The Geotechnical Investigation: Which Method Should We Use?
A geotechnical investigation and report is necessary to avoid costly delays and changes of plans due to underground conditions.
A geotechnical investigation may seem like a waste of time due to the pressure to get moving with a trenchless project. However, the value of a geotechnical investigation should not be underestimated as it yields vital information that will ensure the project can be successfully planned and executed.
In a standard construction project involving excavations, the conditions that contractors will face become apparent as the excavation progresses. Changes in ground conditions can be visually inspected, existing underground pipes are exposed and decisions can be made based on what can be seen.
Trenchless projects are executed blindly, in the sense that the whole project happens out of sight, and special techniques must be used to determine what the conditions are like underground.
Only a licensed geotechnical engineer should perform a geotechnical investigation for a trenchless project. This will ensure that accurate information is made available and that the right decisions are made. (Read on in "Why a Detailed Geotechnical Report Means Success for Your Trenchless Project.")
Geotechnical Investigations for Trenchless Construction
Ground Penetrating Radar
Ground penetrating radar (GPR) is a method that employs specialized equipment used to identify where there are underground pipes, utilities and cables in an area where a trenchless project will be implemented. This is very important and once the locations of existing infrastructure are marked on the ground, decisions can be made about the routing of the new installation. (Learn more in "The View Underground: Ground Penetrating Radar.")
The principle behind ground penetrating radar is that of high frequency radio signals. A signal is transmitted into the ground, and a receiver measures the time taken for the signal to return due to reflection by objects in the ground. Depending on the type of ground conditions, GPR can measure between 3 and 30-feet underground.
A limitation of GPR is its lack of penetration in soils with a high clay content and the need for trained personnel to accurately use the equipment and interpret the results.
While costs to perform GPR vary depending on equipment being used and the contractor, the Federal Highway Administration estimates that every $1 spent on GPR before highway construction saves about $5 in construction costs.
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Where there is some uncertainty of exactly where existing underground piping and infrastructure runs, potholing is used to excavate a small hole so that a visual inspection can be done. A vacuum excavation technique is used to perform the excavation at the point where a suspected intersection may occur.
Bores and Trial Pits
Bores and trial pits are dug along the proposed route of a trenchless construction project in order to identify the ground conditions and groundwater levels. This information is critical to determine which trenchless method is most appropriate for the conditions.
A geotechnical investigation is mostly done in an effort to identify the soils and ground conditions contractors will be working with. For this, soil sampling is a key element. Geotechnical engineers and geologists can sample soil cores to identify the type of soil being worked in (sandy, clay, cohesive, cohesionless.) and whether or not hard rock is present, as well as determine the sizes of the rocks below.
Depending on this information, different drilling tools and bits must be used for hard, medium and soft ground, as well as for soils that contain small rocks, medium rocks or large underground rocks.
Soil sampling also enables contractors to plan the correct formula for the drilling fluid that will be used to stabilize and condition the ground during drilling. (Also read "Ground Improvement Requirements When Working With Sandy Soil.")
Geotechnical Investigations for Trenchless rehabilitation
Pipe Penetrating Radar
Pipe penetrating radar (PPR) is like GPR except it happens inside a non-metallic underground pipe. A robot mounted on tracks is remotely controlled as it travels along a concrete pipe measuring the condition of the pipe wall and voids behind the wall. The information collected from PPR is used to scope a maintenance repair and select the most appropriate and cost-effective method for trenchless rehabilitation.
PPR can also identify where geological formations are infiltrating a pipe through a crack. (Learn more about PPR in "A Look at Pipe Penetrating Radar.")
Pigging is another means of non-destructive testing for underground pipes. PIG is an acronym for a Pipeline Inspection Gauge, but it also gets its name from the squealing sound the PIG makes as it passes through a pipe. Intelligent pigs are fitted with sensitive electronic devices, which use ultrasonic or magnetic flux leakage to measure the integrity of the pipe.
Reports generated by intelligent pigs highlight corrosion, erosion, cracks and metal loss, thus giving a comprehensive evaluation of a scope of repair work for trenchless rehabilitation. Pigs can be used while the pipeline is in service, saving downtime while the geotechnical investigation is in progress.
Closed Circuit Television (CCTV)
Closed Circuit Television (CCTV) is used to do a visual inspection of the internals of an underground pipe. An operator watches a screen while the camera makes its way through the pipe. The camera can be stopped at any time, rotated to any position in the pipe and retrace its steps as instructed by the operator.
Using CCTV enables experienced engineers to identify problem areas visually and plan repairs appropriately.
Things That can go Wrong Without a Geotechnical Investigation
Unexpected ground conditions like rock or clay sections can have a major impact on the success of a trenchless project. Horizontal Directional Drilling is not suitable for clay soils as the clay clogs up the drill, but once a project is started, it is very difficult and costly to change methods halfway through. High levels of groundwater can also have a major impact, as water can seep into the tunnel and cause a collapse of the soil above.
Without a geotechnical investigation, trenchless projects can collide with existing underground infrastructure. This is very costly, can result in unplanned downtime, extra costs and delays.
Without a geotechnical investigation in trenchless rehabilitation, assumptions are made about the condition of the pipe underground and then repair plans are actioned. If the piping is not as damaged as was assumed unnecessary repairs will be carried out leading to a waste of money and resources.
On the other hand, if the piping is assumed to be in better condition than it actually is, insufficient repairs will be executed. The pipe is likely to fail in the near future as the repair will not completely resolve the problems.