Due to the shallow hard bedrock observed throughout the peninsula, it was decided that it was not worth drilling the site. Although there was a thin (less than 1-foot) layer of gravels in some areas, they are not uniformly across the surface and bedrock was clearly visible throughout the peninsula. Therefore, this site is not a good candidate for fill material due to the shallow shale which is poor quality as potential fill material.
The Iron Creek Boat Ramp Borrow Area (Map 4, Appendix A) is located on the west side of the boat ramp in an area containing willows and grasses. We understand from the CPW that this small hillside erodes onto the boat ramp, covering it with mud, making it a maintenance issue. Therefore, removing some of this hillside will make the boat ramp more usable. The following photographs show the nature of this hillside and the potential borrow area.
Iron Creek Boat Ramp Borrow Area: Photo 9 (left) is a view up to the borrow area and Photo 10 (right) is a view looking north at BH#1 in the borrow area.
Borehole BH#1 was drilled to 26.5 feet and we found 1 foot of light brown sandy gravel underlain by silty/clayey fine sand to 6 feet and clayey silt to 7 feet. A layer of clayey gravel with sand was encountered from 7 to 13 feet. From 13 to 26.5 feet is silty clay with some gravel. Standard Penetration Tests (SPT) blow counts of 12 to 36 blows per foot (bpf) were recorded from 10 to 25 feet in 5-foot increments. This indicates stiff to hard conditions. No shale bedrock or groundwater was found to the depth of 26.5 feet.
In the causeway boreholes (BH#2 and BH#3), we found different conditions in the two holes. In BH#2 (south hole) we found 2 feet of gravel and cobble fill material due to its position in the road shoulder and soft silty sand fill material to 4 feet. From 4 to 11 feet is brown, stiff, dry, clay with gravels. Groundwater was encountered at 11 feet where the clay transitioned to formational Mancos Shale. From 11 to 21.5 was Mancos Shale that was brown, moist to wet, hard, highly weathered shale with abundant salts and clay. Some of this shale contained thin layers of weathered siltstone and sandstone, but it was mostly claystone. The southern borehole was terminated at 21.5 feet without reaching shale. N-values in the entire column ranged from 17 to 33 bpf. In BH#3 (north hole), we found 1 foot of dark brown topsoil underlain by brown, moist, fine sandy/clayey silt to silty/fine sandy clay to 10.5 feet. A gravelly
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clay layer extended from 10.5 to 13 feet and from 13 to 21.5 feet was clayey sand and sandy clay with some gravels below 18 feet. No shale groundwater was found in this borehole. N- values of 7 to 24 bpf were recorded in 5 foot increments from 5 to 20 feet. This indicates firm to very stiff conditions. The following photographs were taken of the soil and shale from BH#2 and BH#3.
Causeway Trail soils: Left photograph (Photo 11) shows the clayey soils found in the causeway from BH#3 at 20 feet (north side of causeway) and right photograph (Photo 12) shows the weathered shale that underlies the clay in BH#2 at 15 feet (south side of causeway). Note the abundant salts in the shale layers of Photo 8.
6.1.3 Campground Roadway
Boreholes BH#4 and BH#5 were located at the south and north ends of the loop road of the Clear Fork Campground, respectively (Map 5, Appendix A). Both boreholes were drilled adjacent to the asphalt edge and there was brown, moist, fine sandy silty clay to clay in the upper 4.5 feet. In BH#4 (south), this clay extended to 11.5 feet. In BH#5 (north), the clay transitioned to highly weathered Mancos Shale with clay soil to 10.5 feet. No groundwater was found in either hole to these depths. N-values of 11 to 21 bpf were recorded in the silty clay to clay soil, while N-values of 53 bpf to 75 blows per 10 inches were recorded. The following photographs show the nature of the pavement at the two drilling sites for roadway evaluation.
Campground Loop Road: Photo 13 (left) is at BH#4 and Photo 14 (right) is at BH#5. Note the “alligatored” and failing pavement.
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We understand that the Clear Fork Campground has received pavement as available from the County, so existing subgrade and base preparation may not have been constructed to a specific standard and the pavement thickness appears to be variable. It is the intent to have the entire campground loop road repaved.
6.1.4 Campground Bathroom/Shower Facility
Borehole BH#6 is located at the proposed bathroom/shower Facility (Map 5, Appendix A). At this location, we found 8 feet of moist, stiff, silty clay to clay until highly weathered formational Mancos Shale was reached at 13 feet. The weathered shale continued to 20.5 feet and it contained clay soil throughout. N-values in the shallower clayey soil were 9 to 13 bpf, while the shale had N-values of 75 and 79 bpf 10 and 20 feet and 55 blows in 5 inches at 15 feet. No groundwater was encountered to 20.5 feet. The following photographs show the nature of the bathroom/shower facility site.
