Foundation Piling as a Potential Conduit r DNAPL Migration

James W. Waymn " G&E Engineering, Inc.

Tiusville, Florida

Richard B. Adams G&E Eriiineering, Inc.

Baton Rouge, Louisiana

Robert G. Adams Occidental Chemical Corporation

Niagara Falls, New York

For Presentation at the 86th Annual Meeting & Exhibition

Denver, Colorado June 13 - 18,1993

INTRODUCTION Along the heady industridzed gulf coast, where the subsurface is typically a "sandwichn of

alternating water-bearing and confining strata, plant expansion or new construction is frequently planned for areas beneath which the shailow groundwater has been historically contaminated by petroleum products, organic solvents, and other non-aqueous phase liquids (NAPLs). The foundation scheme for industrial ptant construction often c~nsists of piles that, in order to obtain adequate load bearing capacity, are driven through shallow water-bearing and confining strata to deeper water-bearing strata. Particularly for dense NAP& (DNAPLs), which are heavier than water, concern has been raised that a driven pile which penetrates a shallow contaminated water-bearing stratum and its underlying confining stratum may provide a conduit for vertical migration of the DNAPL to deeper water-bearing strata.

As research into this concern was initiated in the fall of 1989, a thorough search of environmental and geotechnical literature data bases revealed no previous investigations of the potential for contaminant migration specifically related to piIe foundations. With no pre-existing data or information upon which to develop further field research or case studies, a bench model was devised to simulate the driving of piies though a contaminated zone into an underlying uncontaminated zone and observe the effects. The model and tests were specifically designed to observe three potential avenues of contaminant transport:

@ Contamination carried down with the pile as it is driven;

Contaminant migration along the pile-to-soil contact; and

e Contaminant migration through the pile (i.e., wicking), specifically through wood piles.

TYPICAL PILE FOUNDATION AND SUBSURFACE SCENARIO While the stratigraphy of the guIf coast, particularly the Mississippi deltaic deposits, is very complex

on a micro scale, there is a common generalization that more permeable, water-bearing strata alternate with less permeable, confining (or aquiclude) strata. A typical stratigraphic scenario, as shown on Figure 1, includes from the surface down:

A 20-foot thick overburden of interbedded silt, silty sand, and silty clay (with a groundwater level at two feet);

* An upper 10-foot thick water-bearing silty sand stratum;

* A 20-foot thick clay confuting unit;

0 A lower 10-foot thick water-bearing silty sand stratum; and

An underlying silt or clay confining unit.

In this scenario, driven piles obtain their desired load bearing capacity upon reaching the lower water-bearing stratum.

BENCH TEST OBJECTWES The objective of the bench test was to simulate the effects of pile driving in typical gulf coast

stratigraphy (Figure 1) which includes a contaminated upper permeable stratum and an uncontaminated lower water-bearing stratum separated by an impervious confining stratum. A steel rod of circular cross section was driven through the modeled stratigraphy, and the lower water-bearing stratum was analytically monitored over time to observe the effects of initial contaminant dragdown and subsequent contaminant migration along the pilelconfining-layer contact. With these two contaminant transport avenues investigated, a second bench model was constructed, and the experiment was repeated using an untreated wood dowel as the test pile to investigate the potcntiai~for contaminant migration internally through the pile.

THE BENCH MODEL

Difficulties of Time and Scale As with all bench tests, the scaling factors between the modeled and the real, in-situ conditions must

be considered. Scaling of hydrogeologic systems is extremely difficult. SoiI materials smaller than fine sand (which indude very fine sand, silt, and clay) simpIy cannot be scaled down since they already represent the smallest possible particle size exhibiting their representative physical properties; clay, in particular, consists of micro-size particles and exhibits unique physical/chemical properties. Although there is some limited latitude in s&g fluid properties in general, in this situation with the soil particle size unscalable, the physical-chemical properties of naturally mineralized water, DNAPL, and water with dissolved DNAPL compounds are, Likewise, unscalable. The inability to scale the soil particle size also inhibits the ability to scale the thickness and depth of the strata, which obviously cannot be reproduced in true dimension in the laboratory. While not scaled, the overburden pressures and resulting consolidation (compaction) of the natuad strata w be approximately reproduced in the bench model.

