Automatic Cone Penetration Testing: a class of its own
Ing. E. Welling, A.P. van den Berg, Heerenveen Drs. Ir. M.W. Bielefeld, IJB Geotechniek, Lemmer
Introduction
An important activity in the geotechnical design process is the performance of soil investigation. For constructions with traditional building materials such as wood, concrete and steel, demands are set to the characteristics and quality of the building materials. Strength, stiffness and durabilityare a few of those characteristics. Consequently the appropriate materials are ordered when building the designs, like B45 from a concrete supplier or S355 from a steel supplier. The building material "soil" calls for a different approach. It cannot be ordered, but has to be taken as it is. Determining the characteristics of the "building material" below the surface is crucial for the geotechnical engineers before making a technically and economically well-founded design.
Considering their importance, it is surprising that the developments in soil investigation have been so limited in the years past. The present practice of performing soil investigations is usually limited to making a number of soil tests, while the price of the investigation plays a more important role than the extent and quality of it. In this article we outline the developments of Cone Penetration Testing. Especially the latest developments, made tolead to a greater accuracy and reliability of the soil investigation, are highlighted.
Technical Developments in Cone Penetration Testing
The first cone penetration tests were performed in the beginning of the thirties in the previous century. With a hand mechanism a cone with a base area of 10 cm² and push rods were pushed into the soil. Measuring the reaction force on top of the push rods gave information about the soil
resistance. As you can see in picture 1, shortly after WWII cone penetration tests were still done this same way, with the cone testing unit mounted on a truck. In fact these were the first CPT trucks, although they cannot be compared to the modern equipment available nowadays. Important
improvements in the process of cone penetration testing, considering the quality and reliability of the test, have been:
- The introduction, by A.P. van den Berg, of hydraulic power for driving the cylinders. Before the sixties the tests were performed manually, that is, the push rod was pushed into the soil by means of a chain and/or rack-and-pinion drive. By using hydraulic cylinders, testing became much easier for the operator. It was not only an improvement of the labour conditions, but also of the quality of the test. As described later on, testing at a constant, normalised speed is very important for the quality of the test. A hydraulic drive provides a much more constant speed than a manual system, especially when the soil resistance is high.
- Switch from mechanical to electrical testing in the seventies. By inserting sensors in the cone, the introduction of electrical cone testing enabled a direct measurement of the cone tip resistance. This led to a considerable improvement of the quality because the disturbing influences of the internal friction of the push rods was eliminated. This internal friction had a great, yet unknown influence on the test results, so the new electrical performance was much better from a
qualitative point of view. Yet it took a long time before all soil testing companies abandoned the mechanical cone. And strangely enough lately the use of the old-fashioned mechanical cone penetration test is booming again. From a viewpoint of quality and reliability this is an undesirable development...
- In the nineties many variations on the well-known cone penetration truck appeared. Meanwhile the extended range of CPT equipment contains a normal tyre vehicle, crawler, Track-Truck, mini-crawler, etc. For each terrain and each kind of test equipment was created.
- The cylinders have a limited stroke length (1 metres). During the free stroke of the cylinders the test is interrupted. This interruption has a large effect on the soil around the cone. The effect often does not show very well in the graph, because the processing software usually manipulates
and obscures these influences. Also the digital format GEF [4] displays the manipulated data. Extensive studies [1,6] however show that the influence of stopping the test can be considerable. Differences in testing results and also in the pore-water pressure, just before and just after stopping can amount to tens of percentiles, see also frame 1. This knowledge has led to the development of continuous cone testing. A considerable improvement of the quality is the result. - Class division in the cone testing standard NEN 5140 [2][3] had the object to improve the quality of the standard cone test. With the introduction of the classes, a distinction was made for the first time between high-quality and less high-quality cone tests. Since the quality can not be seen on the graphical presentation, it was necessary to introduce a class indication. An earlier initiative had tried to secure the quality of the cone test by means of the KOMO Mark. Although important elements of the standard BRL 2364 [5] belonging to the KOMO Mark were incorporated in the NEN5140, it still does not give a clear insight into the quality of the cone test as supplied. The present practice of cone testing is that classes and quality of the cone test are not a real issue for the parties ordering for soil investigation. And so it seems that all cone test still have the same quality, while in fact this is certainly not the case.
