CA2875185C - Core drill and method of performing in-situ consolidation testing of permafrost soil - Google Patents

Abstract

The method of performing in-situ consolidation testing of permafrost soil can include : core drilling into the permafrost soil using a core drill, the core drill thereby receiving a sample of the permafrost soil; applying heat to the sample of the permafrost soil to thaw the ice contained therein; applying downward longitudinal pressure to and draining water from the sample, thereby consolidating the sample within the core drill; and measuring the consolidation of the sample.

Description

CORE DRILL AND METHOD OF PERFORMING IN-SITU
CONSOLIDATION TESTING OF PERMAFROST SOIL
BACKGROUND
[0001] Permafrost is common in very cold climates and refers to ground which remains frozen all year round. Typically, permafrost is located at a given depth below the ground surface, and the layer of ground above the permafrost can actively freeze or thaw depending on the season.

[0002] Although the layer of ground above the permafrost can have had the occasion to consolidate (i.e. settle down and compress) over the years, the permafrost can contain a significant amount of frozen water which could thaw and drain should the permafrost thaw for a reason or another.

[0003] While it is relatively standard to dig into the soil above the permafrost when the former has thawed (e.g. during summer), excavating the permafrost itself is quite another question since permafrost can be hard as ice. On the other hand, the permafrost normally provides a relatively stable support structure, and is therefore often used as a foundation for northern structures.

[0004] Should, however, the permafrost thaw for a reason or another, the water contained in the permafrost could drain and the remaining solid matter settle, especially when subjected to significant compressive strain, which can have important consequences for any structure (e.g. buildings, pipelines, roads) built thereon.

[0005] Consolidation testing (or rather, thaw consolidation testing to be exact) can be performed on the permafrost using a consolidometer (sometimes referred to as an oedometer), in order to allow civil engineers to assess the amount of consolidation of the ground which could be expected should permafrost thawing occur ¨ which, in turn, can allow to design the structure to be built thereon accordingly. However, former consolidation testing methods suffered from many inconveniences. For instance, permafrost consolidation testing typically involved the removal of a sample from the permafrost and subsequent transport of the sample to a testing facility located hundreds of kilometers away. The extraction of the Date Recue/Date Received 2021-04-01 permafrost sample posed a first challenge in itself given that the permafrost can be located beneath a significant layer of active soil. The transport of the sample posed an entire other issue associated with the preservation of the sample in an unfrozen and relatively unaltered state over lengthy transport, not to mention the costs associated to transporting the sample over such long distances.
SUMMARY

[0006] In accordance with one aspect, there is provided a core drill specifically adapted for obtaining a sample of permafrost.

[0007] In accordance with another aspect, there is provided a core drill with an integrated consolidometer which can do all of obtain a sample of permafrost within the core, thaw the permafrost sample, apply pressure to it, allow the liquefied water to drain, and measure the resulting consolidation on site, without having to bring the permafrost sample to an offsite laboratory.

[0008] In accordance with another aspect, there is provided an associated method.

[0009] In accordance with another aspect, there is provided a core drill having an annular drill bit mounted concentrically at a tip of a hollow shaft of an elongated auger, at least one flight extending helicoidally around the hollow shaft and extending along the length of the auger, and a blade terminating each one of the at least one flight, adjacent to the annular drill bit, the blade having a sloping face leading to a cutting edge and designed to engage the soil surrounding the annular drill bit and to lead the freed soil upwardly into and along the at least one flight when the core drill is rotated.

[0010] In accordance with another aspect, there is provided a core drill having an annular drill bit mounted concentrically at a tip of a hollow shaft of an elongated auger, at least one flight extending helicoidally around the hollow shaft and extending along the length of the auger, an inner wall concentrically positioned inside the hollow shaft and extending along a consolidometer portion of the core drill; and a sample area housed inside the inner wall;
wherein the sample area is aligned with and projects axially from a circular opening in the Date Recue/Date Received 2021-04-01 annular drill bit in a manner that upon operation of the core drill into the ground, a sample of the ground neatly cut by the annular drill bit is received within the sample area.

[0011] In accordance with another aspect, there is provided a method of performing in-situ consolidation testing of permafrost soil, the method comprising : core drilling into the permafrost soil using a core drill, the core drill thereby receiving a sample of the permafrost soil; applying heat to the sample of the permafrost soil to thaw the ice contained therein;
applying downward longitudinal pressure to and draining water from the sample, thereby consolidating the sample within the core drill; and measuring the consolidation of the sample.

[0012] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.

