Landfill Construction through Peat and Organic Silt. James M. Tinjum 1 and Joel V. Schittone 2

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1 Abstract Landfill Construction through Peat and Organic Silt James M. Tinjum 1 and Joel V. Schittone 2 This paper addresses design principles and construction practices used to combat soft ground conditions based on an extensive case history covering 3 years of landfill construction. It includes the characterization of the soft ground at the site using a variety of methods, including cone penetrometer testing (CPT), borings, laboratory testing, and landfill history. Conceptual designs evaluated to improve and/or mitigate the soft ground problems are reviewed. And finally, this paper addresses some of the many difficulties encountered in the field and how problems were resolved. Due to the severity of the soft ground problem, the potential impact to neighboring rail lines, and the available construction time frames, various technologies were implemented. For a 250-linear meter (m) section along the rail line where 85,000 cubic meters (m 3) of soft soil required excavation and replacement at an accelerated schedule, an internally braced, dual-wall sheetpile system was selected. Two sheetpile walls were driven through the soft soil and embedded in underlying stiff clay till. Staged excavation of soft soil between sheetpile walls then commenced, internal bracing was placed, and replacement clay was compacted in place. Remaining soft soil on the landfill side was 'mudwaved' out during placement of a soil buttress. For another section adjacent to the railroad, a soil preload application was employed. This economical approach was selected because the soft soil thickness was less than 4 m and there would be a 2-year construction schedule. The two case histories demonstrate a wide variety of techniques that may be employed to combat soft ground problems. Introduction~Background Landfill expansions are increasingly being located in areas of marginal soil and in areas that are closer to structures potentially impacted by construction. Such was the case for the expansion of a municipal landfill located in the Midwestern ' Member, ASCE, Project Geotechnical Engineer, RMT, Inc., 744 Heartland Trail, Madison, WI Project Director, RMT, Inc., 744 Heartland Trail, Madison, WI

2 SOFT GROUND TECHNOLOGY 389 United States. Full development of available acreage at the active landfill site required footprint expansion to the base of railroad embankments used by freight and high-speed commuter trains. The project site (Figure 1), measuring 68 hectares, has been an active landfill area for over 25 years with 10.5 million m ~ of permitted disposal capacity (i.e., airspace). The site is bounded by railroad lines to the north and east. Figure 1. Plan View of Site Prior to landfill development at the project site, the area was an unfarmed lowland with surficial deposits of fibrous peat, organic silts, and fine sand outwash. As the landfill was developed, these natural soft soil deposits were excavated from areas being developed and stockpiled or placed in disposal trenches. The area to the south of the original landfill (a future area for horizontal expansion) was used predominantly as a clay borrow area during historical landfill operations. As natural clay tills were mined from these southern limits, the excavated peat and organic silt spoils were replaced in these borrow areas. The railroad embankments rise approximately 3 m above existing grade; generally in low areas with some channelized surface water flow, standing water, and groundwater at or close to the surface. It is believed that the railroad embankment was constructed circa 1920 by surcharging ballast through the native deposits, thus displacing the soft deposits and consolidating the remainder. The railroad maintains the embankment through a semiannual maintenance program, which consists of ballast addition and track leveling. Laboratory Testing There are three predominant natural types of soil located on the project site: (1) surficial peat, (2) organic silt and fine sand, and (3) silty clay till. In addition, soft soil spoils derived from a mix of the fibrous peat and organic silt exist in the disposal area to the south of the site. The general origin, composition, and physical properties of these soil types are presented in Table 1.

