Tamarack Ranch SWA - Recharge Pond and Pipelines Phase II - Bid Package (5 of 5).pdf

Tamarack Ranch SWA Recharge Pond and Pipelines - Phase II

DIVISION IV – REPORTS

PROJECT I.D. NO. C16NE6C

RQS: 2017*763

APPR. CODE PC002INFP

MANDATORY PRE-BID CONFERENCE: March 30, 2017 at 11:30 am

BID OPENING: April 13, 2017 at 1:30 pm

TABLE OF CONTENTS

Tamarack Ranch SWA Recharge Pond and Pipelines - Phase II

C16NE6C

• Trechless Feasibility Study including Boring Logs by Lithos Engineering dated April 25, 2016

Engineering from the ground down

TRENCHLESS FEASIBILITY STUDY

INTERSTATE 76 CROSSING AT CROOK COLORADO

CROOK, COLORADO

APRIL 25, 2016

2629 Redwing Road, Suite 150 Fort Collins, Colorado 80526

970.373.3195 www.LithosEng.com

April 25, 2016

Project No. 16005-1

Colorado Parks and Wildlife

Department of Natural Resources

6060 North Broadway

Denver, Colorado 80216

Attention: Mr. Ben Hamann, PE

Project Manager, Capital Development Program

Regarding: Trenchless Feasibility Study

Interstate 76 Crossing at Crook, Colorado

Logan County, Colorado

Mr. Hamann:

This report presents our Trenchless Feasibility Study for the Interstate 76 Waterline Crossing at Crook, Colorado project. This study was conducted in general accordance with the agreement between Lithos

Engineering and the Colorado Parks and Wildlife, dated February 9, 2016. Contained herein are general subsurface conditions, engineering design recommendations, and trenchless construction considerations in support of the subject project.

If you have any questions regarding the contents of this report, please contact the undersigned.

Sincerely, Lithos Engineering

Lance Heyer, PE Robin Dornfest, PG, CPG

Associate President

Colorado Parks and Wildlife Interstate 76 Crossing at Crook, Colorado Page I of II

TABLE OF CONTENTS

EXECUTIVE SUMMARY

INTRODUCTION

PROJECT BACKGROUND

2.1 Site Description

2.2 Proposed Construction

GEOTECHNICAL INVESTIGATION

3.1 Subsurface Investigation

3.2 Geotechnical Laboratory Testing

SUBSURFACE CONDITIONS

4.1 Site Geology

4.2 Subsurface Materials

4.2.1 Eolian

4.2.2 Coarse Alluvium

4.3 Groundwater

EVALUATION OF FEASIBLE TRENCHLESS INSTALLATION METHODS

5.1 Auger Boring

5.1.1 Shaft Considerations

5.1.2 Tunneling Considerations

5.1.3 Other Considerations

5.2 Pipe Ramming

5.2.1 Shaft Considerations

5.2.2 Tunneling Considerations

5.2.3 Other Considerations

5.3 Horizontal Directional Drilling

5.3.1 Shaft Considerations

5.3.2 Tunneling Considerations

5.3.3 Other Considerations

5.4 Tunnel Boring

5.4.1 Shaft Considerations

5.4.2 Tunneling Considerations

5.4.3 Other Considerations

5.5 Microtunneling

5.5.1 Tunneling Considerations

5.5.2 Shaft Considerations

5.5.3 Other Considerations

5.6 Engineers Opinion of Probable Construction Costs

5.7 Trenchless Feasibility Conclusions

GEOTECHNICAL DESIGN RECOMMENDATIONS

6.1 Thrust Block Recommendations

6.2 Lateral Earth Pressures

6.3 Pipeline Modulus of Soil Reaction (E’)

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page II of II

6.4 Pipeline Thrust Restraint Design Recommendations

6.5 Corrosion Potential

6.5.1 Sulfate Attack on Concrete

6.5.2 Corrosion Potential of Steel

CONSTRUCTION CONSIDERATIONS

7.1 Temporary Excavations

7.2 Bedrock or Oversized Material Excavation

7.3 Construction Dewatering

CONSTRUCTION OBSERVATION AND QUALITY ASSURANCE

LIMITATIONS

REFERENCES

LIST OF FIGURES

Figure Number Title

1 Site Vicinity and Boring Location Map

2 Geologic Cross Section Through Proposed Crossing Alignment

APPENDICES

Appendix Title

A Boring Log Key and Boring and Test Pit Logs

B Geotechnical Laboratory Testing Result

C Trenchless Installation Methods Overview

D Engineering Opinion of Probable Construction Costs nielsenh Text Box

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 1 of 21

EXECUTIVE SUMMARY

The following Trenchless Feasibility Report presents the results of our geotechnical investigation, trenchless feasibility and methods discussion, geotechnical design recommendations, and construction considerations in support of the Crook Waterline Crossing project. Lithos Engineering utilized standard geotechnical drilling and sampling techniques to obtain soil samples and used them to describe the subsurface conditions and conduct geotechnical laboratory testing to define soil properties and behavior.