[0013] It will be understood that in the instant specification, the expression upward and downward are used to refer to associated features of the core drill when it is oriented vertically and driven downwardly. It will be understood that such expressions are used solely for the sake of simplicity and common sense since it will be understood by persons of ordinary skill in the art that although it is straightforward to imagine the core drill being used in the vertical orientation, the same core drill can also be used in other orientations. The expressions upward and downward are thus in no way intended to restrict the scope of this specification to the use of such core drills in the vertical orientation or to exclude the use of core drills in other orientations.
DESCRIPTION OF THE FIGURES

[0014] In the figures,

[0015] Fig. 1 is a view of an example of a core drill being rotated and driven into permafrost soil to receive a sample thereof;

[0016] Fig. 2 is an oblique view of the core drill of Fig. 1 shown whole;
Date Recue/Date Received 2021-04-01

[0017] Fig. 3 corresponds to the view of Fig. 2, where the core drill is fragmented to show internal components thereof;

[0018] Fig. 4 is an exploded view of another example of a core drill;

[0019] Fig. 5 is an exploded view of the head of the core drill of Fig.
4;

[0020] Fig. 6 is an exploded view of a consolidometer portion of the core drill of Fig. 4;

[0021] Fig. 7 is an exploded view of a piston driver portion of the core drill of Fig. 4.
DETAILED DESCRIPTION

[0022] Fig. 1 shows an example of a core drill 10 engaging the permafrost soil 12. The soil and the preparation will be explained prior to detailing the core drill 10 itself. The description of the core drill 10 will begin by explaining the drilling and sample-making functionality prior to discussing the ability of the presented core drill 10 example of performing a consolidation test in-situ, without even removing the core drill 10 from the ground 14. It will be understood throughout this description that the operation of the core drill 10 can be done by a drill rig using known techniques.

[0023] As shown in Fig. 1, a significant amount of unfrozen soil 16 can be present above the permafrost 12. This can be the case in the heart of summer, for instance, when the active layer has thawed. In this example, a cylindrical hole was dug in the unfrozen soil 16 above the permafrost 12 using known techniques, and a temporary support tube 18 was engaged in the hole to prevent the soil 16 surrounding the hole from caving in. The support tube 18 was selected to have an internal diameter compatible with the external diameter of the core drill 10. The core drill 10 can then be engaged inside the support tube 18 until it reaches the frozen permafrost soil 12 as shown in the figure. Depending on the geographic location, permafrost 12 can have varying compositions having varying relative percentages of water (ice), rock, gravel, sand, clay, silt, etc. The amount of settling which can be expected to occur upon unfreezing of the permafrost thus does not only depend on the amount of compression stress, but also on the particular composition of the permafrost.
Date Recue/Date Received 2021-04-01

[0024] The core drill 10 can, in this example, be seen to generally include a hollow shaft 20 having a head section 22 at the tip 24. More specifically, the core drill 10 has an annular drill bit 26, which can be a diamond drill bit for instance, which is mounted to the tip 24 of the hollow shaft 20 and which is provided to provide a neat annular cut into the permafrost soil 12 while producing a limited quantity of heat. The hollow shaft 20 has a sample area 28 provided therein which projects upwardly from a circular hole 30 in the annular drill bit 26 in a manner that as the core drill 10 progresses downwardly, a cylindrical sample is formed and progressively engaged into the sample area 28.

[0025] In this specific example, the core drill 10 is further provided with two equidistant helical flights 32 surrounding the hollow shaft 20 along a length thereof, forming an auger 40, and the head section 22 is further provided with two blades 34 ¨ with each one being provided at the end of a corresponding one of the two flights 32. The blades 34 have a sloping face 36 and a cutting edge 38, and are designed to roughly cut and break down the soil surrounding the sample and lead the freed soil upwardly into the path of the flight 32.
.. During operation, the core drill 10, and its associated auger 40, rotates significantly faster than if it were a screw, entraining freed soil upwardly along the flights 32 and out of the hole.
In different types of permafrost soils 12 which were tested, the use of a combination of blades 34 surrounding an annular drill bit 26 was found very effective in achieving the combined desired results of obtaining a workable, unaltered sample of the permafrost soil 12 in the sample area 28 while clearing soil from around the sample 28 to allow unhindered progression of the annular drill bit 26 into the permafrost soil 12 with all the integrated consolidation-testing equipment which will now be detailed.

[0026] As presented above, and although these features are optional, the example core drill 10 presented in Fig. 1 is provided with integrated consolidation-testing equipment (i.e.
an integrated oedometer). Firstly, a piston 42 is slidingly engaged inside the sample area 28 and is adapted to provide compression pressure to the sample. Secondly, a heating element 44 is provided to allow thawing of the sample in-situ ¨ without even removing the core drill 10 from the ground 14. Thirdly, drainage means 46 are provided to allow the evacuation of melted water obtained from the thawing. Fourthly, insulation 48 is provided around the sample area 28 to limit the impact of the heating element 44 on the surrounding soil.
Date Recue/Date Received 2021-04-01

[0027] In this specific example, all these functions are achieved using the shown structure. More specifically, an inner wall 50 is provided concentrically from the hollow shaft 20 outer wall structure, with a spacing between the inner wall 50 and outer auger structure 40. A heating element 44 is provided in the form of a heating wire coiled externally around the inner wall 50, while air or another insulating material can be provided in the spacing between the heating wire and the outer auger structure 40 in order to limit the transfer of heat from the heating wire to the outer auger structure 40.

