When Ground Improvement is Needed During Trenchless Rehabilitation
Ground Improvement and stabilization is necessary when dewatering projects are in place to avoid the collapse of the soil and costly delays.
Our planet is covered with over 70% water. The remaining 30% is where we dig, tunnel through, build and live. However, under that 30% land area, there is plenty of water. Almost anywhere that we dig, we can be sure of hitting water somewhere beneath. Groundwater can be found almost everywhere under our feet. While this water is vital for our sustenance, it can prove to be a big challenge for construction and rehabilitation projects.
Dewatering is a default process that every construction project has to undertake while digging foundations. However; in cases like trenchless construction and rehabilitation, many features are unseen and need to be ascertained before proceeding further. Settlement is one such major cause that calls for ground improvement. Soil consists of about 45% minerals, 5% organic matter, 25% air and 25% water by composition. All of these vary in size and volume. As ground and weather conditions change, the composition of the soil with regard to moisture and air content also changes. Sometimes the soil may contract due to lack of moisture and sometimes it may expand due to excess moisture. (Read more in "Cost-Saving Trenchless Dewatering in Construction Projects.")
Ground improvement is basically done to stabilize soil and prevent it from settling or collapsing especially when a utility pipeline or structure is passing through the area. While working on a trenchless rehabilitation project, it is not uncommon to encounter soil conditions that call for ground improvement before the repair can be carried forward, and water plays a crucial role in determining the prevailing soil condition. One of the major circumstances that call for ground improvement is settlement.
Settlement can usually be observed on the ground over sub-structures like foundations or pipelines. Sometimes, however; it may not be noticeable on the surface due to stratification of soil, with some strata being hard, as when boulders are present. While undertaking trenchless rehabilitation projects, however, different methods of identifying problems within the pipeline are employed. CCTV cameras, Ground Penetrating Radar (GPR), Pipe Penetrating Radar (PPR), boreholes, manual inspection, etc. are just some methods used for pinpointing the problem areas. These inspection methods when employed can also be used to detect deflection of pipeline, voids over the pipeline, cracks due to excess surface soil pressure etc. (Learn more in "The View Underground: Ground Penetrating Radar.") Usually two types of settlements are observed:
Systematic settlement occurs as a result of voids in the annular space between the pipe installation and the soil, either by collapse of soil or due to its elastic deformation. This annular space is important in horizontal directional drilling (HDD) and related methods of trenchless construction to allow for the circulation of drilling fluid that lubricates and cools, and also helps during the pullback process. If the annular spaces are not sealed off properly once the installation is completed, it can result in the collapse of the soil over the pipeline that continues upward to the surface until a trough like depression is noticed on the surface. Systematic settlement can be avoided by proper soil investigation and selection of appropriate depths for installing pipeline, maintaining reasonable overcut radii, using appropriate drilling fluids in sufficient quantity and filling the annular space with grout on completion of installation.
Large settlement usually occurs as a result of loss of ground due to over-excavation. This can create voids and sinkholes above the borehole. This can be avoided by undertaking appropriate geotechnical investigation which will help in the selection of the right methods and tools for undertaking the project. Understanding the soil condition can help the contractor to utilize ground improvement methods like jet grouting, groundwater cut off, dewatering, freezing etc. that will help stabilize the soil to control the ground conditions. For this, a geotechnical report is needed.
Identifying Problem Areas
To prevent accidents due to settlement and collapse, testing of pipelines and field monitoring are essential requirements. Usually, the need for ground improvement is only understood once the trenchless rehabilitation process has been initiated. However; advanced technological means of exploring pipelines can help identify these problems before work is started. Video inspection, PPR, and GPR are excellent methods for the detection of problems within and outside of the pipe. Places that need ground improvement in the vicinity of a pipeline can also be visually identified. Reduction in the rate of flow in the pipeline, backflows, blockages, pools of slowly seeping ground water, unusual growth of vegetation, and collapse of sections of ground is some indicators of a deeper problem within.
Using Tech to Identify Ground Improvement Needs
During a CCTV inspection, a camera is mounted on crawlers and connected by cables to a monitor in the control room and guided through a pipeline. It is advisable to clean the pipeline prior to launching for accurate results. The speed of the instrument should be ideally set at 6 meters per minute for pipes 200-mm. or less in diameter, and 12-meters per minute for larger pipes. Lighting should be good enough to illuminate 2-meters ahead of the camera. Modern innovations in video inspection of pipes allow for tilt and pan for closer inspection, fish eye lenses that enable 180° view at any point, laser systems to measure the pipes profile and sonars for flooded pipe sections or inverts. Cameras can identify defects such as stress deformation, cracks, corrosion, infiltration, exfiltration, etc.
Ground Penetrating Radar (GPR)
Ground penetrating radar has an electromagnetic antenna that emits radio waves which can penetrate through concrete, soil and rock and provide a 3 dimensional image of the area under survey. GPS systems can pinpoint the exact location and alignment of utilities. Passive sound detection method is used to detect leakage along pipes, on hydrants etc. by using a highly sensitive ground microphone. Voids, settlements, and other anomalies can be detected via GPR. (Learn more in "The Science of Getting it Right: Locating Underground Utilities.")
Pipe Penetrating Radar (PPR)
For PPR, a robot carrying the PPR travels through the pipe under inspection. As the pulse from the PPR travels through the material of the pipe, any sharp change in the property of the material, like at the interface of pipe and water, is reflected or refracted. The greater the difference in the properties of the materials, the greater is the amount of energy that is reflected back. A receiving antenna detects these reflected waves and is recorded as a single trace. As the pulse travels, a profile of the reflected waves is created. In sewer rehabilitation, this method can be used to detect liner bonding, voids outside pipe walls by detecting variation in bedding conditions, etc. Identifying any problems using PPR can help understand the structural stability of the pipe, can prevent sinkholes, and save cost on costly rehabilitation by early detection. PPR is one of the recent and most accurate methods of detecting the need for ground improvement.
Trenchless rehabilitation has brought with it great relief to the woes of traditional dig and repair methods. Thanks to continuing technological advances in the trenchless industry, it is becoming easier to detect problems before they become one. Rectifying problems whether by traditional methods or by using trenchless rehabilitation methods are always time consuming and expensive because of the vast network of pipes and cables that run underneath. As more contractors are becoming aware of the numerous benefits of trenchless technology, repairs and up-gradation of pipelines is becoming hassle free and non-disruptive to the public. Silently it is also helping us gradually create a stress free environment by eliminating traffic jams and annoyed and stressed people.
Written by Tabitha Mishra | Civil Engineer, Technical Content Writer
Tabitha has a Bachelors Degree in Civil Engineering from Mumbai University, India, and is currently freelancing as a technical content writer. Prior to writing, she has worked as a site engineer and site manager for various building construction, building rehabilitation, and real estate evaluation projects.
Tabitha is also certified as a Primavera project management professional and is well versed with Auto CAD. In her spare time, she does private consultation for small-sized home builders and assists with plans and permissions.