Construction Dewatering Means and Methods Presentation

Construction Dewatering Getting Your Project on Firm Ground… …Before It Starts

Introduction • David Giles – Co-Founder/Managing Partner: TerraFirma Earth Technologies, Ltd.

• Overview of Construction Dewatering – Giving guidance to those directly or indirectly involved in the planning, design, supervision, construction and operation of dewatering systems.

Topics of Discussion • Construction Dewatering – What is it? – Why do it?

• The Consequences of Improper Dewatering • Design Considerations • Subsurface Investigations and Soil Borings • Methods of Groundwater Control • Case Studies

Topics of Discussion • Construction Dewatering – What is it? – Why do it?

Dewatering: What is it? Intercepting/minimizing groundwater seepage from entering an excavation.

Depressing the piezometric water surface to a point below the excavation.

Topics of Discussion

Dewatering: Why do it? Short Term Objectives Long Term Objectives (Temporary Systems) (Permanent Systems) • Intercept Seepage • Reduce or eliminate lateral and/or uplift – Increase slope stability pressures – Prevent loss of material • Achieve waterproofing • Reduce lateral/uplift objectives pressure • Improve the excavation and the backfill characteristics of the excavation to allow construction to proceed in a safe and

Topics of Discussion • Construction Dewatering – What is it? – Why do it?

• The Consequences of Improper Dewatering

Improper Dewatering The Consequences • Blows, Rendering Excavation Subgrade Unstable • Boils/Piping, the Creation of Voids Rendering Slopes and Subgrades Unstable • Excavation Support Systems Rendered Unstable • Future Settling • Lost Time • Decreased Worksite Safety

Improper Dewatering The Consequences • Blows, Rendering Excavation Subgrade Unstable • Boils/Piping, the Creation of Voids Rendering Slopes and Subgrades Unstable • Excavation Support Systems Rendered Unstable • Future Settling • Lost Time • Decreased Worksite Safety

Improper Dewatering The Consequences • Blows, Rendering Excavation Subgrade Unstable • Boils/Piping, the Creation of Voids Rendering Slopes and Subgrades Unstable • Excavation Support Systems Rendered Unstable • Future Settling • Lost Time • Decreased Worksite Safety

Improper Dewatering The Consequences • Blows, Rendering Excavation Subgrade Unstable • Boils/Piping, the Creation of Voids Rendering Slopes and Subgrades Unstable • Excavation Support Systems Rendered Unstable • Future Settling • Lost Time • Decreased Worksite Safety

Improper Dewatering The Consequences • Blows, Rendering Excavation Subgrade Unstable • Boils/Piping, the Creation of Voids Rendering Slopes and Subgrades Unstable • Excavation Support Systems Rendered Unstable • Future Settling • Lost Time • Decreased Worksite Safety

Improper Dewatering

Rio Grande River Crossing, Albuquerque, NM

42” Gas Pipeline, Blythe, CA

Improper Dewatering

Proper Dewatering

Topics of Discussion • The Origins of Dewatering • Construction Dewatering – What is it? – Why do it?

• The Consequences of Improper Dewatering • Design Considerations • Subsurface Investigations and Soil Borings • Methods of Groundwater Control

Design Considerations

Design Considerations

Design Considerations

Depth to Water & the Piezometer

Design Considerations

Hydraulic Conductivity (K) “K” can simply be defined as the ease at which water moves through soil. More specifically defined by D’Arcy’s Law

Q=KA(h/L) L

Q = Quantity of Water Flow K = Permeability of Soil A = Cross-sectional Area h = Friction Loss in Distance

Estimating Permeability Visual Classification (USCS)

•

J. Patrick Powers

Design Considerations Dominant Considerations Location and Geologic Environment Size and Depth of Excavation Groundwater (Piezometric) Level Soil & Aquifer Characteristics (Hydraulic Conductivity) • Proposed Excavation Method and Excavation Support • Proposed Schedule • Avoiding Undesirable Side Effects • • • •

– Proximity to Existing Structures, Potable and/or Irrigation Wells

Design Considerations

Design Considerations

Design Considerations

Topics of Discussion • Construction Dewatering – What is it? – Why do it?

• The Consequences of Improper Dewatering • Design Considerations • Subsurface Investigations and Soil Borings • Methods of Groundwater Control • Case Studies

Subsurface Investigations

Typical Bore Log for Pump House, Fulton, Arkansas

Topics of Discussion

Dewatering Methods • Open Pumping • Pre-Drainage – Wellpoints – Deepwells – Eductors

• Methods of Cutoff – Soil Bentonite – Interlocking Steel Sheet Pile – Concrete Caisson

Open Pumping

Favorable Conditions: • Dense, Well Graded Soils, Some Cementation/Cohesiveness • Firm Clays, Few Sand/Silt Lenses Unconnected Hydraulically • Hard Fissured Rock • Low Dewatering Head • Remote Source of Recharge • Open Cut, Relatively Flat Slopes, Minimal Depth Below GW Table • Steel Interlocking Sheet Pile, Concrete Caisson

Dewatering Methods Open Pumping •Pre-Drainage •

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Wellpoints Deepwells Eductors

Methods of Cutoff • •

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Soil Bentonite Interlocking Steel Sheet Pile Concrete Caisson

Vacuum Wellpoints Key System Components: • • • • •

Vacuum Pump Suction Header Wellpoint Swingjoint Discharge Pipe

Accessories: wellpoint, swingjoint, header pipe

Traditional Rotary Lode Vacuum Wellpoint Pump 8”, Diesel Driven

Vacuum Wellpoints

Vacuum Wellpoints Favorable Conditions: Wide Ranging Soil Types Impervious Clay/Rock At or Near Subgrade Highly Stratified Soil Wide Ranging Permeability Remote or Proximate Sources of Recharge Excavations