Campground Loop Road: Photo 15 (left) is a view to the north and Photo 16 (right) is a view to the east of the building site. Note the gentle slope to the west.
6.2 Laboratory testing
Laboratory tests were performed on the predominant native soil types to evaluate the range of plasticity and particle size characteristics (Appendix C). These results are also summarized on each of the borehole logs (Appendix B). Five Atterberg limits tests were performed on samples of the clayey soils. The five samples have liquid limits (LL) of 31 to 52, plastic limits (PL) of 16 to 18, and plasticity indices (PI) of 13 to 34. A soil with a PI of less than 15 is considered to have a low potential for swelling when wetted and shrinking when dried, a soil with a PI of between 15 and 30 is considered to have moderate potential for swelling, and a soil with a PI over 30 can be expected to have a high probability of swelling. These ranges of PI indicate a wide range of clay behavior from low to high plasticity.
Gradation analyses (Particle Size Distribution) performed on six samples indicate that the clayey soils are composed of 56 to 93% silt and clay, which means that their behavior is dominated by fine-grained soil materials. One soil sample, taken from a depth of 10-11.5 feet in the Iron
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Creek Boat Ramp Borrow area, is more granular and is composed of 48% gravel, 35% sand, and 17% silt and clay (sample DS2). Based on these laboratory test results, most of the fine- grained soils classify as lean clays (CL) with variable amounts of sand and gravel and one (sample DS18 from BH#6) classified as a fat clay (CH), according to the Unified Soil Classification System (USCS). The more granular sample (sample DS2 from BH#1) classified as clayey gravel with sand (GC).
Three swell/consolidation tests were performed on samples taken from BH#5 (north roadway) and BH#6 (proposed bathroom/shower facility site). Under a seating pressure of 100 pounds per square foot (psf) and left at their in-situ moisture contents of 10 to 19%, the samples compressed 0.4 to 0.9%. When inundated with water at constant stress, the samples swelled 1.5 to 4.8%. Upon the addition of progressively increasing pressures to 2,000 psf, the samples consolidated a total of 4.1 to 4.9%. The initial dry densities of these samples were 107, 109 and 119 pounds per cubic foot (pcf). The estimated constant volume swelling pressures generated within the samples are 680 to 2,390 psf. These indicate moderate to high swell potential and swelling pressure.
Soil strength and moisture-density (Standard Proctor) tests were performed on bulk samples obtained from BH#3 at the north end of the causeway and from BH#5 at the north end of the campground loop road. The Standard Proctors were 115.4 pcf at 15.2% moisture content and 116.4 at 11.4% moisture content, respectively. The California Bearing Capacity (CBR) test result performed on the roadway sample from BH#5 was 2.4 at a 95% Maximum Dry Density (MDD) of 110.6 pcf.
A series of geochemical tests were conducted on soil sample DS14, taken from a depth of 5 to 6.5 feet in BH#4 (south loop road). The soil sample had a water soluble sulfate concentration of 0.32%, a chloride content of 0.008%, an electro-conductivity of 1,293 µS/cm, and a pH of 7.3. The water soluble sulfates content and electro-conductivity are indicative of corrosive soil. Recommendations for addressing the corrosive nature of the soil are presented in the Recommendations Section of this report.
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ENGINEERING ANALYSIS
7.1 Borrow Material Suitability Analysis
As discussed in Section 6.1.1, it was determined that the Peninsula Boat Ramp borrow area (Map 3, Appendix A) was unsuitable as a borrow area due to shallow shale bedrock. However, the Iron Creek Boat Ramp borrow area (Map 4, Appendix A) has good potential for embankment fill material due to its cohesive clay component and some rock content. Rocks appear to be mostly gravel-sized with little potential for over-sized material, except for the existing rip-rap. Shale is greater than 26.5 feet and groundwater should not be an issue if it is excavated in the fall or winter while the reservoir level is low.
7.2 Causeway Trail Slope Stability and Retaining Wall Design Parameters
Two proposed options for the causeway trail consist of constructing the trail by widening the road embankment with a 2H:1V (horizontal:vertical) slope or constructing the trail on an engineered retaining wall supported on the existing slope. This analysis checks the global slope stability and does not include the internal stability or retaining wall design.
Two conditions consisting of a full reservoir and a simulated rapid drawdown scenario were checked for each scenario. The highest embankment of approximately 30 feet was selected to represent the most susceptible profile. Generalized slope soil properties were developed from test borings’ BH#2 and BH#3 data, laboratory testing, and engineering judgement. The existing road embankment is assumed to be locally sourced clay like that encountered in the test borings. A sample was remolded to 95% of Standard Proctor with an unconfined compressive strength test to determine the undrained shear strength (Appendix C). Test results indicate at 5% strain, the undrained shear strength will be more than 1,000 psf. The existing clay was correlated using split spoon blow counts and the underlying weathered claystone bedrock was very hard and modeled with a cohesion of 5,000 psf. Table 1 presents the properties used in the models (Appendix D). A vehicle surcharge of 250 psf was applied at the top of the road embankment.
