Environmental and Engineering Geoscience; August 2007; v. 13; no. 3;
p. 255-264; DOI: 10.2113/gseegeosci.13.3.255
© 2007 Association of Engineering Geologists
Investigation and Geogrid Reinforced Repair of the Foxridge Drive Landslide, Mission, Kansas
JAMIE MARTENS1
1 URS Corporation, 8300 College Boulevard, Suite 200, Overland Park, KS 66210
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ABSTRACT
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A 180-ft-wide (54.4-m-wide) landslide occurred along the north side of Foxridge Drive (Mission, Kansas) in July 2001. The earthen mass had moved laterally and downward approximately 2 ft (0.6 m). Additional movement during the following months indicated the landslide was threatening stability of the north edge of the roadway. In addition, the road was scheduled to be widened by approximately 10 ft (3.0 m) in the near future. Therefore, repair of the slope was critical to the stability of the road and the future widening. A phased subsurface investigation revealed subsurface conditions generally consisting of fill overlying alternating layers of limestone and shale. An inclinometer was installed to monitor further slope movement and evaluate the location of the slip surface. Exploratory drilling, inclinometer monitoring, and modeling of the slope indicated the most probable slip surface was positioned at the contact between the fill and underlying shale. The landslide was likely caused by a rise in the groundwater level resulting from a wetter than normal spring. A geogrid reinforced earth system was designed to repair the slope. The base of the repair consisted of crushed rock backfill with welded-wire baskets at the face to provide an independent toe wall. The upper portion of the design section consisted of reinforced clay and crushed shale backfill. Topsoil was brought in to provide a medium for re-vegetation.
Key Words: Engineering Geology • Geotechnical • Geogrids • Landslides
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Introduction
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Foxridge Drive is a two-lane roadway situated at approximately mid-height on an overall 20% slope in Mission, Kansas. The general location of the site relative to the Kansas City metropolitan area is shown on Figure 1. Figure 2 is a generalized cross section of the slope illustrating pertinent relatively flat-lying sedimentary geologic units. Turkey Creek is positioned at the toe of the slope, and the roadway is situated on an approximately 45 ft& ( (13.7 m) wide bench approximately 30 ft (9.1 m) above Turkey Creek. The grade of the lower slope is approximately 2.3 Horizontal (H):1 Vertical (V). The ground surface rises at a shallower gradient of 3H:1V south of Foxridge Drive.
Bedrock consists of alternating layers of limestone and shale of Pennsylvanian age (O'Connor, 1971). The Argentine Limestone crops out at the top of the slope, and the Iola Limestone is exposed approximately 8–10 ft (2.4–3.0 m) above the roadway level (Figure 2). Bedrock outcrops below Foxridge Drive are generally obscured by dense vegetation. The Westerville Limestone outcrops along the banks of Turkey Creek at the toe of the slope.
City of Mission (City) Public Works personnel received notification of a possible landslide along the north edge of Foxridge Drive in early July 2001. At that time, city personnel observed that an approximately 150 ft (45.7 m) long section of the north edge of the roadway had moved laterally and downward approximately 2 ft (0.6 m). The soil material north of the curb exhibited the most displacement evidenced by separation from the concrete curb and vertical displacement of guardrail posts and signposts (Figure 3). The maximum lateral extent of the head scarp was not visible as the cracks extended beneath the curb and asphalt section of road. City crews immediately barricaded the northern portion of the road to eliminate vehicular traffic over the potentially unstable section of roadway. In addition, a temporary asphalt curb was constructed to divert surface water run-off away from the head of the slide, and plastic sheeting was draped over the head of the slide to minimize water infiltration.
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Phase I Site and Subsurface Investigation
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The subsurface investigation was conducted in a phased approach. Because of the high-profile nature of the affected roadway, the City needed quick, preliminary information to assess the current slope stability and to outline future investigations if warranted. Subsurface information concerning the downhill portion of the slope within the landslide was limited, as the north slope between Foxridge Drive and Turkey Creek was heavily vegetated with grass, trees, and brush. In addition, the underlying geologic units were not visible at the ground surface because they were covered by fill.