Although the physical/chemical properties of the soil and fluids canr~ot be scaled in the model, the natural degree of consolidation of the soil can be reproduced in the model. Therefore, the natural permeability of the soil materials relative to the fluids, and hence, the fluid migration rate, are also reproduced in the model. Considering that the natural strata thicknesses must be radically reduced in the model while the fluid migration rate is not scaled down, the time required for fluid migration within the model is proportionatePy less than under natural conditions.

Simuiatioo Sincc a truly scaled model was not feasible, the experiment attempted to simulate a set of conditions

which wouId explore qudtatively the three aforementioned probable transportlmigxation avenues associated with a driven pife. The simulated conditions were selected to be extremes that would definitively reveal the presence or absence of contaminant movement from an upper to lower permeable layer associated with a pile-like penetration of an intervening impermeable layer. Hence, the mode1 simply investigates the migration potential without a definite quantitative relationship to any particular in-situ situation.

Model Design and Construction The bench model experiment was conducted in two sequential phases. The first phase explored the

contaminant transport potential of a steel pile penetration; the second repeated the experiment with a wood pile penetration. To eliminate any cross interference, the model was completely reconstructed between the two phases. As with most prototype construction, various improvements were made in the second model.

Soil Profile, For the purpose of the simulation model, the upper and lower permeable strata were constructed of clean quartz sand (UniF~ed Soil Classification Group SP material) so that they would be highly permeable and allow rapid and complete pore volume replacement. A relatively coarse ASTM 20/30 size gradation was -ucd with ti fine sand (ASTM 40/60 gadation) buffer b contact with the codning stratum.

A commercialJy,availabie potter's clay was used for the confining stratum. This material is a lean day of moderate plasticity (Liquid Limit = 42 and Plasticity Index = 24) and is easily molded at a moisture content a t 25 to 30 percent by dry weight. I'

The, sequence of strata within the model was constructed as follows:

Thickness (inches) Material Stratum

20/38 Sand Upper Fluid Bearing 4/60 Sand H * It

Clay Conhing Zone 40/60 Sand Lower Fluid Bearing 20/30 Sand " " "

Overburden Pressure and Consolidation. The soil profile in the model was subjected to a laterally- confined ver t id compressive force of 2,000 pounds per square foot (psf) which is equivalent to an overburden pressure of 40 to 45 feet of normally-consolidated clay, silt, and sand with a water table at a depth of two feet. This simulates the conditions roughly at the mid-point of the confirning stratum in a typical gutf coast stratigraphy (see Figure 1). Upon applying the load, the vertical consolidation was monitored over time and compared to the expected settlement at an applied load of 2,W psf as determined from a laboratory consolidation test on a sample of the clay. Measured consolidation was allowed to reach 90 percent of that expected before the piie tests proceeded. The overburden pressure was maintained throughout the experiment.

Interstitial Fluids. The upper stratum, which is the simulated shallow contaminated water-bearing stratum, was saturated with a mixture of two common DNAPLs, tetrachloroethylene (PCE) and trichloroethylene (TCE). The proportion was two parts PCE (specific gravity >1.6) to one part TCE (specific gravity > 1.4). There was no water introduced into the upper stratum, only DNAPL.

The lower stratum was saturated with analyte-free deionized (DI) water. (As discussed in a following section, the vessels were constructed to allow complete purging and displacement of the lower stratum fluid.)