Table 1. Generalized Slope Soil Properties Unit Weight Estimated Friction Cohesion Soil Unit (pcf) Angle (Φ) (psf) Riprap 150 40 0 Free-Draining 125 36 0 Structural Fill Existing Road 115 0 1,000 Embankment Existing Clay 105 0 1,200 Weathered Claystone 125 0 5,000 Bedrock *As specified by the design engineer on project documents or in accordance with local municipal requirements.
Widened Slope. The widened slope stability with a full reservoir has a Factor of Safety (FS) of 3.3. A simulated rapid downdraw scenario has a FS of approximately 2.0.
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Retaining Wall. The retaining wall slope stability with a full reservoir is approximately 3.6. A simulated rapid downdraw scenario has a FS of approximately 2.0.
A sensitivity discussion is warranted given the test borings were only performed at the embankment fill margins due to accessibility issues and soil variations are possible. Soil variations towards silty soils such as silt or silty sand will increase the friction angle and reduce or remove cohesion which may lower the FS closer to 1.0 (less than 1.0 is a state of failure) and increase the likelihood of slope failure. We assumed 95% of Standard Proctor for the road embankment. Lower compaction will result in less undrained shear strength and lower FS.
Bearing Capacity. Bearing capacity to support the retaining wall design was calculated assuming the bottom of foundation was buried 2 feet or deeper on a slope 2H:1V or less steep. The foundation was assumed to be a strip footing 4 feet or wider. The supporting soil was assumed to be Existing Road Embankment material (Table 1) with an estimated undrained shear strength of 1,000 psf. The allowable bearing capacity for supporting the retaining wall is presented in the Recommendations section.
Sliding. Based on the UCS testing, the tolerances for strain will potentially control the acceptable adhesion at the bottom of the retaining wall. If 5% strain or more is acceptable, then 1,000 psf undrained shear strength provides a maximum adhesion of 750 psf following NAVFAC guidelines along the bottom of the retaining wall. Engineering judgement suggests that reducing this further to 500 psf adhesion, equivalent to NAVFAC’s soft to medium stiff cohesive soil, would reduce the strain potential and provide for soil variations that are likely at this site.
7.3 Bathroom/Shower Facility
The bathroom/shower facility will be a roughly 800-900 square foot, single story structure located at the north end of the Clear Fork Campground near BH#6 (Map 5, Appendix A). The soils at this site are clays with moderate to high swell potential underlain by highly weathered formational claystone at 8 feet. These soils are likely to have high potential for heaving a lightweight slab or shallow footing, such as is typical for a small structure like this. Exterior flatwork will also be subject to significant heave pressures. Therefore, it is recommended that the footprint of the building be kept as a simple square or rectangle so that it can be made rigid to resist flexure and that 2 feet of the clayey native soil is removed from under the bottom of footing and replaced with compacted structural fill. This removes the foundation components from direct contact with expansive soil, which reduces the potential for differential foundation movement. Frost depth is anticipated to be 36 inches at this site.
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7.4 Pavement Design
7.4.1 ESAL Calculation
To estimate average daily traffic, we consulted the ITE Trip Generation manual (8th edition) for a state park (Land Use Code 413). We assumed a total park area of 734 acres based on information provided by Colorado Parks and Wildlife staff. ITE estimated peak day (Sunday) ADT of 1.1 per acre or approximately 807 ADT for the roads and parking lots. Assuming that 95% of traffic to the Crawford State Park will be passenger vehicles and 5% will be single axle delivery trucks and recreational vehicles, we estimated the 18-kip ESAL’s as follows:
95% of ADT as passenger vehicles = 0.95 x 807 = 767 ADT 5% of ADT as single-axle trucks = 0.05 x 807 = 40 ADT 18-kip ESAL’s = 767 x 0.03 + 40 x 0.249 x 365 days per year x 20 years = 240,681
Therefore, 18K ESAL’s = 250,000 for the 20-year life span of flexible pavement for the site. Section 7.4.3 summarizes the calculated structural sections for an ESAL value of 250,000. To be conservative, we rounded down the CBR of 2.4 to 2.0 for the clayey native soil which is the subgrade under the proposed parking lots and roads. Using a general conversion equation for Resilient Modulus (MR) as follows:
MR = CBR x 1500 Where MR = Resilient Modulus The estimated MR for the subgrade is 3,000 psi.
7.4.2 Subgrade Support Characteristics
Table 2 (below) summarizes the typical subgrade support characteristics for the soils encountered. Assumptions for choosing the subgrade characteristics listed in Table 2 are:
(a) The pavement structures on site will be exposed to moisture levels approaching saturation more than 25% of the time and that these areas of potential saturation will have a “good” quality of drainage. No typical drainage information was available for the soils encountered on the site at the time of this report, therefore a CDOT recommended drainage coefficient of m1 = 1.0 was used.