As part of the preliminary phase of the investigation, three exploratory borings were drilled on July 25 and 27, 2001, in Foxridge Drive at the head of the slide to characterize the subsurface materials (Figure 4). The Phase I exploratory borings were drilled with a truck-mounted CME-75 drill rig and advanced with 4 in. (10 cm) outer diameter solid stem augers. Materials were logged in the field based on drilling characteristics, auger cuttings, and soil samples collected using 2 in. (5 cm) diameter California lined samplers, a 2 in. (5 cm) diameter split-barrel sampler in conjunction with the Standard Penetration Test, or 3 in. (8 cm) diameter thin-walled samplers. The borings encountered 13 to 13.5 ft (4.0 to 4.1 m) of fill material overlying approximately 8 ft (2.4 m) of slightly weathered, hard limestone. The limestone was underlain by 6 ft (1.8 m) of shale. The borings terminated in another lower limestone unit at a depth of 27 ft (8.2 m) corresponding to the approximate base level of Turkey Creek located at the toe of the slope.
One issue that proved to be critical at this early step in the investigation and would continue to be problematic throughout the duration of the project was the high density of utilities. Key borings could not be drilled at the immediate head of the slide as desired because of the presence of overhead power lines running parallel with the north side of the roadway. At one location, the power line was sheathed to position a boring as close as possible to the maximum probable extent of the slide. During the drilling at this location, a 24 in. (61 cm) diameter forced sewer main was encountered. Other utilities that were identified included a 4 in. (10 cm) diameter gas line and buried telephone line located immediately north of the road and a 12 in. (30 cm) diameter forced sewer main located below the southern portion of the roadway.
The preliminary phase also included surveying to locate and map pertinent features of the landslide. Three cross sections extending from the south edge of Foxridge Drive to the north side of Turkey Creek were developed for use in future field reconnaissance and slope stability analyses. Field reconnaissance and surveying indicated that the actual width of the landslide was approximately 180 ft (54.9 m) with the eastern edge of the slide located approximately 240 ft (73.1 m) west of the intersection of Foxridge Drive and Lamar Avenue. Examination of the slope did not reveal an obvious toe bulge or lateral scarp. Review of the alignment of the slope with respect to Turkey Creek indicated that the slide was situated on a cut bank side of a meander bend of the creek. Outcrops of limestone corresponding to the lower limestone encountered in the preliminary exploratory borings were visible in the banks and bottom of Turkey Creek.
Based on the boring information and geologic mapping, a preliminary subsurface profile was developed. The Argentine and Iola Limestone units were previously identified on the southern hillside above Foxridge Drive. The exploratory borings located in Foxridge Drive encountered fill material, weathered shale, and two limestone units separated by approximately 6 ft (1.8 m) of shale. Based on the elevations obtained from the survey data, the upper limestone encountered in the borings was the Cement City Limestone, underlain by the Quivira Shale and the Westerville Limestone (lower limestone). The Cement City Limestone Member is typically blue-gray to gray, massive to slabby limestone that weathers light gray to tan. The average thickness of this unit in the Kansas City area is approximately 9 ft (2.7 m) (URS Corporation, 1995). The Cement City Limestone at the site was described as light gray to brownish gray, medium hard to hard, and slightly weathered to fresh. The full thickness of the unit measured at boring BH-1 was 8 ft (2.4 m). In general, the Quivira Shale underlying the Cement City Limestone is highly variable. In one of the Phase I borings, the shale was described as light olive gray to gray and fresh. The shale typically weathers rapidly to a fat clay and is generally not sound for engineering purposes. The measured thickness at the boring location was 6 ft (1.8 m), which is consistent with the average thickness in the area (URS Corporation, 1995). Typically, the Westerville Limestone is highly variable in the upper portion and may consist of oolitic limestone, interbedded shale and limestone, or cherty limestone. The lower portion is more uniform and consists of light gray, usually unit bedded limestone measuring 3 to 4 ft (0.9 to 1.2 m) thick (URS Corporation, 1995). One boring encountered the upper foot of the limestone; however, the drilling methods (augering) provided limited data on the material.