The following summarizes our general conclusions, trenchless installation methods discussion, and construction considerations:

1. The subsurface conditions encountered do not preclude the proposed trenchless construction as we understand it.

2. Subsurface materials encountered during the subsurface investigation are uniform and consist of silty sand and poorly-graded sand with silt. Groundwater as measured during drilling was encountered below the proposed construction envelope and is not anticipated to impact the planned construction.

3. The subsurface materials tested are not corrosive to steel and contain low levels of soluble sulfates with expected low risk of sulfate attack on concrete.

4. Five trenchless installation techniques were identified as potential methods to install the proposed crossing: auger boring, pipe ramming, horizontal directional drilling, tunnel boring, and microtunneling.

5. Horizontal directional drilling is recommended as the most cost effective installation technique.

6. Lithos recommends shortening the length of the proposed trenchless installation to the extent possible and orienting the waterline perpendicularly to Interstate 76.

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 2 of 21

INTRODUCTION

Colorado Parks and Wildlife Department (CPW) intends to construct a 24-inch diameter waterline as part of the Tamarack Recharge and Wildlife Phase-II Project. The project site is located approximately 2.5 miles southeast of the town of Crook in Logan County, Colorado. The 24-inch diameter waterline will be installed under Interstate 76 and will extend from Colorado Parks and Wildlife right-of-ways on either side of the interstate.

PROJECT BACKGROUND

The proposed waterline will convey water between water holding and recharge basin facilities on the north and south sides of Interstate 76. The 24-inch waterline will be pressurized, therefore required line and grade tolerances are expected to be lower in comparison to a gravity scenario. Our subsurface investigation and discussions included in this report are based upon a preliminary alignment produced by the CPW. This report presents our evaluation of the portion of the alignment that includes the proposed trenchless crossing under Interstate 76. The conclusions and recommendations outlined in this report do not pertain to the anticipated open cut portions of the proposed alignment extending north of STA 22+08 and southeast of STA 27+65.

2.1 Site Description

The project site includes both CPW and Colorado Department of Transportation (CDOT) right-of-way. As proposed, the trenchless crossing will extend from within CPW right-of-way and all surface construction and excavation will take place within CPW right-of-way as well. In general, the site topography slopes from northwest to southeast with the Interstate 76 embankments extending approximately 10 feet above native grades. The CDOT right-of-way length extends approximately 425 feet laterally from CPW fence lines at the proposed crossing orientation. Interstate 76 includes an approximately 100-foot-wide median between east and westbound travel lanes. The eastbound travel lane is situated approximately 2 feet higher than the westbound travel lane.

2.2 Proposed Construction

In general, the proposed waterline slopes at approximately 1 percent downward from north to south. The proposed crossing is approximately 550 feet in length and extends from pipe bends at STA 27+65 and

22+08 along the provided alignment. CPW intends to install a casing and carrier pipe for serviceability and to accommodate CDOT requirements. The alignment continues toward water holding facilities to the north and southeast of the proposed crossing. These sections are anticipated to be open-cut and were not considered as part of our scope of work. The crown of the waterline is situated approximately 4 feet below the existing ground surface at the proposed shaft or pit locations. The crown of the proposed waterline is situated approximately 13 feet below the median ground surface. The eastbound and westbound interstate pavement surface are situated approximately 18 and 14 feet above the proposed waterline, respectively.

GEOTECHNICAL INVESTIGATION

Lithos conducted a subsurface investigation near the crossing location in February, 2016. The subsurface investigation included geotechnical drilling and a subsequent geotechnical laboratory testing program.

The geotechnical investigation provided the subsurface information presented in Section 4, and aided in

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 3 of 21 producing our opinions on feasible trenchless installation methods, geotechnical design recommendations, and construction considerations provided in Sections 5, 6 and 7, respectively.

3.1 Subsurface Investigation

Two geotechnical borings were drilled on February 22nd, 2016 to investigate the general subsurface conditions near the trenchless crossing location (Figure 1).

Geotechnical drilling was conducted by Vine Laboratories of Denver, Colorado utilizing a Central Mining

Equipment (CME) 55 truck-mounted drilling rig. Drilling and sampling procedures were conducted in general accordance with ASTM D1586 – Standard Test Method for Penetration Test and Split-Barrel

Sampling of Soils. Continuous-flight solid-stem augers were used to advance the borings to a depth of 60 feet below the existing ground surface. Lithos considers the trenchless envelope or area of focus to extend one diameter above and below the proposed alignment. During drilling, modified California barrel

(1.9-inch inner diameter) and split-spoon (1.4-inch inner diameter) samples were obtained in 2.5-foot intervals within the proposed trenchless envelope and 5-foot intervals outside of the envelope. The modified California barrel sampler was utilized to obtain relatively undisturbed samples of cohesive materials suitable for laboratory testing. The split-spoon sampler was used to collect disturbed samples of coarse material suitable for index testing. The number of blows by a 140-pound hammer falling 30-inches required for 12 inches of sampler penetration (recorded in 6 inch increments) are presented on the boring logs (Appendix A). Blow counts with less than 6-inches of penetration are presented showing the number of blows for the resulting depth of penetration (50/2” = 50 blows to drive the sampler 2-inches).

3.2 Geotechnical Laboratory Testing

A geotechnical laboratory testing program was conducted on representative samples collected during the subsurface investigation. A laboratory summary table and graphical testing results are provided in

Appendix B. Laboratory tests conducted in general accordance with associated ASTM standards are presented in Table 4.1, below.