- With the introduction of a fully automated cone testing robot, all possible disturbing human influences on the cone test are eliminated. The cone testing process, performed continuously, is entirely computer controlled, resulting in the highest possible quality of the cone tests. The operator can dedicate himself entirely to the geotechnical assessment of the cone test to be performed, thus contributing to a further improvement of the quality. Repeatedly screwing on and off the CPT rods by hand belongs to the past, so that the latest CPT truck also complies fully with the strict legislation with respect to labour conditions in the Netherlands.
Disturbing influences on the cone test
A traditional electrical cone test is strongly influenced by the operator and the cone test equipment. As explained above, the best quality cone test is acquired by testing at a speed as constant as possible (20 mm/s according to the standard), without interruptions. Causes of errors in a "normal" electrical cone test are:
- deviation from the standard cone testing speed of 20 mm/s, the speed itself as well as a variation on it.
- interruption during the free stroke of the cylinder, every metre of the cone test - extra interruption while adding new push rods
In order to further improve the cone test the disturbing influence of the operational aspects, like screwing on the push rods, the manual manipulation of the test speed, etc., must be further eliminated.
In order to improve the quality of the cone test and the labour conditions of the operator, an automated CPT system has been developed.
New development Cone Penetration Testing System
In a joint project, manufacturer of CPT equipment A.P. van den Berg in Heerenveen and geotechnical investigation company IJB Geotechniek in Lemmer have realised a new Cone Penetration Testing Truck with a fully automated cone penetration testing system.
The course of development consisted of two partial projects: 1. development of wireless cone penetration testing 2. development of cone penetration testing robot Ad 1.
The introduction of electrical cone testing made an electrical connection between the cone with incorporated sensor, situated at lower end of the CPT string, and the data acquisition system above the ground. This measuring cable is an awkward element during cone testing operations. While adding on a new push rod, the operator has to lead the cable through the push rod. Since each push rod weighs somewhat over 7 kilo's, the operator is at constant risk of developing RSI. The success of further development of automated cone testing is determined by the possibility of eliminating the
measuring cable. Both companies mentioned abovehad separately set steps in the direction of this development in the eighties. Activities at that time even led to a patent for wireless cone testing. The electrotechnical sciences at that time were however not sophisticated enough to be able to further develop the system into a practically applicable one. Only recently, after many developments in micro-processors, data communication and storage, it became possible to develop such a wireless cone testing system. The wireless system uses light as a transmission medium (see frame 2 and picture 4). Ad 2.
The modern operator has a complex task: he or she must operate the system. It is important to have knowledge of hydraulics and of testperformance. In fact, the operator should even have a
geotechnical background. After all, he or she is the eye of the geotechnical advisor. With the automation of the cone testing process, the cone test has become fully automatic. This gives the operator time for the things that really matter: a first evaluation of the extent and quality of the soil investigation. He or she has to judge whether the soil investigation is representative enough for the client in order to serve as a basis for the economical design of the geotechnical advisor.
The newly developed cone penetration testing robot takes care of the automatic performance of the cone test. The robot consists of a positioning unit, that positions the push rods above each other, a placement unit, that adds on or removes the rods, and a storage turret that contains a sufficient amount of push rods, see frame and picture 5.
On to reliable soil investigation
The primary goals of the development of the automated cone testing system are improvement of the quality of the soil investigation and improvement of the labour conditions of the operator. The
continuous cone test is an absolute must in the present building practice. The standard with division in cone testing classes has never become operational because the differences between the practice and the standard are too large by far. While introducing the standard, the quality of the soil investigation was the main object. However in reality the way to do a testis determined by economic aspects. It’s cheaper to provide less quality and the resulting financial profit usually does not go to the clients ordering the soil investigation. From a viewpoint of quality and reliability mechanical cone testing should no longer be applied in the Netherlands. The traditional electrical cone test has proven to acquire more reliable test results. The same goes for cone tests made with an automated system which result in a higher class cone test, because the operation has no impact on the cone test results. Quality must therefore be clearly recognizable on the type of cone test, and the introduction of a new class division is obvious. The class division is based on an increasing reliability and quality which are the result of a diminished operational handling:
- electrical
- continuous testing - automated testing
Mechanical cone testing no longer belongs on this list.
