CPTs EXECUTED IN DIFFICULT CONDITIONS USING CPTWD (CONE PENETRATION TEST WHILE DRILLING) AND ITS FUTURE DEVELOPMENTS

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1 CPTs EXECUTED IN DIFFICULT CONDITIONS USING CPTWD (CONE PENETRATION TEST WHILE DRILLING) AND ITS FUTURE DEVELOPMENTS M.Sacchetto A.Trevisan S.P.G. s.r.l. via dell artigianato Adria (Ro) Italy Abstract: During the last few years we have faced difficult challenges several times, among them: -execution of deep and very deep CPTU tests in non-penetrable soils, in which penetration with standard methods was not possible i.e.:- tests in soils where penetrable layers are alternated to non-penetrable ones, -execution of CPTU tests down hole, alternated with undisturbed sampling and different kinds of testing in the same hole, execution of deep CPTU tests from jack-up and simply floating barges. The results of the above experiences are shown and commented upon. Moreover, looking to the offshore market, where it is not possible to use jackup barges, due to the high level of water, we purposely designed new equipment for pushing using the barge as a reaction, and allowing compensation of the heave.. Moreover we designed some wireline tools which, together with CPTWD, can cover a wide range of in-situ tests. 1. INTRODUCTION The most popular way to perform Cone Penetration Testing is using standard cones (mostly piezocone, Ø 35,6 mm) pushed by a static penetrometer at a standard Rate of Penetration of 2 cm/second; the penetrometers push several tonnes and therefore they need reaction, which is given by dead weight (ballasted penetrometers) or augers driven into the soil as deep as is required for developing the reaction. Therefore the standard CPT to be performed needs to fulfil at least three basic conditions: a) the point of application of the thrust on the rods needs to be somehow joined to the soil to be tested (the point of application of the thrust cannot be floating) b) the reaction to the thrust must be proportional to the resistance of the soil, in terms of sum of resistance of the tip and total friction resistance along the whole drillstring (this is one of the reasons for so-called refusal ); if the reaction is poor it is not possible for the rods to be pushed since the penetrometer cannot fully develop its thrust c) the rods and the whole structure should be strong enough to bear the thrust applied by the penetrometer (commonly the standard Ø 35,6 mm rods cannot bear more than tonnes). To overcome problems (b) and (c) we developed the CPTWD (cone penetration test while drilling); actually the reaction needed is fairly low compared to the one needed by a standard cone, the lateral friction being dramatically lower since the lateral area to be pushed into the soil is just 737 cm 2 (50 cm of standard rods instead of several meters). Besides the rods are the drillstring used for conventional drilling, it means a Ø 127 mm rods 10 mm thick, therefore the limitation related to the strength of the CPT-rods is not determinant. CPTWD can also be used in floating conditions (with some problems), nevertheless it would be better to develop a dedicated device for heave compensation so as to have a system which would be perfectly suitable for carrying out CPTu in almost any conditions. One more advantage of using CPTWD would be to carry out CPTu in soils where penetrable and notpenetrable layers are alternated. Furthermore CPTWD is just one of the tools which can be used for sampling and testing the soil using the same drillstring, in a way which can be described as plug-ins. 2. CPTWD (CONE PENETRATION TEST WHILE DRILLING) AND INFLUENCE OF THE RATE OF PENETRATION IN CPTU The CPTWD system (Cone Penetration Test While Drilling) is an integration between a standard CPTU and a wireline coring system; sometimes the MWD (monitoring while drilling) recording is also added, so as to have a matrix with all the CPTU parameters and the MWD ones (Flow, Torque, thrust, Rop, etc) all together versus depth every 2 cm. The CPTWD also allows to alternate CPTU strokes when/with sampling, coring, down-the-hole testing; at the end of the test, since the hole is always cased with rods whose inner diameter is 107 mm, there is

