Post-construction Evaluation of HDD Installations - Part 4
December 1, 2007
Moisture content is defined as the ratio of the weight of the water to the weight of the solid particles in a soil medium. The moisture content in sands generally lies between 10 percent and 30 percent, while in clay it can range from less than 5 percent to more than 300 percent. The impor-tance of moisture content in a soil mass cannot be understated, as it can have a significant effect on some of the characteristics and behavior of a soil. For example, in fine-grained soils such as clay, high moisture content can greatly reduce the shear strength. At every excavation of the field tests conducted, five samples were collected from each installation to determine moisture content of the annular space and the respective soil me-dium.
The moisture content of the clay averaged 27 percent, while the annular space averaged 38 percent up to four weeks after installation. Analysis of the annular space after one year revealed a decrease in moisture content to 32 percent. This indicates that equalization of the annular space and the native clay seemed to have occurred, as evidenced by the 6 percent reduction in moisture content exhibited in the annular space.
The moisture content of the sand averaged 5 percent, while the annular space averaged 22 percent up to four weeks after installation. Similar to the clay site, the analysis of the annular space from the 1-year excavation revealed a decrease in moisture content to 12 percent. It appears that, over time, an equalization of the annular space and the surrounding soil medium occurs, thereby increasing the strength properties of the annular space.
Clay Site AnalysisAt each cross-sectional excavation, photographs were taken to visually capture the in-situ state of the annular space over time. As observed, the HDPE pipe generally is centered within the annular space region with no evidence of voids. Additionally, clay, being a cohesive soil, enabled the drilling fluid to remain within the boundaries of the annular space, thereby permitting the fluid to set up/solidify. In-situ analysis of the unconfined shear strength of the drilling fluid revealed that the properties increase over time within the annular space region.
Comparing the one-week excavation to the one-year excavation reveals that the visual annular space actually decreases over time, and is relatively nonexistent in the one-year excavation. It appears that, over time, the amount of water present in the annular space decreases, and the texture of the annular turns into a more solid state. This is similar to previous findings that the moisture content in the annular space is only slightly higher than that of the surrounding soil after two years. This may be attributed to equalization between the annular region and the native soil formation. In addition, there is no evidence of the existence of voids in any of the cross sections. These findings also were evident in 4-inch and 12-inch installations.
Sand Site AnalysisFor the pipe installations in sand, it was difficult to determine whether voids existed in the annular space. Because of the disturbance created when excavating and cutting the pipe, the slurry in the fragile and very liquefied annular space had a tendency to flow out of the cross section that was just created. Once the slurry stopped flowing and the annular space was intact, a clear and evident sign of a slight void was present in the 8-inch installation. In each excavation, the voids always occurred beside the pipe at 90 degrees, and never below or directly above. It is difficult to assess whether these voids existed before the cross sections were excavated or whether they resulted from the disturbance that was created when the cross section was made. Some questions were answered once closer inspection of the annular space with the void was made.
When examining the void, it was evident that the void was not local to that cross section, but, in fact, spread continuously through the formation. This void was prominently found only in the 4-inch and 8-inch installations, indicating that this may be an isolated incident caused by the actual soil medium encountered. Additionally, it is suspected that these voids may have been formed due to the fluid permeating into the surrounding co-hesionless sand formation. The one-year excavation of the 8-inch pipe revealed no presence of voids whatsoever in the annular space. Therefore, the presence of voids is not thought to be a problem during HDD installations in sand; however, any short-term voids that may be present will more than likely be eliminated through redistribution of the soil with the annular space over time. It is important to recognize that more solids are present in the annular space region, and they provide support to the installed product pipe. Also, the presence of small voids is not a cause for alarm, as the surrounding native soil retains its compactive effort, which is difficult to achieve using traditional open-cut trenching methods of pipe installa-tion.
In comparison to the clay installations, the analysis revealed that the HDPE pipe in all three of the sand installations tended to settle in the upper region of the annular space. This is a result of buoyancy forces acting on the pipe in the cohesionless sand, resulting in the pipe “floating” upward. In such installations, particularly when crossing under a body of water, it often is the practice to weigh down the pipe by filling it with water to counteract the buoyant upward forces.