(b) From CDOT’s Table 5.6 Drainage Quality (2014 Edition), it was assumed that water would be removed from all pavement structures within one day and so an overall drainage quality of “good” was used.
(c) A reliability factor of 75% was assumed due to the conservative ESAL count in Section (1) above. CDOT’s Table 1.3 – Reliability (Risk) recommends a range of reliability of 50-85 % for roads. However, 75% reliability was used to represent an acceptable long-term service life.
(d) A standard normal deviate (ZR) of -0.674 and a standard deviation of 0.44, as required by CDOT for all designs, was used.
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(e) Per CDOT recommendations, initial and terminal Serviceability Indices were assumed to be 4.5 and 2.5 respectively, thus a Design Serviceability Loss (ΔPSI) of 2.0 was calculated by subtracting the terminal serviceability index from the initial Serviceability Index.
Table 2. Pavement Thickness Design Factors Parameter Subgrade Resilient Modulus (psi) 3,000 Drainage coefficient 1.0 Reliability (%) 75 Standard Normal Deviate (ZR) -0.674 Standard Deviation 0.44 Serviceability Loss 2.0 Strength coefficients: HMA 0.44 ABC 0.12 SOIL-CEMENT 0.13 SUBGRADE 0.10
CDOT Equation 3.2 was used to calculate the required pavement section for flexible (asphalt) pavement. Equation 3.2 states: SN = a1D1 + a2D2m2 + a3D3m3 Where: a1, a2, a3 = structural layer coefficients D1 = thickness of bituminous surface course (inches) D2 = thickness of base course (inches) D3 = thickness of sub base (inches) m2 = drainage coefficient of base course m3 = drainage coefficient of sub base
The minimum recommended structural numbers (SN), were calculated to be 3.44 for 250,000 18 kips ESAL’s and 2.97 for 100,000 ESAL’s for the gravel road sections.
7.4.3 Pavement Section Selection
Based on the design criteria and calculations presented above, we identified two options for the flexible pavement section for the Crawford State Park parking lots and roadways. Alternative #1 is considered a more typical paving section and is suitable for the high number of anticipated turning movements in the parking lots at the park. Alternative #2 deletes the layer of Class 1 Aggregate Base Course (ABC) in favor of a thicker section of Class 6 ABC and is also appropriate for the high anticipated number of turning movements in the parking lots but has a slightly lower structural number than alterntative#1 in a thinner section. Both flexible pavement sections are considered suitable for use on the parks roads and parking lots and are recommended equally depending on the project costs of materials for the two alternatives. Alternative #3 is for a gravel road section that is suitable to use in non-paved areas of the park.
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Table 3. Pavement Section Alternatives Class 1 Sub-base Total Pavement HMA Class 6 Base Course Section Section SN Thickness Course Thickness (ABC) Thickness Alternatives (in.) (in.) Thickness (in.) (in.)
Alternative #1 3.50 4 6 9 19
Alternative #2 3.44 6 12 16
Alternative #3 3.00 - 10 16 26
Pavement section construction recommendations are presented in the Recommendations section of this report.
RECOMMENDATIONS
Based upon our limited site evaluation and results of our subsurface testing, it appears that the proposed improvements are feasible with special attention to foundation subgrade preparation, foundation design, causeway trail fill and associated retaining wall placement and design, general site preparation and drainage design. The following recommendations are offered to enhance the long-term performance of the foundation soils, foundations and site improvements. It should be noted that the measures offered address only the construction at the proposed building site. They cannot and will not arrest or prevent large-scale geologic processes that may be on-going elsewhere on the property and within the Crawford area.
8.1 General Design Criteria
1. Shallow components of the foundation system should be extended into the soil a minimum depth below finished grade as prescribed by the local building official to reduce the negative effects of frost heave.
2. In the context of climate change, geologic hazards such as flooding, debris flows, prolonged drought (causing plant stress, wildfires, disease and insect infestations), super rain and snow storms (i.e. 1,000 year events), rockfall, avalanche, etc. may become more common, intense, and destructive than previously observed and/or documented in recent history. Therefore, we encourage the owner to consider design measures that are more conservative and protective than have been historically employed and incorporated into jurisdictional “design standards.” Prediction of such events is beyond the scope of this report and as such, available information and studies which DOWL relies upon, may only meet what would be considered the current minimum standard of care of the local geotechnical engineering industry.
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8.2 Seismic Design Criteria
In accordance with Section 1613.5 of the 2006/2009 International Building Code (IBC) and our knowledge of the site, we recommend that this site be designated as Site Class C (very dense soil and soft rock with N>50). This classification is based on shallow exploratory data which encountered soft rock (N>50) at 10 feet below the ground surface, and assumes similar conditions for a depth of 100 feet. For Site Class C, the maximum spectral response acceleration at short periods (0.2 second, SMS) is 0.498g and at one second (SM1) is 0.144g. These values are taken from the USGS website based on the latitude and longitude coordinates for the site.