Table 4.1 – Geotechnical Laboratory Testing Geotechnical Test ASTM Standard

Grain Size Distribution D 422

In-Place Moisture and Density D 2937

Direct Shear D 3080

Atterberg Limits D 4318

If field determined soil descriptions differed from results indicated by laboratory classification testing, the boring logs presented in Appendix A were amended to reflect laboratory testing results. Geotechnical laboratory testing results were utilized in developing engineering design and construction recommendations presented in this report.

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 4 of 21

SUBSURFACE CONDITIONS

Subsurface conditions were determined based on the subsurface investigation, discussed previously. Soil descriptions noted on the boring logs and below are in general accordance with ASTM D 2487 – Standard

Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) and D 2488

– Standard Practice for Description and Identification of Soils (Visual-Manual Procedure). Boring logs and a supplementary boring log key presenting details in addition to discussions included in this geotechnical engineering report are provided in Appendix A. A generalized geologic cross section in the vicinity of the proposed trenchless crossing based on our subsurface investigation and the proposed waterline alignment provided by CPW is provided on Figure 2.

4.1 Site Geology

The project site lies in the Great Plains Physiographic Province of North America. The area is dominated by the erosional valley created by the South Plate River where it has incised the Ogallala Formation. The valley of the South Platte River is approximately 5 miles wide and up to 400 feet deep in the northeastern portion of the State. The South Platte River has shifted southward through Quaternary time as many streams were bringing water and sediment from the northwest and few from the southeast. This sediment aggraded the South Platte River and pushed it against its southern valley wall. A lack of major tributary streams coming into the South Platte from the south, resulted first from the south Platte River staying close to the bank and later from eolian sand covering the slope between the bank and the river. Eolian

(windblown) deposits impose a strong southeastward-trending geomorphic pattern on the area and dominate the project site. Many of the eolian sand features parallel the direction of the prevailing winds coming out of the northwest. The dunes range in age from Pinedale Glaciation to modern and some dunes are still moving (after Scott, 1982 and Bjorklund and Brown, 1957).

4.2 Subsurface Materials

Primary materials encountered during the subsurface investigation include: eolian sands and coarse alluvium. Groundwater was encountered during drilling in both borings LE-1 and LE-2.

4.2.1 Eolian

Surficial eolian (windblown) deposits consisting of silty sand (SM) and poorly-graded sand with silt (SP-

SM) were encountered in both borings and extended to depths ranging from 6.5 feet to 9 feet below the existing ground surface. The eolian deposits were noted as loose and dry.

4.2.2 Coarse Alluvium

Underlying the eolian deposits, the borings encountered coarse alluvium consisting of poorly-graded sand

(SP) to the maximum depth of exploration. Coarse alluvium was noted as loose to medium dense within the proposed pipe alignment and increased to dense to very dense with depth. The material was noted as dry to moist above the groundwater table and wet below the groundwater table and did not emit an apparent odor.

4.3 Groundwater

Groundwater was measured in both LE-1 and LE-2 at depths of 56.5 feet and 48.0 feet below the ground surface during the subsurface investigation on February 22nd, 2016. Groundwater should be expected to fluctuate based on precipitation, site development, water levels in the adjacent ponds, river levels, and

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 5 of 21 seasonal variations but is not anticipated to affect the design, construction, or long term performance of the waterline.

EVALUATION OF FEASIBLE TRENCHLESS INSTALLATION METHODS

Five trenchless techniques have been identified as potentially feasible methods for the installation of the proposed 24-inch diameter waterline: Auger Boring, Pipe Ramming, Horizontal Directional Drilling, Tunnel

Boring, and Microtunneling. The following sections describe each technique including the potential risks and benefits. Discussions in the following sections are based upon the proposed crossing alignment as presented by CPW and materials encountered during the subsurface investigation except for the HDD method, which uses the same alignment but a different grade as discussed below. Appendix C includes a summary table identifying relative benefits or risks of each technique.

5.1 Auger Boring

Auger boring is a trenchless installation method which simultaneously jacks casing pipe along the proposed alignment while a continuous helical auger string within the casing pipe removes spoil material and carries the material back to the launch shaft. A cutting head can be attached to the first set of helical augers and sits outside, flush with, or inside of the casing depending on the anticipated subsurface conditions.

Figure 5.1 – Auger boring installation (after International Society of Trenchless Technology)

5.1.1 Shaft Considerations

A jacking frame assembly sits within the constructed launch shaft and provides the necessary thrust and torque to advance the auger string and casing pipe through the subsurface. The jacking frame slides on rails that in turn, transfer thrust into structural restraint situated near the back of the shaft. Thrust

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 6 of 21 restraint can consist of a combination of steel plates, driven piles, a concrete thrust block, or drilled shafts depending on the depth of the shaft, the subsurface conditions, and the anticipated jacking loads.

