agency of transportation office memorandum...

AGENCY OF TRANSPORTATION OFFICE MEMORANDUM

To: Paul Libby, Project Manager, Highway Safety & Design

From: Eric Denardo, Geotechnical Engineer via Christopher C. Benda, P.E., Soils and

Foundations Engineer

Date: March 14, 2014

Subject: Wallingford STP 019-8(11) – Preliminary Geotechnical Recommendations

1.0 INTORDUCTION

It was requested of the Materials and Research Section to evaluate a slope stability issue on VT Route 140 east of Wallingford, Vermont. The area of concern lies 325 feet southwest of mile marker 3.0 continuing east along Route 140 for approximately 1400 feet. The failing slope is along the northern side of Route 140. Flooding of the Roaring Brook, which runs adjacent to the southern side of Route 140, during major storm events in recent years, is believed to have contributed to the slope failure. Figure 1 below displays a plan view of the area of concern. Contained herein are our site visit findings, and subsequent geotechnical recommendations.

Figure 1. Plan View of VT 140 and Slope

2.0 SITE INVESTIGATION

Based on site visits and a survey of the site, several important factors regarding the geology and geometry of the site were noted. The area of concern has a steep slope which extends from a drainage ditch at the bottom, up to the tree line, where the grade becomes less steep. The upper portion of the slope is heavily vegetated mostly with trees of varying diameter. Starting at the western end of the area of concern, extending 500 feet, the slope has a steep drop of approximately 20 feet from the tree line to the drainage ditch. This portion of the slope has been stripped of vegetation as seen in Figure 2. The slope to the east of this section is less steep but still has grades steeper than 1V:1.5H and may experience future failures. Possible boulders, cobbles, or bedrock

WALLINGFORD STP 019-8(11) of 12 outcrops can be seen in the area of the slope that has been stripped of vegetation as shown in Figure 2.

Figure 2. Slide Area ‒West End

The slides that have occurred in the past appear to be surficial in nature. The main cause of the slides is believed to be erosion of the toe of the slope during major storm events. The soil on the surface of the slope appears to be granular in nature. Currently the slope is as steep as 60 degrees in some areas, which means the material will continue to slough into the ditch until the soil on the face of the slope reaches its natural angle of repose. Material on the slope as well as trees near the scarp at the top of the slope will continue to become unstable and fall into the ditch at the toe of the slope. The top of the slope is currently stabilized by the vegetation and shallow grade. A factor that could be influencing the surficial sliding of the slope is an excess of ground water that funnels down the slope in defined channels. Figure 3 shows water flowing in a drainage channel. It appears as though the water could be seeping out of the face of the slope and into the channel which is also contributing to the stability issues.

WALLINGFORD STP 019-8(11) of 12

Figure 3. Water Drainage Channel

3.0 RECOMMENDATIONS

Due to the surficial nature of the slides, the steep grade, and the vertical height of the slope, we recommend a balanced approach be taken to address these slopes utilizing a few different remedial measures. Multiple options for remediation of the slope to an acceptable factor of safety were explored. Laying back of the slope was considered initially, however the large amount of excavation required to achieve 1V:1.5H was deemed not feasible. In addition, laying back of the slope would require removal of vegetation at the top of the slope which is currently aiding in the surficial stability of the upper portion of the slope. The next solution considered was the implementation of a retaining wall. A retaining wall system would involve the installation of a wall at the toe of the slope with fill graded to 1V:1.5H behind it. A modular block gravity retaining wall system is the recommended option for the less steep portions of the slope where the geometry is feasible. In sections where the height of modular block retaining walls required to re-grade the slope to 1V:1.5H exceeds 5 feet, other options should be implemented.


3.1 RETAINING WALLS Sta. 206+40-Sta. 208+20, Sta. 210+60-Sta. 212+60, Sta. 213+40-Sta. 215+00 Three sections of the slope will have a retaining wall maximum height of 5 feet, from approximately Sta. 206+40 to Sta. 208+20 (180 feet), Sta. 210+60 to Sta. 212+60 (200 feet), and Sta. 213+40 to 215+00 (160 feet). A typical cross section is shown in figure 4. These three sections together total 540 feet of proposed retaining wall. Due to the proximity of the slope to the right of way of Route 140, a modular block retaining wall that does not require a concrete footing should be used. This type of system will minimize excavation near the right of way of Route 140 and of the toe of the slope. The wall should be constructed on a one foot thick crushed stone leveling pad the bottom of which should be placed three feet below the ground surface and backfilled with free draining fill. A perforated drain pipe should be installed behind the retaining wall within a drainage aggregate and geotextile envelope. The slope should then be re-graded with granular borrow to 1V:1.5H up to the crest of the slope and surfaced with 1 foot of type I stone fill on top of the granular borrow. Based on the sections taken along these three areas, 5 feet should be the maximum height for the retaining walls to be used for the slope. We recommend two borings be taken for each section of retaining wall with continuous sampling for 12 feet. The six boring locations are noted in table 3.1.

