Scope of Discussion. Foundation Related Issues. Foundation Preparation and Treatment During Dam Construction 10/14/2014

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1 Foundation Preparation and Treatment During Dam Construction John W. France, PE, D.WRE URS Corporation Denver, Colorado October 14, Scope of Discussion Broad discussion of treatment of foundations applicable to: Dam rehabilitation / modification New dam construction Topics addressed: Treatment of exposed rock surfaces Grouting (limited treatment) Treatment of soil foundations (for other than seepage) Foundation seepage reduction Foundation seepage collection and control 2 Foundation Related Issues Support for overlying structures Settlement of overlying structures Foundation seepage Quantity Internal erosion (piping) Uplift pressure Most foundation treatment efforts are aimed at addressing seepage issues 3 1

2 Embankment Foundation Seepage Concerns -1 Foundation filter Filtered exit Unfiltered exit Filtered exit Unfiltered exit Open-work gravel layer Filtered exit Bedrock, open-jointed 4 Embankment Foundation Seepage Concerns -2 5 Embankment Foundation Seepage Concerns

3 Embankment Foundation Seepage Concerns -4 Core Material Cavity pr ogresses to reser voir / high per meabl e zone Coarse downstream shel l Material erodes into downstream em bankment or exits at toe Genera l se epag e gradi ent fro m foundation into embankment Jointed rock foundation 7 Treatment of Exposed Rock Surfaces Foundation shaping Surface treatment Cleaning Dental concrete Slush grouting Shotcrete 8 Shaping of Rock Surface Reduces cracking potential Limits differential settlement Limits stress concentrations Facilitates compaction 9 3

4 Surface Treatment Prevents internal erosion of embankment soil into open joints/discontinuities Prevents internal erosion along the foundation contact Prevents the loss of joint filling materials 10 Rock Treatment Varies With Zones Most critical beneath core and filters Less critical but beneficial beneath shells 11 Foundation Surface Preparation for Earthfill Core and Filter/Drain Zones Fill with dental concrete Fill with slush grout or dental concrete Remove promontories Remove overhang at slope of 0.5:1 or flatter; or fill with dental concrete at a slope of 0.5:1 or flatter Remove loose material and clean Typical Foundation Treatment Details 12 4

5 Shaping and Surface Treatment in Core Trench 1 1 (min) Remove overhangs and steep slopes Remove or shape rock protrusions Fill with concrete Remove loose blocks and backfill with concrete 6 (typical) Fill with concrete 1 Clean depressions and backfill with 2 concrete for depressions greater than 6 Surface Treatment and Shaping in Core Trench Transverse Direction (min) 1 6 (typical) Remove overhangs 2 1 Remove or shape rock protrusions Fill with concrete Surface Treatment and Shaping in Core Trench Longitudinal Direction 13 Details for Foundation Shaping Remove overhangs 0.5H:1V or flatter 6 (min) Greater than 1 foot, excavate OR place dental concrete within core zone (outside core zone higher than 5 feet) 0.5H:1V or flatter 45⁰ (min) 6 (min) 14 Foundation Shaping Remove overhangs Shape to facilitate fill placement Methods Excavator Hoe Rams Jack Hammers Trim Blasting 15 5

6 Foundation Shaping 16 Overhang Formwork 17 Overhang Concrete 18 6

7 10/14/2014 Shaping Facilitates Compaction: Abutment Rolling 19 Shaping Facilitates Compaction : Wheel Rolling 20 Foundation Cleaning- Blade Cleaning Removes majority of loose materials prior to blowing with air/water 21 7

8 Foundation Cleaning- Hand Work Labor Intensive Shovels Jackhammers Picks Crow Bars Rock Hammers Vacuums 22 Foundation Cleaning: Brushing 23 Foundation Cleaning-High Pressure Air / Water Effective in moving and dislodging loose debris form irregular surface 24 8

9 Foundation Cleaning-High Pressure Air / Water 25 Dental Treatment Three types Backfill concrete Dental concrete Slush grout 26 Backfill Concrete for Leveling 27 9

10 Backfill Concrete for Shaping Used to treat steep ledges on abutments 28 Backfill Concrete for Shaping 29 Dental Concrete Used to treat larger discontinuities (joints, fractures, etc.) and smaller irregularities 30 10

11 Dental Concrete 31 Dental Concrete Note Dental concrete placed to just below rock surface to minimize thin edges 32 Dental Concrete 33 11