Auger bores up to 300-ft in length are common and have been completed routinely in ground conditions similar to those encountered during our investigation. However, the proposed crossing length of 550 feet exceeds the upper limit of auger bore technology by most industry standards. Given the uniform ground conditions encountered in our subsurface investigation, Lithos believes auger boring up to 300 feet to be within the ability of contractors typically performing auger bore installations in Colorado. Therefore, if auger boring is considered for the proposed waterline installation, a central median shaft between the east and west bound lanes of Interstate 76 is recommended. The median at the proposed crossing location is approximately 100-feet wide, providing substantial work and laydown area for an auger boring installation. Given the medians relatively central location between CPW right-of-way, auger bore distances below both the east and westbound lanes could be shortened to between 200 and 250 feet if a central shaft was utilized. The central shaft is expected to extend approximately 15 feet below the existing ground surface, substantially deeper in comparison to the launch and/or receiving shafts on the north and south sides of Interstate 76. In order to limit construction traffic and work within the median, Lithos recommends the central median shaft receive the auger bore from launch shafts located to the north and south of Interstate 76.

5.1.2 Tunneling Considerations

The primary risk in selecting auger boring is the possibility of overcutting the dry eolian and alluvial sands and creating surface settlement or sinkholes. Given the subsurface conditions, the risk of over-excavation and the creation of voids above the crown of the tunnel is relatively high. Auger boring is most likely to cause settlement of Interstate 76 in comparison to the other methods presented in this section. Ground modification such as compaction or permeation grouting is commonly utilized prior to trenchless excavation to stabilize subsurface materials prone to voids and subsequent settlement. Ground modification is often time consuming and costly, specifically for smaller diameter applications such as the proposed.

A general rule is that an accuracy of about one foot of variation should be expected per 100 feet of length.

If a single drive with an auger boring were attempted, the exit may be as much as five to six feet off the planned location given the proposed crossing length. If this variability is unacceptable, then guided auger boring should be considered. Guided auger boring utilizes a steerable pilot tube, which is pushed prior to auger boring on the correct line and grade. The auger head uses a special attachment to keep the jacked casing pipe on the same line and grade as the pilot tube. Even with a pilot tube, auger bore line and grade tolerances are limited in comparison to tunnel boring and microtunneling. Finally, a 36-inch diameter steel casing would likely be required for auger boring.

5.1.3 Other Considerations

After the completion of trenchless excavation and the installation of the final carrier pipe, Lithos recommends grouting the annular space between the casing and carrier pipes, called backfill grouting, as well as injecting grout between the casing pipe and ground, called contact grouting. Contact grouting fills any remaining voids on the outside the casing pipe, limiting long term settlement potential. Backfill

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 7 of 21 grouting stabilizes and provides long term protection for the carrier pipe. Occasionally, backfill grouting will not be installed to allow service of the carrier pipe.

In general auger boring requires less setup and has a larger pool of local contractors perform auger boring in comparison to tunnel boring and microtunneling. In terms of relative construction durations, auger boring can be considered similar to pipe ramming, should be expected to take longer than horizontal directional drilling, and should be expected to be completed faster than tunnel boring and microtunneling.

The auger boring process does not contain common environmental concerns and risk of loss of slurry fluid utilized in horizontal directional drilling and microtunneling.

5.2 Pipe Ramming

Pipe ramming is an open-ended casing pipe trenchless installation technique often utilized where problematic soil conditions exist such as the dry eolian and alluvial sands at the proposed site. The carrier pipe is driven along the desired alignment with a pneumatic hammer situated in a launch pit or shaft. The pneumatic hammer must be able to overcome soil frictional forces on the internal and external circumference of the rammed pipe in addition to displacing some soil directly in front of the pipe wall.

During ramming or after the casing has been rammed along the desired alignment a continuous helical auger string or compressed air can be utilized to remove material within the installed pipe. Partial soil removal can be completed during the installation process if skin friction is of concern, however, enough material must be kept near the casing face to ensure stability.

Figure 5.2 – Pipe ramming installation (after International Society of Trenchless Technology)

5.2.1 Shaft Considerations

The pneumatic hammer is typically attached to the casing pipe with an increased wall thickness collar allowing for the ramming process to translate to the product pipe without damage. Product pipe is

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 8 of 21 rammed in sections initially along a rail system within the launch pit or shaft, similar to auger boring.

Different than other jacked pipe systems discussed in this section (auger boring, tunnel boring, and microtunneling) is the lack of required thrust restraint.

Although pipe ramming installations have been successfully completed for lengths over 300 feet, Lithos believes the proposed 550-foot crossing to surpass standard pipe ramming capabilities for the necessary

36-inch diameter casing pipe. Therefore, if pipe ramming is selected as the preferred installation method for the proposed crossing, a central shaft within the median of Interstate 76 is recommended. Based on our experience with CDOT, the approximately 100-foot wide median at the proposed crossing location, and standard pipe ramming installations, a central shaft can be accommodated. To lessen construction activity in the median, Lithos recommends the central shaft receive the casing pipe from two launch pits or shafts at the outer crossing extents. Use of two pipe rams with a central shaft is another reason why the line and grade tolerance essentially will require a guided installation. Attempting to marry two sections of pipe in a 20-foot receiving shaft that are several feet apart can be time consuming and costly.

Pipe ramming and auger boring are the two alternatives considered for the proposed installation where a central shaft is recommended. If pipe ramming is selected as the preferred installation technique, Lithos will provide a general shaft construction layout.