The cone testing speed must be made clear to be an important parameter for the quality of a test for the users of tests: the average speed as well as its deviation.
In that frame the performance of a fully automated cone test is a class of its own.
Conclusion
With the introduction of a new generation cone penetration testing systems, a new impulse is given to the quality of the cone test, which is no longer influenced by operational aspects, therefore leading to a more constant and higher quality and reliability of the soil investigation. The advantage benefits both geotechnical designers and their clients. Higher quality leads to fewer uncertainties and therefore to a more economical design. Designs based on a better quality soil investigation carry fewer risks. A higher quality soil investigation pays itself with the resulting design and construction.
Literature
[1] Campanella, R.G. & Robertson, P.K., "Current status of the piezocone test, penetration Testing", ISOPT-proceedings, Balkema
[2] NEN 5140, Nederlandse Norm Geotechniek,"Bepaling van de conusweerstand en de plaastelijke wrijvingsweerstand van grond. Elektrische sondeermethode", 1996 (Dutch Standard for Geotechnics, "Determination of the cone tip resistance and the local friction resistance of the soil. Electrical cone penetation testing method")
[3] Article Land en Water, "VOTB leden leggen klassen uit" ("VOTB members explain classes")
[4] Van Niekerk, W.J., Nohl, W.A., Van de Graaf, H.C., Bielefeld, M.W., Jonker, A., "Gegevens die iedereen kan lezen. Sonderen in digitaal standaardformaat". Land en Water, nr. 7/8, 1998
("Data that everyone can read. Cone penetration testing in digital standard format")
[5] BRL 2364 "Elektrisch Sonderen", KIWA 1994, 2001 ("Electrical cone penetration testing")
[6] Lunne, T., Robertson, P.K., Powell, J.J.M., "Cone Penetration testing in geotechnical practice", par. 5.1.5, Blackie Academic & Professions, 1997
Pictures
Picture 1. Cone penetration testing shortly after WWII: a manually operated cone penetration testing system at the back of an old army truck
Picture 3. Interior of the latest type CPT truck: fully automated cone penetration testing system, for maximum reliability of the soil investigation
Picture 4. Installing the new CPT system in a CPT truck with integrated tracks (Track-Truck) makes (almost) any terrain accessible.
Picture 5. Electrical cone with Optocone Module. Micro-electronics inside the cone: a full-fledged processor, digitizing the measuring data, storing them and transmitting them to the CPT truck up above.
Picture 6. Overview of the CPT robot, consisting of a positioning unit, that positions the push rods above each other, a placement unit, that adds on or removes the rods, and a storage turret that contains a sufficient amount of push rods
Picture 7. While the CPT robot performs the cone test automatically, the operator can concentrate on the first interpretation of the measuring results, ensuring that the soil investigation has the required level of quality.
Figure 1. Raw, unmanipulated CPT graph. Clearly visible are the 1 metre intervalls, when the cone stands still for a short while during the free stroke of the cylinders. Also the difference in the cone values just before and after the interruption is clearly visible.
Frame 1. Continuous cone penetration testing
Why continuous cone penetration testing?
Most important reason to apply continuous cone penetration testing, concern the disturbing influence that the interruption of the test has on the soil around the cone. The stand-still every one metre causes undesired dissipation effects during a normal cone test. By testing continuously, without interrruptions, measuring disturbances over 2 cm up to even 50 cm are prevented [1]. Also the stand-still every metre causes peaks in the graph during normal cone tests. Release of the tension on the cone causes peaks in the graph every metre, or more often. These disturbing effects are partly cosmetically removed, but this is a rough arithmetical adaption that has no relation with the true influence on the measuring results.