12 Slush Grout Used to treat Small discontinuities (joints, fractures, etc.) Typically a sanded mortar Individual discontinuities Dig out and clean Place by pouring, pushing, rodding, funneling Do not allow to spread out on rock surface Will need to be chipped off rock surface 34 Slush Grout Multiple closely spaced fractures Broom with stiff bristle broom Place on foundation directly ahead of initial core placement 35 Slush Grout 36 12

13 Slush Grout 37 Slush Grout 38 Shotcrete Used to treat Highly fractured shattered zones of rock Shotcrete Non-fiber reinforced Fiber reinforced Stronger Less likely to crack 39 13

14 Shotcrete Wet-Mix Shotcrete Process Cement, aggregate and water mixed by mechanical methods Air introduced at nozzle Nozzleman has no control over shotcrete mixture Dry-Mix Shotcrete Process Cement, aggregate and air are blown to nozzle where water is added Nozzleman controls amount of water in shotcrete 40 Shotcrete Surface Preparation Place on cleaned approved foundation free of water Surface dampened before shotcrete application Install ground wires or pins on surface to guide placement thickness Placement Discharge from nozzle between 0.5 and 1.5 m from surface Buildup to specified thickness using several passes Cure similar to backfill concrete 41 Slaking Protection: Hard 42 14

15 Slaking Protection: High Rough Excavation 43 Question Session No Grouting Reduce hydraulic conductivity of foundation and thereby seepage under the dam by intersecting and filling factures and voids within the rock mass. By its nature, the extent of the work required for grouting cannot be known until excavation is complete and grouting program is implemented 45 15

16 Grouting Grouting is a specialty construction process and must be directed by people with experienced personnel 46 Grouting Cautions Limited soils for which grouting is effective Soils (and soft rock) can be hydrofractured In rock, grout only penetrates water-and airfilled features Grout may deteriorate over time; or at least effectiveness of a grout curtain may deteriorate over time 47 Types of Grouting Blanket Grouting Low pressure shallow holes located in a pattern across the foundation surface Curtain Grouting Low pressure shallow hole grouting and intermediate to high pressure grouting of deeper zones in rows of holes Stitch Grouting Grouting of fault or shear zones in a pattern of holes designed to intersect and cross the zone at various selected depths 48 16

17 Example Grouting Layout 49 Grouting Program Elements Spacing Depth Number of lines (grout curtain) Sequence Pressure Grout mixes Monitoring and control 50 Foundation Grouting Holes oriented to best intersect defects depends on geology Hole pattern; primary, secondary, tertiary Pressures limited to preclude hydraulic fracture Grouting of bore holes in stages Upstage or ascending stage Downstage or descending stage 51 17

18 Foundation Grouting Grout mixes A range of mixes Start thin and thicken as needed Balanced, stable mixes essentially no bleed Reference points for uplift check Verification water test holes after grouting Procedures highly dependent on nature of foundation Grouting as final exploration 52 Split Spacing Method Primary, secondary, tertiary, etc. to closure P S T T P P S T T P P S T T P First Second Third 53 Stage Grouting Ascending Stage Grouting Grout hole drilled complete to bottom Grout starting with bottom stage and work up to top of hole Descending Stage Grouting Grout hole drilled stopped due to partial or total loss of circulation, artesian flow, caving conditions, or grout flow to adjacent holes Grout stage, washout and continue to drill 54 18

19 Groutability Readily groutable k > 1 x 10-3 cm/sec (100 lugeons) Marginally groutable k ~ 1 x 10-4 cm/sec (10 lugeons) Barely groutable k < 1 x 10-5 cm/sec (1 lugeon) Effective grouting likely does not reduce rock mass permeability to less than about k < 1 x 10-5 cm/sec (1 lugeon) 55 Grouting Drill Rigs 56 Access Challenges 57 19

20 Monitoring / Recording Computerized Real Time Monitoring and Assessment System Records, displays, stores and analyzes Water pressure testing data Grouting data Measures Flow Pressure Lugeon value Cumulative grout volume A number of other parameters 58 More Information on Grouting ASDSO Webinar October 9, 2012 Introduction to Grouting for Dams 59 Question Session No