5.2.2 Tunneling Considerations

Pipe ramming lengths are limited by frictional forces, as discussed previously, and line and grade tolerance. Both limitations are governed by anticipated soil conditions, in this instance primarily eolian and alluvial sands. A general rule for pipe ramming is that an accuracy of about two feet of variation should be expected per 100 feet of length. This potential deviation is significant and should not be tolerated. However, as with auger boring, a pilot tube may be used to guide the rammed casing pipe along the proposed line and grade. If done properly, a guided pipe ram can expect accuracies of less than one inch from the proposed line and grade.

The primary risk of pipe ramming is the potential for ground settlement through consolidation of the eolian sand. Often, wind deposited material has a low density and actions with large vibrations can densify the material leading to surface settlement. Lithos believes the risk of ground settlement initiated by the pipe ramming process to be significantly less of a risk in comparison to auger boring. Another drawback is the need for the central shaft, which will be significantly deeper than the launch and/or receiving shafts on either side of the freeway embankment. There is some small risk of surface heave near the edges of the trenchless crossing where the ground cover is minimal. However, a small amount of ground heave will likely not be considered to be a major problem at these locations. Similar to auger boring, a 36-inch diameter steel casing would likely be required to utilize pipe ramming.

5.2.3 Other Considerations

As with auger boring, backfill grouting is recommended; however, contact grouting is not typically needed with pipe ramming because voids are not created outside the casing pipe and the overcut is relatively small. The construction duration should be considered similar to auger boring, longer than horizontal directional drilling, and shorter than tunnel boring and microtunneling. The lack of thrust restraint in comparison to auger boring, tunnel boring, and microtunneling is expected to yield a materials cost savings given the relatively shallow launch shaft depths. After the completion of trenchless excavation

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 9 of 21 and the installation of the final carrier pipe, Lithos recommends grouting the annular space between the casing and carrier pipes, called backfill grouting. Backfill grouting stabilizes and provides long term protection for the carrier pipe. Occasionally, backfill grouting will not be installed to allow service of the carrier pipe.

5.3 Horizontal Directional Drilling

Horizontal Directional Drilling (HDD) is a commonly used trenchless installation method utilized for crossing locations where third parties, constructability, and/or environmental concerns will not allow for traditional open-cut construction. During the HDD installation process, a pilot hole is initially drilled along the proposed pipeline grade and alignment. After the installation of the initial pilot hole, a reamer is attached to the drill string and pulled back through the pilot hole. The reaming process continues to enlarge the hole in single or multiple steps as required to accommodate the desired casing or carrier pipe diameter. The carrier pipe is installed with the last reaming step. A graphic displaying a typical HDD installation procedure is presented below.

Figure 5.3 – Typical HDD installation procedure (after Valley Directional Drilling, Inc.)

5.3.1 Shaft Considerations

Entry and receiving pits are typically excavated at each end of the proposed HDD alignment. Pits serve to collect drilling fluid to be reclaimed. Drilling fluid typically consists of a mixture of water and bentonite or polymer and is continuously pumped to the cutting head. Drilling fluid aids in the removal of cuttings, Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 10 of 21 stabilizing the bore hole, cooling the cutting head, and works to lubricate the product pipe and reduce sidewall friction. Entry and receiving pits necessary for HDD installations are much less comprehensive and require substantially less support in comparison to Auger Boring, Tunnel Boring, and Microtunneling techniques. The proposed length of installation is well within general HDD installations and can accommodate an alignment from CPW right-of-way to right-of-way, not requiring the inclusion of a central shaft in the median between eastbound and westbound travel lanes.

5.3.2 Tunneling Considerations

In general, the presence of substantial gravels and cobbles are not ideal for HDD installations. Based on subsurface investigation subsurface materials within the proposed waterline alignment consist primarily of eolian and alluvial sands. Therefore, Lithos believes the proposed waterline installation to be within the ability of local contractors typically performing HDD installations of this diameter.

An HDD installation technique will not alone necessitate the need for both a casing and carrier pipe in order to complete the proposed 24-inch diameter waterline crossing. However, we understand that CPW would like to install both a casing and carrier pipe for serviceability and to satisfy CDOT utility crossing requirements. Since a casing and carrier pipe are both proposed, a larger diameter casing can be installed with HDD prior pulling the smaller diameter product pipe through the casing. For the proposed application, we recommend a 36-inch diameter casing is installed prior to pulling the 24-inch diameter carrier pipe through the larger pipe.

Given the ground conditions present and the anticipated installation depth, Lithos does not recommend a drill path of closer than 2.0 feet to any existing utilities, as measured from the outer edge of the proposed borehole to the nearest existing utility outer edge. The location of all existing utilities to be crossed during the HDD installation should be visually confirmed by exposing them near the crossing location prior to drilling. In addition, the location of parallel utilities within 5.0 feet of the proposed borehole alignment should be potholed and confirmed prior to drilling. The accuracy of HDD line and grade along a drilling alignment is typically not as great in comparison to alternative techniques discussed in this section, however, the specific exit location is typically very accurate.

