2. Amber (alert): Measurements approaching design limits (typically 70–80% of the maximum allowable value). Increase monitoring frequency, notify the shoring engineer, review the next construction step.
3. Red (action): Measurements at or exceeding design limits. Stop excavation immediately, evacuate the excavation if there is any indication of imminent wall failure, notify the shoring engineer and site management. Do not resume work until the engineer has assessed the situation and provided written direction.

14. Modern Erosion & Sediment Control

Erosion and sediment control (ESC) on a dewatering project goes far beyond rolling out a silt fence around the perimeter. The volume and velocity of water discharged from a dewatering system can overwhelm basic controls. Here is what you need to know about modern ESC practices.

Beyond Basic Silt Fence

• Sediment control logs / silt socks: Mesh tubes filled with compost or wood chip mulch. They filter runoff more effectively than silt fence because the fill media traps fine particles. Placed at the perimeter, around catch basins, and across drainage swales. More durable than silt fence and conform better to uneven ground.
• Turbidity curtains: Floating barriers used in open water (ponds, rivers, lakes) to contain turbid water near the discharge point while sediment settles. Required when dewatering discharge enters any body of water.
• Dewatering bags (filter bags): Large non-woven geotextile bags (typically 3 m × 5 m or larger) that you pump directly into. The water filters through the fabric, leaving sediment inside. The bag can be reused until full. This is the single most effective sediment control measure for pump discharge on construction projects. A dewatering bag can reduce TSS from 500+ mg/L to below 25 mg/L in a single pass.
• Settling tanks / sediment traps: Steel tanks or excavated ponds where water velocity drops and particles settle out. For fine particles (silt, clay), settling alone may not be sufficient — the water may remain turbid for hours. This is where flocculants come in.

Polymer-Enhanced Sediment Control

Anionic polyacrylamide (PAM) flocculants are widely used in construction dewatering to dramatically accelerate sediment settlement. PAM causes fine clay and silt particles to clump together (flocculate) into larger, heavier particles that settle in minutes instead of hours.

• Application: PAM is mixed with the pump discharge water at a dosage of 1–5 mg/L (ppm) using an inline dosing system or a drip/sprinkle system at the settling pond inlet.
• Anionic vs. cationic: Only anionic PAM should be used. Cationic PAM is toxic to aquatic organisms and is prohibited for use near waterways. Always verify the product is anionic before use.
• Effectiveness: Properly dosed PAM can reduce TSS from 1000+ mg/L to below 10 mg/L. It is often the only way to meet stringent discharge limits near sensitive waterways.

Automated Turbidity Monitoring

On projects discharging near sensitive receptors (fish-bearing streams, wetlands, municipal water intakes), automated turbidity sensors should be installed on the discharge line. These sensors provide real-time turbidity readings (in NTU — nephelometric turbidity units) and can trigger alarms or automatically shut down dewatering pumps if turbidity exceeds the permit limit. Typical permit limits range from 25 NTU (sensitive fish habitat) to 75 NTU (general waterway discharge).

Regulatory Framework

• Ontario Water Resources Act (OWRA): Prohibits discharge of any material that may impair water quality. This is a broad prohibition — turbid water with no chemical contamination still violates the OWRA if it impairs a waterway.
• Federal Fisheries Act, Section 36(3): Prohibits the deposit of a “deleterious substance” into water frequented by fish. Sediment is considered a deleterious substance. Penalties include fines up to $1,000,000 and/or 3 years imprisonment. This is a federal offence — the Ministry of the Environment is not the only agency watching.
• Conservation Authority regulations: Most sites near waterways or wetlands are within a regulated area of a Conservation Authority (Grand River CA, Credit Valley CA, etc.). A permit under Ontario Regulation 150/06 (or its successor, O. Reg. 41/24 under the Conservation Authorities Act) may be required for any work that alters a waterway or discharges into a regulated area.

15. Putting It All Together

As a superintendent on a church construction project, you will rarely design a dewatering or shoring system — that is the engineer’s job. But you are responsible for understanding how the system works, monitoring its performance, recognizing when something is wrong, and making the call to stop work if safety is at risk. Here is a checklist for every project with significant excavation:

1. Before excavation begins:
   • Confirm PTTW or EASR is in place (check the posted permit number)
   • Review the geotechnical report — know the soil type, water table depth, and recommended shoring/dewatering systems
   • Review the shoring drawings and dewatering plan with the engineer
   • Confirm pre-construction surveys of adjacent structures are complete
   • Verify monitoring instruments are installed and baseline readings are taken
   • Confirm standby pump is on-site and tested
   • Verify ESC measures are installed and discharge point is approved
2. During excavation:
   • Monitor dewatering discharge for clarity every shift
   • Check pump operations daily (flow rate, running hours, fuel/power)
   • Review monitoring data after each excavation lift
   • Verify lagging is installed before the next lift proceeds
   • Attend tieback stressing and review test results
   • Inspect ESC measures daily and after every rain event exceeding 10 mm
3. After excavation is complete:
   • Maintain dewatering until backfill is above the water table
   • Do not remove shoring until the structure can support the lateral earth loads (typically after the first floor slab is poured and cured)
   • Decommission wells and wellpoints per the PTTW conditions (typically grouting wells closed)
   • File final monitoring report and retain records for minimum 5 years

Groundwater and unstable soils are the two most dangerous forces on a construction site. They are invisible until they are not, and when they reveal themselves, people can be killed. Every dewatering pump that runs, every tieback that is stressed, every lagging board that is installed exists to keep crews safe and buildings sound. Understand these systems. Respect the water. Build it right.