geotechnical evaluation of sediments in support of
TRANSCRIPT
Geotechnical Evaluation of Sediments in Support of Remedial DesignElle Anzinger, Michael Paquette, PE, Michael
Franceschina, PE, and Tim Sattler (Langan, Doylestown, PA, USA)
The Site – Shallow Tidal Basins/Creek
The Site – Metrics
Reach Area(acres)
AverageDepth (ft)
Tidal Range (ft)
ResidenceTime (days)
NorthernTidal Basin 30.8 2.4 0.2 37
Central Tidal Basin 2.8 2.0 0.3 3
Southern Tidal Basin 3.9 2.5 0.7 ~ 2 ‐ 5
Challenges• Access Issues! Shallow water (~ 2 ft in basins), substantial
mudflats, inconsistent bathymetry; tidal flows greater than net flow; wind driven currents, no linear flows (except near culverts).
• Sampling! Very soft, flocculent sediment, layers of higher density solids; high suspended solids & visually impacted (VI) material (normal split‐spoons and Shelby tubes won’t work!).
• Modeling! Super soft material not following normal properties within model.
• Other! Remedial Design considerations.
Geotechnical Investigation Approach • Previous investigations had challenges with
traditional soil borings with SPT testing and undisturbed sampling.
• SPT testing indicated “weight of rod” and “weight of hammer” material.• Difficult to develop geotechnical design parameters this
low on the strength “spectrum” using state‐of‐practice methods for soil and geo‐materials.
• Undisturbed sampling was difficult for the very soft sediments near the surface.• Material was too soft and moisture content too high to
reliably collect undisturbed samples.• Material was highly sensitive to disturbance after
extraction (during transport to lab).
Geotechnical Investigation Approach • Cone Penetration Testing
• Integrated electronic piezocone penetrometer pushed through the subsurface
• Measures • Tip resistance • Sleeve friction • Pore pressure• Seismic response (via accelerometer and geophone)
• Traditional piezocones not sensitive enough to precisely measure properties of very soft sediment• Properties of very soft sediment and relatively stronger underlying
soils desired • Both traditional and high sensitivity cones implemented
Geotechnical Investigation Approach • Challenging access and maneuvering
• Limited access & launch locations• Variable & constantly changing water depth • Creek investigation areas separated by
physical barriers/land
• Marsh Buggy• Amphibious rig capable of operating over
land & shallow water • Very low track bearing pressure to traverse soft
surfaces• Capable of float to traverse deeper pockets of
water • Flexible access/no need for “boat launch”• Standard 20‐ton CPT ram
Geotechnical Investigation• 15 CPTs in three tidal basins
• Target depth of 35 feet • Shallow refusal at some locations
Tip Resistance, qt (tsf) Sleeve Friction, fs (tsf) Tip Resistance, qt (tsf) Sleeve Friction, fs (tsf)
Geotechnical Analysis• Evaluation of the “free‐field” conditions representing dredging
in the expansive marsh area;• Evaluation of dredging along the natural shoreline;• Evaluation of dredging against critical areas including the
railroad embankment, roadway embankment, and the landfills.
• Dredging scenarios were analyzed using PLAXIS 2D, a commercially‐available two‐dimensional finite element method (FEM) program for soil and rock analysis.
Geotechnical Analysis• Generally three subsurface conditions in the tidal basins were
noted: a layer of very soft organic solids with high water content at the mud line, underlain by very soft to soft clay and silt with organics, further underlain by medium to dense sand.
Stratum Number Stratum Name Sediment
Internal Friction Angle (f)
Cohesion (psf)
Unit Weight (pcf)
1 Very Soft Organic Solids Clay Zero 20 80
2 Clay and Silt Clay Zero 300 87
3 Sand Sand 37 Zero 130psf = pounds per square footpcf = pounds per cubic foot
Geotechnical AnalysisFree‐Field Dredging• Modeling of the free‐field condition considered dredging of the
suspended solids and creek sediment in the expansive marsh. The main conclusions from the FEM model are:• The suspended solids zone will behave similar to a liquid.• Water provides a stabilizing force and we estimate that the stiffer clay and
silt will be temporarily stable at inclinations of up to approximately 1:1 (horizontal to vertical)
• The clay and silt below the suspended solids zone will likely not be stable when dredged in the dry (no water pressure stabilizing force) and will likely result in stability failures wider than the 1:1 zone predicted in the wet condition.
