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flash floods that, as indicated by Mesa County Public Works, have over-topped the roadway inthis area. High velocity runoff can erode surface sediments and scour channel walls, causingwidening and/or down-cutting of drainages. Vegetation in the area consists of junipers,rabbitbrush, and other native high desert plants. This sparse vegetation does not provideuniform protective cover from erosion as evidenced by many small rills and gullies in the loose,soft soils and weathered sandstone. Protecting these soils from erosion near the roadway andwalls both during and after construction will be important to reduce soil loss and roaddegradation over time.4.2Flooding/Debris FlowsThe Atwell Gulch channel is dry for much of the year, but can carry large volumes of water inflash flood events. These flood events also carry muddy debris consisting of rocks, vegetation,and other loose materials picked by running water. The following photographs show the natureof the channel in Atwell Gulch.Photo 5 (left) is a view to the north at Atwell Gulch as BH#3 was being drilled. Notethe muddy and rocky sediment in the channel in this straight section of the arroyo.Photo 6 (right) is a view of the channel downstream of BH#5 where the stream hascut into sandstone bedrock.4.3RockfallGiven the uneven, weathered, and benched nature of the sandstone bedrock in this area, thereis potential for rockfall. As seen on the Site Plan, 45.5 Road hugs a rock outcrop on the westside of this road segment between BH#2 and BH#6. However, rockfall hazard from this areacurrently appears minimal due to the nature of the rock outcrops and distance from theroadway. Since no road widening on the west side of 45.5 Road is proposed with theimprovements, there is no increased risk from rockfall hazards and no mitigation is necessary.Other geologic hazards are not known to be present near this segment of 45.5 Road.Mesa County 45.5 Road Improvements MP0.6 FINAL geotech report.docxProject #7122.74818.01Page 3 of 19

SOIL CHARACTERISTICS5.1Field EvaluationA total of eight boreholes were drilled at this site using a tracked CME 55 rig, with four holesaugered near the road in embankment or native soils (BH#1, BH#2, BH#7, BH#8) and fourholes cored into the streambed of Atwell Gulch (BH#3 - BH#6), as indicated on the Site Plan(Appendix A). The locations of the borings were selected to evaluate the soils for retaining walldesign, determination of slope stability, and pavement design. The boreholes were drilled with a4-inch solid stem continuous flight auger for the roadway embankment soils and HQ core barrelwas used for coring in the streambed. Soil samples were obtained at discrete depths bywithdrawing the 4-inch drill string and inserting a standard 1.375-inch inside diameter (I.D.)split-spoon sampler without liners to perform in-situ Standard Penetration Tests (SPTs) ingeneral accordance with ASTM Standard D-1586. The number of blows required to drive thesampler 12 inches in 6-inch increments were recorded (SPT “N” penetration resistance values)and, when properly evaluated, indicate the relative density or consistency of the soils. For thecoring, upon reaching sandstone, the center drill string was removed and a HQ wireline coringassembly was lowered into the hole. Five-foot-long continuous core samples were retrieved asthe coring assembly progressed into the sandstone.The soil, bedrock, and groundwater conditions were logged and the borehole and core logs arepresented in Appendix B. Representative samples of soil and rock materials were tested in ourlaboratory with the results presented in Appendix C. Our embankment augered boreholesreached depths of 19.2 to 24.3 feet and the streambed cores reached depths of 5 to 14 feet.Our findings and recommendations are based on materials found within these depths and soilconditions may change between boreholes and below these depths. The following photographswere taken of the native soil and rock retrieved from BH#4, BH#5, and BH#8.Photo 4 (left) shows the soils collected from BH#8 at 19-20.5 feet, which isrepresentative of the slope colluvium and weathered bedrock of the area. Photo 5(right) shows the cores of sandstone from BH#4 (top) and BH#5 (bottom), which aretypical of the bedded nature of the sandstone in the bed of the drainage.In the embankment boreholes (BH#1, BH#2, BH#7, BH#8) we found light brown, tan to white,dry, moderately dense to very dense, silty to clayey sand with gravel and some cobbles. Ofthese four holes, all were dry except for BH#7, where a seep was recorded at a depth of 14Mesa County 45.5 Road Improvements MP0.6 FINAL geotech report.docxProject #7122.74818.01Page 4 of 19