First Test Vessel, The first test vessel was constructed of PVC, TygonTM, and steel as shown in Figure 2. The body of the vessel was a four-foot tall, 12-inch diameter PVC pipe (with a 3/8-inch wall thickness) mounted to a PVC base flange, The base flange was tapped with a fa pipe slotted near the top of the lower stratum and a purge pipe slotted near the base; this arrangement allowed for essentially complete pore yolume replacement of the water in the lower stratum, Manometer tubes were tapped into the PVC pipe at the base of the upper and Iower strata. All construction was completed with tapped and threaded connections, bolted gasket connections, or pIastic welding; no mastic, which could influence analytical results, was used. Although it was certainly recognized that the DNAPLs are solvent to PVC, it was assumed that over the anticipated four-week to six-week duration of the test, there would be no structural deterioration of the vessel. However, at the conclusion of the first pile test, the PVC vessel opposite the DNAPL-saturated upper stratum had lost some rigidity. This prompted the construction of another type of vessel for the second pile test.

The soil materials were placed in the vessel in carefulIy controlled layers, and were lightly compacted with a tamping tool; the bottom foot of the tamper was specially designed to fit the circular interior of the vessel. A steel plate was placed on the top of the upper stratum for confinement, and the assembled vessel was placed on a hydraulic jack within a load frame. (The top piate was configured with a center hole and guide for the later insertion of the test pile.) The Iower stratum was saturated with DI water, and the overburden stress was applied. Upon achieving 90 percent consolidation, the upper stratum was saturated with DNAPL, and the hydrostatic head in the upper stratum and lower stratum were equalized. This eliminated any vertical hydraulic gradient between the permeable strata; of course, there was a potential density gradient from the DNAPL in the upper stratum relative to the DI water in the lower stratum. The apparatus was now ready for proof testing and the piie driving experiment.

Second Test Vessel. The vessel for tRe second test was constructed to the same dimensions as the first vessel but was entirely of steel components. The method of applying and maintaining the overlburden

pressure was also modified. The hydraulic system used in the first vessel experienced a slow bleed-off of pressure as the result of continued time-dependent consolidation of the clay confining stratum, and the system required near-constant monitoring throughout the test duration to maintain constant loading. For the second vessel, a passive dead load (sand bags) and cantilever were used to maintain the required constant load. Otherwise, the vessel was configured, filled, and preloaded in exactly the same manner as the first vessel. The second apparatus is shown on Figure 3.

Pile Driving. As previously mentioned, the load plate placed on the surface of the upper stratum was provided with a center hole, and the load frame was configured with supporting guides to accommodate a ?4- inch diameter steel rod or wood dowel "test pile." The pile-driving hammer was a modified soil compaction test drop hammer equipped with a socket to fit over the %-inch diameter test pile. The drop hammer and pile guide were designed to allow the test pile to be advanced without wobbling or experiencing a lateral load.

Proof Testing the Model After the loaded vessel had been subjected to the conf~ning pressure and the clay had achieved the

primary consolidation, the DI water in the lower stratum was purged and concurrently replaced by a volume of fresh DI water equal to the pore volume of the lower stratum. (approximately one galion). The purged DI water was sampled, and the sample was laboratory analyzed by EPA Methods 601 and 602 for purgeable and volatile aromatic hydrocarbons, including PCE and TCE. None of the compounds was detected at a laboratory detection limit of 10 micrograms per liter (pg/L).

STEEL PILE TEST The test pile consisted of a %-inch diameter carbon steel rod with a conical tip. With successive

blows from tbe drop hammer, the steel rod was advanced (1) through the DNAPL-saturated upper stratum, (2) through the clay c o n f i g stratum, and (3) five inches into the six-inch thick lower stratum. Immediately after the steel test pile was driven, the pore volume in the lower stratum was purged and concurrently replaced with fresh DI water. The purged pore water was sampled and analyzed for the presence of dissolved PCE and TCE. This purging and sampling process was repeated 12 times over a 37-day period after the pile was driven.

The combined PCE/TCE concentration in the first pore volume replacement after the steel pile driving was 35 pg/L. The PCE/TCE concentration reached a maximum of 102 gg/L in the third pore volume replaced, decreased to the detection b i t of 10 pg /L by the seventh pore volume replacement, and fell below detection limit after the eighth pore volume replacement on the 13th day. The PCE/TCE concentration remained below detection through the 12th and frnal pore volume replacement on the 37th day. The results are shown graphically on Figure 4.

