Dewatering, Shoring & Excavation Support

Dewatering Method Selection

Method	Best Soils	Max Drawdown	Cost
Sump pumping	Clay, silt	Exc. depth	$
Wellpoints (single)	Sand, gravel	4.5–6 m	$$
Wellpoints (2-stage)	Sand, gravel	10–12 m	$$$
Deep wells	All types	20+ m	$$$
Eductors	Silt, fine sand	25–30 m	$$$$

Wellpoint Spacing

Soil	Spacing
Coarse sand/gravel	0.5–1.0 m
Medium sand	1.0–1.5 m
Fine sand	1.5–2.0 m
Silty sand (marginal)	1.0–1.5 m

Shoring System Selection

System	Max Depth	Water Cutoff	Vibration
Soldier pile & lagging	15+ m	Poor	Low–mod
Sheet piling	12–18 m	Good–exc.	Mod–high
Secant pile wall	25+ m	Excellent	Very low
Tangent pile wall	20+ m	Poor–fair	Very low
Soil nail wall	12–15 m	None	Very low
Braced (struts)	20+ m	Varies	Low

Soldier Pile Specs

• H-piles: W200×46 to W310×97 at 1.8–3.0 m c/c
• Lagging: 75–100 mm timber or 38 mm plywood
• Excavate in 1.0–1.5 m lifts; lag before next lift

Vibration Limits (PPV mm/s)

Structure	Limit
Heritage / historic	5–12
Residential (good cond.)	12–25
Commercial / industrial	25–50
Reinforced concrete	50
Buried utilities (concrete)	50
Buried utilities (plastic)	100

Pre-construction survey required within 30 m of pile driving. Seismograph must be calibrated within 12 months.

Regulatory Thresholds

Water Taking (OWRA)

Volume	Permit Required	Processing
≤50,000 L/day	None	—
50K–400K L/day	EASR (low-risk)	Immediate
>400K L/day or high-risk	Full PTTW	90–120 days

Tieback Testing

• Proof test (100%): 133% design load, 10 min hold
• Performance test (5%): Incremental to 133%
• Creep test: <2 mm/log cycle time at 133%
• Load cell alert: >20% drop from lock-off = investigate

Monitoring Triggers

• Amber: 70–80% of max allowable → increase frequency
• Red: At/exceeding limit → stop work, evacuate, call engineer
• Inclinometer alert: 25 mm cumulative or 10 mm between readings

Code & Permit References

• O. Reg. 213/91 s. 222–242: Excavation safety, shoring requirements
• O. Reg. 213/91 s. 228/234: P.Eng. design >6 m or near structures
• OWRA: Water taking permits (PTTW/EASR)
• Federal Fisheries Act s. 36(3): Deleterious substance prohibition
• CSA A23.1: Concrete in the dry; bearing surface protection
• OHSA s. 43: Worker right to refuse unsafe work

Fines: $100K/day individual, $1M/day corporation (OWRA). Up to $1M + 3 yrs (Fisheries Act).

Discharge & Sediment Control

• Dewatering bags: Best single measure; TSS 500+ → <25 mg/L
• PAM flocculant: Anionic only; 1–5 mg/L dose; TSS → <10 mg/L
• Storm sewer discharge: Municipal permit + <25 mg/L TSS
• Turbidity limits: 25 NTU (sensitive fish) / 75 NTU (general)
• Visual clarity check: 2×/shift minimum; record in site diary