California Department of Transportation
 

DIB 83-02 - 9.1 Replacement

DESIGN INFORMATION BULLETIN No. 83-02
CALTRANS SUPPLEMENT TO FHWA CULVERT REPAIR PRACTICES MANUAL

9.1 REPLACEMENT
  9.1.1 Repair vs. Replacement
  9.1.2 Replacement Systems
    9.1.2.1 Open Cut (trench) Method
    9.1.2.2 Trenchless Excavation Construction (TEC) Methods
      9.1.2.2.1 Pipe Jacking
      9.1.2.2.2 Microtunneling
      9.1.2.2.3 Pipe Bursting and Pipe Splitting
      9.1.2.2.4 Trenchless Replacement References
    9.1.2.3 Other Considerations for TEC
         

9.1 REPLACEMENT

9.1.1 Repair Verses Replacement

Refer to FHWA Culvert Repair Practices Manual Volume 1, Chapter 7, pages 7-27.

Choosing whether to repair or replace the deficient culvert depends upon several considerations:

  • The condition of the culvert and its suitability to repair or rehabilitation
  • Current and future loading conditions in the area and for the roadway served by the culvert.
  • Alignment and other physical factors related to the culvert. Significantly changed (or planned) roadway geometry or embankment depths may indicate a culvert replacement rather than a simple repair. Conversely, for relatively short culverts with smaller diameters under shallow cover on rural highways with low ADT, it may be more cost-effective to replace.
  • Ability to conform to current standards.
  • Availability of funding, fabrication, construction expertise of local contractors, or construction capabilities of maintenance forces.
  • User costs and out-of-service costs during either repair or replacement.
  • Environmental demands or aesthetic considerations.

Certainly, the choice between repair and replacement should be based upon a consideration of all of the factors. A simple, arbitrary, and un-researched blanket decision should be avoided. The costs of repair and continued operation versus the costs and ultimate operation of a replacement culvert may be significant and the alternative should be chosen with this significance in mind. A worksheet similar to Table 7.6 in FHWA Culvert Repair Practices Manual Volume 1, Chapter 7, page 7-27 is suggested as a systematic approach to deciding whether to repair or replace. As previously discussed under the ‘Caltrans host pipe structural philosophy’ (see Index 6.1.1), if the host pipe is not capable, or being made capable of sustaining design loads, it should be replaced rather than repaired.

9.1.2 Replacement Systems

If the decision has been made that replacement will provide the most satisfactory solution to the problems being encountered at the culvert site, various replacement methods can be considered.

For some large culvert replacements, options may include consideration of bridge construction. However, for most locations replacement will consist of installing a new culvert.

Generally, the culvert replacement will fall under the categories of

  • open trench construction, or
  • trenchless construction.

9.1.2.1 Open Cut (Trench) Method

The open cut trench method is the most commonly used method for replacing a culvert. The general procedure is to excavate a trench and remove the existing culvert, prepare the appropriate bedding for the new culvert, install the new culvert and fill the trench around the pipe with either slurry/flowable type material or with compacted lifts of soil. The pavement is then patched to reasonable limits beyond the edge of the pipe trench. For detailed guidelines for installing culverts in a trench, see Caltrans Standard Plans A62D, A62DA, A62E, A62F, Sections 19 and 61 through 67 of the Standard Specifications, Chapter 850 of the HDM, and Section 2 of this D.I.B. for physical standards. Also see appropriate Standard Special Provisions (SSP’s). Controlled low-strength material (CLSM) is described in FHWA Culvert Repair Practices Manual Volume 1, Chapter 7, page 7-34 and Slurry Cement backfill is described in Section 19-3.03F of the Standard Specifications. A memo dated 9/27/01 by Caltrans Corrosion Technology Unit recommended allowance of placing both slurry and CLSM as backfills with both aluminum and aluminized (type 2) pipe.

The flow chart under Index 2.1.1 outlines the general thought process and factors involved in determining which type of material to select for replacement using the open cut (trench) method.

9.1.2.2 Trenchless Excavation Construction (TEC) Methods

Trenchless excavation construction (TEC) methods include all methods of installing culverts below grade without direct installation into an open-cut trench. To date, the majority of trenchless work for the department has been accomplished by utility owners through the permit process with the design and construction responsibilities and liability placed on the utility owner. However, for culvert replacement, trenchless excavation is usually a preferred option over open trench construction when very high roadway fills and/or high traffic volumes exist without the availability of a reasonable detour route. No one method is suitable for all types of soil and site conditions. The selection of compatible methods is site specific and highly dependent on subsurface conditions. In addition to adequate specifications and guidelines for contractors to follow, a thorough soils investigation and an accurate underground utility location plan are critical for minimizing subsequent construction problems and claims. At the present time, there are no standard specifications or standard special provisions developed for most of the TEC methods presented herein.

Per Index 7.1.6.2, it is strongly advised to contact the appropriate headquarters units for assistance when considering replacement using the TEC methods that are referenced in this section. The importance of early communication with the Geotechnical Design specialist from the Division of Engineering Services (DES) and coordination with District and/or headquarters Permits cannot be over-emphasized.