UNTREATED WOOD PILE TEST -. I ne test pile consisted of a %-inch diameter, ii;i:rea:cb, hicktnory WOO^ dour::! ~ 4 t h . a mr?icd tip. The

wood test pile was driven in the same manner as the steel rod. Immediately after the wood test pile was driven, the pore volume in the lower stratum was purged and concurrently replaced with fresh DI water. The purged gore water was sampled and analyzed for the presence of dissolved PCE and TCE. This purging and sampling process was repeated seven times oyer a 23-day period after the pile was driven.

The combined PCE/TCE concentration in the first pore volume replacement after the wood pile driving was 127 pg/L. The PCE/TCE concentration reached a maximum of 2,220 pg/L with the fifth pore volume replaced, which occurred one week after the wood pile driving. The PCE/TCE concentration remained on the order of 2,000 P ~ / L with the sixth and seventh pore volume replacements at two and three weeks duration, respectively. The results are shown graphically on Figure 5.

W~cking Test The ability of PCE and TCE to migrate axially through the hickory wood dowel by capillary force (in

opposition to gravity) was tested independently of the driven pile experiment. The bottom four inches of a 12-inch long, %-inch diameter dowel was placed in a container of the DNAPL, mixture. The top of the container was sealed around the dowel to prevent volatilization of the DNAPL. Upward wicking of DNAPL was visually evident on the exterior surface of the dowel. After one day, the DNAPL had migrated 2% inches, and after four days, 4% inches. After four days, a sample of the non-immersed end of the dowel was analyzed and found to contain 10.1 milligrams per kilogram of PCE and TCE.

CONCLUSIONS

Tip Dragdown A finite quantity of DNAPL was carried down from the upper stratum to the lower stratum as the

piles were driven. This finite quantity is referred to as "dragdown." The dragdnm effect was most evidciit with the steel pile experiment since eight replacements of the pore water were successful in flushing the dragdown to below detectable concentrations in the lower stratum. Since the average PCEITCE concentration was determined for each of the pore volume replacements (approximately one gallon each), the total dragdown volume could be determined. The %-diameter steel pile carried approximately 3.9 x 10.' gallons of PCEITCE to the lower stratum as dragdown.

In the wood pile experiment, the dragdown could not be quantified due to interference, possibly from the wicking transport mechanism. However, the appearance of PCE/TCE in the lower stratum immediately after driving the wood test pile certainly suggests that it, too, carried down an initial dragdown quantity, probably of sirnilar magnitude to the steel pile.

Pife-to-Soil Contact Migration The steel pile experiment demonstrated that driving the test pile through the clay confining stratum

resulted in a tight pile-to-soil contact. This is consistent with the well-established behavior of cohesive clay to remold under c o n f i g pressure which, immediately after pile driving, acts to create and maintain a positive pressure contact. Over the duration of the experiment, the tightness of the pile-to-soil contact in the c o n f i i g stratum was demonstrated by the ability to flush the lower stratum free of the initial dragdown and by its remaining free of detectable DNAPL concentrations.

Wclring Effects in Wood Biles The untreated wood test pile provided a conduit for downward migration of the DNAPL. This is

not surprising since the wicking test confirmed upward capillary movement of DNAPL within an untreated wood test pile. The downward migration appeared to reach a relatively steady rate; however, no estimate of the migration rate was attempted.

Implications for Pile Foundations Penetrating Shallow Ce.n.h,r?Z.n.a),o~: Based on the results of this very basic model experiment, which did not investigate all potential

modes of DNAPL transport which may be associated with driven piles, impervious piling (e.g., steel and probably concrete) can be expected to impact lower strata with a finite, one-time quantity of dragdown. The dragdown in the model was 2.8 x lo4 gallons per square foot of pile cross section area. This suggests that the contamination impact resulting from dragdown may be small. For example, if the construction of a new plant process area requires 145, 10-inch square precast concrete or steel piles, then the total effective pile tip area is 100 square feet, and the dragdown based solely on the model results is 0.028 gallons of DNAPL. If this were introduced into a lower 10-foot thick water-bearing stratum (porosity of 25 percent) underlying a plant process construction area of, say, 250,000 square feet, then the contaminant concentration impact to the water-bearing stratum under the process area would be approximately 6.0 parts per billion. This is approximated as the volume of dragdown, assuming it is pure contaminant, divided by the volume of water assuming complete dissolution of the contaminant. While such a direct extrapolation of the model to a real

situation is arguably questionable, this example may provide insight as'to the general order of magnitute of dragdown impact.

