This research was undertaken with two primary objectives:
• Document the current practices and applications of trenchless technology in the United States and, particularly, in Iowa
• Evaluate the effects of trenchless construction on surrounding soil and adjacent structures
To fulfill these objectives, a literature review was first conducted to assemble information on the current practice of trenchless technologies. The literature review examined the rationale for trenchless technology and introduced the major trenchless construction and rehabilitation methods. Soil investigation methods for trenchless projects, QC/QA, the effects of trenchless technologies on surrounding soil, and design processes were all discussed.
To gain addition insight regarding trenchless technology, trenchless practitioners were surveyed and interviewed. Three separate surveys, with each survey targeting a different geographic region, were sent to professionals. The surveys targeted Iowa, the Midwest, and the entire United States. These surveys and interviews focused on the following four major topics:
• Method familiarity
• Observed pavement distress
• Reliability of methods
• Future improvements
The Iowa survey garnered 34 respondents, 60% of whom were public employees and 40% of whom were contractors and consultants. The survey results indicated that HDD, auger boring, pipe jacking, and cured-in-place pipe are considered to be the most common trenchless
construction methods used in Iowa. The respondents also reported that pipe ramming and pipe bursting are the least favorably viewed of the common methods because of perceived risks associated with these methods. Respondents were asked if they had seen pavement distress or other problems occur as a result of trenchless installations, and 47% reported that they had.
A shorter survey was sent to professionals around the Midwest, and it garnered 32 respondents. Of these respondents, 22% were public employees and 78% were contractors and consultants. These respondents reported that the most common trenchless methods used in the Midwest are HDD, cured-in-place pipe, pipe jacking, and localized repairs. Of these respondents, 29% reported observing pavement distress or other problems resulting from trenchless technologies.
Questions from the Midwest survey were included in a larger survey that was conducted by Dr. Mohammed Najafi of the University of Texas at Arlington and sent to state department of transportation employees across the United States. The 12 respondents reported that sliplining, HDD, pipe jacking, cured-in-place pipe, and localized repairs were the methods that they had encountered most.
Many additional comments were collected in response to these surveys. Researchers also
conducted interviews as part of field and office visits. As a result of these activities, the research team collected many comments related to trenchless technology. Professionals commonly expressed the following general comments:
• There is a desire for cost-effective QC/QA standards to reduce risk.
• Encountering unmarked utilities is a major problem.
• More soil testing could be useful because many projects currently use no soil testing.
• Heave or subsidence due to trenchless construction can cause ground movements of up to about 2 feet.
Survey and interview results indicate that the frequency of pavement distress and other
trenchless-related problems is an ongoing problem in the industry. Inadequate soils information and QC/QA are partially to blame.
A field investigation was performed that involved observing 19 trenchless construction projects. Research activities included documenting construction procedures, noting successes and
challenges, interviewing personnel, obtaining soil samples for laboratory testing, and measuring stress changes in the soil near the borehole during construction. Soil samples were tested in the laboratory to better understand the types of soil involved in the trenchless installations. Finally, the results were analyzed and discussed.
The trenchless construction projects that were studied in the field work were classified as “Site Visits” and “Field Monitoring.” The “Site Visits” portion (Section 4.2) of the field work
involved observing 13 trenchless construction projects. For these projects, researchers visited the jobsites, observed and documented construction practices, and noted successes and challenges. Soil samples were tested in the laboratory to evaluate soil properties. “Field Monitoring,” as described in Section 4.3, involved the same tasks as “Site Visits” but also included additional soil testing and measuring soil stresses during construction. This additional soil testing made these project investigations more thorough compared to the investigations in the “Site Visits” in Section 4.2. Undisturbed soil samples were recovered and tested in the laboratory. When the appropriate samples and testing equipment were available, samples were tested using confined and unconfined compression, consolidation, and multistage consolidated-undrained triaxial tests.
Soil stresses in the field were measured during the six “Field Monitoring” projects by installing push-in pressure cells in the ground near the bore path before the boring began. These pressure cells provided readings of soil pressure increases experienced as the boring equipment passed the instruments. Soil samples were analyzed in the laboratory to correlate observations and pressure readings to soil properties.
Pipe sizes installed during the observed projects ranged from 0.75 inches in diameter up to a 10- foot by 5-foot box culvert. Installation lengths ranged from 24 to 495 feet. These projects were all successfully completed but, in two projects, the HDD caused frac-out and surface heave. The trenchless methods used for the 19 total projects included 1 pipe jacking, 1 tunneling, 1 impact moling, 5 auger boring, and 11 HDD.
The first observed project to experience a surface heave and a frac-out involved an HDD installation of two 4-inch-diameter HDPE pipes in one borehole. The installation depth was 17 feet, and the bore length was 400 feet. Researchers summarized that the cause of this failure was probably incorrect mismatch between the drilling fluid mixture and the soil type encountered. A drilling fluid that was high in bentonite was used because of the contractor’s concern that granular soils would be encountered; bentonite drilling solutions reduce the possibility of borehole collapse in granular soils. When clay was encountered, the lack of a clay-inhibiting polymer in the drilling fluid apparently caused the drilling fluid to stick to the gravelly clay soil so that the borehole sealed shut. Fluid pressures rose because drilling fluid was being pumped into the borehole and could not escape. The borehole eventually fractured along the planes of greatest weakness and drilling fluid seeped to the surface. This hypothesis was later supported by the research team’s subsurface soil investigation in which permeable sandy soil was observed around the area of frac-out and less permeable clay was observed around the area of heave. A better knowledge of subsurface conditions may have encouraged the contractor to use a different drilling fluid mix and possibly avoid this problem. This example suggests that additional field testing is desirable for HDD projects in which the contractor is uncertain of the subsurface conditions.
The second project that experienced a frac-out was an HDD installation of 8-inch HDPE pipe at a depth of 6 feet and a length of 495 feet. The frac-out may have been caused by one or a combination of the following factors: (1) a lack of soil cohesion may have enabled the drilling fluid to more easily breach the borehole walls, (2) a mismatch between drilling fluid mix and the soild and construction procedure, (3) a lack of borehole stability, and/or (4) the speed at which the reamer was pulled through the borehole (possibly too fast). Researchers could not definitely conclude the cause of the frac-out. The research team noted that most of the observed projects did not utilize a soil testing program. Contractors generally believed that they had sufficient experience in the local area and that soil testing was an unnecessary expense.
Calculations were made using cavity expansion theory to predict the pressure increases that could be expected.
Future research could provide trenchless project participants with a better understanding of several trenchless construction methods and of how to avoid pavement damage and other problems. Soil pressure monitoring of other trenchless methods in addition to HDD and impact moling could allow a better understanding of how these methods interact with soil to possibly cause pavement damage and other problems. Also, additional FE modeling of various installation procedures could improve understanding of the conditions that increase the risk of pavement damage and other problems. Additionally, an improved knowledge of the causes of HDD drilling fluid pressure buildups that lead to heave and frac-out could lead to a decreased risk during HDD installation. Based on survey comments and interview results, surface heave is considered to be the most common concern regarding the use of HDD.
Because the projects observed by the research team were successful overall, trenchless technologies appear to be effective methods for installing utility pipe in areas where open- trenching is undesirable. However, the contractor’s experience level is very important, and it is
also important to conduct soil testing in areas of uncertain subsurface conditions. It is expected that trenchless technologies will become more popular as project participants gain experience and technology improves.
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