2 the possibility to install almost any geotechnical instrumentation (piezometers, extensometers, inclinometers, etc) Sometimes it is not possible with CPTWD to push at a constant rate of Penetration of 20 mm/s, due to the way the CPTWD works and due to the soil conditions. Actually the CPTWD needs a combination of thrust, rotation, water flow and moving the cuttings up which could be difficult to get in some cases; therefore in those cases the correct RoP (20 mm/s) is somehow sacrified for performing the test and for getting some continuous data. This means that the CPTu parameters should be somehow corrected in order to have the correct soil profile and interpretation. For this reason we have performed several comparative tests at different rates of penetration and we compared the results, also using different methods (standard CPTu wìth penetrometers and deep tests with CPTWD). In both cases (deep and shallow) the results confirm expectations: Qc decreases proportionally to the decreasing of RoP, Fs does not seem to be significantly affected by RoP, nor is U. The variations of U seem to depend more on other factors, like saturation and/or the natural variation of the soil. It has been found that the evaluation of geotechnical parameters (such as Cu, Ø, etc) made with the usual correlations and the usual software is normally more affected by several other factors than the slight variation of Qc, Fs, U with a slower RoP than standard. Therefore it could be useful for Geotechnicians to also know the RoP in order to correct if necessary the parameters according to different Rates of PenetrationFollowing is a figure showing a tentative correlation between Qc and Rate of Penetration, distinguishing between different types of soil. The ordinate is the ratio between a certain Qc and the Qc 20 (with standard RoP) and the x-axis is the Rate of Penetration. Upper graph is for Clay, lower graph is for sand, the medium is in average 3. CPTU PERFORMED IN DIFFICULT SOILS AND FROM FLOATING BARGE 3.1 CPTu in very stiff silt and clay A CPTu test was performed in a site (using CPTWD) where stratigraphy is extremely peculiar: from 0 to 13 m sandy gravel, and from m to 160 m very dense silt, sometimes more or less sandy or clayey, but apparently uniform looking at the cores. In that site, after a pre-hole 13.4 m deep, we tried hard to push with both the CPTU cone and the Marchetti dilatometer standard methods without any result, no penetration at all also after having anchored the drillrig to the soil (like a penetrometer). Then we tried with CPTWD with extremely low RoP, and we were able to get continuous data from 17 to up to 30.5 m and we could have tested deeper Figure 2: CPTWD carried out at very low rate of penetration in very stiff silt and clay The results show that the Qc is significantly very high (constantly over 25 Mpa, as well as the Fs); the U seems to detect the percentage of clay/sand in the silt; this CPTWD test was carried out close (10 m) to a continuous core BH with samples and in situ (SPT, pressiometer, permeameter) testing; the CPTWD results are in very good accordance with the stratigraphy and results of other tests Fig 1: tentative parametric correlation between RoP and Qc

3 3.2 CPTu and SPT in the same hole (from floating pontoon) with alternate layers of penetrable soils and gravel In a Northern Italian harbour some tests were done in a site where the stratigraphy is very variable and above the bedrock there are alternate layers of sand/silt/clay and gravel. Only a floating pontoon was available, anchored with piles in locations where the water depth on average was 12 m. In order to have some geotechnical parameters of the gravel it was preliminarily asked to run standard CPTu tests down to the top of the gravel, then DPSH tests down to the bottom of the gravel and then CPTu until the deeper layer of gravel, then DPSH again, etc down to the top of the bedrock. Since the Jack-Up barges were not available we decided to perform the tests with CPTWD, replacing DPSH with continuous SPT tests. It has to be noted that in this case the harbour was well-protected and the pontoon (although simply floating) was well-fixed on four piles: we tried to run CPTu tests with a standard 20- ton penetrometer and the rods broke once the cone touched the top of the gravel, although the penetrometric rods were prevented from bending by a heavy-duty casing pipe for the first 14 m. Modern rigs have a hydraulic circuit called buoyancy which allow a manual, limited adjustment of the upward/downward movement of the rotary head; this arrangement makes pushing down the drillstring while mantaining a relatively constant rate of penetration easier. The buoyancy circuit is commonly manually-operated, so the Driller has to take care of the fine adjustments of height of rotary head according to heave (a sort of manual heave compensation). This adds one more difficulty to the already-difficult task of pushing down the cone while drilling, and therefore in these cases the results depend much on the skill of the operator. Following is the graph of the CPTU test, there are blank intervals, corresponding to the SPT performed in gravel Figure 3: CPTWD executed from floating pontoon alternating CPTu and SPT CPTU results Figure 5: CPTWD executed from floating pontoon alternating CPTu and SPT SPT results