feet. N-values of 10 blows per foot (bpf) to greater than 50 bpf were recorded in this colluviumand slope alluvium. The variability in density was due to the variable amount of rock within thesoil matrix. Weathered and poorly cemented sandstone was found underlying the colluvium at adepth of 9 to 22 feet at the north end of the study area and 14 feet at the south end.In the streambed cores (BH#3 – BH#6) we found variable amounts of alluvial gravels andsandstone fragments overlying sandstone bedrock of the Mesaverde Formation. This rock is atan, white, light brown to light green, poorly cemented, kaolinitic sandstone with some pebbleconglomerate. As seen in the core logs (Appendix B), total core recovery (TCR) was excellent(86-100%) and Rock Quality Designation (RQD) ranged from 0% to 95%, which is a measureof core segments greater than 4 inches. High-quality rock has an RQD of more than 75% andlow-quality rock has an RQD of less than 50%. In general, the core runs from BH#3 (includingsample CS1) and BH#4 (sample CS2) were higher quality than the core runs from BH#5(sample CS3) and BH#6. The core logs show that the fracture orientation, count, and typeindicate mostly bedding related (low angle) fractures with a Strength Index of R2 to R3 andWeathering Index of W1 to W2. In addition to the weak cementation and bedding structure,discontinuities caused by pebbles and joints added to the variable nature of the sandstonebedrock.Existing road sections were exposed with a shovel and measured at three locations:1. At the north end of the segment uphill of BH#3 – 6-inches of asphalt (2 layers of 3inches on 3-inches) over approximately 10-inches of Class 6 roadbase (3/4” minus withsome fines).2. Uphill of BH#4 – 4-inches of asphalt (single layer) over approximately 6-inches of Class6 roadbase over sandy native soil.3. At the south end uphill of BH#6 – 4-inches of asphalt (single layer) over 9-inches ofunknown gravelly fill.Bulk samples of the native soil material underlying the exposed road sections #1 and #2 abovewere blended and used for determining the California Bearing Ratio (CBR) and Standard Proctor(see Section 5.2). This material was determined to be representative of the native soilunderlying the existing roadway.5.2Laboratory testingLaboratory tests were performed on the predominant native soil types to evaluate physical andchemical characteristics. Individual laboratory results are included in Appendix C and aresummarized on the borehole logs (Appendix B). Atterberg limits tests determined that samplesfrom BH#1 and BH#2 were non-plastic, while samples from BH#7 and BH#8 have liquid limits(LL) of 21 and 25, plastic limits (PL) of 14, and plasticity indices (PI) of 7 and 11. A soil that isnon-plastic has very low potential for swelling and shrinking with changes in moisture contentand has little or no cohesion, while soil with a PI of less than 15 is considered to have a lowpotential for swelling when wetted and shrinking when dried. Gradation analyses performed onthese four samples indicate that the soils are composed of 10 to 24% clay, 16 to 31% silt, 41 to55% sand, and 4 to 30% gravel (samples DS1, DS5, DS11, DS15). Based on these laboratorytest results, these soils classify as silty/clayey sand to silty/clayey sand with gravel (SP, SC, SM)Mesa County 45.5 Road Improvements MP0.6 FINAL geotech report.docxProject #7122.74818.01Page 5 of 19