However, for major projects involving trenchless technology, depending on complexity, it may be more efficient to use a consultant where in-house expertise is not available for planning and design.

These guidelines go beyond the Encroachment Permit Manual tunneling requirements, which the designer should also be familiar with.

For a description of the various trenchless excavation construction (TEC) techniques, see FHWA Culvert Repair Practices Manual Volume 1, Chapter 7, pages 7-38 through 7-45. The following tables were copied with permission from NCHRP Synthesis of Highway Practice 242 (Iseley, T. and S.B. Gokhale):“Trenchless Installation of Conduits Beneath Roadways”, Transportation Research Board, National Research Council, Washington, D.C., 1997, and supplement the TEC information given in FHWA Culvert Repair Practices Manual Volume 1, Chapter 7, pages 7-38 through 7-45:

Classification systems for trenchless methods
*New Austrian tunneling method is shotcrete-supported tunneling

DESCRIPTION OF TRENCHLESS CONSTRUCTION METHODS

Method Type
Method Description
I. Techniques Not Requiring Personnel Entry—Horizontal Earth Boring (HEB)
Auger Boring (AB) A technique that forms a borehole from a drive shaft to a reception shaft by means of a rotating cutting head. Spoil is transported back to the drive shaft by helical wound auger flights rotating inside of steel casing that is being jacked in place simultaneously. AB may provide limited tracking and steering capability. It does not provide continuous support to the excavation face. AB is typically a 2-stage process (i.e. casing installation and product pipe installation).
Slurry Boring (SB) A technique that forms a borehole from a drive shaft to a reception shaft by means of a drill bit and drill tubing (stem). A drilling fluid (i.e. bentonite slurry, water, or air pressure) is used to facilitate the drilling process by keeping the drill bit clean and aiding with spoil removal. It is a 2-stage process. Typically, an unsupported horizontal hole is produced in the first stage. The pipe is installed in the second stage.
Microtunneling (MT) A remotely controlled, guided pipe-jacking process that provides continuous support to the excavation face. The guidance system usually consists of a laser mounted in the drive shaft communicating a reference line to a target mounted inside the MT machine’s articulated steering head. The MT process provides ability to control excavation fact stability by applying mechanical or fluid pressure to counterbalance the earth and hydrostatic pressures.
Pipe Ramming (PR) A technique for installing steel casings from a drive shaft to a reception shaft utilizing the dynamic energy from a percussion hammer attached to the end of the pipe. A continuous casing support is provided and over excavation or water is not required. This is a 2-stage process.
Soil Compaction (SC) This method consists of several techniques for forming a borehole by in-situ soil displacement using a compacting device. The compacting device is forced through the soil, typically from a drive shaft to a reception shaft, by applying a static thrust force, rotary force and/or dynamic impact energy. The soil along the alignment is simply displaced rather than being removed. This is a 2-stage process.
II. Techniques Requiring Personnel Entry
Pipe Jacking (PJ) A pipe is jacked horizontally through the ground from the drive shaft to the reception shaft. People are required inside the pipe to perform the excavation and./or spoil removal. The excavation can be accomplished manually or mechanically.
Utility Tunneling (UT) A 2-stage process in which a temporary ground support system is constructed to permit the installation of a product pipe. The temporary tunnel liner is installed as the tunnel is constructed. The temporary ground support system can be steel or concrete tunnel liner plates, steel ribs with wood lagging, or an all-wood box culvert. People are required inside the tunnel to perform the excavation and/or spoil removal. The excavation can be accomplished manually or mechanically.


CHARACTERISTICS OF TRENCHLESS CONSTRUCTION METHODS

Type a
Pipe/Casing
Installation Mode
Suitable b
Pipe/casing
Soil
Excavation
Mode
Soil
Removal Mode
AB Jacking Steel Mechanical Auguring
SB Pulling/Pushing All types Mechanical and Hydraulic Hydraulic, Mechanical Reaming and Compaction
MT Jacking Steel, RCP, GFRP, PCP, VCP, DIP Mechanical Auguring or Hydraulic (Slurry)
PR Hammering/Driving Steel Mechanical
Auguring, Hydraulic, Compressed Air, or Compaction
SC Pulling Steel, PVC, HDPE Pushing Displacement (in-situ)
PJ Jacking Steel, RCP, GFRP Manual or Mechanical Augers, Conveyors, Manual Carts, Power Carts, or Hydraulic
UT Lining Steel or Concrete Liner Plates, Ribs w/Wood Lagging, Wood Box Manual or Mechanical Augers, Conveyors, Manual Carts, Power Carts, or Hydraulic

a AB–Auger Boring; SB–Slurry Boring; MT–Microtunneling; PR–Pipe Ramming; SC–Soil Compaction;PJ–Pipe Jacking; UT–Utility Tunneling.
b Steel–Steel Casing Pipe, RCP–Reinforced Concrete Pipe, GFRP–Glass-Fiber Reinforced Plastic Pipe, PCP–Polymer Concrete Pipe, VCP–Vitrified Clay Pipe, DIP–Ductile Iron Pipe, PVC–Polyvinyl Chloride Pipe, HDPE–High Density Polyethylene Pipe.


