CPTWD (Cone Penetration Test While Drilling) a new method for deep geotechnical surveys

Transcription

1 CPTWD (Cone Penetration Test While Drilling) a new method for deep geotechnical surveys Authors : Massimo Sacchetto-Engineer-: S P G drilling company -Adria (Ro) Italy Kjell Elmgren Engineer ENVI Environmental mechanics Alingsas Sweden Annalisa Trevisan Engineer S P G drilling company -Adria (Ro) Italy Kenth Melander Engineer ENVI Environmental mechanics Alingsas Sweden ABSTRACT: CPTWD represents integration of: wire-line drilling system, standard and modified piezocone, MDW-monitoring while drilling. During the CPTWD test the cone is protruding in front of the drill bit during drilling in the same way as a corer; CPTU data are stored in memory unit. At the same time as the CPTU data are logged, drilling parameters (MWD) are also recorded. The system allows for the change between CPT testing, continuous core drilling, down-hole testing and non coring drilling with MWD. The combination of CPT parameters and drilling parameters can be a very powerful basis for interpretation of the data. The advantages of this system compared to the other down hole type CPTU is that much longer strokes than the normal 3 m can be made. In addition, the information from the drilling parameters is very useful, especially in hard formations where CPTU cannot be performed. It is expected that future development work will improve the method in terms of the test procedures and in terms of capabilities of the system itself, by adding new tools (e.g. field vane test, Permeameter, fluid sampler, thin wall samplers, dilatometric DMT). 1 INTRODUCTION The CPTWD is the acronymous of Cone Penetration Test While Drilling. Basically it can be considered as an integration between a wire-line system (core barrel, non coring device, cable recovery system, etc), piezocone and the MWD (Monitoring While Drilling) system. Prior to the introduction of the CPTWD, it is necessary to briefly outline every part of the system : the wire-line system, the piezocone, the MWD. 2 DESCRIPTION OF THE SYSTEM 2.1 WIRE-LINE SYSTEM The recovery by cable or wire system (hence the name wire-line ) is based on the use of a core barrel which, integrated with the casing, makes a unique body. Pushing and rotation are conveyed by the same casing. The system is essentially composed of: *Drilling rods (or "casing") inside which, at the bottom terminal part, there is an element (usually a slot) where the core barrel is lowered down by the wire and takes place inside, so allowing the drilling rods to rotate and push. *Core barrel or drilling tool: this is placed inside (throughout the interior of) the drilling rods and has a hook up system. In the following Picture 1, the particulars of a four hooks system are shown: Picture 1 The drill mud will flow in the annular space between the external wall of the core barrel and the internal wall of the rods and lubricates the walls of the hole while removing the cuttings. Most of the time the mud is recycled and stored on the surface before it is put back into circulation. The following Picture 2 shows a typical installation for a wire-line c.c.drilling: Picture 2 There are different types of core barrels as well as different types of tools which can be placed inside the drillstring (rods). The following Picture 3 shows 1