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Preliminary Analysis
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The available survey, field reconnaissance, and Phase I subsurface data were used to develop typical cross sections depicting the possible landslide geometry. Key findings from the Phase I site and subsurface investigation included the presence of fill overlying the native shale and limestone bedrock and the absence of groundwater in the borings. Furthermore, the upper materials sampled did not reveal slickensided or disturbed materials. This information suggested that the landslide did not appear to extend a significant distance southward beneath the existing roadway. The most probable location of the slip surface was postulated to be at the contact between the fill material and the Quivira Shale encountered in the Phase I exploratory borings. This conclusion was based on the available data and more than 35 years of experience by senior level project team members investigating and working with various shale units in the Kansas City area. In particular, the Quivira Shale can be problematic because of its rapid degradation and ability to transmit groundwater (Retter, 2002).
In order to refine the data needs for the Phase II investigation, initial slope stability analysis concentrated on recreating the potential slide geometry to match the conditions observed in the field and back calculate the strength of the materials. Using the generalized subsurface stratigraphy developed from the boring information and the geometry of the slope through the approximate center of the landslide obtained from the initial surveying efforts, slope stability analysis was conducted using UTEXAS3 software (Wright, 1990). The purpose of the initial analysis was to evaluate the residual strengths of the materials and the sensitivity of the model to various changes in the assumed conditions. These changes included the northern extent of the Cement City Limestone, elevation of the groundwater table, and the thickness of the fill material overlying the Quivira Shale.
The slope stability analysis was conducted with an assumed groundwater level at the contact between the fill and the Quivira Shale and drained shear strength values (i.e., cohesion = 0). Given the limited subsurface data within the body of the landslide, a circular search routine was employed while varying parameters one at a time. The analysis produced two potential slip surfaces, each extending to different depths but consistent with the available data. One potential failure surface corresponded to a deeper slide along the contact between the fill and shale and exiting near the toe of the slope. The other failure surface was slightly shallower and daylighted near the middle of the slope. The calculated residual strength (friction angle) of the fill and shale materials for both failure surfaces was comparable and ranged between 15° (shallow surface) and 18° (deep surface). The variable critical to further defining the location of the potential failure surface was the lateral extent of the upper Cement City Limestone.
The results of the Phase I investigation and analyses were discussed with the City and were used to define the objectives of the next phase. The Phase II investigation was designed to address the following data needs: 1) lateral extent of the upper Cement City Limestone toward Turkey Creek, 2) presence and elevation of groundwater, and 3) contact between the fill material and the underlying native shale.
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Phase II Site and Subsurface Investigation
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The second phase of the investigation included limited clearing of the lower hillside to facilitate drilling six additional borings and installing an inclinometer within the slide mass. A track-mounted Acker drill rig was used to access the hillside and drill the borings on September 19–20, 2001. Borings were advanced with 4.5 in. (11.4 cm) inside diameter hollow stem augers through the fill and shale. A pneumatic hammer was used to advance through limestone bedrock.
The six additional borings provided valuable information concerning the lateral extent of the Cement City Limestone and the depth and condition of the fill material. The Cement City Limestone was found to extend between 6 and 14 ft (1.8 and 4.3 m) beyond the north edge of Foxridge Drive. Where present, the limestone measured 9 ft (2.7 m) thick and was described as light to medium gray, hard, fresh, and finely crystalline. Variable amounts of fill were encountered and generally consisted of low plasticity clay with weathered shale and limestone fragments. Laboratory testing of the fill indicated a liquid limit between 43 and 47 percent with a plasticity index ranging between 27 and 31 percent. In most borings, the fill was in direct contact with in situ Quivira Shale. The Westerville Limestone was encountered at the base of several borings at elevations consistent with the Phase I borings and the outcrops in the banks and bed of Turkey Creek. Furthermore, groundwater was detected in two of the borings near the contact of the fill and the Quivira Shale.