3 390 SOFT GROUND TECHNOLOGY Soil Type Peat Table 1. Summary of Laboratory Test Results (Ranges Recorded) Moisture Content (%) Unit Weight (IcN/m 3) Fiber Content (%) Organic Silt Clay Till Organic Specific Content (%) Gravity The underlying clay at the site consists of a gray or green-gray silty clay glacial till. Thin, gravelly sand horizons occur sporadically throughout the unit, which extends to a depth of approximately 50 m below ground surface (bgs). Cone Penetration Testing To complement various field soil boring investigations, cone penetration tests with soil electrical conductivity (CPT-EC) measurements were performed to provide information onsite stratigraphy, and in particular, to delineate very soft peat/organic layers. A track-mounted CME 850 drill rig was used to deploy the CPT-EC equipment. While this approach provided very good information relating to delineation of soil stratigraphy economically, the ability to correlate data to engineering properties was limited. Penetrometer soil resistance data obtained from the peat/organic layers were often very low, at the limits of accuracy of the penetrometer. Cone end bearing resistances, a total stress measurement, were often not much different from those expected from static water pressure; thus, soil shear resistances were extremely low (generally less than 20 kpa). Very poor strengths were thus expected from the peat/organic layers. Correlations were attempted for evaluating the peat/organic soil. However, due to the extreme softness of some layers, no correlations could be developed. It is noted that there appears to be a very limited data base for correlating CPT-EC data to engineering properties for fibrous, organic soil (Olsen and Farr 1986). For design purposes, conservative engineering properties for peat were selected based on reported values in the literature (Edil and Wang 2000, Dhowian and Edil 1980, Mesri et al. 1997) in conjunction with the estimated properties developed from CPT-EC interpretation and laboratory test results. Project Constraints During the 1998 construction season, a horizontal and vertical landfill expansion was constructed. Part of the expansion abutted directly against a railroad embankment used by both freight and high-speed commuter trains (Figure 1). Construction limits were within 7 m of the rail lines. Geotechnical field investigations indicated that a large bowl-shaped deposit of peat/organic soil existed in the vicinity of the railroad tracks. The deposit consisted of fibrous peat underlain by soft organic silt and fine sand of thicknesses to 10.5 m total depth (Figure 2). The thickness of this deposit decreased as it approached the main cell footprint. Due to the inability to investigate subsurface conditions immediately below the railroad embankment, it was not known how much organic soil existed immediately below the ballast comprising the embankment. In order to construct the landfill expansion, it was necessary to excavate and then replace 85,000 m 3 of soft soil while maintaining the integrity of the railroad tracks/embankment at all times during construction.

4 SOFT GROUND TECHNOLOGY FUTURE ROAD WORKING ~ I~.ATFORM ~/ i_ ~, \ / / <5-..,i,. j :.+. / ORQ~IIC SILT UNWEATHERED TILL Figure 2. Section through Soft Soil and Railroad Embankment (dimensions in meters) The ultimate design approaches had to satisfy a variety of constraints. The chosen design had to be structurally capable of restraining the railroad embankment to strict tolerances while achieving a regulatory hydraulic conductivity requirement of less than lxl0 "9 figs. Also, per regulatory requirements, the landfill liner could not be constructed on soft soil or other unsuitable material which provided insufficient strength to meet foundation and mass stability requirements. Finally, the chosen approaches had to be constructable within one construction season for the phase with the large bowl-shaped peat deposits, and within 2 years for the 170-meter section along the tracks with shallower soft soil deposits. Because of the potential for differential settlement, one of the major concerns for the railroad owner was that the water levels be maintained at current levels in the area immediately adjacent to the railroad embankment. The water levels could be impacted by construction excavation and associated dewatering. Analysis of Alternatives Several engineering solutions were initially developed to solve the soft ground engineering problems at the landfill site. The alternatives were reviewed by a team consisting of the owner, the contractor, the subcontractors, and the site engineers. A partial list of conceptual designs reviewed is outlined in Table 2. For the area where the soft soil deposits were the deepest, the construction was closest to the tracks, and the construction was required to be completed within one year, the contractor proposed the internally braced, dual-wall sheetpile system on a means and methods type of contract. The contractor also took on the responsibility of monitoring the adjacent railroad tracks. For the following phase of landfill build-out (Year 1999/2000 construction), the existing conditions were considerably less severe. The soil preload alternative was chosen when stability analyses indicated it could be constructed without failure. This economical approach was selected because the soft soil thickness was less than 4 m, the railroad embankment was farther removed from the cell footprint, and available cell space allowed for a 2-year construction schedule.

