Vacuum Consolidation Dewatering in Sediment Remediation

Transcription

1 Vacuum Consolidation Dewatering in Sediment Remediation Anatomy of contaminated sediment volume Vacuum Consolidation Dewatering (VCD) Underwater dewatering for volume reduction. VCD application modes and advantages Daekyoo Hwang, Ph.D., P.E

2 Nature of Contaminated Sediment Recent deposits Settled in water Contaminated Sediment (organic silt/clay) Very loose Solids vol << Water vol Remove water Volume reduction Key to sediment remediation (Not appreciated in current practice) 2

3 Anatomy of Sediment Volume Env l engineers - Solids content, 35% of total weight Solids = 35g (35%) Water = 65g (65%) Geotech engineers - Water content,186% of solids weight Solids = 35g (100%) Water = 65g (186%) Geologists - Porosity, 82% (for Gs = 2.5) S = 18%) Water = 82 % (Total vol = 79 ml) Porosity shows how much volume reduction is feasible for sediment remediation. 3

4 Vacuum Consolidation Dewatering Underwater sediment consolidation (dewatering) using horizontal drains and vacuum loading Settled sediment Horizontal drain Vacuum pump - Overconsolidation by vacuum - Used for 25 years in Japan and Europe 4

5 Horizontal Drains Wick Drain Detail Tube Drain Roll Wick Drain Roll 5

6 Horizontal Drain Installation Japanese Way from Chiba, et. al, 1990 To reduce sediment volume Belgian Way from Van Impe, et. al, 2001 To secure shipping channel 6

7 Magnitude of Vacuum Loading Vacuum loading = Atmospheric pressure 14.7 psi (2,117 psf) Field vacuum 70% effective = 2,117 x 0.7 = 1,482 psf Pressure from 1 foot of loose, moist sand = 100 psf Sediment application (submerged) ( ) = 37.6 psf Field vacuum is equivalent to 1,482/37.6 = 39 ft of submerged sand load! Compare with typical 2.5 ft of in-situ capping load. Other vacuum loading advantages No heavy truck traffic for fill loading/removal No extended time for fill loading/removal Inherent 100% overconsolidation 7

8 Magnitude of Settlement by VCD Solids content of organic silt/clay sediment 35% Porosity = 82% 18% solids + 82% water After VCD, water content of soft clay = 40% Porosity = 50% 50% solids + 50% water S = 18 ml Water = 82 ml S = 18 ml W = 18 ml Water = 64 ml removed Post-VCD sediment volume = 36% of the original volume Field performance may be 40 ~ 45% VCD Settlement ~ 50% of sediment thickness 8

9 Application 1 In-Situ Capping Replacing Off-Site Disposal Water depth In-situ capping is acceptable. In-situ capping No loss of water depth In-situ capping Loss of water depth is not acceptable. Dredging + off-site disposal + capping $$$$$ VCD + In-situ capping No dredging, no off-site disposal $ Plus, low disturbance, leachate removal, overconsolidation 9

10 Application 2 On-site Disposal without Land Space Dam Disposal cell Contaminated Sediment Area A, volume V Enhanced cap 0.05 ~ 0.08A Disposal vol 0.35 ~ 0.5V Buffer cover from cell excavation Makes the target sediment disappear (almost) No loss of water depth + small footprint Cost saving from no off-site disposal $$$$ $ 10

11 Application 3 High Cost Capping vs. VCD Large capping area VCD Problem - Performance questions multilayer cap high unit cost high total cost VCD solution Enhanced performance (vacuum OC) Small footprint (5~8% ), low cost Cost saving from small footprint 11

12 Application 4 - Waterfront Development Traditional Method Heavy retaining walls Long sequential activities Large settlement, large footprint Large surcharge fill and removal VCD Cost Saving Light retaining structures Parallel activities Smaller disposal area Smaller surcharge fill/removal Retaining structure Surcharge Sediment consolidation with wick drains and surcharge Sediment consolidation with horizontal drains and vacuum Cost saving from no import of surcharge 12

13 Application 5 On-site Treatment In-situ treatment performance - Heterogeneous, thin, and large area - Questions of reagent delivery/contact - Problems of repeat treatment VCD solves all these problems - Homogenized sediment during dredging - Uniform delivery via horizontal drains - Extraction, injection and recirculation - Fully contained, long-term operation Extraction lines Injection lines Injection pump Extraction pump Cost saving from no off-site disposal by treatment 13

14 VCD Summary Only underwater dewatering and volume reduction method. Can eliminate most off-site disposal remedies by replacing with in-situ/on-site remedies. Enhances the performance of in-situ/on-site remedies. - Overconsolidation and minimal O/M - Leachate removal and low disturbance - Small footprints and minimal truck traffic Reduce the cost of in-situ/on-site remedies. Primary cost saving Secondary cost saving A New Sediment Solution Platform - Creative, versatile, and cost-effective. 14