21 Treatment of Soil Foundations Reasons for soil foundation treatment Control settlement Provide stability (static or seismic) Control seepage this will be dealt with separately later in the presentation 61 Settlement Soils of concern: Generally compressible clays and silts For very high dams, loose sands or even soft rock could possibly be a concern, but unusual Treatment options for compressible soils Remove and replace Wick drains and / or staged construction 62 Excavate and Replace Dewatering often required 63 21

22 Wick Drains 64 Wick Drains Installed on patterns to promote drainage Used to accelerate consolidation and strength gain in staged embankment construction Used to enhance benefits of improvement methods (dynamic compaction, vibroflotation, vibrocompaction piles, compaction grouting) in finer-grained soils Stability Static stability not typically a concern except for soft, plastic clays or silts Seismic stability can be a concern for a wider range of soils particularly for loose, cohesionless soils (liquefaction) Treatment options to improve stability Remove and replace Increase strength with in-situ treatment Reinforce 66 22

23 In-Situ Treatment to Increase Strength Dynamic compaction Vibroflotation Vibro-replacement (stone columns) Compaction grouting For all methods verification testing would be required, and often test sections are needed. 67 Dynamic Compaction Dropping weights to create vibrations that densify the soil Weights are dropped on a controlled pattern Effectiveness verified indirectly through before and after investigations Dynamic Compaction Reference: Densification, Inc

24 Dynamic Compaction - Limitations Effectiveness diminishes with depth practical limit is about 30 feet Most effective in clean, cohesionlesssoil; less effective as fines content increases Not effective in plastic soils Vibroflotaion Densifyingsoil in place with a vibratory probe with water injection Probes completed on a controlled, relatively closely spaced pattern Effectiveness verified indirectly through before and after investigations Vibroflotation Reference: Hayward Baker

25 Vibroflotation Vibroflotation- Limitations Most effective in clean, cohesionlesssoil; less effective as fines content increases Not effective in plastic soils Vibro-Compaction Piles (Stone Columns) Similar to vibroflotation, except that gravel fill is placed in the probe hole Can be done with water (wet method) or without water (dry method) Effectiveness in finer-grained soils can be enhanced with wick drains Effectiveness verified indirectly with before and after investigations

26 Vibro-Compaction Piles Reference: Hayward Baker Vibro-Compaction Piles - Limitations Most effective in clean, cohesionlesssoil; less effective as fines content increases Not effective in plastic soils Mixing of gravel with in situ soil, rather than densification has been observed in some soft or very loose finer-grained soils Compaction Grouting Injection of low mobility (low slump) grout to densify soil Completed in holes drilled on a pattern Effectiveness verified indirectly through before and after investigations

27 Compaction Grouting Drill Rod Compaction Grout Casing Casing Casing Compaction Grouting - Limitations Does not include vibration, so effectiveness in cohesionlesssoils may be limited May have more effectiveness in finer grained soils, but very low permeabilities may limit effectiveness Reinforcement Shear walls Replaced or modified blocks Structural elements (e.g. piles, anchors, MSER) Composite strengths Can be combined with berms and buttresses

28 Shear Walls Shear Wall Construction Methods Cement deep soil mixing (CDSM) Jet grouting Cement Deep Soil Mixing (CDSM*) Mixing in situ soils with cement grout or other slurries Using multiple shaft mixing tools with cutting heads and mixing paddles, or Using wheels on horizontal axis or trenching techniques Equipment illustrated later in seepage barrier wall discussion * aka DSM, DMM

29 Jet Grouting Can limit treatment to layers at depth. 85 Jet Grout Blocks WickiupDam, OR In Situ Reinforcement - Piles Reinforcing Piles Thin Weak Layer Piles essentially act as shear pins Used at Sardis Dam, MI for seismic rehabilitation

30 Composite Strength Plan View Reinforcing Elements In Situ Soils Composite Strength If: Strength of in situ soil is c i + σ tan φ i Strength of reinforcement is c r + σ tan φ r Replacement ratio = R r = A r / A tot A r = area of reinforcement elements A tot = total area Then: Composite strength: c i + (c r -c i ) R r + tan φ i + (tan φ r -tan φ i ) R r Based strictly on an area ratio Strain incompatibility could be an issue Question Session No

31 Foundation Seepage Cut Off Methods Grouting discussed earlier Low Permeability Blankets Barrier Walls Note: Cut off is in quotes, because it is very difficult to truly cut off seepage. 91 Low Permeability Blankets Upstream blanket Soil Blankets Geomembranes Blankets may need to be very long 92 Limited Effectiveness of Upstream Blankets Reference: Cedergren(1989) 93 31