2 the terminal part of a core barrel suitable for soft soils Picture 3 The tool is lowered inside the drillstring (casing) and recovered by a cable or wire (therefore called wireline ) moved by a special hoist. The wire has a fishing-tool (called overshot ) attached at its end. Such an overshot is designed in a way which allows the hooking/unhooking operation in an automatic way. The wire line drilling operations are essentially executed as follows (referred to as c.c.drilling): * The drill rig pushes and rotates the casing inside which the core barrel is placed (the shape and length of the core barrel depends on the type of soil). Normally, mud circulation inside the drillstring is used. The mud leaves the terminal part between the core barrel and the casing without interfering with the inside of the core barrel. *When the drilling is completed, the rotary head of the drill rig is moved away laterally and the overshot is lowered down into the drillstring by the wire, driven by a high-speed hoist. The overshot hooks and the core barrel can be recovered. * The core is taken out of the core barrel, which is cleaned and prepared for the next operation, * The core barrel is lowered inside the drillstring again and one more rod is added at the top of the drillstring, the rotary head is connected to the rods, mud is injected and the drilling starts again. The advantages of the wire-line system, compared to the traditional method are: better quality of the cores, higher percentage of recovery, faster execution, less power required from the drill rig and the possibility to easily change between different types of tools. 2.2 PIEZOCONE CPTWD For the experimental CPTWD system, a standard piezocone, having a memory data storage capability, has been used. ( Ref. 1) This type of cone, conforms with the following standard: "International Reference Test Procedure for CPT/CPTU from ISSMGE Rev.3". The accuracy class is class1. In its "standard" version it is a system which measures Q c, F s, U, (point resistance, local friction, pore pressure and inclination of the drillstring every 2 cm of penetration (versus time in case of use in memory mode, without cable) During the sounding, two data files are produced: one in the internal microprocessor inside the CPTU cone, and the other in the data collector; Picture 4. Picture 4 After recovering the cone, it is connected to the data collector and its CPTU data will be downloaded. Proper software in the data collector will allow for the synchronization between the memorized data as a function of time (inside the cone), and those as a function of depth (inside the data collector). In this way, it is possible to obtain a normal data-file (Qc, Fs, U, incl.) versus depth. In the special "CPTWD" version, Picture 5, an additional pressure sensor for the evaluation of the U in one more point of the piezocone (U 3 location according to the International Standards) and a rotation sensor have been added. The latter allowing one to determine whether the piezocone has been subject to rotation during the sounding, or not. The reason to detect U 3 was to determine whether or not the U is affected by the overpressure caused by the injection of mud. 2

3 Besides this, the range of U has also been increased to 50 Bar in order to achieve test depths down to 300 m. Picture MWD (Monitor While Drilling) It is a commonly and widely used method employed mainly in the deep oil drillings. It has been defined in several different ways, according to the company who built it, but is internationally known as MWD (Monitor While Drilling). In order to monitor the reaction to the penetration of the soil in the drillrig, pressure sensors in the hydraulic oil circuitry, depth transducers, volume transducers, etc are applied. In the specific case of the CPTWD, the sensors signals are conditioned and amplified in the data collector, where they are stored and printed out in real time while drilling. The measured parameters could be all the ones during the drilling. Still, they normally are: - Bit-load (thrust pressure) - Torque ( rotation pressure) - Mud pressure - RPM (number of revolutions per minute ) - ROP ( rate of penetration) - Mud flow (volume of injected mud / min.) 2.4 THE CPTWD METHOD Basically, the CPTWD (CONE PENETRATION TEST WHILE DRILLING) system is an integration of the techniques described above. A wire-line core barrel has been modified, allowing it to keep a CPTU cone inside in the centre in such a way, that the cone is protruding from the bottom of the drill- bit by at least 35 cm. A MWD data recording system is connected to the drill rig. In this case it is practical if the data collector is capable of handling both the drilling parameters and the CPTU data at the same time. Practically, the performance of such system are the following: Continuous core drilling, using the common core barrels, samplers, etc, but with the possibility of monitoring even the c.c.drilling with MWD; That is, to have a registration of all the implemented parameters even while carrying out a wire-line c.c.drilling. MWD with non coring drilling: A non coring tool, for example a tricone (always wire-line) is fixed inside the drillstring so that during the drilling the data collector records all the relevant drilling parameters every 2 cm. In this way, it is possible to (together with the monitoring of the cuttings) establish the stratigraphy. CPTU+MWD test at the same time: The CPTU cone-core-barrel is placed inside the drillstring. While the drillstring moves forward (at a speed as close as possible to the standard, 2 cm/s) the MWD system is always in function. After a substantial penetration, the CPTU cone is recovered, the data is downloaded to the data collector and subsequently synchronized with the corresponding MWD data. As a result, at the end of the drilling, it is possible to obtain a data matrix which includes (for every 2 cm of depth) for example: Qc, Fs, U 2, U 3, Bit load, Torque, Mud pressure, ROP, Mud volume and RPM. During an interruption of the sounding it is possible at any time in order to carry out dissipation tests. The evaluation of the drilling parameters in real time can be useful to the operator who eventually decides when to interrupt the penetration of the piezocone (i.e. because the soil is too dense). (Ref 3). It is useful to use the formula of the specific energy. This gives the Energy needed to penetrate one cubic meter of the soil material. (Ref. 2) For this reason (MWD), the CPTWD should not be 3