FACTORS AFFECTING THE SELECTION AND USE OF TRENCHLESS TECHNOLOGY (TT) ALTERNATIVES

Factors
Description
Diameter of Drive Need to identify which methods are suitable to install the pipe required for the drive from project scope. As the diameter increases, the complexity and risks associated with the project also increase. Some methods are unsuitable for some diameters.
Length of Drive Need to identify which methods are suitable for installing the pipe for the drive lengths required by the project scope. As the length increases, the complexity and risks associated with the project also increases. Length of drive may rule out certain methods or result in cost penalties for mobilization for short distances.
Abandonment Under what conditions should the work be stopped and the line abandoned. What will be the abandonment procedures?
Existing Underground Utilities Need to determine location of all existing underground utilities and underground structures so that the likelihood of obstruction or damage can be addressed for each TT alternative. Actions need to avoid obstruction should be identified for each prospective method.
Existing Above Ground Structures The likelihood of ground movement caused by the proposed TT alternatives should be evaluated. A possibility of heaving the roadway of causing ground subsidence should be evaluated. The parameters to be monitored to ensure minimum effect on adjoining structures must be identified.
Obstructions The likelihood of encountering obstructions (either naturally occurring or manmade) should be evaluated. The proposed equipment must be able to handle the anticipated obstruction. For example, some techniques might permit steering around or crushing obstacles up to a certain size.
Casing Is a casing pipe required? Or can a product pipe be installed directly? If a casing pipe is required, does the annular space between the product pipe and the casing pipes need to be filled? If so, with what materials? Does the casing pipe need to have internal and/or external coatings? What distance should the casing extend beyond the pavement edge?
Soil Conditions Need to accurately determine the actual soil conditions at the site. Is the proposed TT equipment compatible with the anticipated soil conditions? Where is the water table? Can the equipment function in unstable ground conditions? Or, will the soil conditions need to be stabilized prior to the trenchless process being employed? If so, how? For example, will the soil need to be dewatered? Is dewatering reasonable at the specified project site? Are contaminated soils or groundwater anticipated? What is the likelihood of ground heaving or settlement? Need to establish allowable limits for ground movement and need to determine how ground movement will be measured.
Drive/Reception Shafts Need to make sure adequate space is available at the project site to provide the required space for the shafts. The working room available may limit the length of pipe segments that can be handled. For example, 12 m (40 ft) steel pipe segments will minimize field-welding time and may be desirable from a construction perspective, but may not be achievable due to site constraints. These constraints need to be identified early in the process.
Accuracy Need to determine alignment and grade tolerance desired for the installation. Typically, the tighter the tolerance, the higher the cost of installation will be. How will this level of accuracy be measured?
Steerability What level of sophistication is needed to track the leading edge of the cutting head and being able to steer it? If the system gets off line and grade, what limits need to be placed on corrections to prevent overstressing the drill stem or pipe.
Bulkheads Bulkheads are used to provide end seals between the casing and product pipe. Need to determine if they should be required. If so, what should they be made of?
Materials Need to determine what materials the casing and product pipe should be (i.e., Steel, RCP, PVC, GFRP, HDPE, etc.) and joint requirements. Selection must be based on use, environmental conditions, and compatibility with the trenchless method.
Ventilation/Lighting Under what conditions will ventilation and/or lighting be required. How will adequate ventilation and/or lighting be determined?
Measurement/Payment How and who will determine the measurement by which the contractor will be compensated? What are the conditions of payment?
Submittals What information is going to be required for the contractor to supply? Who will receive the submittal information? What are the qualifications of the reviewers? What are the construction risks and who will accept these risks (contractor or owner)?

OVERVIEW OF TRENCHLESS METHODS

Method
Primary
Applications
Depth
Length
Diameter
Type of Pipe
Accuracy

Auger Boring (AB)

Crossings
(All types)

Varies

40-500 ft

8-60 in

Steel

Medium

Microtunneling (MT)

Sewer Installations

Varies

80-750+ ft

10 in-10+ft)

Steel. RCP, Fiberglass, GFRP, DI, VCP, PVC

High

Maxi & Midi Horizontal Directional Drilling

Pressure lines, water, gas, cable

<160 ft

400-6000 ft

3-54 in

Steel, HDPE

Medium

Mini-Horizontal Directional Drilling (Mini-HDD)

Pressure lines, water, gas, cable

<50 ft with walkover system

40-600 ft

2-14 in

Small diameter steel pipe, HDPE, DI, PVC, Copper service lines, cable

Medium

Pipe Ramming

Crossings

Varies

40-200 ft

4-42 in

Steel

Low

Pipe Jacking (PJ)

Sewers, Pressure Lines, Crossings

Varies

No theoretical limit-
1600 ft
spans achieved

42-120 in

RCP, Steel, Fiber glass

High

Utility Tunneling

Sewers, Pressure lines, Crossings

Varies

No theoretical limit

> 42 in

Cold formed steel plates, pre-cast concrete segments

High

OVERVIEW OF TRENCHLESS METHODS (cont.)

