Massimo SacchettoAnnalisa Trevisan

CPT EXECUTED IN DIFFICULT CONDITIONS USING CPTWD AND FUTURE DEVELOPMENTS

BRASILSeptember 2012Porto De Galinhas

CPTWD: Cone Penetration Test While Drilling

“CPTWD” stands for Cone Penetration Test While Drilling; it can be considered as an integration between: a standard CPTU, a wireline coring system, and sometimes the MWD (monitoring while drilling)

The CPTWD allows to alternate CPTU strokes with sampling, coring, down-the-hole testing (i.e. SPT), and it’s possible to install geotechnical instrumentation (diameter of the hole ≥ 130 mm)

CPTWD• However CPTWD is NOT a “down-the-hole” CPT system;

basically it’s a standard piezocone with memory (it’s not possible to use with cable) which is embedded into a wireline corebarrel; the piezocone protrudes down the drill bit for 50 cm, so the CPTU data normally are not affected by rotation and pushing; of course the cone does not rotate thanks to bearings. Differently from “down-the-hole” systems the CPTWD is pushed by wireline rods and penetrates the soil as fast as the drillstring does. After the test is done or whatever is required to take a sample or if gravel/bedrock/not penetrable soil is encountered, the cone (the CPTWD corebarrel) is retrieved by a “overshot” driven by a cable (WIRELINE) .

CPTWD

1) CPTWD in very stiff soil carried out at very low rate of penetration

A CPTu test was performed in a site (using CPTWD) where the stratigraphy is : from 0 to 13 m gravel, and from 13 m to 160 m very stiff SILT. 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 much deeper.

NO PENETROMETER!!

EXAMPLES

CPTWD IN VERY STIFF SOIL

THE BLUE LINE IN Qc GRAPHSHOWS 30 MPa

Qc=30 MPa

Qc Fs U Fr%

2) CPTWD carried out from FLOATING barge simply anchored to seabottom, with 8÷14 m water depth (Adriatic sea, Venice Mobile Dams Project)

CASE HYSTORIES

CPTWD from floating barge

CPTWD from floating Barge

• It has been necessary to FILTER the graphs for taking away (as much as possible) the oscillations/fluctuations transmitted to the cone by the vertical movement of the barge and drillrig.

CPTWD in VENICE- DISSIPATION TEST -

The small “oscillations” in the shape of the dissipation curve are probably due to waves !!

3) CPTu and SPT in the same hole (from floating pontoon) with alternate layers of penetrable soils and gravel

It was available only a floating pontoon, anchored with piles in locations where the water depth in average was 12 m. For soil characterization we should have switched the penetrometer (for CPTU) and the drill rig (for coring and SPT) 4÷5 times each location. Since was not available the Jack-Up barge we decided to perform the tests with CPTWD using the available floating pontoon and a small drill rig (4 ton weight).Were carried out 2÷3 tests per day down to -35 m with single shift.

WATER depth 12,3 m

CPTU

CORING GRAVEL+SPT

CPTU

CORING GRAVEL+SPT

CPTU

bedrock

HEAVE COMPENSATION

The aforementioned jobs were made with CPTWD in good weather conditions, with low heave or using a very big barge well anchored. The results show that the data might be affected by the vertical movement of the barge, even if the CPTWD system of pushing is much more “flexible” than the standard penetrometric pushing.For this reason we designed a simple system for HEAVE COMPENSATION which could be easily used with standard rigs (and penetrometers)

HEAVE COMPENSATION

NEW DEVELOPMENTS

CPTU is not an “universal” test giving all the possible results, therefore we thought to widen the range of tests that would be carried out in the same hole, with the same drillrig, without changing the drillstring. Therefore we designed (and in some case made) special geotechnical and environmental wireline tools to be used with the same philosophy of CPTWD

NEW DEVELOPMENTS

• Wireline DILATOMETER

• Wireline VANE TEST

• Wireline PERMEAMETER

• Wireline WATER-SAMPLER AND PERMEAMETER

• Wireline thin wall Samplers (shelby and osterberg type)

USED IN VERY STIFF SILTDown to -180 mIt is a wireline “down-the-hole water well” to be installed in a prehole

Wireline permeameter/sa

mplerIt’s made up of a tube containing a under-vacuum vial connected to a filter; the bottom of the tube has an auger which penetrates the uncontaminated soil at the bottom of the hole; once the auger penetrated one meter the drillstring is pulled up and the filter is mechanically opened, so the vial starts to be filled. A pressostat on the top of the vial measures the variation of pressure versus time, so to get a permeability coefficient

WIRELINE SAMPLER-PERMEAMETER

STEP 1: CLOSED

WIRELINE SAMPLER-PERMEAMETER

STEP 2: OPEN

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 floatingbarge/platforms, etcThese 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 matterof anchoring the penetrometer and pushing, but it’s acombination of calibrating thrust, rotation andtorque, water flow in the drillbit, flushing cuttingsaway in order to avoid the stuck of drillstring.

CONCLUSIONS

In order to improve the use of CPT rigs and CPTWD equipment offshore we designed a relatively- simple reactive system which allows heave compensation and it could be applicable (as a matter of principle) with some adaptations to standard rigs.

CONCLUSIONS

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 nearshore and offshore geotechnical deep surveys.

CONCLUSIONS

BRASILSeptember 2012Porto De Galinhas