4 considered a blind system. Several tests have been carried out and it has been noted that the operator can easily handle the change of stratigraphy (i.e. stop the drilling or decide when to change the tool) within a range of cm. In order to increase the safety of the cones, there are two alternative ways to go: 1) alteration of the length of the piezocone in front of the drillstring system and 2) a safety system which allows for the return of the piezocone back into the core barrel when the maximum value of resistance has been reached. CPTU, as described above but without the detection of the MWD parameters, can also be used. In that case, the data collector connected to the drill rig only detects the depth versus time, in order to synchronise the CPTU data with the depth data, exactly like the normal CPTU tests in memory mode. * DOWN-HOLE CPTU: the whole drillstring is lifted m; a special core barrel having the memocone protruding for the same length (i.e m) is lowered and hooked inside the drillstring; then it is pushed down by the drill-rig (without rotation) for the same length. This allows to check and calibrate the CPTWD having some pure (not affected by rotation and/or mud injection) CPTU data. * DOWN HOLE tests of other sort: at any depth it is possible to lift the core-barrel and perform any type of test (SPT, Permeability test, Dilatometric, Vane test, etc). The different options exposed above can be selected according to the type of soil as well as the scope of the bore-hole. The drillstring actually remains the same and the tools related to the three options are interchangeable any time the operator decides to do so. 2.5 TECHNICAL DETAILS OF THE CPTWD The general scheme of the CPTWD prototype is shown below: Picture 6 The system is basically made up of: Casing tubes. Those are of the same type used in c.c. drilling. For the prototype, the external diameter was 127 mm, which allows them to collect Ø 86 mm cores and Ø 88.9 mm (standard) undisturbed samples. The bottom part of the casing is equipped with a specially designed drill- bit. Hooking system. There are normally four hooks placed inside the core barrel, shaped in a way which allows them to get into (once they are unhooked from the overshot) the slot placed inside the casing, at a distance equal to the length of the core barrel. Core barrel. In this case it is not quite appropriate to use the definition core barrel because it is not used to carry out c.c.drilling operations, but it is basically a wireline barrel used to carry out drillings in gravel, which 4

5 has been modified in a way to make it suitable to keep a CPTU cone, a security system, etc., inside itself. The so called corebarrel, is of the "fixed head" type. It rotates with the casing so that the bottom part breaks up the soil. It is similar to the type used in gravel. At the bottom, there is a drillbit of special design. The following,picture 7, shows how the mud flows (at a maximum pressure) through the openings of the drill-bit and between the drillbit and the internal side of the casing: Picture 8 Safety device. Inside the "core barrel", the CPTU cone is assembled in a device which allows it to return into the core barrel when the resistance is greater than a pre-set maximum value. Picture 9,.shows the CPTU cone after its retrieval at the end of the test. Picture 7 the following scheme shows (left) the principle of the wire-line drilling system in general and of the CPTWD in particular: Picture 9 3 RESULTS OBTAINED WITH THE CPTWD 3.1 Parma zone (July 2000) 5