5.3.3 Other Considerations

The injection of drilling fluid has been known to cause surface heave or settlement if proper care is not maintained during the installation. Given the proposed depth of installation relative to the existing

Interstate 76 pavement surface, Lithos does not believe the risk of surface heave to be high. A pipe stress analysis should be performed prior to final selection of pipe and HDD installation. In addition, an inadvertent fluid loss or hydraulic fracture (frac-out) assessment should be completed to determine potential environmental risks due to loss of drilling fluid. HDD is expected to be most rapid and by far the most cost effective method of installing the proposed waterline

5.4 Tunnel Boring

Tunnel boring is another jacked pipe system that utilizes a rotating cutter head, called a tunnel boring machine (TBM), or a steerable shield to excavate material. The cutter head or shield can take on a variety of configurations depending on the anticipated subsurface and groundwater conditions. Based on the subsurface conditions encountered, a comprehensive pressure balance system is not anticipated as

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 11 of 21 necessary. TBMs and shields are manned and can make regular line and grade adjustments. Similar to auger boring and microtunneling, the jacked pipe aspect of tunnel boring necessitates thrust restraint and a jacking frame.

Figure 5.4 – 54-inch diameter TBM (after Akkerman)

The primary differences between tunnel boring and microtunneling, discussed below, relative to the proposed waterline crossing are: (1) tunnel boring machines are operated from within the machine assembly; (2) the ability to modify, replace, or inspect the cutter head as a whole or individual pieces that comprise the cutter head assembly; and (3) the lack of slurry recirculation and pressure injection used to stabilize the face and excavate material. The manned entry aspect of the TBM or shield will necessitate a bigger casing pipe relative to all other methods discussed in this section. The standard minimum diameter manned entry TBM or shield is approximately 54 inches, creating a substantial annular space and additional material excavation in comparison to other discussed methods.

5.4.1 Shaft Considerations

The proposed crossing length can be installed with a TBM or shield without the need to shorten the crossing length or utilize a central median shaft as is needed for auger boring and pipe ramming. The jacked pipe system will require thrust restraint, similar to auger boring and microtunneling, due to the

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 12 of 21 shallow proposed launch shaft, the crossing length, and the typical minimum 48-inch diameter casing pipe. Tunnel boring will likely require the largest carrier pipe in comparison to all other considered methods, increasing excavated materials and thrust restraint competency. Thrust restraint can consist of a combination of steel plates, driven piles, a concrete thrust block, or drilled shafts depending on the depth of the shaft, the subsurface conditions, and the anticipated jacking loads.

5.4.2 Tunneling Considerations

Due to the uniform eolian and alluvial materials expected throughout the crossing, an open face shield could be considered as a more cost effective alternative in comparison to a TBM. If a shield is utilized the material will be mined from face with a modified digger arm or by hand depending on the casing size selected and available working space. Sand shelves and an overhanging shield are recommended given the relatively coarse, low cohesion material expected to be encountered during the installation. Lithos believes the risk of creating voids exists if tunnel boring is selected for the proposed installation. The benefit of tunnel boring in comparison to auger boring relative to void creation and settlement is the manned entry aspect and the ability to visually monitor the ground immediately in front of the tunnel as the installation progresses.

Tunnel boring as it is described in this section is the only method eliminating all of the following concerns:

(1) the ability to install the proposed crossing under the entire CDOT right of way section and keep proposed work activities within CPW right of way; and (2) a lack of slurry recirculation minimizing environmental concerns; (3) high accuracy line and grade control; and (4) disposal of HDD drill fluids.

5.4.3 Other Considerations

Similar to other methods where a casing and carrier pipe is anticipated, after the completion of trenchless excavation and the installation of the final carrier pipe, Lithos recommends grouting the annular space between the casing and carrier pipes, called backfill grouting, as well as injecting grout between the casing pipe and ground, called contact grouting. Contact grouting fills any remaining voids in the ground outside the casing pipe, limiting long term settlement potential. Backfill grouting stabilizes and provides long term protection for the carrier pipe. Occasionally, backfill grouting will not be installed to allow service of the carrier pipe.

5.5 Microtunneling

Microtunneling is a specific subset of tunnel boring that utilizes a remotely-controlled, guided pipe jacking system that provides continuous mechanical and/or fluid pressure support to the face of the system to stabilize ground and water pressures. The primary benefits of microtunneling include: (1) the ability to provide continuous face support different from other trenchless installations discussed in this report; (2) accuracy of installations line and grade; and (3) a remotely controlled system such that workers do not need to be in the excavation continuously. Microtunneling is a jacked pipe technique requiring thrust restraint and jacking frame to advance carrier pipe as shown in the illustration below.

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 13 of 21

Figure 5.5 – Typical microtunneling installation procedure (after Bothar Boring and Tunneling, Inc.)

5.5.1 Tunneling Considerations

The proposed crossing length and subsurface conditions encountered during our investigation are well within the abilities of standard microtunneling installations. As discussed previously, groundwater is situated well below the proposed construction envelope and is not anticipated to impact the proposed construction. However, the relatively clean, loose existing eolian and alluvial soils can be prone to creation of voids during tunneling if tunnel excavations are not controlled appropriately. Although all trenchless installation methods discussed in this section can cause voids, Lithos believes the risk of creating voids due to microtunneling for this scenario to be low, typical for this type of installation.