Shear Strength TestA pocket penetrometer was used in analyzing the unconfined shear strength of the hole’s annular space. The pocket penetrometer only was utilized at the clay site, because it only is applicable in fine-grained soil. For each cross-sectional excavation that occurred in clay, the pocket penetrometer was used to determine the unconfined shear strength. Comparisons subsequently were made between the different pipe installations to assess the change in shear strength over time. The pocket penetrometer field test was performed at various locations in the annular space surrounding each installation. The unconfined shear strength of the annular space material increases over time. This increase may be explained by the consolidation of the surrounding soil medium within the annular space over time and the slow hydration of the native clay in the slurry, thereby increasing the strength properties around the installed pipe.
Conclusions/RecommendationsObservations of the annular space region at various time intervals after installation indicate that small voids may initially be present in cohesionless soils due to the permeation of fluid into the surrounding native material. However, it is important to realize that more solids are present in the annu-lar space region, thereby providing support to the installed product pipe. No voids were detected in any of the installations in the cohesive soil.
The shear strength of the annular space is dependent on the characteristics of the native soil and its reaction with water. As was evident when com-paring the clay and sand installations, the state of the clay annular space was far more solidified than the sand installation. It also exhibited strength and cohesive characteristics while the annular space in sand was fluid-like. In addition, the sand installations did exhibit initial voids, which is an-other sign of non-cohesion. Measures of the unconfined shear strengths of the annular space in the clay soil medium indicate that it reaches about 70 percent to 80 percent of the native soil after one year. This is an important consideration, because these measures would be difficult to achieve from open-cut installation methods.
Even within the same soil site, or even the same installation, there are differences between every cross-section. Because the soil naturally exhibits different stratums or pockets of compositions, strengths and moisture content, and the annular space reflects this as well. The annular space was discovered to change in shape, texture, composition, shear strength and moisture content from cross-section to cross-section. In all cases, the di-ameter of the annular space region decreased over time to the point that it equalized (or consolidated) with the native soil. The moisture content of the clay averaged 27 percent, while the annular space averaged 38 percent up to four weeks after installation. Analysis of the annular space from the one-year excavation revealed a decrease in moisture content to 32 percent. The moisture content of the sand averaged 5 percent, while the annu-lar space averaged 22 percent up to four weeks after installation. Similar to the clay site, the analysis of the annular space from the one-year exca-vation revealed a decrease in moisture content to 12 percent. It appears that, over time, an equalization of the annular space and the surrounding soil medium occurs, thereby increasing the strength properties of the annular space.
The primary and most important function of the post-installation annular space is to behave like the native soil and provide security to the installed pipe. In-situ tests and visual assessments validate this notion, as all of the pipes that were installed remained secure, with no evidence of any poten-tial movement. Surface monitoring points placed along the installations revealed that no ground settlement occurred at the surface during installa-tion. Additionally, no ground settlement was observed over time.
Much information may be obtained from field studies of construction processes, providing contractors, owners, manufacturers, engineers and oth-ers interested in directional drilling with a better understanding of the influence that HDD installations have on the surrounding medium. The six pipes installed and the 28 cross-sections that were excavated and analyzed support the opinion that the annular space does provide the necessary attributes for the short-term and long-term success of a pipe installation using horizontal directional drilling.
Recommendations for future research include expanding the scope of research to include other soil mediums, pipe diameters and pipe materials. This could include drilling in mixed soil conditions along a continuous borepath. Additionally, it would be beneficial to assess the annular space using different mixtures of drilling fluid and at different depths of installation. Continued long-term evaluation of the annular space after installa-tion to assess the unconfined shear strength over a longer time horizon is recommended. This could provide additional validation of the long-term integrity of the installed pipe. Furthermore, more elaborate strength tests or other geotechnical in-situ or laboratory tests to measure properties of the annular space could be utilized. Assessment of the annular space in installations made below the water table is suggested for comparison with the results from this research.