5 392 SOFT GROUND TECHNOLOGY Internally Braced, Dual-Wall Sheetpile Design and Construction Access Haul Road In order to provide access to the construction area (a wet, marshy area) for a 90-metric ton crane, 32-metric ton off-road haul trucks, and miscellaneous support equipment, a multiple layer geogrid/crushed aggregate working platform/access road was required. The platform/access road consisted of 0.2 m of general fill leveling layer, overlain by two layers of geogrid and crushed limestone gravel. As construction proceeded, timber skids were brought to the site and used by the crane and excavators to better distribute loads over the haul road. S_heetpile/Barrier Wall System The temporary double-row, internally braced sheetpile wall was then constructed to allow for excavating soft native soil and backfilling the excavation with select clay fill between the rows of sheetpiles (Figure 3). The sheetpile walls were constructed in phases, with a spacing of approximately 12 m on center. 1.2-m-wide pile sections were driven using a vibratory hammer suspended by a 90-metric ton crane. Per design requirements by the sheetpile subcontractor, the sheeting was driven a minimum of 2.7 m into the underlying stiff clay till layer. Internal bracing (one or two levels, as necessary) was provided with I-beam walers and 0.6-m-diameter pipe struts at 5.6 m on center. Excavation of the soft native soil between the sheeting rows proceeded in tandem with the placement of the internal bracing system described above. Excavation was conducted with a standard excavator and a 'long reach' excavator (see Figure 4). No more than two bays (i.e., 5.6-m sections between the internal pipe struts) were completely excavated at a time. Excavation proceeded into the stiff clay till until a minimum soil strength of 150 kpa was recorded as measured by a pocket penetrometer. A key trench, approximately 1.5-m-wide by 1.1-m-deep, was excavated into the stiff clay till to provide additional resistance to sliding along the base of the barrier wall and to provide additional hydraulic cutoff at the interface of the existing stiff clay till and recompacted fill. Figure 3. Sheeting Design Section (dimensions in meters)

6 SOFT GROUND TECHNOLOGY 393 Table 2. Alternatives Analyses Partial Peat Removal with Soil Surcharge (Mudwave) 1. Construct a nonstructural soil/cement slurry wall along the outside edge of the perimeter access road to control groundwater. 2. Excavate the upper "crust" of desiccated and/or fibrous peat, and preload remaining soft soil with an engineered fill. 3. Begin excavating the toe of the soft soil area, allowing the soft soil under the preload to 'mudwave' out. 4. Continue to place soil near the slurry wall while displacing underlying peat, and continue excavation at the toe. Advantages 9 Relatively inexpensive. 9 Relatively fast construction sequence. Disadvantages 9 Potential mudwave could propagate toward the tracks. 9 Soft soil would remain under the preload thus (1) regulatory agency may not approve and (2) soft soil remaining could present a longterm slip surface once landfill build-out is complete. Peat Consolidation with Soil Preload 1. Place geotextile separation layer prior to placement of compacted structural fill in staged, thin lifts. 2. Allow soil preload to consolidate/strengthen underlying soft soil deposits (over a 1-year plus time frame). 3. Excavate soft soils from landfill side in staged increments, and replace with a structural buttress. Advantages 9 Most economical engineered solution. 9 Simple construction sequence where underlying soft soils are nominal and soil preload is distant from railroad embankment. 9 If soil preload fails in areas, revert to a "mudwave" operation. Disadvantages 9 Requires two construction seasons to complete. 9 Soft soils will remain under the preload. 9 Potential for soil preload to fail during staged excavation. 9 Cutoff of water flow from railroad side may not be adequate. Complete Peat Removal with Soil/Cement Slurry Wall (Embedded 2.7 m into stiff clay) I. Construct a structural soil/cement slurry wall along the outside edge of the perimeter access road. 2. Incrementally remove peat and replace with compacted structural fill. 3. If peat or soft soils "flow" during excavation, install sheet piles perpendicular to the slurry wall. Advantages Disadvantages 9 Slurry wall would provide good groundwater 9 Heavy equipment access needed. control. 9 Questionable ability to control 'mudflow' at 9 All peat/soft soil would be excavated and toe of excavation. replaced. 9 Moderate cost without sheeting, very 9 Structural slurry wall would resist movement expensive with sheeting. of railroad embankment. Internally Braced~ Double Sheetpile Wall 1. Install parallel rows (along railroad embankment) of sheetpiles, 2.7 m into stiff clay. 2. Excavate peat/soft soil between sheeting walls, placing internal pipe braces (one or two revels) as excavation advances. 3. Place recompacted, low-permeability clay between walls, extracting struts as fill progresses upward. 4. Mudwave and excavate soft soils on landfill side, constructing structural buttress as a result; remove piling. Advantages 9 Double sheetpile wall would cut off flow of water from tracks. 9 Structurally sound alternative for maintaining railroad stability. 9 Compacted clay would act as a perimeter vertical hydraulic barrier, eliminating need for bentonite slurry wall after construction. Disadvantages 9 Heavy equipment access needed for sheeting operation. 9 Excavating and placing fill in a 'tight' environment. 9 High construction cost.