[0028] The drainage can be achieved in various ways. In the illustrated embodiment, the drainage is achieved by the use of a porous filtration element such as a cylindrical, porous, stainless steel filter (e.g. such as manufactured under the tradename SIKARTM
by GKN
sinter metals) being provided within the inner wall 50 and snugly receiving the piston 42 therein, which allows migration of liquid water around the piston 42 and into the spacing above the piston 42 while keeping the solid material in the sample area 28 below the piston 42. Alternately to a porous filtration element such as illustrated, drainage apertures can be provided axially across the piston 42 to allow liquid water to flow across it, for instance.

[0029] As can be perhaps best understood referring to Fig. 3, the circular opening 30 provided inside the annular drill bit 26 projects upwardly to the sample area 28. In the described embodiment, the annular drill bit 26 has an annular wall which has a given thickness which can, for instance, correspond to roughly the thickness of the inner wall 50 surrounding the sample area 28; however, the combined presence of the inner wall 50, stainless steel filter 46, heating element 44, hollow shaft 20, and insulation spacing 48 leads to a greater overall thickness than the thickness of the annular wall of the annular drill bit 26.
In the illustrated embodiment, the additional thickness around the consolidometer portion 52 of the core drill 10 is accommodated by the presence of an optional frusto-conical section 54, which has a diameter which narrows between the consolidometer portion 52 of the core drill 10 and the tip 24, where the cutting edge 38 is received. It will be noted that the external diameter of the flights 32 stays the same along the frusto-conical section 54 than the external diameter of the flights 32 along the consolidometer portion 52 of the core drill 10.

[0030] Still referring to Fig. 3, it will be seen in this example that an additional portion of the core drill 10 extends above the consolidometer portion 52, opposite the frusto-conical Date Recue/Date Received 2021-04-01 section 54. This additional portion will be referred to herein as the cylinder portion 56 of the core drill 10 with reference to the fact that it houses, in this specific embodiment, a piston-driving cylinder 58 firmly held in a fixed position relative to the hollow shaft 20. Furthermore, in this embodiment, a shaft 60 which drivingly connects the piston 42 to the driving cylinder 58 extends fully across the driving cylinder 58 in this case and out the other end, in a manner to protrude axially from the hollow shaft 20 of core drill 10. This feature allows an operator to visually inspect the movement of the shaft 60, and thus infer movement of the piston 42, during operation and/or allow to measure the consolidation of the sample by measuring the movement of the shaft 60.

[0031] In this specific example, the diameter of the hollow shaft 20 was maintained constant above the frusto-conical section 54 not only along the consolidometer portion 52 of the core drill 10, but also along the cylinder portion 56. Accordingly, the portions 52, 56 of the core drill 10 which extend above the frusto-conical section 54 will be referred to collectively as the main section 62 in this specification. In alternate embodiments, the diameter of the hollow shaft 20 can narrow back down above the consolidometer portion 52 of the core drill 10, to name one alternate example.

[0032] Referring now to Fig. 4, an alternate example of a core drill 110 is shown. This alternate example of the core drill 110 is relatively similar to the first example described above and illustrated in Figs. 1 to 3. In the example shown in Fig. 4, a double ball joint 164 arrangement is used between the piston 142 and the shaft 160 in order to free the piston from lateral (transversal) strain while allowing its longitudinal (compressive) effort onto the sample.

[0033] As can be understood from Fig. 4, the core drill 110 can be provided in the form of several longitudinally assemblable components, which can be useful in the maintenance operations of the core drill 110, for instance. More specifically, the frusto-conical portion 154 is provided separately from the consolidometer portion 152, and from the cylinder portion 156. In this specific embodiment, a coupling head 166 is also provided and can be assembled to the top of the cylinder portion 156 for driving the rotation, for instance. In the other figures showing this second example, Fig. 5 shows an example construction for the frusto-conical portion 154 and head portion 122, Fig. 6 shows an example construction for Date Recue/Date Received 2021-04-01 the consolidometer portion 152, and Fig. 7 shows an example construction for the cylinder portion 156. In alternate embodiments, the core drill 110 can be provided in a greater or lesser number of components, as will be understood by those having ordinary skill in the art.