32 Seepage Barrier Walls: Foundation and Embankment Embankment Pervious Foundation Impervious Foundation 94 Seepage Barrier Walls: Foundation and Partial Embankment Embankment Pervious Foundation Impervious Foundation Wall needs to connect to water barrier in the embankment 95 Seepage Barrier Walls: Foundation Only Embankment Pervious Foundation Impervious Foundation Wall needs to connect to water barrier in the embankment 96 32

33 Seepage Barrier Walls: Partial Cutoff Embankment Pervious Foundation 97 Limited Effectiveness of Partially- Penetrating Cutoffs Reference: Cedergren(1989) 98 Seepage Barrier Walls Continuous Trench Walls Soil Mix Walls Element Walls (Panel) Jet Grouting Walls (not discussed further) Sheet Pile Walls 99 33

34 Continuous Trench Barrier Walls 100 Continuous Trench Barrier Walls Salient Features Slurry Supported Excavation No Backfill Restrictions No Backfill Joints Non-Structural Low Permeability Typical Backfills Soil-Bentonite (SB) Cement-Bentonite(CB) Soil-Cement-Bentonite (SCB) 101 Excavators Long Boom / Long Stick Excavator (to 80 to 90 ft) Clamshell / Grab Trencher (with slurry)

35 Backfill Equipment Bulldozer and Excavator Mixing Box CB Mix Plant No end stops No tremie (except for great depths) 103 Continuous Wall Benefits Low cost Rapid Construction Low permeability Applicable to most soil types No joints 104 Cement Deep Soil Mixing (CDSM*) Mixing in situ soils with cement grout or other slurries Using multiple shaft mixing tools with cutting heads and mixing paddles, or Using wheels on horizontal axis or trenching techniques To depths exceeding 100 ft * aka DSM, DMM

36 RSW (triple auger) 106 Cutter Soil Mixing (CSM) Cutter Soil Mixing (CSM) Herbert Hoover Dike

37 TRD Method (Trench Cutting RemixingDeep Wall Method) Soil-Cement- Bentonite 109 TRD Wall Construction -Herbert Hoover Dike TRD Grout Batch Plant 110 Top of Exposed TRD Wall -Herbert Hoover Dike Top of Wall

38 Element Barrier Walls Constructed of individual elements, connected together to provide continuous wall Different types of equipment Constructed in primary / secondary sequence Practical to large depths, 250 feet plus directional control becomes very important at depth 112 Primary Elements 113 Secondary Elements

39 Element Wall Configurations Primary Elements Installed First Secondary Elements Fill Between P S P P S P S P P S P S P Grab or Clam Shell 116 Hydromill(Hydrofraise)

40 Hydromill(Hydrofraise) 118 Sheetpiles Conventional sheetpiles can be used for temporary or permanent seepage barriers Sheetpiles can be either steel or plastic (vinyl) Issues include Corrosion of steel sheetpile Buckling of steel or plastic sheetpile Lack of advancement Interlock separation Interlock leakage

41 Foundation Seepage Collection and Control Methods Foundation Drains Relief Wells Approaches shown are to address only foundation seepage, not embankments; often combined with embankment treatments 121 Shallow Toe Drain For limited shallow foundation seepage Likely requires dewatering for rehabilitation May require reservoir lowering for rehabilitation 122 Deep Toe Drain For more extensive, deep foundation seepage Almost certainly requires dewatering for rehabilitation Likely requires reservoir lowering for rehabilitation

42 Toe Drain and Berm Berm provides resistance for pressure and improved stability 124 Trench Toe Drain Trench and Berm Drain trench With slurry methods, trench drain can be constructed without dewatering Potential construction issues 125 Relief Wells Low permeability layer Confined aquifer with seepage pressure xx x xx x x

43 Relief Wells Relieve pore pressures and lower piezometric surface within confined pervious foundation strata Reduce uplift and improve stability Control exit gradients and reduce piping potential Maintenance required Drain trench or toe drain may be better alternative 127 Question Session No Post Event Evaluation & Quiz Please click the following link to take the Seminar Evaluation and Quiz: t.php?id=14074 You must complete the Seminar Evaluation and Quiz to receive PDH credit hours