Challenges of Building Your Filter or Drain

Federal Energy Regulatory CommissionChallenges of Building YourFilter or DrainJim McHenry, P.E.

Federal Energy Regulatory CommissionDesign is Done…Construction to Begin…• Pitfalls to avoid, problems to consider• As owner, have your engineer engaged thruconstruction• A properly designed filter or drain could beworthless unless properly built…

Federal Energy Regulatory CommissionUnder seepage….• Original Dam Foundation – 40 feet alluvial & windblown sands, no cutoff• Post-Construction 1948 - Piezometric levels 10 – 15 feet above ground• Add second collector drain and 239 relief wells• Seepage monitoring – 56 piezometers• Collector Drain Flow (1500 gpm at conservation pool – 23 ft below flood pool)• Cost for cleaning/ evaluation of wells & piezometers $250K/5 yrs

Federal Energy Regulatory CommissionEven with wells…wet groundPhreatic surface at or near ground level – conservation pool

Federal Energy Regulatory CommissionNew Seepage Berm and Toe DrainBermDamToe Drain15 ft deep, 11,000 lf

Federal Energy Regulatory CommissionBerm Construction – Surface WaterHigh water pressuresoutcrop at ground surface

Federal Energy Regulatory CommissionToe Drain – 2 Stage Sand and Gravel

Federal Energy Regulatory CommissionTwo-Stage Filter & Drain Installation• Problem – Natural soils, requires two materials for filter & drainage• One Solution - Doghouse

Federal Energy Regulatory CommissionTwo-Stage Filter Drain Installation• Steel or wood construction – multiple pieces needed

Federal Energy Regulatory CommissionTwo-Stage Filter Drain InstallationRoof closed – install outside filter

Federal Energy Regulatory CommissionTwo-Stage Filter Drain InstallationRoof open for inner drain material

Federal Energy Regulatory CommissionFinger Drain tie-ins – old to new

Federal Energy Regulatory CommissionConstruction Submittals – Toe Drain Pipe• Design vs. ASTM vs. What’s available …• Design – largest allowable slot for coarse aggregate 0.25 in.• ASTM F949 pipe 24-36 inch - slot size 0.125 in. – design OK• Manufacturers – for larger pipe – 0.375 in holes• Problem – find manufacturer willing to do special order• Solution – 11,000 lf 24-36 in pipe gang drill 0.25 slotsLessons:• Manufacturer’s may not produce according to ASTM standard• Not all will do special order• Check prior to bidding

Federal Energy Regulatory CommissionDrain System MaterialsThoughts:• Several material and manufacturer choices for pipe, manholes.• Depending on your project, all or some may work• Key factor – how they work together to maintain flow• Dissimilar materials or manufacture may not work well together• Coordination with contractor, supplier, engineer/owner

Federal Energy Regulatory CommissionSurvey Control & Camera Inspection• Longer the drain, flatter the slope – survey control more critical!• Review contractor survey plans for constructionExpertise, Qualifications• Your Construction QA - insure grade & elevations start right• Once it’s in, it needs to work• Therefore, if critical drain, don’t approve system as final withoutcamera inspection

Federal Energy Regulatory CommissionFilters/Drains - Shoring• Deep drains & filters require shoring design• Thorough review/approval important• Clear specification requirement forenforcement• Unusual approaches and outcomes caninfluence rest of work• Two contractors, two approaches

Federal Energy Regulatory CommissionShoring Style #1- Driven SSPResult of too small hammer• Engineer and Contractor should anticipate driving conditions

Federal Energy Regulatory CommissionShoring Style #1- Driven SSP• What will shoring system allow? How safe at bottom of hole?• Alternative compaction method proposed for first lifts

Federal Energy Regulatory CommissionShoring Style #1- Compaction Test Section• Alternative compaction method tested vs. spec compaction