[0034] The core drill described above and illustrated can thus allow to not only obtain a workable sample of permafrost soil, it can further be used to perform consolidation testing thereof in-situ, without even having to remove the sample of permafrost soil from the ground.
This can allow performing an additional consolidation test in the following manner : a first sample is obtained to fill the sample area up to the maximum position of the piston, the rotation is stopped, consolidation is performed by heating, compressing and draining the sample; the consolidation ratio can be measured; and the piston can then be raised and the portion of the sample area freed by the consolidation can be used to receive a further portion of the permafrost which can then be consolidated as well, etc.

[0035] Alternately, a core drill such as described above, or a variant thereof, can be used to simply extract a sample from the ground and then either test the variant nearby, or send it off to a distant laboratory. In some embodiments, it can be advantageous to provide the sample area in the form of removable cartridges which can be interchanged and removed from the remainder of the core drill for sending off to a distant laboratory, for instance, and it will be understood that such an embodiment does not require a piston, drainage means, heating means, nor insulation.

[0036] The examples described above and illustrated are intended to be exemplary only.
For instance, the examples described above and illustrated can be adapted for housing samples having ¨30 cm in length, though persons of ordinary skill in the art can adapt the teachings provided herein for obtaining samples having other sizes.
Furthermore, the embodiments described above were found particularly well adapted to obtaining samples in various compositions of permafrost soil having rocks of less than about 20-30 mm in width contained therein, and the core drill can be adapted specifically in view of different types of soil. It will also be understood that the combined action of the blades and annular drill bit can be useful to other ends than that of obtaining a permafrost sample. The coring means described above were found to be more suitable for a portable drilling installation as lightweight drill rig can be used and no drilling fluid are required. It will also be understood Date Recue/Date Received 2021-04-01 that the principles of the in situ thaw consolidation testing described above can be integrated with coring means used in alternative drilling technologies such as, for instance, drive sampling, diamond, or sonic drilling, with or without drilling fluid, to provide still further alternate embodiments. The scope is indicated by the appended claims.
Date Recue/Date Received 2021-04-01

Claims (8)

WHAT IS CLAIMED IS:

1. A core drill having an annular drill bit mounted concentrically at a tip of a hollow shaft of an elongated auger, at least one flight extending helicoidally around the hollow shaft and extending along the length of the auger, an inner wall concentrically positioned inside the hollow shaft and extending along a consolidometer portion of the core drill;
and a sample area housed inside the inner wall; wherein the sample area is aligned with and projects axially from a circular opening in the annular drill bit in a manner that upon operation of the core drill into permafrost soil, a sample of the permafrost soil cut by the annular drill bit is received within the sample area wherein the inner wall is separated from the hollow shaft by a radial insulation spacing; further comprising a heating element extending externally along at least a portion of the length of the inner wall and operable to thaw the sample of the permafrost soil contained therein, and a piston slidingly engaged inside the inner wall, the piston being operable to apply a downward longitudinal pressure against the sample for draining and consolidating the sample.

2. The core drill of claim 1 wherein the inner wall is removable from the hollow shaft collectively with a sample contained therein for transportation of the sample to a consolidation test facility.

3. The core drill of claim 1 further comprising a porous filtration element housed within the inner wall, wherein the piston is snugly received inside the porous filtration element and wherein the porous filtration element is adapted to allow migration of drainage water obtained from the consolidation of the sample upwardly, above the piston.

4. The core drill of claim 1 wherein the piston is provided with at least one drainage aperture extending longitudinally therealong.

5. The core drill of claim 1 further comprising at least one flight extending helicoidally around the hollow shaft and extending along the length of the auger, and a blade terminating each one of the at least one flight, adjacent the annular drill bit, the blade having a sloping face leading to a cutting edge and designed to engage the permafrost Date Recue/Date Received 2021-04-01 soil surrounding the annular drill bit and to lead freed soil upwardly into and along the at least one flight when the core drill is rotated.

6. The core drill of claim 5 wherein the hollow shaft has a main section and a frusto-conical section having a diameter which narrows from the main section to the tip.

7. A method of performing in-situ consolidation testing of permafrost soil, the method comprising :
core drilling into the permafrost soil using a core drill, the core drill thereby receiving a sample of the permafrost soil;
applying heat to the sample of the permafrost soil to thaw the ice contained therein;
applying downward longitudinal pressure to and draining water from the sample, thereby consolidating the sample within the core drill; and measuring a movement of a piston applying the downward longitudinal pressure to the sample, thereby measuring the consolidation of the sample.

8. The method of claim 7 further comprising continuing the core drilling into the permafrost soil subsequently to the consolidating of the sample within the core drill, thereby receiving a further sample portion in the core drill.
Date Recue/Date Received 2021-04-01