Method
Working Space Required
Compatible Soil Type
Operator Skill Requirements
Chief Limitations

Augur Boring (AB)

Entry & Exit bore pits. Length 26-36 ft
Width 8-12 ft. Room for storing augers, casing etc.

Variety of soils conditions (see Table 9)

High

High capital cost for equipment, high set-up cost (bore pits); cannot be used in wet runny sands, soil with large boulders.

Microtunneling (MT)

Primary Jacking Pit: 13 ft long, 10 ft wide, smaller retrieval pit, room for slurry tanks, pipe storage.

Variety of soil conditions including full-face rock and high groundwater head.

High to operate sophisticated equipment

High capital cost and set-up costs, obstructions.

Horizontal Directional Drilling (HDD)

Access pits not required. Space for set up of rig and drilling fluid tank: 400 ft x 200 ft

Clay is ideal.  Cohesionless sand and silt require bentonite. Gravel and cobbles are unsuitable.

High degree of knowledge of downhole drilling, sensing and recording. Training essential.

Requires very high degree of operator skill.  Not suitable for high degree of accuracy such as gravity sewer application.  Can install only pipes with high tensile strength e.g., steel, HDPE.

Mini-Horizontal Directional Drilling (Mini-HDD)

Equipment is portable and self-contained. Requires minimal area.

Soft soils, clay and sand. Unsuitable for rocks and gravel.

Same as HDD

Accuracy dependent on range of the electromagnetic receiver < 50 ft

Pipe Ramming

Large surface area required to accommodate bore pit, excavated soil, air compressor, pipe to be installed, etc.

Almost all soil types. Earthen plug formed at the leading edge of casing preventing soil flowing into pipe.

Fair skill & knowledge required to determine initial alignment, make decisions on open or close faced bore, lubrication requirements, etc.

No control over line and grade. A large piece of rock or boulder can easily deflect pipe from design path. Pipe has tendency to drop and/or come up to the surface. For larger pipe diameters equipment cost increases substantially. Specialized operation requiring great deal of planning and coordination. High capital cost.

Pipe Jacking

Jacking pit is a function of pipe size. Pit sizes vary from 10-30 ft

Stable granular and cohesive soils are best. Unstable sand is least favorable. Large boulders cause frequent work stoppage. Method can be executed with any ground condition with adequate precautions.

This is a specialized operation requiring a great deal of skill and training. Line & grade tolerances are usually very tight and corrective actions can be very expensive.

Specialized operation requiring great deal of planning and coordination. High capital cost.

Utility Tunneling

Smaller surface area as compared to PJ due to compactness of the liner system.  Access pit size varies from 9 to 25 ft.

Same as PJ

Same as PJ

High capital and set-up cost. Carrier pipe is required to carry the utility and the space between the carrier pipe and liner has to be grouted to provide adequate support unless a permanent lining system is used.

APPLICABILITY OF TRENCHLESS TECHNIQUES IN VARIOUS SOIL CONDITIONS

Soil type
Cohesive Soils (Clay)
Cohesionless Soils (sand/silt)
Soil and Groundwater
N Value (Standard Penetration Value as per ASTMD 1452)
N<5
(soft)
N=5-15
(firm)
N>15
(stiff-hard)
N<10-30
(loose)
N=10-30
(medium)
N>30
(dense)
High Ground Water
Boulders
Full-Face Rock
Applications
Auger Boring (AB)
o
*
*
o
*
*
x
< 33%Þ 1
< 12ksi
Microtunneling (MT)
*
*
*
*
*
*
*
< 33%Þ 1
< 30ksi
Maxi/Midi-Horizontal Directional Drilling (HDD)
o
*
*
o
*
*
o
o
< 15ksi
Mini-Horizontal Directional Drilling (Mini-HDD)
o
*
*
o
*
*
o
x
x
Impact Moling/Soil Displacement
o
*
*
x
*
o
x
x
x
Pipe Ramming
*
*
*
*
o
o
o
< 90%Þ
x
Pipe Jacking (PJ)
W/ TBM
o
*
*
o
*
*
o
o
< 30ksi
W/ Hand Mining (HM)
x
*
*
o
*
*
x
< 95%Þ
o
Utility Tunneling (UT)2
W/ TBM
o
*
*
o
*
*
o
o
< 30ksi
W/ Hand Mining (HM)
o
*
*
o
*
*
o
< 95%Þ
*
*: Recommended o: Possible x: Unsuitable
(This table is based on the assumption that experienced operators using proper equipment perform work)
1 Size of largest boulder versus minimum casing diameter (Þ)
2 Ground conditions may require either a closed face, earth pressure balance, or slurry shield.