7 394 SOFT GROUND TECHNOLOGY Figure 4. Photo of Excavation into Retaining Structure Immediately upon completion of the excavation, placement and compaction of select clay fill commenced. Select clay fill was end-dumped into the barrier wall excavation and was spread in thin lifts by the excavator and a small dozer. Compaction was completed by a wedge-foot compactor. All field moisture-density test results indicated that the compacted fill was in conformance with the minimum requirement of 90 percent of the Standard Proctor maximum dry density, at a moisture content greater than optimum. Because the clay barrier wall was also to function as a hydraulic barrier, hydraulic conductivity testing was conducted on thin-walled samples to establish its equivalency to a bentonite-soil slurry wall. All laboratory tests indicated that the lxl0 9 m/s regulatory hydraulic conductivity requirement was met. Soil Buttress/Mudwaving Program Once the southern 125 m of the barrier wall had been completed, placement of the soil buttress/mudwaving operation was initiated. Clay soil being excavated from the adjacent landfill cell construction was placed as surcharge to displace underlying soft, wet soil deposits. Displaced peat and other unsuitable soil was excavated near the toe of the soil buttress. The soil buttress was compacted only to the extent possible by soil and machinery loads. A sheepsfoot compactor and a dozer were typically used to place and compact the soil buttress fill. As the soil buttress operation progressed, the contractor found that the properties of the soft soil being removed improved (e.g., lower water content, higher strengths) to the point where a clean excavation/replacement operation could be conducted. This allowed for the complete removal of soft soil throughout the northern 125 m of the barrier wall. It is believed that the improving conditions were tied to the completion of the sheetpile installation into clay soil at the end of the construction limits, thus effectively cutting off the majority of water flow into the excavated deposits. Perimeter Landfill Berm Construction The final step involved the construction of the perimeter landfill berm on top of the recently constructed clay barrier wall and soil buttress. This perimeter berm was constructed with compacted fill in 0.3-m-thick lifts. While the design did not call for this perimeter berm to be a low-permeability structure, it essentially was because the fill material used was a lean clay. Because the construction season