8.3 Bathroom/Shower Facility Foundation
Due to the potential for differential movement of the native subgrade expansive soils, we recommend a minimum of 2-feet of over-excavation and replacement with compacted structural fill (defined below in the Site Preparation and Grading Section). The following recommendations are provided to guide foundation design and construction.
1. The thickened edge slab-on-grade foundation should be placed on a minimum of 2 feet of imported structural fill should be designed using an allowable bearing capacity (qa) of 2,000 psf.
2. Below the thickened-edge and slab should be excavated to a minimum depth of 2-feet below bottom of the thickened-edge grade and with compacted structural fill. The excavation should extend a minimum distance equal to one-half the depth of over- excavation horizontally from the edge of the footings. Subgrade soils disturbed by excavation operations should be re-compacted prior to placement of the structural fill. If soft or yielding soils are encountered, DOWL should be contacted to assess the soil conditions and recommend remedial measures. Typical procedures involve removing soft/yielding subgrade soils to firm material and replacing them with compacted structural fill. A geotextile fabric for separation should be placed on the subgrade prior to placing the structural fill to reduce the migration of fines into the structural fill. The structural fill (Table 5), should be compacted in 6-inch lifts until the desired footing elevation is achieved.
3. Once the excavation is exposed, but prior to placement of any fill or footing forms, a representative of DOWL must be called out to verify the nature and density of the foundation excavations and cut slope materials, to ensure that relatively uniform soil conditions are present and to confirm that our recommendations are consistent with actual conditions. If we do not verify the soil conditions, DOWL cannot be held responsible for recommendations that may be inconsistent with actual conditions.
4. Observation and testing during construction is essential to ensure that the geotechnical recommendations are consistent with conditions and that the project is constructed in general conformance with project design and specifications. Any geotechnical observations or testing will be provided at additional charge and we should be contacted
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at least 2 days in advance for scheduling site visits. In addition to excavation observations, we can provide observation and testing of soil density, concrete and grout, foundation forms and rebar, pile installation, steel, welds, grading features, and drain systems.
5. All concrete used in foundation components at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.
8.4 Bathroom/Shower Facility Slab-On-Grade Considerations
A thickened-edge slab-on-grade foundation is recommended to control differential movement due to swelling clay soil. These are slab considerations to help guide design.
1. To provide an adequate bearing surface, topsoil and organic material should be stripped. The subgrade material should be proof-compacted and soft spots removed and replaced with washed rock or structural fill. The 2-foot over-excavation and replacement with structural fill must extend below the entire slab to reduce differential slab movement. If additional fill is needed to elevate the slab area to the desired foundation grade, this can be accomplished using structural fill.
2. To provide a capillary break, the slab-on-grade should be placed on 4 inches of ¾-inch to 1½-inch washed rock on the prepared subgrade. Where moisture-sensitive interior floor finishes are applied to the slab, an unpunctured vapor barrier between the gravel and the floor slab is also recommended.
3. Under-slab plumbing should be avoided to minimize the potential for leakage under the slab. Where necessary, under-slab plumbing should be provided with flexible couplings and should be leak-tested prior to being placed in service.
4. All concrete slabs used at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.
8.5 Campground and Causeway Retaining Structures
Retaining walls are not anticipated in the campground, but are an option, and are being considered along the Causeway. The following recommendations should be followed for retaining structures. Additional recommendations are provided in the Causeway Trail Stability Section.
1. Walls acting to restrain soil should be designed using the lateral earth pressures given in Table 4 below. These values assume a level backslope (slope behind the walls) or outboard slope (slope below the toe of wall), no hydraulic pressures behind the wall, the use of “free-draining” structural fill, and no surcharge loads applied within the backslope zone. Native clays and shale soils are not recommended for backfill due to their low permeability and swell potential. We should be contacted to recommend
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modified lateral earth pressure values for increased backslope angles, decreased outboard slope angles or loading within the backslope zone.
Table 4. Lateral Earth Pressures
Structural Fill Active Earth Pressure 34 pcf* Passive Earth Pressure 400 pcf* At-Rest Earth Pressure 54 pcf* Unit weight of soil 120 pcf** Coefficient of Friction 0.32 ***
* pounds per cubic foot (fluid equivalent) ** pounds per cubic foot *** concrete on dry soil conditions
2. Retaining walls should have provisions for drainage so that hydrostatic pressures are relieved. This is usually accomplished by providing free-draining granular backfill between the wall and retained soil, with a collection drain provided at the bottom of this granular zone, and/or the use of weep holes through the face of the wall. The drain system should be continuous and have a positive outfall which releases the collected water well away from the wall in a manner that minimizes the erosive energy of concentrated flow. The design engineer should ensure that drainage design is compatible with design assumptions.