COST RANGE FOR TRENCHLESS CONSTRUCTION METHODS (Based on Midwest Cost Indices, 1996) 1, 2

TT Method
Cost Range
Installation Comments

Auger Boring  (AB)

$3-4/D.I./LF

Line and grade not critical

 

$4-6/D.I./LF

Line and grade critical

Slurry Boring  (SB)

$1-3/D.I./LF

Line and grade not critical

Microtunneling  (MT)

$13-20/D.I./LF

Line and grade critical

Horizontal Directional Drilling (HDD)3

 

 

      Maxi

$200-500/LF

Line and grade not critical

      Midi

$50-200/LF

Line and grade not critical

      Mini

$5-50/LF

Line and grade not critical

Soil Compaction

$1-2/D.I./LF

Line and grade not critical

Pipe Ramming (PR)4

$3-6/D.I./LF

Line and grade not critical

Pipe Jacking

 

 

      W/ TBM

$5-9/D.I./LF

Line and grade critical

      W/ Hand Mining (HM)

$6-15/D.I./LF

Line and grade critical

Utility Tunneling

 

 

      W/ TBM

$6-10/D.I./LF

Line and grade critical

      W/ Hand Mining (HM)

$7-16/D.I./LF

Line and grade critical

TBM: Tunnel Boring Machine, D.I.: Per Inch of Pipe Diameter, LF: Per Linear Foot of Pipe, D.MM: Per 100 MM of Pipe Diameter, and M: Per Meter of Pipe.

1) Cost includes cost of installation, mobilization, de-mobilization and planning.  Does not include casing/carrier pipe material cost, cost for preparing entry/exit pits and shafts, or dewatering costs.

2) Costs assume good ground conditions (i.e., sandy clay, sand, silt), moist ground, and fairly firm soils (N = 6-20) with shafts 20 ft deep, and bore length 50 ft.  Does not include mixed face condition or soil with significant rock formation or boulders.

3) Horizontal Directional Drilling is not so much a function of the pipe diameter as it is the length of the bore for small diameters, e.g. in a Mini-HDD it costs the same to install a  2 inch pipe as it costs to install a 6 inch pipe provided the length remains the same.  For diameters larger than 10 inches, the cost is a function of both the diameter and length of the installed pipe. This method is primarily used by the utility industry for small diameter bores; therefore, consider using other TEC methods.

4) Pipe Ramming requires a heavier pipe to sustain the dynamic loads.  This will affect the material costs.

9.1.2.2.1 Pipe Jacking

Pipe jacking is a trenchless method for installing a pipe through the ground from a drive shaft to a reception shaft. The pipe is propelled by jacks located in the drive shaft. The jacking force is transmitted through the pipe to the face of the pipe jacking excavation.

The pipe jacking method may be used to install reinforced concrete or steel pipe with diameters ranging from as low as 18 inches to as great as 132 inches. However, both excavation and spoil removal processes usually require workers inside the pipe during the jacking operation. Therefore the minimum recommended diameter is 42 inches in order for workers to have access through the pipe to the leading end. This method is widely used, particularly where deep excavations are necessary or where conventional open excavation and backfill methods may not be feasible.

During the jacking process, soil is removed either mechanically or manually from the leading end of the pipe. Either an auger or conveyor can be used to transport the excavated material back to the jacking pit. See pictures below:

Pipe jacking method in progress

Pipe jacking method in progress

Once the jacking process is started, it typically is specified that the process be continued uninterrupted until completion so as to keep the pipe from "freezing" in place. Lubricants often are applied to the exterior of the pipe to be jacked to reduce frictional resistance.

Two types of loads are imposed on pipe installed by the jacking method:

  • the axial load due to the hydraulic jacks,
  • and earth loading due to overburden. This vertical load generally becomes effective only after the installation is complete.

The axial or thrust jacking loads are transmitted from one pipe section to another through the joint surfaces. It is essential that the pipe ends are parallel so that there will be a relatively uniform distribution of forces around the periphery of the pipe. Specifying a higher class of pipe provides little or no gain in axial crushing resistance.

As with any trenchless excavation construction method, the feasibility of pipe jacking for a given site must be established before construction through exploratory soil borings or other information relating to the composition of the soil likely to be encountered. Pipe jacking requires that the soil be relatively uniform in composition and free from large boulders or rock outcroppings.

The local variations in pressure on the leading section can result in damage to the culvert sections, misalignment, and voids in the fill. Similarly, jacking through groundwater bearing strata may present difficulties, especially in sandy soils as the saturated soil may flow into the pipe. This can lead to reduced soil densities above and around the pipe.

RCP Pipe Jacking example
RCP Pipe Jacking example

For long pipelines and culverts, it may be necessary to establish intermediate-jacking stations, so that predetermined jacking force limitations will not be exceeded. The location of intermediate jacking pits is decided after consideration of several factors. Since a primary advantage of this method is the elimination of traffic impacts, intermediate jacking pits also should be located to minimize traffic disturbance. Storage of materials and equipment is also a concern and may require temporary guardrail or traffic barrier to shield traffic from the site. Diversion of stream or overland flow will also be necessary to prevent flooding of the jacking pit.