8 SOFT GROUND TECHNOLOGY 395 was nearing completion, this 9-m-high berm was constructed in a period of several weeks. This ultimately led to some (approximately for a 30-m section) lateral 'bulging' of the perimeter berm. The likely cause of this movement was suspected to be due to the build up of excess pore pressure. Monitoring and Alarm Program Meetings with railroad officials were conducted early in the project to determine what movements were tolerable and to establish an adequate monitoring program. Based on these consultations, a series of reflecting prisms were established as reference points along the railroad tracks. A robotic electronic distance meter (EDM) was mounted on a concrete pier to measure x, y, and z coordinates of the prisms. Collected data were compiled and presented in a program on a computer system contained in an enclosed building housing the EDM. As various action levels were recorded, a communications system automatically called the construction superintendent or his backup. Readings were obtained every 15 minutes during construction. Throughout the entire construction period, cumulative movements of greater than 2.5 centimeters (as compared to established baseline) were not recorded, thus construction was never required to be suspended. Per the requirements of the railroad owner, water levels were maintained in the area of the railroad embankment at or above the level recorded prior to construction. Shallow water level monitoring was conducted with two piezometers installed between the tracks and the barrier wall construction. Water levels in these piezometers were manually measured daily during construction. Water levels were maintained throughout the construction period with minimal additional effort beyond directing the discharge from dewatering pumps into the area between the barrier wall and the railroad embankment. Soil Preload Design and Construction Construction Sequence As previously discussed, a soil preload design was selected for a second area of landfill build-out adjacent to the railroad (Figure 5). To allow for the soil preload to function as designed (and not to create an unplanned surcharge/mudflow), soil fill was placed and compacted in stages. First, a nonwoven geotextile was placed along the preload construction limits to act as a separation and base foundation layer. Then, settlement base plates were set along the centerline of the soil preload. Fill was then placed and compacted in 0.3-mthick uncompacted lifts. No more than 0.6 m of preload was placed in any given day, and placement was halted if any lateral movement of underlying soft soil was observed emanating from the preload. As a further effort to minimize the potential for creating a mudflow toward the concurrent excavation activities in the main body of the soil spoil areas, excavation limits were established a minimum of 60 m from the toe of the soil preload. The baseline water level between the preload and the railroad embankment was established prior to construction activities. After the soil preload was constructed, monitoring of the settlement plates commenced. An evaluation of the data from this monitoring will be conducted prior to allowing the remainder of the cell to be excavated, a staged soil buttress to be placed, and the perimeter access road to be constructed. These activities are planned for early summer 2000.

9 396 SOFT GROUND TECHNOLOGY Figure 5. Soil Preload Section Performance to Date Leading into the second year of construction in the soil preload area, performance has been adequate. The primary performance objective, maintenance of water levels between the soil preload and the railroad embankment, has been achieved. In addition, monitoring results of the settlement plates have indicated that 0.22 to 0.46 m of consolidation occurred 9 months after the soil preload was applied. Figure 6 shows the measured amount of settlement for the first several months after placement of the soil preload. Figure 6. Settlement Data for the Soil Preload Soft soils did 'bulge up' in one 10-m segment between the soil preload and the railroad embankment. However, monitoring of the control prisms on the railroad tracks did not indicate any corresponding movements of the railroad embankment. In addition, one 20-m segment of the preload did partially "slip"