6 The CPTWD system has been employed near Parma (Italy) in the geotechnical surveys for the project of some TAV (high speed train project) works on sites characterized by an alternance of fine soils (clay, silt, sand) and gravel layers often interlayed by clay intervals of little thickness (about a decimetre). The execution of traditional static penetrometric tests would have been impossible due to the frequent presence of gravel intervals of different depth. The CPTWD test has been carried out as follows: the drilling starts with the use of the CPTU cone and MWD down to the top of the gravel; the core barrel with the CPTU cone was recovered at regular intervals (of about 1-2 m) in order to check the reliability of the data as well as the condition of the cone. -As soon as the gravel was encountered, that is, as the Q c was reaching 50 MPa, the "safety device" was trigged and the piezocone was retracted in to the core barrel. At the same time, the Operator had a way to realize not only the retraction of the cone, but also the variations of the drilling parameters, constantly monitored in real time, (every 2 cm), indicating a very dense layer. -Once the cone was retracted, the core barrel with the piezocone was recovered and the data related to the last interval before getting to the gravel was down loaded and stored. After recovering the data, the CPTU cone was again installed in the core barrel and the cylinder of the "safety device" was again pressurized so that the CPTU cone could again be placed in position. -After this, a non coring-tool (tricone) that made the non coring drilling possible, was installed and with simultaneous detection of the drilling parameters every 2 centimetres, drilling was carried out until another "penetrable" layer (clay, silt, sand) was found and then the CPTU+MWD started again. Results are shown in the following graph: Picture 10. Picture 10 From left to right are Q c, U, F r, ROP (rate of penetration), Torque, Mud pressure. The CPT graph is interrupted where there are layers of gravel. These are where the CPTU has been interrupted and MWD non coring drilling has been carried out. 3.2 PIACENZA ZONE (FEBRUARY-MARCH 2001) The system has been used for the execution of two deep tests, carried out using a rig on a small jack-up barge, in order to investigate mainly sandy soils with gravel intervals. The drillings have been located on the two projected supports of the new tall bridge TAV (high speed train project) on the river Po. Different from the work previously described, more advantage has been taken of the potentials offered by the wire-line recovery system. In these soils, the CPTWD method has not been used continuously, but instead intermittent with c.c. drilling, non coring drilling and the withdrawal of undisturbed samples, always using the same basic system and without changing the casing. Picture 11 is an example of a penetrometric graphic (Qc, Fs, U, Fr) from 60 to 120 m and one of the c.c. drilling operations in the same hole. 6

7 Picture 11 Picture TREPORTI (VENICE, ITALY) (JULY 2001) As a matter of experimentation, one vertical CPTWD, was carried out down to 110 m, in mainly sandy zone, starting from 50 m. Due to very high resistance of the sand (sometimes Q c greater than 50 Mpa), some intervals were carried out at continuous core drilling (also in order to verify and calibrate the static test) and non coring drilling with MWD. In order to keep a constant rate of penetration, having high density sandy intervals, the Operator sometimes increased the pressure of the mud injection at intervals in order to allow a 2 cm/s rate (Ref 5 ). Therefore, in order to evaluate the potential effects of such pressure in the penetrometric parameters, another pressure sensor has been added to the current model, just above the friction sleeve ("U3" according to the recent nomenclature). The diagram below shows the penetrometric graphic (Q c, F s, U, F r ) from 0 to 110 m. 4 COMMENTS ON THE RESULTS Actually, the amount of current available data to be compared with CPTWD is relatively small and at present, comparative tests with other testing methods have not been carried out. Accordingly, it would therefore be necessary, to perform a more extensive validation of the obtained results with the following procedures: * To execute comparative tests with CPTU and CPTWD at a depth which is achievable by the normal penetrometers. Such tests should be carried out in different types of soils in order to establish the correct testing procedures, in terms of the length of the cone, thrust calibration, pressure and volume of the injected fluid. * To establish the cone s optimum length below the drill-bit by testing in different types of soil also at great depth, analyzing the variations of U 2 and U 3 as 7

