Geotechnical Investigation by Directional Core Drilling

The need for development and improvements of infrastructure is always present, often including tunnels and other geotechnical constructions. Many of these tunnels are constructed in low quality ground conditions and often below rivers, lakes or cities. Such conditions give rise to complicated planning and difficult construction, while the consequences of mistakes can be severe. To reduce the risk of accidents thorough geotechnical investigation becomes necessary.

A technique that is being used more and more for geotechnical investigation is directional core drilling. With directional control over a borehole, it is possible to drill along a defined trajectory, for instance a planned tunnel alignment, while collecting a core sample over the full borehole length. This core gives firsthand information about the rock quality near the borehole, as well as structural information and indications of the amount of water present.

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Directional Core Drilling

The directional core drilling technology was developed by the Norwegian company Devico AS and is provided by Tech Directional Services and International Directional Services in the United States and Canada. The technology has been used in the mining and tunneling industry all over the world the last 20 years.

The equipment consists of an 18-ft long wireline operated core barrel that replaces the standard core barrel when directional control is necessary. The core barrel is operated under the same parameters as a standard core barrel and requires no additional equipment or adjustments to the drill rig or drill string.
By the use of the Devico equipment, a borehole may be started at high angles from the tunnel alignment before it is guided to follow parallel with the alignment, as illustrated in Figure 1. This is achieved by slowly changing the direction of the borehole. The rate of change is kept to approximately 9 degrees per 100 ft, which means that a borehole starting 20 degrees off the tunnel alignment will be parallel after about 215 ft of directional drilling.

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When the borehole has reached the parallel of the tunnel alignment it is common to switch back to standard core drilling for completion of the hole. If the tunnel alignment is curved, or the borehole deviates from the path, additional corrections of varying length may be performed.

Benefits with Directional Core Drilling

Standard core drilling is frequently used for geotechnical investigation, but usually has limitations in densely populated areas and where tunnels are planned underneath rivers, harbors, areas with restricted access, etc. Difficulties finding a suitable position for the drill rig will often occur, affecting the start direction of the drill hole, and resulting in much of the drilling being performed in rock formations far from the tunnel alignment.

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When the borehole is controlled with directional coring, the start position and start direction is of less importance, as the borehole quickly can be guided toward and along the planned tunnel alignment. Since the hole can be kept within a short distance from the centerline of the tunnel, the properties of the collected core are highly relevant for describing the ground properties that will be encountered during construction.

The greatest benefit with directional coring is the rock samples that is collected from the complete borehole trajectory, and lets the tunnel engineers get a visible impression of the ground conditions. As seen in Figure 2, the location and orientation of geological anomalies, fractures, fault zones, etc. can easily be determined with high accuracy, while mechanical properties can be tested directly on the core. Since the direction of the hole and the tunnel is nearly identical, the magnitude and the properties of the various ground structures seen in the core will be similar during excavation.

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The borehole itself may also be used to reveal information about the ground conditions. A variety of probes and cameras have been specially designed for use in boreholes. Among them are borehole radars for structural analysis, probes and tools for determination of water inflow or outflow, fracturing devices for rock stress measurements, etc.

After the core and borehole have been analyzed, it forms a good basis to estimate the amount of grouting and stabilization necessary, as well as production rates and material properties. As a result, the construction can be performed in a more secure manner and the stabilization can be better adapted to the geological properties.

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Detroit Upper Rouge Tunnel

During the last years, directional coring has been used most frequently in Norway and in Hong Kong. There are many reasons for this, but in Norway it is mainly due to all the fjord crossings with sub-sea tunnels requiring careful geological investigations. The popularity in Hong Kong is primarily caused by the population distribution and the rapid development in the area, where any delays or accidents during construction can give severe consequences. Boreholes with lengths up to 4,000 ft have been drilled to secure detailed geotechnical investigations.

A few projects have also been completed in the United States, in Boston and New York, and, most recently, in Detroit at the Upper Rouge Tunnel project.

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The Upper Rouge project was intended to be a combined sewage overflow control system for the upper Rouge River area in Detroit. Previous tunneling at nearby locations had at times been subject to high groundwater pressure and inflow, as well as occurrence of natural gases. Thorough geotechnical investigation programs were therefore undertaken to get a clear picture of what to expect during the construction.

The initial investigation programs located a low quality rock formation near the middle of the tunnel alignment. This led to an alteration in the construction plans where the proposed tunnel alignment was split into two tunnel sections at a different altitude, with a connection shaft between them. Such a solution would not be optimal for construction and operation, but more secure in the troublesome rock formation.

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During the final design phase in 2006-2007, a further investigation was conducted by Jacobs Engineering and Somat Engineering, to get as much data from the low quality formation as possible. Included in the program were two directional boreholes with core sampling, performed by International Directional Services.

The two directional boreholes, with lengths of 1,500 and 2,000 ft, respectively, were started with an inclination of 30 degrees before they were bent to follow the horizontal tunnel alignment. The tunnel was planned with a length of seven miles, but since this is too long to drill the boreholes were focused on the low quality zones. Here, they went through most of the geological formations existing along the tunnel alignment and also the most severe joints and fractures expected.

The main subject of the boreholes was to identify vertical joints that could produce high groundwater inflow during excavation. Another important factor was the continuous core samples collected over the entire borehole length, which would be very helpful for the bidders in their planning of the tunnel construction.

Several tests, including acoustic televiewer, single and double packer tests and hydraulic fracturing, as well as mechanical tests on the rock core, were performed in the directional boreholes to determine the hydraulic properties of the geological formations encountered.

When the investigation program was completed in 2007, the core analysis and borehole tests showed that the rock formations were more competent than expected, and the few vertical joints located were mainly healed. On this basis it was concluded that significant groundwater inflow during construction would not occur. The proposed split tunnel alignment was therefore rejected and replaced with the original single bore tunnel suggestion that would be more preferable for construction and post-construction operation.

Tender documents for construction were prepared in 2008, but delayed and cancelled a few months ago due to financial issues caused by the worldwide economic downturn.

Directional core drilling as an exploration method is still gaining popularity with ongoing projects in North-America, Europe and Asia, and one soon to be initiated in South America.

Rune Lindhjem
is a project manager with Devico AS, Melhus, Norway.

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