3. Excavations for retaining and foundation walls should be laid back a minimum of 35° from the vertical prior to backfilling against retaining structures. For safety, excavations should also be in accordance with OSHA Regulations 29 CFR 1926. Consequently, gentler excavation faces may be required.
4. Fill material placed behind the walls should consist of free-draining granular material compacted as per the design engineer’s specifications. Native soil should not be used as backfill due to the predominance of fines and their expansive qualities. Compaction to 90% of Standard Proctor maximum dry density is typically used to minimize post- construction settlement of the backfill. Over-compaction of the backfill should be avoided so that excessive pressures are not placed against the retaining wall. Unless expressly approved by the design engineer, only hand-operated light-duty compaction equipment should be used within three feet of the wall. The upper one foot of backfill should consist of clayey (i.e., less permeable) soil to create a barrier against infiltration of surface runoff.
5. All concrete used for retaining structures at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.
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8.6 Bathroom/Shower Facility Foundation Drainage and Ventilation
It is important to prevent moisture penetration into the soil beneath or adjacent to the structure. Moisture can accumulate due to poor surface drainage, drywell and infiltration systems, over-irrigation of landscaped areas, waterline leaks, melting snow, subsurface seepage, or condensation from vapor transport.
1. Provisions should be made to direct water away from foundations and under slabs. This may be accomplished using conventional footing drains. Alternatively, consideration may be given to using concrete forms that facilitate both dewatering and the removal of radon gases.
2. Perimeter foundation drains should be constructed (including a discharge to daylight) as soon as the foundation excavation is complete. This will minimize the accumulation of standing water in the excavation which can soften and otherwise weaken foundation soils.
3. Roof drainage should be captured by eave gutters. Downspouts should be fitted with extensions to discharge a minimum of 10 feet away from the structure or piped into a closed underground drain system and evacuated off-site. In no case should the downspouts be directed into the perforated foundation drain system. These points of discharge should be identified in the site drainage plan so that water is readily removed from the site.
4. All foundation drains should be integrated into the site drainage plan as discussed below for final disposal from the building site. In no case should surface or roof drainage be introduced into the foundation drain system.
5. Floor systems and confined areas above concrete floor slabs should be properly ventilated to allow for the release of radon gas.
8.7 Site Preparation and Grading
1. The site drainage plan, in tandem with the landscape and grading plans, should ensure that the construction does not impede natural drainage patterns. Surface water should be directed away from the building foundation both during and after completion of construction. This includes water from landscaped areas, patios, decks, drywell systems, infiltration galleries and roofs. Drainage plans should ensure that precipitation, snowmelt, and runoff are conveyed around and away from the building as well as the driveway. This runoff should be dispersed (not concentrated) in a manner consistent with the natural, pre-construction drainage pattern.
2. Final grading around the perimeter of the foundation should slope positively away from foundation areas. Concrete flatwork adjacent to the foundation should slope away at a grade of at least ¼-inch per foot.
3. Development should utilize "best practices" for design and construction so that on-site erosion is minimized. This may include selective thinning of vegetation, construction of
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temporary diversion ditches, silt fencing, and/or dust suppression. The local building official will be able to provide specific details regarding these requirements. Fill needed on sloped areas should be placed on benched keyways to allow adequate placement and compaction of fill material. DOWL should observe cut slopes for visual evidence of seepage or planes of weakness and offer remedial measures as appropriate.
4. Grading of all permanent cut and fill slopes should not exceed 2H:1V. Existing or created permanent slopes greater than 2H:1V and over 3 feet in vertical height upon which permanent improvements are constructed and/or where retention or enhancement of current slope stability is desired, should be restrained by an engineered retaining structure/system.
5. Irrigation of landscaped areas should be kept at a distance of at least 5 feet from the perimeter of the building and sprinkler heads should be set to spray away from and not towards the foundation. Xeriscape landscaping practices are recommended for this site.
6. Backfill placed in utility trenches leading to the structure should be densely compacted in accordance with project specifications to inhibit surface water infiltration and migration towards the foundation, as well as minimize post-construction settlement of the trench backfill. We recommend low-permeability check-dams be installed in the trenches at the lot line and the structure to inhibit water flow along any utility trenches.
7. Fill used at this site should meet the gradational and compaction requirements listed in Tables 4 and 5 below. Fill should be placed and compacted in maximum 6-inch lifts, unless otherwise directed by the design engineer. Structural fill should not be placed on frozen or wet existing soil or fill material. The foundation excavation should be open a minimal amount of time to avoid degradation of the foundation soils.