4 3.3 deep CPTu tests using CPTWD and a Drillrig mounted on a floating barge, simply anchored cone by the vertical movement of the barge and drillrig. In front of the location of the Northern inlet of the Venice Mobile Dams, some CPTu tests were carried out down to a depth of m from the seabed, water depth in open sea being on average 11 m. No jackup barges were available at that time, and due to time constraints we placed an Atlas Mustang A66 drillrig mounted on a truck on a big barge, to be anchored at any location by means of four dead weights moved by a crane. Since the barge did not have any moon-pool we had to install a cantilever for the drilling operations. fig 6 CPTWD from a barge Adriatic sea, Venice - fig 8 results of one of CPTu tests executed from a floating barge. 4 IMPROVEMENTS and DEVELOPMENTS of TESTING for OFFSHORE DRILLING fig 7 drill rig used for the CPTWD from a floating barge The following graph shows one of the tests run on a water depth of around 11 m and a depth of 60 m; it can be noted that the graphs show slight value oscillations, with a frequency which is the same as the wave. It means that the Driller could not avoid or dampen the small waves by using the buoyancy circuit of the rig and therefore the heave slightly affected the values (both resistances and U). It was necessary to filter the graphs to take away (as much as possible) the oscillations transmitted to the 4.1 HEAVE COMPENSATION Focusing on the execution of CPTu tests in very difficult conditions from the above-outlined examples, it can be understood that CPTWD can easily overcome some heavy limitations of the standard way of carrying out CPTu (by pushing with a static penetrometer), but mostly regarding the depth of execution, drilling through hard layers, sampling and testing in the same hole of CPTu, executing CPTu in almost non-penetrable soils (sometimes altering the Rate of Penetration compared to the standard), executing deep CPTu with a very low reaction (the friction on the rods actually is extremely low if com-

5 pared to that of the standard CPT tests) and executing CPTu without problems of bending of the CPTrods. On the other hand the CPTWD can help in the execution of tests from floating barges (offshore, nearshore) but it doesn t solve the problem of fixing the pushing device to the soil to be tested, meaning that the measurement of the Rate of Penetration should be related to the soil which is penetrated (in brief any test performed from a floating barge needs to have heave compensation) Besides the CPTWD needs a combination of several parameters in addition to simple pushing as with the standard penetrometers: water flow, thrust, rotation, flowing the cuttings away from the bottom of the hole and (in case of operations from a barge) calibration of the buoyancy if possible, and the drillrig has to be perfectly equipped for wireline operations. The above-listed operations need an unconventional level of skill of the drilling Team and a perfectly-equipped drillrig. In offshore operations where the height of the waves can be in meters, the only way of performing CPT testing at the present is to use a heavecompensated vessel with down-hole system, latching internally of drillstring and pushing out the cone by a piston (therefore using the drillstring as reaction); the majority of heave-compensated rigs do not have an efficient system for pushing or do not have any at all, giving the weight of the rods the task of applying thrust. Not to have thrust or not to have a reliable way to push the rods is a problem both for the execution of good quality coring and sampling and for eventual execution of continuous tests as CPTWD. For this reason we have designed a heavecompensation system which can be adapted to a standard drillrig. The system is made up of two or four hydraulic pistons connected in such a way that once the rotary head is pushed or pulled the force is transmitted to the barge in the opposite direction (hence the barge giving reaction) and the rotary head is allowed to move only related to the soil, the upward and downward oscillations created by the waves don t matter and nor do their amplitude/frequency. The system is not active but reactive, it works only if a push/pull force to the rotary head is applied. Fig. 9 schematich sketch of heave compensation system 4.2 IMPROVEMENTS-DEVELOPMENTS Further improvement of the system is possible by adding several wireline tools. We have aldready designed and in some case already tested the following wireline tools (they work mostly at the bottom of the hole and not in a continous way as CPTWD does): -wireline flat dilatometer (WL-MEDUSA): instead of CPTwd is placed inside the drillstring a wireline latching system which holds a barrel in which is placed an electronic flat dilatometer, developed together with Mr.Marchetti. -wireline Permeameter and Sampler: the permeameter is a sort of slotted pipe (like a filter of a water-well) put inside a prehole (made with wireline small diameter core barrel); in the slotted pipe is run the permeability test inside the drillstring; the wireline sampler works with the same principle of BAT but the filter of the sampler is screwed into the soil instead than pushed (as the standard BAT) and there is no needle creating communication between the vial and the filter; connecting a pressostat with a datalogger to the top of the vial is possible to run a permeability test at the same time of the sampling (but without the severe limitations of BAT due to the needle); this method has been successfully tested down to -180 m.