Geotechnical AnalysisNatural Shoreline Dredging
• Modeling of dredging along the natural shoreline considered vertical dredging cuts made immediately adjacent to the shoreline.
• The natural shorelines have relatively shallow slopes (around 10H:1V), so global slope stability issues are not anticipated.
• The failure mode at the shoreline is likely to be a sloughing near the dredging face. We estimate that vertical dredging cuts of 1 to 2 feet will result in settlement and lateral displacement extending about two to three times the depth of cut (i.e. a 3H:1V zone of sloughing) into the upland area.
Geotechnical AnalysisCritical Area Dredging
• Two embankments cross over the tidal basins: • 1) Railroad with two twin 36” diameter pipe culverts• 2) Road way with a single 48” pipe culvert and tide gate.
• These areas were modeled to evaluate vertical cuts because the orientation of the embankments perpendicular to the deep center channel’s dredging would not allow for sloped dredge cuts. The analysis included a surcharge from a locomotive. • Confirmed remedial method of hydraulic dredging.
Analytical & Modeling ChallengesModel Cases ‐ FEM• Two general cases were evaluated based on site conditions. • Additionally, wet and dry models were run to assess slope stability in cases
under low tide or dry areas during summer months.
Analytical & Modeling ChallengesMaterial Parameters used in Modeling• Material Parameters were estimated from available subsurface exploration
data (Borings, CPT, site visits). Mohr Coulomb material model was used in modeling based on correlations from CPT data.
• Elastic modulus is most crucial factor of determining material deformation under stress.
MaterialUnit
Weight (pcf)
Elastic Modulus (ksf)
Cohesion (psf)
Friction Angle (deg)
Poisson Ratio
Floc 79 3.55 20 0 0.25
Clay 87 6.27 300 0 0.25
Sand 130 187.97 0 37 0.25
Analytical & Modeling ChallengesMaterial Parameters ‐ Challenges1. Low Modulus of Elasticity and Unit Weight of stepped excavation adjacent to Floc material led to immediate failure
of Models in PLAXIS and FLAC (Geotechnical Modeling Softwares).2. Floc is understood to behave more like a fluid than a soil. Any changes in water pressure or adjacent slope
geometry destabilize model and cause large displacements.
Floc layer is removed from FEM analysis, and model is revised to determine slope stability during dredging operations of up to 8 feet of dredging.
Model fails immediately after 2 ft cut in wet model(FLAC 8.0)
Analytical & Modeling ChallengesModeling Output• Case 2 (Natural Shoreline Dredging)
• Based on dredging logistics along the shoreline, deep dredge cuts are not expected adjacent to a shoreline sloping 10H:1V. In an unlikely dredge depth of 4 ft adjacent to an embankment, lateral displacements are on the order of less than 2 inches (in clay). Therefore there is minimal disturbance and Case 1 governs.
• Case 1 (Free Field Dredging)• Dredge machinery will not allow for vertical cuts. Analysis will determine the
maximum slope to reach 8 ft dredge depth under 2 ft lifts. • Water pressure provides stability to the stiff clay slope. Dry conditions were evaluated
in a 2 ft lift, as it is unlikely deep dredge areas will be dry.• A slope of 1H:1V, with a horizontal 2 ft offset is recommended based on analysis in
wet and dry conditions.
Analytical & Modeling ChallengesModeling Output• Under 1H:1V with 2 ft offset, maximum lateral displacements are about 1 inch
for an 8 ft total dredge. • Dredging will need to be performed slowly with minimal disturbance to soft
clay layer.
Lateral Displacement Contours
• Sediment softness (complete lack of strength) and slope stability for hydraulic dredging and shoreline treatment.
• Limiting factors: wetland and riparian zone impacts.• Conservative protective assumptions for remedial design
(horizontal and vertical cut ratios).• Ultimately, free‐field (open water) cuts had to be accounted
for by an increased cubic yardage of material to be removed.
What does it mean for Remedial Design?
Original Total 51,103
Less Shoreline Benching 1,261
Additional Open Water Edges 4,258
6" Overdredge 11,736
Grand Total (Cys): 65,836
Remedial Design