During the jacking procedure, care should be given to personnel safety. Hydraulic jacks that can cause breakage of materials exert heavy pressures. Hydraulic hose lines also may rupture and cause injury.

Illustration of typical equipment setup for jacking concrete pipe

A typical equipment setup for jacking concrete pipe is as shown schematically above.

A variation on pipe jacking is noted in a process involving a steam-powered hammer (much like a pile driver) instead of a pneumatic jack. Because of the energy involved with each blow, a mandrel must precede the driven pipe. Similar to the jacking process, the hammering process requires the removal of displaced soil and material as the pipe moves into the embankment.

A viable alternative to using RCP or steel pipe for pipe jacking is fiber reinforced polymer concrete pipe (FRPC) or reinforced polymer mortar (RPMP), which is about a third of the weight per foot of precast RCP. See FHWA Culvert Repair Practices Manual Volume 1, page 2-27 and Index 2.1.1.1.3.1 of this D.I.B. Also refer to Caltrans HDM, Section 829.8 for specific procedures, limitations and other considerations for Jacking Pipe.

The installation of underground utilities within State highway right-of-way is performed by the use of a trenchless technology, in most cases. The requirement for encasement of utility installations is for the protection of the traveling public and to minimize the amount of disturbance to the structural integrity of the roadbed. See Index 623.1 “Bore and Jack” of the Encroachment Permits Manual: http://www.dot.ca.gov/hq/traffops/developserv/permits/pdf/manual/Chapter_6.pdf.

9.1.2.2.2 Microtunneling

No universally accepted definition for microtunneling (MT) exists. However, MT can be described as a remotely controlled, guided pipe jacking process that provides continuous support to the excavation face. MT is a trenchless construction method for installing culverts beneath roadways in a wide range of soil conditions while maintaining close tolerances to line and grade from the drive shaft to a reception shaft. The most common way to categorize MT is by the spoil removal system (i.e., slurry or auger). A slurry system is more capable of handling wet, unstable ground conditions. Both auger and slurry MT systems have five independent systems:

  • Microtunneling boring machine
  • Jacking or propulsion system
  • Spoil removal system
  • Laser guidance and remote control system; and
  • Pipe lubrication system

The most common materials used for MT are RCP, ductile iron, welded steel, and fiber reinforced polymer concrete pipe (FRPC) or reinforced polymer mortar (RPMP). The range in diameter experienced in the U.S. is from 12 inches to 144 inches, however, the most common range is from 24 inches to 48 inches.

Settlements typically associated with microtunneling, or other tunnel construction methods, include two types: large settlements and systematic settlements. Large settlements occur primarily as a result of over excavation by the tunneling or microtunneling machine leading to the loss of stability at the face and the creation of voids above the installed pipe or tunnel. Large settlements are almost always the result of improper operation of the machine, or sudden unexpected changes in ground conditions. Large settlements must be avoided through geotechnical investigation and good workmanship by the Contractor. The importance of a skilled and experienced machine operator cannot be over-emphasized.

Systematic settlements are primarily caused by the collapse of the overcut, or annular space, between the jacking pipe and the excavation, and to a lesser extent by elastic deformations of the soil ahead of the advancing tunnel. The overcut is necessary in microtunneling and pipe jacking to allow lubrication to be injected, to decrease jacking forces to reasonable levels, and to facilitate steering of the microtunnel boring machine (MTBM). During tunneling, or after the tunnel is completed, the soil may collapse or squeeze onto the pipe, resulting in settlements at the surface. Systematic settlements can be controlled by limiting the radial overcut the contractor is allowed to use, as well as filling the annulus with bentonite lubricant during tunneling, and with cement grout after tunneling is completed. Systematic settlements generally decrease with distance above the crown of the pipe and with lateral distance from the centerline of the pipe. Systematic settlements decrease as the annular overcut decreases, and as soil consistency (density, stiffness) increases. Systematic settlements also decrease as pipe diameter decreases. See Index 9.1.2.3, settlement monitoring, under other consideration for TEC.

For machine tunneling with steel or concrete segments used as temporary supports, an overcut or gap is created between the excavated bore and the support ring outside diameter as the supports are erected and bolted into place inside the tail of the shield. The tunnel boring machine (TBM) is propelled off the previously installed supports, and as the support ring exits the shield, a gap is created. For segmental steel or concrete rings, the ring can be expanded against the soil surrounding the bore as the rings exit the shield. In this case, a special spacer segment is used to fill the gap in the circumference created by the expansion of the rings against the soil. The remaining gap is then grouted.