8 function of the pressure and the volume of the fluid injected at the bottom of the drillstring. * To execute CPTWD tests at different penetration rate, within 1 and 2 cm/s, and compare the results * Broad comparison between the penetrometric results and those obtained with other testing methodologies in situ (field vane test, SPT, etc) and in laboratory (where possible). * To compare, when feasible, and economically possible, the penetrometric results obtained at great depth. For instance, comparing the CPTWD data with other penetrometric tests (wire line CPT downhole used for offshore surveys) or penetrometric tests carried out with pre-drilled holes. * To evaluate not only with in situ tests, but also with laboratory tests and/or with the help of mathematic models, the influence of the rotation and fluid injection on the penetrometric data. In this way, it would be possible to calibrate, in a more scientific way, the MWD parameters. * Since, for the first time, it is possible to compare results of MWD and CPTU from the same soil mass, there should be a possibility to carry out accurate calibrations of the drilling parameters and to get more objective and reliable data and possible correlations between the MWD and CPTU in soft soil. * In case the CPTWD is evaluated in sufficiently documented and monitored cases, there is hope for the verification throughout "back-analysis", of the data obtained 5 POSSIBLE APPLICATIONS The following are possible cases where the use of the CPTWD system could be competitive in relation to the traditional type of survey: * Sites with alternating layers of non penetrable soil (i.e. gravel, cemented sands) and penetrable intervals of geotechnical interest (Ref. 4). * Presence of compact overburden with subsequent penetrable layers. * Geotechnical deep drilling (in particular the OFFSHORE drillings) where the cost/production ratio is important. * Unavailability of the static penetrometer rig, providing the possibility to carry out static penetrometric tests with any kind of drillrig, at least at conventional depths. * Geotechnical and environmental deep drillings: - at the end of the c.c.drilling/cptwd, it is possible to install a piezometer and/or geotechnical instrumentation inside the bored hole. * Deep surveys and main presence of sandy soils in which it is not possible to withdraw sufficiently undisturbed samples and the normal tests in situ (i.e. SPT) would not be economically efficient or reliable. * The opportunity to have a more complete data matrix than with a normal penetrometer, which allows other kinds of interpretations. In particular, the fact of having in the same row (every 2 cm) the data of: Q c, F s, U 2, U 3, thrust, torque, rate of penetration, RPM, volume of injected fluid. 6 CONCLUSIONS The CPTWD method lends itself to future improvements and developments, not only from a practical point of view, but also in a theoretical and interpretative area. The same general principle can be used for the implementation of other kind of tests with other kind of sensors, even when the continuous (that is, simultaneously with the drilling process) process to obtain the data is prerogative to the static penetrometric test. Still, with the idea of adapting other instruments to a core barrel wire-line, the time has been reached for the studying of further applications complementary to the CPTU test, for example: * down hole wire-line field vane test. In contrast to CPTWD, this type of test is "discontinuous", that is, it is not possible to carry out scissometric tests during the drilling. * execution of tests with Permeameter and sampling of fluids. Such applications has already been performed by NGI (Norwegian geotechnical institute) for the execution of offshore tests: a wireline application of a Permeameter and gas sampler at great depth (D.G.S.: deep gas sampler) has been implemented by the NGI essentially for the petroliferous/oil drilling, but it is also applicable at 8

9 shallow depth and is based on the same operating principle as the BAT/GEON. * DMT (dilatometric) tests: the implementation of an application of a down-hole wire-line electronically operated Marchetti s dilatometer. * It is possible to adapt other types of sensors to the cone, i.e. sensors measuring chemical parameters. * From a theoretical and interpretative point of view, the use of the CPTWD in soft soils, together with the MWD, will allow one to correlate, in a more efficient way, the mechanical characteristics of such soils with the data obtained by diagraphy. * With the help of the right interpretative and mathematic models it might be possible to extend correlations which were found for soft soils also to coarse or rocky non penetrable soils. The future improvements of the CPTWD system will include a combination between a drill rig and a pushing system of penetrometric type, possibly anchored in the ground (or to the floating craft in the case of offshore tests) for a higher capacity of pushing. This will allow a better control of the drilling parameters, (Ref. 5) leaving to the drill-rig the unique task of rotating the drillstring and injecting the mud. 7 REFERENCES 1) Lunne et al. CPT: Cone Penetration Testing in Geotechnical practice 2) Fabio Fortunati & Guido Pellegrino: The use of electronics in the management of site investigation and soil improvement works: Principles and applications. ISC98, volume 1. 3) Kazuo Tani: Importance of instrumented drilling. ISC98, volume 1. 4) J. Peuchen : Commercial CPT profiling in soft rocks and hard soils. ISC98, volume 2. 5) W:G:B: te Kamp: The influence of the rate of penetration on the cone resistance qc in sand. ESOPT II, Amsterdam ) Rolf Larsson: Use of a thin slot as filter in piezocone tests. CPT 95 Linkoping ) Kjell Elmgren: Slot type pore pressure CPTu filters. Behaviour of different filling media. CPT 95, Linkoping ) T.Lunne: In situ testing in offshore geotechnical investigation. In Situ 2001 Bali 9