Federal Energy Regulatory CommissionShoring Style #1- Vibratory Plate on Excavator• Acceptable compaction andcoverage for bottom liftsLessons:• Excavation and Shoring Plansubmitted/reviewed per spec• Issue of bottom of the holepersonnel safety not vetted inplan – construction needs forcompaction not fully considered• Lesson: tighter spec, tighterreview• Ask the right questions

Federal Energy Regulatory CommissionShoring Style #2 - Box and Plates• Cut Bench, excavate, install trench box, drive steel plates outside box

Federal Energy Regulatory CommissionShoring Style #2 - Box and Plates• Raise Trench Box as fill placement, compaction progresses

Federal Energy Regulatory CommissionShoring Style #2 - Box and Plates• At some point toe drain filter material and pipe is in, and bench is shallow, trenchbox removed

Federal Energy Regulatory CommissionShoring Style #2 - Box and Plates• It may not be beautiful…

Federal Energy Regulatory CommissionShoring Style #2 - Box and Plates• But can still get the job built…

Federal Energy Regulatory CommissionShoring Style #2 - Box and Plates

Federal Energy Regulatory CommissionWhy you may need dewatering control 24 hrClassic heave offoundation sand intoexcavation

Federal Energy Regulatory CommissionProtection of WorkTrench flooded – sediment flushed inRemoval of damage in progressNote south end open to runoff

Federal Energy Regulatory CommissionProtection of Work – Clarify Responsibility in Specs

Federal Energy Regulatory CommissionFilter/Drain Specifications - Caveat• Guide spec:“Toe drain filter shall be washed sand, sand and gravel,crushed stone, crushed stone screenings, or slag composedof hard, tough, durable particles free from adherentcoatings.”• First 75%+ filter delivered good river sand• What happens when source runs low, or can’t keep up on alarge job?• Contractor finds new source – less $ - crushed limestone

Federal Energy Regulatory CommissionFilter/Drain Specifications - Caveat• Problem:• Crushed Limestone Screenings – Carbonates• Screenings with groundwater – cementation!• Effectiveness of drain compromised• Can’t accept these materials as drain or filter• Relied on “adherent coatings” to disallow• Lesson –Do specification review with regard to mineralogy of yourmaterial sourcesWrite out what you don’t want

Federal Energy Regulatory CommissionIt can all come together…

Federal Energy Regulatory CommissionOlder Dams & Filters – Conditions Change…Filter Sand placed aroundconduit, from core to outlet1960’s designFilter Sand material contactsbedding and waste rock

Federal Energy Regulatory CommissionConditions similar for 40+years…just seepageRt. Wall of OutletConduit –Waste RockOutlet of Spillway Conduit

Federal Energy Regulatory Commission40+years later – sand movement…Right spillway wallJoint in right wall

Federal Energy Regulatory CommissionNew staged filter designed....multi-layersOriginal sand filter around conduitNew filter materials

Federal Energy Regulatory CommissionConstruction Access – confined working space..Coarse Agg BSand A

Federal Energy Regulatory CommissionConstruction Access and Protection….Scraping around walls….Working around conduits….

Federal Energy Regulatory CommissionConstruction Access and Protection….Low flow operates 24 HR• Working around existing facilities• Low flow requirements must be met at all times

Federal Energy Regulatory CommissionConstruction Protection….Good progress until…SandCoarseBedding

Federal Energy Regulatory CommissionConstruction Protection….First rain…Bedding lostdownstreamContaminatedmaterials

Federal Energy Regulatory CommissionConstruction Protection….or start overRunoff down access roadinto excavation

Federal Energy Regulatory CommissionDewatering system responsibilities…• Design• Monitoring• Review and Approval of Dewatering PlansParty responsible for decision is authority on decision?• Know your contractor’s strengths or weakness• Methods for dewatering problems during construction

Federal Energy Regulatory CommissionDewatering system redesignsMore wells needed – number & location? Single or joint responsibility?

Federal Energy Regulatory CommissionBe it large or small…it’s your dams’ safety belt