Jacking pit for 48 inches RCP Microtunneling project under American River, Sacramento
Jacking pit for 48 inches RCP Microtunneling project under American River, Sacramento

9.1.2.2.3 Pipe Bursting and Pipe Splitting

Pipe Bursting (for brittle materials) and Pipe Splitting (for ductile materials) are processes in which the trenchless pipe replacement is carried out by pulling a new pipe (typically fusion welded HDPE) behind a cone ended bursting tool. The bursting tool is pneumatically or hydraulically driven and effectively hammers its way through the host pipe, displacing the fragments into the surrounding soil, while simultaneously pulling the new pipe into place behind it. Pipe Bursting is the only trenchless method that allows for the upsizing of the original pipe

Pipe bursting can be used on almost any type of existing pipe except ductile iron or heavily reinforced concrete. Segmental replacement pipe can be used in lieu of fused pipe, but requires jacking equipment to force it in behind the bursting unit. Currently the applicable size range is limited to between 2-54 inches, with larger units becoming available. The typical length of pipe replaced by pipe bursting is slightly over 330 feet, but greater lengths have been done. In addition, depth, soil conditions, peripheral utilities and service connections will dictate whether pipe bursting is appropriate.

9.1.2.2.4 Trenchless Replacement References

NCRHP Synthesis 242 – ‘Trenchless Installation of Conduits Beneath Roadways’.
http://www.usroads.com/journals/rmej/9804/rm980403.htm

AASHTO Highway Drainage Guidelines/Volume XIV, 5.1.4.4 Pipe Bursting (Existing pipe material must be clay, RCP, cast iron or PVC).

ASCE Standard Construction Guidelines for Microtunneling, December 28, 1998.

So.Cal. APWA and the AGC of California in their STANDARD SPECIFICATIONS FOR PUBLIC WORKS CONSTRUCTION-MICROTUNNELING 2000 EDITION.

No-Dig Engineering Journal, published by Trenchless Technology Incorporated

9.1.2.3 Other Considerations for TEC

The following guidelines are for trenchless projects where the potential for subsidence (loss of ground) and risk is high.

High-tech methods do not necessarily mean safer methods. The amount of risk depends on the contractor's experience in addition to a number of factors that require engineering judgment such as: depth of cover, diameter of tunnel, proposed methods, tunnelman's classification of materials to be tunneled (cohesionless sands, gravels, and cobbles or boulders below groundwater surface are probably the worst) and potential obstructions.

In house designs should consider the following four categories. Depending on complexity, it may be necessary to hire a consultant to perform the design:

1. Geotechnical Investigation
2. Settlement Monitoring
3. Contractor Submittals
4. Contract Inspection

Items 2 and 3 should be addressed in the Plans and Specifications and should be based on the results of Item 1. Geotechnical Design within the Division of Engineering Services (DES) should review the Geotechnical Reports and the Plans and Specifications prior to bid. If cohesion less materials below the ground water table or "running or flowing ground" conditions are identified, special precaution should be taken in the permit review. The Contractor Submittals to the Engineer required in the Contract Documents should be provided for Caltrans review prior to starting work. The Contract Inspection should depend on the proposed trenchless methods, project complexity, and risk to the public.

1. Geotechnical Investigation

A minimum of two borings, one on each side the highway crossing is recommended. An additional boring should be made in the median if practical. This can be increased or reduced depending on risk and variability of tunneled materials.

2. Settlement Monitoring

The ground movements caused by trenchless pipe installation techniques can have a significant effect on adjacent services and road structures.

Surface settlement is mainly a result of loss of ground during tunneling and dewatering operations that cause subsidence. During microtunneling, loss of ground may be associated with soil squeezing, running, or flowing into the heading; losses due to the size of overcut; and steering adjustments. The actual magnitudes of these losses are largely dependent on the type and strength of the ground, groundwater conditions, size and depth of the pipe, equipment capabilities, and the skill of the contractor in operating and steering the machine. Sophisticated microtunneling equipment that has the capability to exert a stabilizing pressure at the tunnel face, equal to that of the insitu soil and groundwater pressures, will minimize loss of ground and surface settlement without the need for dewatering.

In general, the subsurface monitoring points should be installed at 5 ft and 10 ft above the crown of the proposed tunnel near the jacking shaft, above utilities, and on shoulders of roadways, to evaluate the Contractor’s operations before proceeding under critical locations.  Additional points at non-critical locations should be monitored to gain an early indication of Contractor workmanship.

Simple subsurface monitoring points (see below) that consist of a length of steel rebar installed inside a cased borehole that extends to the desired height above the tunnel crown are recommended.

Illustration of settlement monitoring point detail

The materials needed are 1/2- to 3/4- inch diameter rebar and 2-inch diameter, Schedule 40, PVC pipe installed in a vertical borehole drilled to the desired depth of the settlement point.  The casing should be covered with a cap to protect it from the weather and a road box can be used if the point is installed inside a traffic area.  The casing is installed at 5 feet or 10 feet above the proposed tunnel crown, and the rebar is inserted into the casing and driven 6 inches to 12 inches below the bottom of the casing, into undisturbed soil.  In this way, the response of the ground can be monitored very closely as the microtunneling or tunneling machine passes beneath the point. These simple settlement points have been shown to perform more reliably than surface points and more complicated and expensive multiple-point borehole extensometers, which may tend to bridge over settlements until heavy loads pass over the affected areas.