The cone penetration testing unit
The performance of continuous cone penetration tests calls for an adapted cone penetration test unit. A double stroke unit is needed instead of the usual single stroke. While one set makes its free stroke back to its starting position, the other set takes over. The cone penetration test unit consists of two hydraulic cylinders, with two hard-chromed piston rods positioned in a vertical downward position. The piston rods are fixed to the underbeam. The underbeam is fixed to the subframe. The piston rods are connected to each other by means of an upperbeam.
Both piston rods are longer than necessary for completing the 1 metre stroke. On these extended piston rods two extra cylinders are connected; in this case with a continuous piston rod. These two cylinders are connected to each other be means of an inbetween beam, comparable to the upperbeam.
In the underbeam a dirt wiper clamp for push rods Ø36 mm is installed, that removes most of the dirt from the rods during pulling.
Both beams work together to push the rods into the soil continuously. Pulling is done with intervals.
Advantages
Continuous cone penetration testing eliminates the uncertainties connected to the stand-still of the cone. The continuous cone penetration test is a class of its own.
Frame 2. Wireless data transmission
As of old data transmission between the cone and the data acquisition equipment in the CPT truck
goes through a measuring cable. Almost just as old is the wish to eliminate the cable and replace it by a means of wireless data transmission. The new system uses light as transmission medium. The convertion to light is done directly behind the electrical cone. At the point where the measuring used to be
connected, now a module is placed that converts the analogue data signal that is supplied by the cone, to a digital signal. The digital signal is then converted into a ray of light. The ray of light is transmitted to the surface through the push rods by means of light transducers. The light transducers make sure that the optical data transmission is not interrupted when the cone with push rods deviates by a certain angle in a certain direction in the soil. In principle the maximum reachable depth is unlimited, thanks to the possibility to amplify the optical signal at one or more points in the CPT string.
Once the light signal has reached the surface it is caught by a camera that converts the reay of light back into a digital signal. Thus the original digital signal can be processed by the data acquisition system. The data acquisition system combines the measured values with the
corresponding test depth and transmits them to a laptop for instance, where the operator can read the test results online on screen.
Electrical cone AD conversion
CPT rods with light transducers
Depth registration
Computer + DAS Camera
Frame 3. Automatic cone penetration testing process
The cone penetration testing cycle is entirely automated by robotizing the process of adding on and removing the push rods. The technique is suitable for both a traditional or, preferably, a continuous CPT unit. In both cases wireless data transmission is a must. The CPT robot consists of the following parts:
- a screwing device for adding on and removing the push rods
- a turret for storage of the push rods
- a pick-and-place unit, that takes the rods from the turret and positions it for the screwing device
The screwing device
The screwing device is installed on the upper beam. It consists of a worm
gearbox with a hollow shaft, a spring chuck and two vertical pneumatic cylinders. The shaft is driven by a hydraulic motor. The pneumatic cylinders compensate for the weight of the screwing device during screwing.
Storage turret and pick-and-place unit
The storage turret consists of a toothed ring bearing with a plate with 40 bushes installed on it. In every bush a CPT rod is placed. The turret is driven by means of a hydraulic motor.
The pick-and-place unit consists of a pivoting arm to which a vertical pneumatic cylinder, two horizontal cylinders and two grabbers are connected.
Working of the system
The upper beam pushes the push rods into the soil. When the upper beam is almost at the end of the stroke the pick-and-place unit takes a rod from the turret and positions it above the screwing device. The screwing device goes up, the spring chuck closes and the rod is screwed on to the previous rod. The middle beam takes over pushing the rods. When the screwing is finished the upper beam goes up again. When the middle beam is almost at the end of its stroke, the upper beam takes over pushing the rods.
The result is a continuous cone penetration test, fully automated and with a very sophisticated data transmission. The ultimate system with respect to quality as well as labour conditions.
middle beam upper beam rod screwing device Continuous CPT unit
Pick-and-place unit