Inclinometer casing was installed at one of the boring locations (BH-106) after completion of drilling activities. The location of the inclinometer corresponded to approximately the center of the landslide mass and beyond the extent of the upper Cement City Limestone. The inclinometer casing was socketed into the Westerville Limestone to provide a competent anchor. The annular space was backfilled with a cement grout similar in strength to the surrounding fill and shale materials. The inclinometer was read a total of six times within the following 2 months to monitor for additional movement of the slide mass. Minimal additional deflection was detected; however, an obvious "break" in the inclinometer graph of deflection versus depth was noted at an elevation corresponding to the contact between the fill material and the Quivira Shale.
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Further Analysis and Probable Causes of Failure
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The initial slope stability analysis was revisited based on the additional subsurface data obtained concerning the lateral extent of the Cement City Limestone and the contact elevation between the fill and the Quivira Shale. Figure 5 (refer to Figure 4 for the plan location of the section) illustrates the revised subsurface conditions and the probable slip surface based on the additional boring information and stability analysis. Determination of the exact location of the failure plane and the specific trigger causing the landslide was not possible with the data obtained during the investigations. One of the major obstacles of the investigation was obtaining subsurface information within the body of the landslide. Truck-mounted drill rigs could not access the slope because of dense vegetation and the grade of the slope. The track-mounted rig could access the slope with extensive clearing; however, drilling and sampling methods were limited. Despite the challenges, the probable depth and location of the failure plane and likely causes could be formulated. The deep failure surface appeared to better match the geometry of the slope, the surface expression of the failure, and the subsurface conditions. Specifically, the deep failure surface followed the anticipated contact between the fill and native shale material. In addition, the deep failure surface was more susceptible to changes in groundwater conditions when compared to the shallow surface.
The spring and early summer of 2001 were wetter than normal. Rainfall totals recorded at a nearby weather station located approximately 2.3 mi (3.7 km) southwest of the site indicate higher than normal precipitation for the months of April, May, and June. Monthly totals for the recording station retrieved from http://www.stormwatch.com were 5.16 in. (13.1 cm), 7.52 in. (19.1 cm), and 9.84 in. (25.0 cm) for April, May, and June 2001, respectively. Historical averages for the official recording station of Kansas City located at the Kansas City International Airport (http://www.wdaftv4.com) are 3.38 in. (8.6 cm), 5.04 in. (12.8 cm), and 4.72 in. (12.0 cm). Thus, the surplus for the 3 months before the failure was more than 9 in. (23 cm). The landslide was likely caused by a rise in the groundwater level resulting from a wetter than normal spring.
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Development of Repair Options
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In general, basic methods of repairing landslides include the following: 1) reduce the driving forces at the head of the slide, 2) increase the resisting forces, and 3) reduce hydrostatic pressures. Several unique features of this project needed to be considered and incorporated into the selected repair option. First, the future widening of Foxridge Drive would increase the loading near the head of the slope. In addition, the continued encroachment of Turkey Creek at the toe of the slope and the desire to minimize altering the floodplain greatly reduced the possibility of buttressing the toe of the slope. The heterogeneous nature of the slope materials, the inconsistent nature of the groundwater level, and the close proximity to Turkey Creek would hinder efforts to dewater the slope. Finally, the City desired an environmentally friendly repair that would promote the ability to re-vegetate the hillside to match the remaining sections of the slope.
Several repair options were developed for presentation to the City. All the options assumed that the deep failure was the most probable and that any repair would be constructed on top of the lower Westerville Limestone unit. The depth of the excavation would be dependent on the materials encountered but was also assumed to extend to the top of the Westerville. The lateral extent of the excavation at the head of the slope would be dependent on the continuity of the upper Cement City Limestone unit.