Surface settlement monitoring points may be used to supplement the subsurface points. However, surface points only indicate gross settlements at the surface after subsurface ground loss has occurred. Due to the shear strength of soils, and the rigidity of pavement and other structures, voids created at depth may not appear at the ground surface for days, weeks, or even months after the tunnel has been completed. By monitoring ground movements much closer to the tunneling operations, at strategic locations before passing beneath the critical features, ground losses, if any, can be detected in time to fill voids quickly before surface facilities are affected, and more importantly, to alert the contractor to alter their procedures to prevent further ground loss.

Once installed, the monitoring points should be surveyed prior to tunneling to establish the baseline.  Surveying should then proceed at least once a day, or every 50 feet of advancement, whichever is more frequent.  In addition to daily monitoring by survey, the points should be checked at more frequent intervals by the onsite inspector using a tape measure as the tunneling machine or MTBM approaches and passes beneath the points.

 

SETTLE MONITORING POINTS
FREQUENCY
ACTION LEVEL*
MAXIMUM ALLOWED**
Surface

Hourly when heading is within 23 feet, otherwise daily

1/4 inch

1/2 inch

Surface (in traffic lanes)

Before and after tunneling

-----------------

1/4 inch

Subsurface

Hourly when heading is within 23 feet, otherwise daily

1.5 inches

2.5 inches

* Corrective action taken (filling voids and alerting contractor to alter their procedures: Systematic settlements can be controlled by limiting the radial overcut the contractor is allowed to use, as well as filling the annulus with bentonite lubricant during tunneling)
** Mitigation such as grouting required

An independent Instrumentation Specialist should install and monitor the settlement monitoring points.  The survey accuracy of the settlement monitoring points should be to 0.005 foot.

Calculations of expected systematic settlements can be made to determine whether changes in pipe depth and spacing of multiple pipes are needed, or whether changes to construction methods or ground improvement are necessary to prevent damage to existing surface facilities. Settlements may be evaluated using methods developed by Birger Schimdt and Peck (1969). This approach models systematic settlements as an inverted normal probability curve, or settlement trough, with maximum settlements occurring directly above the centerline of the tunnel, and with settlements decreasing with distance from the tunnel centerline. The approach actually has no theoretical basis in soil mechanics, but has been adopted based on empirical correlations with observed settlement magnitudes and distributions. The equations and diagram for the calculations are shown in Figure 1.

Systematic settlement diagram

3. Contractor Submittals

The following submittal requirements are presented below as an example and are specifically for microtunneling (see Index 9.1.2.2.2), however, similar information is required for other types of boring.

1) Manufacturers' data sheets and specifications describing in detail the microtunneling system to be used.

2) Detailed description of similar projects with references on which the proposed system had been successfully used by contractor/operator.

3) Description of method to remove and dispose of spoil.

4) Maximum anticipated jacking loads and supporting calculations.

5) Description of methods to control and dispose of ground water, spoil, temporary shoring, and other materials encountered in the maintenance and construction of pits and shafts.

6) Shaft dimensions, locations, surface construction, profile, depth, method of excavation, shoring, bracing, and thrust block design.

7) Pipe design data and specifications.

8) A description of the grade and alignment control system.

9) Intermediate jacking station locations and design.

10) Description of lubrication and/or grouting system.

11) Layout plans and description of operational sequence.

12) A detailed plan for monitoring ground surface movement (settlement or heave) due to microtunneling operation. The plan shall address the method and frequency of survey measurement. At minimum, the plan shall measure the ground movement of all structures, roadways, parking lots, and any other areas of concern within the calculated settlement trough of all microtunneling pipelines. A description of how settlements will be monitored and excessive settlements will be avoided and contingency plan should also be required to establish how the Contractor will mitigate any excessive settlements. A pre-construction survey should also be required in the Contract Documents and conducted by the Contractor, accompanied by the Engineer and Owner representatives, to document pre-construction conditions and protect against frivolous claims.

13) Contingency plans for approval for the following potential conditions: damage to pipeline structural integrity and repair; loss and return to line and grade; and loss of ground.

14) Procedures to meet all applicable OSHA requirements. These procedures shall be submitted for a record purpose only and will not be subject to approval by the Engineer. At a minimum, the Contractor shall provide the following:

a) Protection against soil instability and ground water inflow.

b) Safety for shaft access and exit, including ladders, stairs, walkways, and hoists.

c) Protection against mechanical and hydraulic equipment operations, and for lifting and hoisting equipment and material.

d) Ventilation and lighting.

e) Monitoring for hazardous gases.

f) Protection against flooding and means for emergency evacuation.

g) Protection of shaft, including traffic barriers, accidental or unauthorized entry, and falling objects.

h) Emergency protection equipment.

i) Safety supervising responsibilities.

15) Annular space grouting plans if required by Contract Documents.

4. Contract Inspection

If in house expertise is not available, it may be necessary to have full time inspection performed by an underground construction-engineering firm specializing in the tunneling methods to be used. This covers other possible methods, which can be evaluated from the Contractor Submittals.

This page last updated August 20, 2011