Microtunneling has been performed successfully on pipes as small as approximately 10 inches and as large as approximately 12 feet in diameter. Due to the range of product pipe diameters accommodated by microtunneling, annular space and grouting could be lessened substantially, especially in comparison to tunnel boring. In addition, smaller casing pipe diameters will lessen skin friction and reduce excavated muck. Microtunneling installations are routinely completed with very high line and grade accuracy in comparison to other methods discussed. As discussed in Section 6.6, microtunneling is substantially more expensive in comparison to other methods considered.

5.5.2 Shaft Considerations

The proposed crossing length can be installed with a microtunnel without the need to shorten the crossing length or utilize a central median shaft as is needed for auger boring and pipe ramming. The jacked pipe system will require thrust restraint, similar to auger boring and microtunneling, due to the shallow proposed launch shaft and the crossing length. Thrust restraint can consist of a combination of steel plates, driven piles, a concrete thrust block, or drilled shafts depending on the depth of the shaft, the subsurface conditions, and the anticipated jacking loads.

5.5.3 Other Considerations

Similar to HDD, slurry recirculation is utilized by the microtunneling process to remove and transport excavated muck material. Due to the slurry injection system, risk of environmental contamination exists

Colorado Division of Wildlife Interstate 76 Crossing at Crook, Colorado Page 14 of 21 and should be further evaluated is microtunneling is ultimately selected. Based on the ground conditions encountered during the subsurface investigation Lithos believes the risk of environmental contamination from slurry loss to be low.

5.6 Engineers Opinion of Probable Construction Costs

In order to assist the CPW with evaluating and understanding the various methods of trenchless discussion presented in this report, Lithos prepared a preliminary opinion of probable construction cost for the five methods. A detailed table including an itemized list of considered costs for each potential installation method is included in Appendix D. The cost opinions assume a trenchless length of 550 feet except for

HDD, which assumes a bore length of 720 feet and a finished steel casing and carrier pipe length of 550 feet. The longer length of the HDD is due to a curved grade such as is typical of HDD installation. Each cost opinion includes a 20 percent contingency to account for unknowns of the project at this time.

Table 6.1 – Engineers Opinion of Probable Construction Costs Method Cost

Horizontal Directional Drilling $661,781

Auger Boring $952,011

Pipe Ramming (includes pilot tube guide) $1,151,492

Tunnel Boring Machine $1,255,257

Microtunneling $2,361,678

5.7 Trenchless Feasibility Conclusions

A summary of the major costs and benefits of each considered trenchless technique, in addition to our general design and layout recommendations are provided below:

Five potential trenchless installation techniques were identified based on the proposed crossing length and diameter, the subsurface conditions anticipated, and industry standard capability for each individual technique:

o Auger boring:

The proposed length is greater than the upper end of standard installations;

A central shaft will be required in the median of Interstate 76;

Creation of voids and subsequent settlement is most likely utilizing auger boring in comparison to other considered methods; and

A casing pipe will be installed in addition to the carrier pipe.

o Pipe Ramming:

The proposed length is greater than the upper end of standard installations;

A central shaft will be required in the median of Interstate 76;

Thrust restraint within the launch shaft is not necessary;

Creation of voids and subsequent settlement is least likely in comparison to other considered methods; and

A casing pipe will be installed in addition to the carrier pipe.

o Horizontal Directional Drilling:

Costly entry and exit shafts are not required;

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Most cost effective alternative considered;

Highest risk of environmental impact due to drilling fluid loss;

Disposal cost of HDD drilling fluids;

A casing pipe will be installed in addition to the carrier pipe.

o Tunnel Boring Machine:

An oversized casing pipe will be installed in addition to the carrier pipe;

Very accurate line and grade; and

Substantially more expensive in comparison to auger boring, pipe ramming, and horizontal directional drilling.

o Microtunneling:

Substantially more expensive in comparison to auger boring, pipe ramming, and horizontal directional drilling and more expensive than tunnel boring; and

Risk of environmental impact due to drilling fluid loss.

A casing pipe is preferred by CPW to allow for service of the waterline and meet CDOT requirements, Lithos recommends the proposed crossing be installed utilizing HDD techniques.

Lithos recommends limiting the proposed crossing length to the extent possible, within CDOT right-of-way. Consideration should be given to negotiating with CDOT to work within their right-of-way to reduce the overall tunnel length. Any of the trenchless methods considered will be more expensive in comparison to traditional open cut construction and numerous utility installations have temporarily and successfully utilized CDOT right of way.

Orient the crossing perpendicularly to Interstate 76 to shorten the overall crossing length and to ease permitted with CDOT. CDOT has historically been uncomfortable with skewed utility installations.

Depending on the alternative selected based on our report and future conversations with CPW, Lithos anticipates considering specific geotechnical and geostructural design aspects relative to the proposed installation. The additional design aspects will be included in future deliverables to

CPW.

GEOTECHNICAL DESIGN RECOMMENDATIONS

The following sections are included primarily for the engineer performing design for the Crook Crossing project. Design recommendations provided include: thrust block, lateral earth pressures, pipeline, water soluble sulfates and corrosion potential of steel. If additional geotechnical design recommendations are necessary, Lithos should be contacted to provide the required information.