10 SOFT GROUND TECHNOLOGY 397 (i.e., one-half of the soil preload shifted down and toward the spoils area excavation area). Additional field exploration indicated that the spoils storage area in this area was oversteepened and very close to the toe of the soil preload. Thus, when excavation commenced approximately 30 m from the preload toe, an unplanned 'mudflow' developed. While this failure did expose a data gap in the initial field investigation, this portion of the soil preload was over 30 m from the tracks and, consequently, had no ill effects on the performance of the railroad embankment. A surcharge was applied to this slip area to 'mudwave' out remaining soft soil spoils beneath the soil preload Lessons Learned One always prefers that construction go according to plan, but that is seldom the case, especially for large subsurface projects with inherent geologic variability. Following is a discussion of some of the difficulties encountered in the field and the resolutions implemented. Geologic Variability While an extensive geotechnical field investigation was conducted prior to the design of the barrier wall project, some field conditions encountered were more severe than anticipated. First, the bottom of the soft soil stratum was unusually nonuniform. The base of this layer often dipped down at a substantial slope (e.g., 1.5 m vertical difference over the 12-m sheetpile spacing). This caused a two-fold problem: the ability of the sheeting to carry the increased active loading without increased embedment in firm native clay tills, and the requirement to place additional levels of internal bracing. Second, a buried stream channel was encountered toward the end of the project, which brought in substantially greater groundwater flows than anticipated, resulting in increased construction dewatering efforts. However, some conditions were better suited for construction than anticipated. For instance, the design originally allowed for only one bay (i.e., 5.6-m section between horizontal pipe struts) to be open at a time. As construction progressed, field conditions allowed for this restriction to be extended to two bays open at any one time. This allowed for a dramatic increase in production, both for excavation and compacted clay replacement. In addition, once the sheetpile walls were in place through the entire 250-linear m construction area, groundwater recharge was largely curtailed. This resulted in a much drier stratum of peat/organic silt on the landfill side of the wall with corresponding strength gain. This ultimately allowed for expedient excavation at much steeper cut slopes than originally anticipated. Haul Road Construction on Top of Fibrous Organics Fibrous peat can be a reasonably strong material due to the intertwining effects of the vegetation mat. This was substantiated during the initial use of the constructed access road. However, problems occurred over an approximate 30-m section of haul road when approximately 1 m of fibrous peat was excavated 2 m from the edge of the haul road, essentially removing lateral support of the haul road. The contractor conducted the excavation in order to allow for full depth sheetpile driving in one stage. However, once the upper crust of fibrous peat was removed, peat began to flow or 'bubble' upward into the excavation, and the haul road began settling. The contractor attempted to repair the haul road by adding more stone and geogrid. In time, the haul road stabilized, but not until up to 3 m of

11 398 SOFT GROUND TECHNOLOGY additional stone and geogrid had been placed at a substantial cost. The problem was corrected prior to further construction by first driving the sheeting to near full depth, excavating surficial peat, and then driving the sheeting to full depth. Conclusion The two case histories described in this paper demonstrate a wide variety of construction techniques that may be employed to combat soft ground problems. To control movement of the railroad embankment during the first phase of the project, contractors drove parallel sheet piling 12 m apart. Staged excavation of soil between sheetpile walls then commenced, internal bracing was placed, and replacement clay was compacted in place. This wall of clay allowed safe construction of the landfill cell without disturbing the railroad tracks, and eliminated the need for a slurry wall around this portion of the site. For the following phase of landfill expansion, a soil preload application was constructed. While excavation is still not complete for this phase of the project, the preload has performed adequately with only one flow-type-failure of a non-critical 20-m section. Acknowledgements The owner of this project forged a team representing the key parties in the project including the earthwork contractor, sheet piling subcontractor, surveyor, construction quality assurance staff, and various design engineering consultants. Representatives from all parties met at the project site weekly and each was empowered to make decisions at the site. Efforts were coordinated intensely, communication was the top priority, and the focus was on correcting problems and keeping the project moving - all necessary ingredients to the overall success of the project. References Dhowian, A.W. and Edil, T.B. (1980). "Consolidation behavior of peats." Geotech. Testing Journal, ASTM, 3(3), Edil, T.B. and Wang, X. (2000). "Shear strength and Ko of peats and organic soils." Geotechnics of High Water Content Materials, STP 1374, ASTM, Fredericksburg, Virginia, Mesri, G., Stark, T.D., Ajlouni, M.A., and Chert, C.S. (1997). Secondary compression of peat with or without surcharging." Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 123(5), Olsen, R.S. and Fan', J.V. (1986). "Characterization using cone penetrometer test." Use of In Situ Tests in Geotechnical Engineering, ASCE Geotechnical Special Publication 6, Blacksburg, Virginia,