The first option was to remove the entire failed area from Foxridge Drive to Turkey Creek. The slope would be reconstructed from the toe to the head with engineered fill. The toe would consist of rock-filled gabions to provide a toe buttress and to resist the erosional effects of Turkey Creek. The gabions would also be used to raise the surface elevation near the toe to decrease the overall angle of the slope. Immediately behind the gabions would be a zone of large diameter granular material to supplement the toe buttress. Engineered fill grading from coarse to fine material would be placed behind the buttress to provide a filter. Coarse, granular material would be used at the base of the excavation to promote drainage of the slope. Finally, compacted clay would be used to cap the slope to minimize surface-water infiltration. The final grade of the slope would be approximately 2.5H:1V. This option would require excavation and storage of the materials from the entire slope. In addition, coordination with the U.S. Army Corps of Engineers (USACE) would be necessary when constructing the gabions on the bank of Turkey Creek.
The second option was formulated in an attempt to avoid altering the creek bank and to limit the extent of the excavation. This option consisted of excavating the material from the head of the slope to within approximately 25 ft (7.6 m) of the creek and backfilling with large diameter rock (shotrock) and capping with clay. This method is frequently employed in the Kansas City area because of the availability of shotrock from local limestone quarries. The construction duration for this method of repair is usually short; however, unless a suitable filter material is placed over the shotcrock, the clay cap can wash out and produce long-term maintenance issues. The reconstructed slope would again be approximately 2.5H:1V. The primary drawback to this option was the possibility of future erosion or additional slides removing the unprotected toe and leaving the engineered fill unsupported.
The third option used geogrid reinforcing to construct a vertical wall section at the base of the slope and a sloping section at the top while leaving the bank of Turkey Creek unaltered. The base of the slope would consist of crushed stone to promote strength and drainage and would be constructed landward of the creek bank. This section would have a nearly vertical face approximately 18 ft (5.5 m) high constructed of welded-wire baskets and geogrid reinforcing. The top section would consist of geogrid reinforced clay and topsoil with a 2H:1V slope. Positive aspects of this option include leaving the bank of Turkey Creek relatively unaltered and potentially excavating less material. In addition, this option enabled the re-use of excavated material in the top section. Furthermore, the base of the reinforced soil mass with its nearly vertical geogrid wrapped face and welded-wire baskets would be essentially free-standing and stable if the outer soil toe was removed due to continued erosion by Turkey Creek. After reviewing the three options, the City authorized the development of design documents for the third option. This decision was primarily based on the minimal disturbance to Turkey Creek, the ability to re-use on-site material, and the desire to construct an aesthetically pleasing repair requiring minimal long-term maintenance.
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Design Highlights
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Final design included a geogrid reinforced section consisting of a lower section of select granular material and an upper section using on-site excavated material. A simplified typical design section is illustrated in Figure 6. Fill for the lower reinforced crushed stone section was specified to be a relatively clean 1.5 in. (3.8 cm) minus crushed stone meeting the following gradation (in sieve size/percent passing): 1-1/2 in. (3.8 cm)/100–95; 3/4 in.(1.9 cm)/95–70; No. 4/65–40; No. 8/46–30; No. 40/22–10; and No. 200/10–2.