Table 5. Gradation Requirements for Fill Material
Type Sieve %Passing, by weight Structural Fill (CDOT Class 6 roadbase) 3/4” (19.0 mm) 100 #4 (4.75 mm) 30-65 #8 (2.36 mm) 25-55 #200 (0.075 mm) 3-12
Structural Fill (CDOT Class 1) 2.5” (63.5 mm) 100 2” (50 mm) 95-100 #4 (4.75 mm) 30-65 #200 (0.075 mm) 3-15
Fill under exterior concrete flatwork 3” (75 mm) 100 #200 (0.075 mm) 0-5
Free-draining fill 3” (75 mm) 100 ¾” (19 mm) 20-90 #4 (4.75 mm) 0-20 #200 (0.075 mm) 0-3 Note: The Plasticity Index for all fill soils should be less than 6.
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Table 6. Compaction Requirements for Fill Material Compaction Application Proctor Moisture Requirement Under footings and slabs 95% max. dry density Modified ±2% of optimum Under exterior flatwork 90% max. dry density Modified ±2% of optimum Road Subgrade 95% max. dry density Standard 0-4% above optimum Road Subbase 95% max. dry density Modified ±2% of optimum Road base course 95% max. dry density Modified ±2% of optimum Behind retaining walls Per project specifications* Utility Trenches Per project specifications* General landscaping Per project specifications* *As specified by the design engineer on project documents or in accordance with local municipal requirements.
8. Any soils containing organics, debris, topsoil, frozen soil, snow, ice, and other deleterious materials shall not be used for anything other than landscaping.
9. A representative of DOWL should be called out to the site to observe placement of structural fill and verify the compacted density. The owner should contact DOWL in advance of the excavations to discuss the specific testing requirements, budget, and scheduling needed for these services.
8.8 Concrete
A water-soluble sulfate test conducted on a sample of the soil from BH#4 showed sulfate concentrations of 0.32% Therefore, we recommend that the cementitious material requirements for Class 2 sulfate exposure in Section 601.04 of the latest edition of the CDOT Specifications for Road and Bridge Construction be consulted and followed.
8.9 Exterior Concrete Flatwork
1. Flatwork may be placed on undisturbed native soil with the topsoil and organic material removed. If fill is needed, it should consist of washed rock or structural fill (see Tables 4 and 5), placed and compacted in accordance with project specifications. As previously discussed, flatwork and other improvements supported on the lightly compacted backfill will likely settle over time. Such improvements should be designed to accommodate such settlement or be founded through the backfill on native undisturbed soils.
2. Flatwork adjacent to buildings should be placed on properly compacted fill. To minimize future settlement and damage to the flatwork and/or adjacent foundations, the fill should consist of approved material placed and compacted per project specifications.
3. Flatwork adjacent to exterior doorways should be dowelled into the foundation to reduce long-term differential movement between the flatwork and structure.
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4. Exterior concrete flatwork should be designed and constructed so that it drains freely away from the structure. Concrete flatwork adjacent to the foundation should slope away at a grade of at least ¼-inch per foot.
5. All concrete used at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.
8.10 Excavation and Shoring
1. Temporary excavations should be in accordance with Occupational Safety and Health Administration (OSHA) regulations and with worker safety in mind.
2. Construction equipment, materials, and soil stockpiles should be located a minimum horizontal distance equal to the height of the excavation from the crest of the excavation unless otherwise approved by the design engineer.
3. Based upon our evaluation, the silty clay to clay found in our boreholes would be most nearly represented by an OSHA Type A soil. We note, however, that the recommended excavation slope angles for this classification do not consider topographic slope angle which must be accounted for when excavating. Our assessment is based upon the soil and groundwater conditions found in our limited evaluation and sampling. The contractor’s “competent person” (defined by OSHA as “an individual capable of identifying existing and predictable hazards…and who has the authorization to take prompt corrective measures to eliminate or manage these hazards and conditions) should evaluate the soil materials exposed during excavation based on composition, structure, and environmental conditions per 29 CFR 1926 and recommend appropriate slope laybacks or shoring, as required. Refer to OSHA’s Technical Manual Section V: Chapter 2 on Excavations: Hazard Recognition in Trenching and Shoring (available on-line at: www.osha.gov) for further excavation guidelines. We can provide these services, as requested.
4. If the excavations will be made or remain open during wet weather, it is recommended that polyethylene sheeting be secured over the excavation face to minimize sediment runoff and deterioration of the foundation soils. Surface runoff above the cuts should be directed away from the excavation using berms or diversion ditches. Water should not be allowed to accumulate and/or pond on the soils and shales in the construction area. It must be removed by gravity or pumped to avoid this condition until permanent drainage systems are operational.
5. We anticipate that the excavation of the site soils can be accomplished by conventional excavating equipment.
8.11 Pavement Section Design
The following recommendations are offered for pavement design based on the analysis presented in Section 7.4 of this report.