Figure 4. Typical Cross Section‒Shallow Section (Retaining Wall)


Table 3.1 Proposed Boring Locations

Boring Station Offset (ft) Ground Elevation (ft) B-101 207+00 -18.0 1190.9 B-102 207+60 -19.0 1191.3 B-103 211+30 -18.0 1203.6 B-104 212+00 -19.0 1206.1 B-105 213+90 -17.0 1221.9 B-106 214+50 -17.0 1226.8

3.2 SLOPE STABILIZATION SYSTEMS Sta. 201+10-Sta. 206+40, Sta. 208+20-Sta. 210+60, Sta. 212+60-Sta. 213+30

3.2.1 WIRE MESH SOIL NAIL SYSTEM

Three sections within the area of concern were found to require retaining wall heights that were neither technically feasible nor cost effective. Alternative systems were considered for these three sections, which total 840 feet along Route 140. The first system considered for the steeper sections of the slope was a soil nail and wire mesh stabilization system such as Geobrugg Tecco Mesh. This system utilizes wire mesh over an array of soil nails. The concept of this alternative is that the nails will be drilled into soil or rock in the slope to aid in global stability while the wire mesh will prevent additional surficial slides that could occur along the slope. The sliding resistance needed is provided in the soil or rock, therefore the length of the nails may vary throughout the slope. Where used, the mesh will be implemented from the toe of the slope to the tree line. Images of what the wire mesh and soil nail stabilization system may look like in the field are included for reference in Figures 7 and 8.


Figure 7. Wire Mesh and Soil Nail Slope Stabilization System

Figure 8. Picture of system installed on a slope adjacent to a river


3.2.2 SOIL NAIL AND SHOTCRETE

Another system considered for implementation is an array of soil nails and shotcrete facing. Soil nails will be installed in the slope leaving the ends exposed. Shotcrete reinforced with rebar mesh will then be installed on the exposed soil nail array. The system will be implemented from the drainage ditch to the tree line. Similarly to the wire mesh system, the soil nails used will help to increase the global stability of the slope and the shotcrete facing will prevent future surficial slides and further erosion of the face of the slope. Horizontal drains are used at the base of the shotcrete to allow water to flow out of the slope. Figures 9 and 10 provide images of what the implemented soil nail and shotcrete system may look like.

Figure 9. Soil Nail and Shotcrete Slope Stabilization System


Figure 10. Shotcrete System Installed on a Slope in Hartford, VT

Because of the visibility of the slope and long sections of shotcrete required, aesthetics may be an important aspect of this system to consider. There are various treatments available for the shotcrete to provide a more natural aesthetic look. As shown in Figures 11 and 12, the shotcrete can be stained and made to look like natural rock or a patterned stone wall.


Figure 11. Shotcrete Sculpted to Look like Natural Rock


Figure 12. Patterned and Stained Shotcrete

As mentioned above, the top of the slope is heavily vegetated. This vegetation aids in surficial slope stability. For this reason, we recommend that any implemented stabilization systems limit removal of trees from the top of the slope. Further investigation is required before construction of the chosen remediation. We also recommend that tests for corrosion potential of the soil such as pH, Sulfates, Chlorides, and resistivity be performed on the soil. Unless aesthetics is a major problem and cannot be addressed with treatments as mentioned above, based on cost, we recommend the use of a soil nail and shotcrete stabilization system for remediation of the steeper portions of the slope. If problems arise while drilling for soil nails, additional borings or test pits may be needed at the discretion of the engineer. 4.0 COST ESTIMATE

A rough cost analysis can be found in Table 4.1. These costs are estimated based on 3 sections of retaining wall totaling 540 feet of the slide area and 3 sections of one of the above mentioned stabilization systems totaling 840 feet.


Table 4.1. Rough Cost Estimates for Remediation Options

Item Description Unit Unit Price Quantity Total

Retaining Walls

Sta. 206+40-208+20 210+60-212+60 213+40+215+00

Block Retaining Wall SF $52 4050 $210,600 Subbase of Crushed Stone CY $30.16 108.32 $3,267 Gravel Backfill for Slope

Stabilization CY $13.38 213.8 $2,861

6" Underdrain Pipe LF $16.63 540 $8,980 Granular Fill CY $9.20 170 $1,564

Geotextile SY $2.53 840 $2,125 Stone Fill Type I CY $40.50 146.312 $5,926

Excavation CY $8.90 560.36 $4,987.20 TOTAL $240,310

Soil Nail and Shotcrete

Sta.

201+10-206+40 208+20-210+60 212+60-213+30

Mobilization LS $7,000 1 $7,000 Up to 20' SuperNails Rebar Mesh,

Plates, and Shotcrete LF $742 840 $623,280

Horizontal Drain Every 10' (as needed) EA $575 84 $48,300

TOTAL $678,580 Tecco Wire Mesh

Sta.

201+10-206+40 208+20-210+60 212+60-213+30

Tecco Wire Mesh, Equipment, Grout, Hardware, etc. SF $101 23275 $2,350,775

The above costs are estimates only and can be refined as the design progresses.

5.0 CONCLUSION

Based on our preliminary analysis, we recommend a combination of modular block retaining walls and a system of soil nails and shotcrete facing to remediate the slope failure along Route 140. The proposed remediation consists of three sections of retaining wall totaling 540 feet and 840 feet of soil nail and shotcrete facing. Before construction, a total of 6 borings, 2 per section of retaining wall, should be completed and soils within the soil nailed area tested for corrosion potential. If a more natural aesthetic look is desired for the shotcrete, available treatments can be implemented.

The options proposed above we see as feasible remediation for this site. As the scope of work for the project becomes better defined, we are available to discuss and provide additional recommendations as needed.

cc: Electronic Read File/WEA Project File/CCB END

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