This corresponds to a locally produced Kansas Department of Transportation (1990) material designated as KDOT AB-1, which was recommended by the geogrid supplier for use with their product. This lower section was bounded by intact rock on the back side (street side) and a geogrid-wrapped, welded wire basket face on the front side (Turkey Creek side). This lower section was 18 ft (5.5 m) tall corresponding to 12 layers of 1.5 ft (0.46 m) x 1.5 ft (0.46 m) baskets. Primary uniaxial geogrids extended 22 ft (6.7 m) from the face of the design section. Secondary biaxial geogrids were used in the lower section to "wrap" the front 4 ft (1.2 m) of each layer. A geotextile filter fabric separated the lower section from the upper section. The upper portion of the design section was approximately 12 ft (3.7 m) tall and consisted of re-compacted clay and shale fill excavated from the site. The material was specified as inorganic clay with a liquid limit less than 60 percent. Rock fragments should not exceed 3 in. (7.6 cm) in the largest dimension and should not exceed 10 percent of the total weight. Clay backfill material should be placed in lifts not to exceed 8 in. (20.3 cm) in thickness and compacted to 95 percent of the Standard Proctor density. The front face of the upper design section was sloped at 2H:1V to tie the existing grade to the new grade incorporating the future widening of Foxridge Drive. The back limit of the upper section was defined by the length of the longest geogrid and the contractor's need to bench the excavation. Various lengths of primary uniaxial geogrids were used in the upper section along with shorter secondary biaxial geogrids at the face to minimize sloughing. An approximately 1-ft-thick (0.3-m-thick) layer of topsoil was specified at the outer edge of the slope to provide a medium for vegetation.
The design section was analyzed with the geogrid reinforcement for long-term drained conditions. The friction angle used in the UTEXAS3 analysis for the lower granular material was 35° and 20° for the upper compacted clay. The calculated factor of safety for the repaired slope was 1.7.
The position of the repair section within the overall slope was carefully selected because of the presence of Turkey Creek at the toe of the slope. The Federal Emergency Management Agency (FEMA) requires that no significant structures be constructed within the 100-year floodplain; thus, the complete design section was positioned landward of the 100-year floodplain. The installation of several outlet pipes to drain the design section was also specified. These pipes extended a minimum of 6 ft (1.8 m) into the lower crushed rock section and daylighted above the normal high water elevation of Turkey Creek. These pipes were not considered significant structures with respect to FEMA, and by discharging above the normal high water mark, a 404 permit was not required. The City submitted the plans to the USACE who provided a "no permit required letter."
As previously mentioned, the high concentration of utilities influenced the design and project specifications. The design section was developed to minimize impact and potential relocation of surface and subsurface utilities to the extent possible. Two utilities (gas line and underground telephone line) were shown within the limits of the design section (Figure 5). Coordination of the removal or relocation of these utilities was specified to be the contractor's responsibility.
Final plans and specifications were submitted to the City on March 6, 2002, and facilitated the pre-bid meeting on March 20. The construction cost estimate of $370,840 was submitted on March 27, 2002. The bid opening was conducted on April 2 with an accepted bid from George J. Shaw Construction Company in the amount of $362,592. Substantial completion of the project was specified on or before August 31, 2002.
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Construction Consultation and Observation
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After an initial coordination meeting, site work began on April 29, 2002. Several field issues requiring design or approach modifications were addressed throughout the course of the construction. The first significant field issue involved encountering a massive block of limestone along the eastern flank of the excavation. The block was discovered during excavation on a Thursday and evaluated on Friday. The block measured approximately 9 ft (2.7 m) thick x 8.5 ft (2.6 m) wide x 50 ft (15.2 m) long. A track hoe was used to scrape the limestone and evaluate the condition and apparent stability of the block. After evaluation, all parties agreed to leave the block in-place and modify the geogrid placement in this area. The decision was based on the size of the limestone block, the lack of movement when subjected to impact loads by large construction equipment, and the knowledge that excavated material downslope of the block would be quickly replaced. Design section construction including basket, geogrid, and crushed rock placement beside and below the block continued on Friday and Saturday. Upon arrival on Monday morning, the block had moved downhill approximately 3 ft (0.9 m). The block was removed, resulting in 134.2 yd3 (102.6 m3) of rock excavation. Additional limestone floaters totaling 355.6 yd3 (271.9 m3) were encountered and removed on the western flank of the excavation. The position of the limestone floaters encountered on both the east and west ends of the excavation were believed to closely correspond to the actual limits of the original failure.