6 Fig. 10 wireline sampler and permeameter

7 -wireline samplers Thin-wall tube type: have been developed and tested several types of samplers with and without front-piston for withdrawing undisturbed samples -wireline Vane Test: the whole device is contained into a wireline core-barrel and there are no rods conneting the vane apparatus to the surface; the rotation is given by an electrical powerpack contained inside the barrel; the shear value are stored inside a RAM managed by a microcomputer. -wireline drilling of pre-holes for pressiometric and rock-dilatometer testing: many tests have been done using this arrangement; in theory could also be possible to make a wireline arrangement of standard Menard pressiometer and/or rock-dilatometer but not with the standard wireline. With the same principle of CPTWD and the other mentioned applications could be adapted almost any tool to the wireline system. In such a way could be made an integrated system which would be capable to run almost any in-situ test down the hole using the same drillstring, in a continous way (as with CPTWD or WL-Medusa) or simply at the bottom of the hole (such as Vane-test, permeability test); alternatively to in-situ test could be done continous core drilling, no coring drilling (placing a wireline no-coring device such as a tricone) eventually with MWD (monitoring while drilling), undisturbed sampling of the soil. 5 CONCLUSIONS In CPT practise some problems occur when there are non-standard conditions, for example when reaction/anchoring of the penetrometer is poor, when there are not penetrable layers, when friction on the whole length of the CPT-rods becomes high, when CPT tests have to be carried out from a floating barge/platforms, etc These problems can be overcome by using CPTWD, but only accepting a noticeable increase of difficulty with the execution of the tests. Actually performing CPTWD is not just a matter of anchoring the penetrometer and pushing, but it s a combination of calibrating thrust, rotation and torque, water flow in the drillbit, flushing cuttings away in order to avoid the stuck of drillstring. Besides that, the drillrig must be suitable for wireline operations and the preparation of the borehole is much more difficult than preparation of the penetrometer for a standard CPTU test. On the other hand CPTWD can be easily used on jackup barges (nearshore) and also on floating barges/platforms but only when the wave height is very low, the barge is well-anchored and the frequency of the waves manageable by the calibration of the pushing circuit by the Driller. In some cases the rate of penetration of CPTWD is not standard, therefore it would be advisable to perform comparative tests in order to assess and better understand how and how much the rate of penetration affects the results of CPT tests; after that it would be advisable to also consider the rate of Penetration in the data elaboration. In order to improve the use of CPT rigs and CPTWD equipment offshore we designed a relatively-simple reactive system which allows us to compensate for heave and it could be applicable (as a matter of principle) with some adaptations to standard rigs. The possibility of having well-manageable thrust on offshore drillrigs would allow not only to perform CPT in a continuous way, but also to increase the quality of boreholes (coring, sampling, testing). In addiction, almost any method of drilling and sampling could in theory be arranged with wireline; we already designed and/or already developed and tested a wide range of tools, among them: corebarrels for any kind of soil, samplers, deep groundwater sampler, permeameter, Vane Test, Flat dilatometer, etc. This developments could bring to an integrated all-purpose system where any tool (for testing, sampling, coring) can be considered as a plug-in, in order to get the widest range of data with the best cost/effect ratio. The development of such integrated system eventually with the possibility to work with a heave compensation would be a breakthrough expecially in offshore geotechnical deep surveys. 6 REFERENCES -Lunne et al. CPT: Cone Penetration Testing in Geotechnical practice -Cestari: prove geotecniche in situ -CPT95 Congress: variuos articles -Sacchetto, M., A. Trevisan, K. Elmgren, and K. Melander, Cone Penetration Test While Drilling, Geotechnicaland Geophysical Site Characterization, Vol. 1 (Proc.ISC-2, Porto, Portugal), Millpress, Rotterdam, The Netherlands, 2004, pp Marchetti, S. Sacchetto, M.: wireline dilatometer WL Medusa DMT2006 Washington -Failmetzger, R, Marchetti, D. Sacchetto, M.: Effective Insitu Tests for Measurement of Soil Properties for Over Water or Deep Investigations Using Wire-line Methods ISC 3 Taiwan Sacchetto, M, Trevisan, A. influence of pushing method and of RoP (Rate of Penetration) on the value of Qc, Fs, U. CPT10 California 2010