6.1 Thrust Block Recommendations

Lithos recommends the following criteria be utilized in designing thrust blocks:

A net allowable bearing pressure of 3,000 pounds per square foot (psf) for foundations bearing on compacted native material. The net allowable bearing pressure assumes maximum allowable total and differential settlements of 1 and 0.5 inches, respectively.

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It is acceptable to increase the net allowable bearing capacity by one-third if the load combination utilized considers infrequent loads such as wind or earthquake. Snow loading is not considered an infrequent load case.

A modulus of subgrade reaction (k) value of 150 psi/in is recommended for properly compacted native material.

A minimum of 30-inches of backfill adjacent to foundation elements is recommended to protect the subgrade from frost action.

6.2 Lateral Earth Pressures

Lateral earth pressures presented in the Table 6.1 should be considered during the structural design.

Lateral earth pressure values are a function of the properties and the geometry of the retained soil and/or bedrock and anticipated magnitude of lateral retention deflection. As discussed in Section 4.3, groundwater is expected to be well below the proposed construction. Therefore, lateral earth pressures for saturated materials are not provided in Table 6.1.

Based on the proposed construction, we anticipate lateral earth pressures to apply to: thrust restraint, soil retention, and casing pipe design. Thrust restraint is anticipated to act primarily as a passive system.

Temporary and permanent soil retention could utilize either active or at-rest lateral earth pressures depending on the selected retention alternative. Lithos can be consulted to provide further guidance on which lateral earth pressure value to utilize for soil retention once an alternative is selected. Below grade pipes are not expected to deflect laterally, therefore at-rest lateral earth pressures should be considered during design.

Table 6.1 - Lateral Earth Pressures

Backfill Material Type Earth Pressure Coefficients Equivalent Fluid Pressure (pcf) 1

Active At-Rest Passive Active At-Rest Passive

Compacted Native Material 0.28 0.44 3.54 34 53 424

Consideration has not been given to vertical loads applied to the backfill surfaces during or after construction as a result of traffic and/or other surcharge loads, sloping backfill, or soil-wall interfacial friction. Standard design values for typical, non-construction related vehicular traffic result in an additional 250 psf vertical pressure.

6.3 Pipeline Modulus of Soil Reaction (E’)

The modulus of soil reaction (E’) is an empirically derived coefficient which is related to the stiffness of the native soils and the nature and degree of compaction of the pipe bedding and trench backfill material.

The modulus of soil reaction is not listed for concrete pipe due to its rigidity and the lack of pipe / soil interaction. Recommended modulus of soil reaction values for various potential pipe types are presented in the following table.

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Table 6.2 – Modulus of Soil Reaction for Buried Pipe

Pipe Type Installation Method Modulus of Soil Reaction

(psi)

Steel Pipe Trenchess Crossing 500

Open-Cut Trenched and Backfilled 1,500

PVC Pipe Trenchess Crossing 600

Open-Cut Trenched and Backfilled 1,800

This E’ values presented for trenches, open-cut sections are applicable when the trench is excavated in soils that are loose/medium stiff or better, typically with blow counts exceeding 4 blows per foot, and where the trench width measured at the pipe elevation is at least two times the pipe diameter. Based on our subsurface exploration, existing site soils are competent and therefore will not require a reduction in

E’.

6.4 Pipeline Thrust Restraint Design Recommendations

Buried, pressurized pipes experience thrust forces at various pipe configurations including tees, reducers, dead ends, valves, bends, and wyes. In order to balance internal hydraulic pressures, thrust blocks or restrained joints are recommended for pipe restraint. For thrust block design, recommended passive equivalent fluid pressures are presented in Table 6.1. In order to mobilize passive earth pressures, thrust blocks should be designed to move 0.04 times the height of the block. In addition, a minimum of 1 foot of cover over thrust blocks is recommended and thrust blocks should be oriented such that the passive pressure influence zones do not overlap for adjacent blocks and do impact other subsurface utilities.

Although not anticipated, if thrust blocks cannot be utilized for restraint due to site constraints, restrained joints may be considered. Various coefficients of friction for potential pipe materials against anticipated trench backfill material are presented below.

Table 6.2 – Coefficient of Friction for Buried Pipe Pipe Type Backfill Material Coefficient of Friction

Concrete Pipe Eolian or Fill 0.35

Steel Pipe Eolian or Fill 0.38

PVC or HDPE Pipe Eolian or Fill 0.30

6.5 Corrosion Potential

The corrosion potential of the site relative to concrete and steel was analyzed based on samples collected during our subsurface investigation. As discussed previously, if auger boring, pipe ramming, tunnel boring, or microtunneling are selected to install the proposed waterline, a steel casing pipe is anticipated. If HDD is selected to install the proposed waterline, a plastic pipe type is anticipated in which case corrosion is not a concern.

6.5.1 Sulfate Attack on Concrete

Water soluble sulfate testing was conducted on two samples collected during the subsurface investigation as presented in Appendix B. Testing indicated water soluble sulfate contents less than 0.001 percent.

According to American Concrete Institute 318, a water soluble sulfate content less than 0.10 percent indicates a negligible risk of sulfate attack on adjacent concrete.

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6.5.2 Corrosion Potential of Steel

Auger boring, pipe ramming and microtunneling techniques are all two-pass trenchless installation…