according to the Unified Soil Classification System (USCS). Due to the low cohesion of the soils,swell/consolidation testing was not performed.Unconfined Compressive Strength (UCS) tests were performed on three sandstone core samples(CS1 from BH#3, CS2 from BH#4, and CS3 from BH#5. These were the only core samples thatwere long enough and intact enough to test using this method. The three sandstone cores hadUCS’s of 4620 psi, 2970 psi, and 2190 psi, respectively.Soil strength and moisture-density (Standard Proctor) tests were performed on blended bulksamples of native soil obtained from a depth of 2 to 5 feet below the road surface above BH#3and BH#4, as previously discussed in Section 5.1. The Standard Proctor maximum density was127.6 pcf at 9.1% moisture content. The California Bearing Capacity (CBR) test result was 17.0at a 95% Molded Dry Density (MDD) of 121.2 pcf.A series of geochemical tests were conducted two soil samples: sample DS3 from 14-15.5 feetin BH#1 and sample DS16 from 14-15.5 feet in BH#7. The soil samples had water-solublesulfate concentrations of 0.01 and 0.03%, chloride contents of 0.008%, electro-conductivities of143 and 79 µS/cm, and pH values of 7.4 and 8.6, respectively. Recommendations foraddressing the potential for corrosive soil are presented in the Recommendations Section of thisreport.5.3Soils SummaryIn summary, the field observations and laboratory testing indicate that the soils to the depthsexplored have moderate to high density, are non-plastic to low plasticity, and are dominated bysand and rock fragments with variable amounts of silt and clay. The bedrock near the roadwayis 9 to 22 feet deep and it is weathered and poorly cemented, but generally less weathered withdepth. No shale or other continuous weak layers were found in the core samples obtained inthe streambed. This material will provide adequate foundation support with weaker, moreweathered surficial layers stripped.ENGINEERING ANALYSIS6.1Pavement Design6.1.1 ESAL CalculationDaily traffic volumes for 45.5 Road were provided in a TRAX Vehicle Type Report, datedJanuary 15, 2008, by Mr. James Nall of Mesa County and were generally broken out as followsfor the two curve sections:Total of 62461%28%11%vehicles for both lanespassenger cars and light trucks (2A-4T) 380 vehiclesTwo-axle, single unit trucks (2A-SU) 175 vehiclesMulti-axle trucks (all other classifications) 69 vehiclesMesa County 45.5 Road Improvements MP0.6 FINAL geotech report.docxProject #7122.74818.01Page 6 of 19

Each of these classification totals was multiplied by the appropriate Colorado Equivalency Factorfor flexible pavement from CDOT Table 1.1 to obtain a total daily ESAL count. Those resultsare:380 passenger car/pickup truck ADT x 0.003 1.14175 single unit truck x 0.249 43.5869 multi-axle trucks x 1.087 75.00The total of these three classifications is 119.72 daily ESAL’s. The daily ESAL total was thenmultiplied by 365 days per year and by a flexible pavement design life of 20 years to get thetotal number of 18-kip ESAL’s.That total is:119.72 x 365 x 20 873,978 18-Kip Total ESAL’sThis ESAL total was multiplied by 0.6 for a lane reduction factor to give a total 20-year designESAL count of 524,387. To account for potentially high multi-axle truck traffic for short periodsof time when the road is used to divert traffic from nearby Interstate-70, the ESAL count wasconservatively rounded up to 1,000,000 for design.Section 6.1.2 below summarizes the calculated structural sections for an ESAL value of1,000,000. Laboratory testing estimated an average CBR of 17 for the SP-SM-SC native soilsubgrade under the roadway. Using a general conversion equation for Resilient Modulus (MR) asfollows:MR CBR x 1500Where MR Resilient ModulusThe estimated MR for the subgrade is 25,500 psi.6.1.2 Subgrade Support CharacteristicsTable 1 (below) summarizes the typical subgrade support characteristics for the soilsencountered. Assumptions for choosing the subgrade characteristics listed in Table 1 are:(a) The pavement structures on-site will be exposed to moisture levels approachingsaturation more than 25% of the time and that these areas of potential saturation willhave a “good” quality of drainage. No typical drainage information was available for thesoils encountered on the site at the time of this report, therefore a CDOT recommendeddrainage coefficient of m1 1.0 was used.(b) From CDOT’s Table 5.6 Drainage Quality (2014 Edition), it was assumed that waterwould be removed from all pavement structures within one day and so an overalldrainage quality of “good” was used.(c) A reliability factor of 75% was assumed due to the conservative ESAL count inSection (1) above. CDOT’s Table 1.3 – Reliability (Risk) recommends a range ofreliability of 50-85 % for roads. However, 75% reliability was used to represent anacceptable long-term service life.Mesa County 45.5 Road Improvements MP0.6 FINAL geotech report.docxProject #7122.74818.01Page 7 of 19