The untreated wood test pile allowed DNAPL migration from the upper to lower fluid-bearing strata in the model. The implications this may have for treated wood piles, a common founding scheme in the gulf coast, are uncertain.

Topics Requiring Further Investigation While several sigmficant conclusions can be drawn from the model experiments, the direct

applicability to real subsurface conditions is unknown. Building upon these very basic model experiments, additional research into several topics would be invaluable in evaluating and mitigating the concerns over pile foundations as potential conduits for contaminant migration. These topics and relevant questions include:

Corofininet Strata ~ l c h e s s and Character, Is there a relationship between the distance a pile i s driven thro* a confining stratum (or strata) and the quantity of dragdown? Is that finite dragdown quantity being stripped away as the pile penetrates confining strata? In other words, is there some threshold thickness of c o a f i g strata beyond which there is no dragdown? Also, do the soil properties of the confining unit affect the quantity of dragdown? To what extent do the chemical properties of the DNAPL m d the c o d i g unit soils interact, and what are the effects of the interaction on dragdown and pile-to-soil contact?

Pile Confiauration. What is the relationship of dragdown to the shape of the pile and pile tip (e.g., blunt versus conical tips, circular versus square versus "W cross section, tapered versus constant cross sectional area, etc.)? Do discontinuities in the pile surface (e.g., joints, splices) affect the pile-to-soil seal?

Wood Pile Preservation and Vertical Continuitv. To what degree do preservatives (e.g., creosote) reduce the wicking potential of wood piles? Can coupling sleeves be designed to create barriers to vertical contaminant migration in wood piles?

Summary The bench model experiment suggests (1) that piles driven through shallow contaminated strata to a

deeper water-bearing stratum will initialIy carry down a f ~ t e and relatively insi@cant quantity of the shallow contamination, and (2) that the pile-to-soil contact through clayey confining strata will not provide a conduit for vertical contaminant migration, even if the contaminants are more dense than water. Untreated wood piles may provide an internal avenue for contaminant migration through wicking action. While the extent to which the experimental results can be directly applied to red situations is unknown, the results do help in identifying DNAPL transport mechanisms and in identifying additional investigative efforts needed to better understand those mechanisms.

r FOUNDATION PILE

m

Figure 1. Typical gulf coast stratigraphy and driven pile foundation,

I SPACER BLOCKS

TOP VIEW

PILE GUIDE

I I STRAIN GAUGE (.001 in.) SPACER BLOCKS

LEVEL GAUGE

12" FLANGE, PVC,

STAN5 PlPE/SCREEN, WELDED TO PIPE

ONE AT TOP OF SAND AND ONE AT BOTTOM 12" BLIND FLANGE OF SAND

ANGLE VALVE

I ! HYDRAULIC JACK

SUPPORT PLATE --I I PRESSURE GAUGE, 0-5000 PSI

HYDRAULIC PUMP

SIDE VIEW

Figure 2. PVC test vessel used for steel test pile.

/ TEST CYLINDER

aAND BAGS

B A s m 4) \\I

TOP VlEW

SIDE VlEW

Figure 3. Steel test vessel used for wood test pile.

LEQENP

a- - -Q PCE (TETRACHLOROETHYLENE)

(3- . - * + TCE (TR~CHLOROE~HYLENE)

&-A TOTAL PCE & TCE

I

5 I

1 2 3 4 6 7 LOWER STRATUM PORE VOLUME REPLACEMENTS

NOTE TO EDITORS Under the new federal copyright law, publiication rights to this paper are retained by the author(s).

Figure 5. DNAPL concentration versus time and gore volume replacements for wood pile experiment.