The drainage characteristics of the lower crushed stone material also necessitated a design modification. The specified crushed stone material for the lower design section permitted between two and 10 percent fines. After observing field performance of the lower compacted layers and conducting qualitative laboratory testing on the permeability of the compacted material, additional drainage provisions were recommended. The minor design modification specified placement of a continuous "clean" crushed rock layer corresponding to the position of the outlet pipes. In addition, the contractor was directed to place a chimney of clean drainage rock at the back of the design section along the excavated shale face. The purpose of the vertical chimney was to capture groundwater at the back of the design section while the horizontal layer would provide connectivity to the outlet pipes. This modification provided improved drainage provisions with minimal cost implications.
After a slow start because of rain, the majority of the earthwork for the reinforced slope was completed by early August 2002, within the requirements of the specifications. Placement of the concrete curb and asphalt roadway section was completed on September 6, 2002. Finally, seeding and placement of an erosion control blanket was completed by September 10 (Figure 7).
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Conclusion
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A landslide occurred along the north edge of Foxridge Drive in July 2001. The two-lane roadway is positioned at approximately mid-height on an overall 20° slope and was scheduled for widening in the near future. Turkey Creek is located at the toe of the slope with the slope positioned on the cut bank side of a meander bend.
A phased subsurface investigation indicated that the subsurface stratigraphy consisted of a variable amount of low plastic clay fill with abundant shale and limestone fragments overlying alternating layers of limestone and shale bedrock. Immediately beneath the roadway and to the north, the fill material was underlain by the Cement City Limestone, Quivira Shale, and Westerville Limestone. Further north on the hillslope, the Cement City Limestone was absent and the fill material was present immediately overlying the Quivira Shale. The investigation also included the installation of an inclinometer near the center of the slide mass to monitor additional movement and evaluate the position of the potential slip surface.
Slope stability analysis using UTEXAS3 software indicated that the probable slip surface was at the contact between the fill and the Quivira Shale. Evaluation of recent rainfall data indicated that the likely cause of the landslide was a rise in the groundwater as a result of the wetter than normal spring. Total rainfall in the 3 months before the landslide was more than 9 in. (23 cm) more than normal.
Three repair options were developed and presented to the City. The chosen option used geogrid reinforcing to construct a vertical wall section at the base and a sloping section at the top while leaving the bank of Turkey Creek unaltered. The base consisted of geogrid reinforced crushed stone for strength and drainage, whereas the top section consisted of geogrid reinforced on-site clay and imported topsoil. This option did not disturb the bank of Turkey Creek and enabled the re-use of on-site material. Furthermore, the reinforced crushed stone base was constructed with a nearly vertical face using welded-wire baskets and geogrid reinforcing to create an essentially free-standing base.
Construction of the repair began in April 2002 and was completed by September 2002. Currently, the repaired slope has a thick grass cover and is performing well. As anticipated, Turkey Creek is continuing to erode the top of the slope; however, the design section has not been exposed.
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ACKNOWLEDGMENTS
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The author wishes to thank Denny Retter, Andrea Prince, and Stephen Weeks for their review and support during the preparation of this manuscript.
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REFERENCES CITED
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Kansas Department of Transportation, 1990, Standard Specification for State Road and Bridge Construction, Kansas Department of Transportation, Division of Printing, Department of Administration Topeka, KS. 1154.
O'Connor, H.G., 1971, Geology and Ground-Water Resources of Johnson County, Northeastern Kansas, Kansas Geological Survey, Bulletin 203, Kansas Geological Survey Lawrence, KS. 68.
Retter, P.D., 2002, URS Corporation Overland Park, KS personal communication,.
Wright, S.G., 1990 (Revised 1991), UTEXAS3: A Computer Program for Slope Stability Calculations, Shinoak Software Austin, TX. 104.
URS Corporation, 1995, Geologic Columnar Section in the Kansas City Area: URS Corporation Overland Park, KS Unpublished work compiled by J. P. Garber,. 1.
Copyright © 2009 by Association of Engineering Geologists
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ABSTRACT
Introduction