(d) A standard normal deviate (ZR) of -0.674 and a standard deviation of 0.44, asrequired by CDOT for all designs, was used.(e) Per CDOT recommendations, initial and terminal Serviceability Indices were assumedto be 4.5 and 2.5 respectively, thus a Design Serviceability Loss (ΔPSI) of 2.0 wascalculated by subtracting the terminal Serviceability Index from the initial ServiceabilityIndex.Table 1. Pavement Thickness Design FactorsParameterResilient Modulus (psi)Drainage coefficientReliability (%)Standard Normal Deviate (ZR)Standard DeviationServiceability LossStrength 001.075-0.6740.442.00.440.120.130.10CDOT Equation 3.2 was used to calculate the required pavement section for flexible (asphalt)pavement. Equation 3.2 states: SN a1D1 a2D2m2 a3D3m3Where:a1, a2, a3 D1 D2 D3 m2 m3 structural layer coefficientsthickness of bituminous surface course (inches)thickness of base course (inches)thickness of sub base (inches)drainage coefficient of base coursedrainage coefficient of sub baseThe minimum recommended structural number (SN), was calculated to be 1.95 for 1,000,00018 kips ESAL’s for 45.5 Road.6.1.3 Pavem ent Section SelectionBased on the design criteria and calculations presented above, we identified three options forthe pavement section for the pavement sections at this project location. All three alternativesuse hot mix asphalt (HMA) flexible pavement. Alternative #2 is a more typical conservativesection and replicates portions of the existing road section, thus this is the recommendedsection. Alternative #1 is a minimally acceptable section, but is not recommended due tothinner HMA which is not likely to be as durable given the heavy truck traffic on this road.Alternative #3 is full-depth asphalt section on native subgrade and is not recommended.Mesa County 45.5 Road Improvements MP0.6 FINAL geotech report.docxProject #7122.74818.01Page 8 of 19

Table 2. Pavement Section AlternativesSNHMAThickness(in.)Class 6 BaseCourseThickness(in.)Class 1 Sub-baseCourse (ABC)Thickness (in.)Total SectionThickness(in.)Alternative #12.646010Alternative #24.32612018Alternative #32.646006PavementSectionAlternativesPavement section construction recommendations are presented in the Recommendationssection of this report.6.2Retaining Wall Global Stability AnalysisIn the DOWL 30% progress plans for this project, it was determined that retaining walls areneeded for two locations in this road segment to protect the road from undercutting by thestream in Atwell Gulch. Adjacent to both retaining walls, grouted rip rap to armor theembankment from scouring during flood flows in the drainage is recommended. In some areas,ungrouted rip rap may be utilized depending on slope to

420 geotextile (separator) (mirafi 180n) s.y. 730 420 geotextile (reinforcement) (miragrid 5xt) s.y. 180 504 toe wall s.f.f. 1365 504 gabion wall s.f.f. 1494 504 rock dowel l.f. 940 504 hilfiker spiralnail l.f. 1760 506 riprap (d50 24 inch) (furnish only) c.y. 685 507 grouted riprap (d50 24-inch) c.y. 685