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1. To provide a stable base for construction of the recommended pavement sections presented above, we recommend that the upper 12 inches of the existing native subgrade soil be scarified and re-compacted to 95% of Standard Proctor (AASHTO T-99) maximum density, at +/- 2% of optimum moisture content. We then recommend the installation of Alternative #1 with 9 inches of Class 1 sub-base, 6 inches of Class 6 aggregate base course (ABC) and 4 inches of hot mix asphalt as a structural section. The Class 1 and Class 6 courses should be compacted to a minimum of 95% of a Modified Proctor maximum density (AASHTO T-180) at +/- 2% of optimum moisture content.
2. Based on material and construction costs and overall section thickness, we recommend that Alternative #1 (See Section 7.4.3), be used for the roadway and parking lot sections and that Alternative #3 be used in non-paved areas of the park. Alternate #2 can be used instead of Alternative #1 if it is desired to have thicker asphalt and thinner fill material.
3. Design and construction of the roadway should promote drainage away from the paved areas. Where needed, roadside ditches should either be constructed or modified to accept road drainage.
4. All paving construction activities should be monitored and tested by a competent civil/geotechnical engineering firm for compliance with the recommendations contained in this report and with the specifications in the latest edition of the CDOT Standard Specifications for Roads and Bridge Construction.
8.12 Causeway Trail Stability
The two proposed trail options are both feasible for construction and are globally stable as discussed in the Engineering Analysis section. The following recommendations present earthwork recommendations, construction considerations, and retaining wall design parameters, however it does not include design of the retaining wall’s internal stability.
1. Areas receiving revetment should be prepared in accordance with the Site Preparation and Grading section.
2. Areas being excavated should be done according to the guidelines presented in the Excavation and Shoring section.
3. For revetment filter design to avoid piping, subgrade material is anticipated to be fine- grained clay represented by the laboratory sieves (Appendix C) from test borings BH#2 (Lab sample DS6) and BH#3 (DS10) with 56 to 80 percent passing the #200 sieve.
4. The retaining wall recommendations presented previously are applicable.
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5. The concrete portion of the retaining wall foundation is anticipated to be over-excavated approximately 2 feet and should be replaced with free draining structural fill meeting the gradation in Table 5.
6. A separation filter such as nonwoven geotextile or designed filter material is recommended between free draining structural fill and the fine-grained embankment to avoid infiltration of fines from piping due to changing reservoir levels. Contamination of the structural fill with fines will reduce its drainage capability and shear strength.
7. Allowable bearing capacity for the Causeway retaining wall may be assumed to be 1,500 psf with a FS of 3.
8. Adhesion to resist sliding along the base of the retaining wall placed on existing road embankment material comprised of clay may use 500 psf.
9. Structural fill in the retaining wall backfill should be freely draining material and a separation filter is necessary to avoid infiltration of fines into the backfill due to piping from reservoir level changes.
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CLOSING CONSIDERATIONS
9.1 Standard of Care and Interpretation of Subsurface Data
This report has been prepared in a manner consistent with local standards of professional geotechnical engineering practice. As previously noted, we did not perform an evaluation of deep subsurface conditions. Evaluation of environmental contaminants was not part of our scope of services performed at this site. The classification of soils and interpretation of subsurface conditions is based on our training and years of experience, but is necessarily based on limited subsurface observation and testing. As such, inferred ground conditions cannot be guaranteed to be exact. No other warranty, express or implied, is made.
Observations of the excavation(s) subgrade by DOWL prior to erection of the foundation system are integral parts of these recommendations. If subsurface conditions differing from those described herein are discovered during excavation, construction should be stopped until the situation has been assessed by a representative of DOWL. Construction should be resumed only when remedies or design adjustments, as necessary, have been prescribed.
9.2 Use of This Report
This report is intended for use by the design team specifically to address the site and subsurface conditions as they relate to the proposed structure(s) described in the Construction Plans Section. Changes to the site or proposed development plans may alter or invalidate the recommendations contained herein.
DOWL retains an ownership and property interest in this report. Consistent with the industry, copies of this document that may be relied upon by the design team are limited to those that are signed and sealed by the Geotechnical Engineer (Standard Form of Agreement Between Owner and Geotechnical Engineer for Professional Services, Engineer’s Joint Contract Documents Committee, 1996). This report together with ancillary data, analyses, test results, and other components and/or supporting parts are not intended or represented to be suitable for reuse by the design team or others on extensions to this project or on any other project. Any such reuse or modification invalidates all aspects of the report and excuses the Geotechnical Engineer for all responsibility and liability or legal exposure.
This report is considered valid for a period of two years from the date of issue provided the site conditions and development plans have not changed from what is referenced in this report. Changes to the site may occur due to development or natural processes. Additionally, technological advances made in construction and changes in legislation may alter the recommendations made herein. Depending upon the site and proposed development changes, DOWL may require additional evaluation (at additional cost) to update the recommendations contained herein.
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