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4TH INTERNATIONAL SYMPOSIUM ON CONE PENETRATION TESTING TU DELFT, 21 AND 22 JUNE 2018

GEOTECHNIEK SPECIAL

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Contents

Cone Penetration Testing in layered soils - 5

Pinpointing tie-back anchors using magnetic CPT investigation - 10

Setting up of a system of standardprocedures for geotechnical investigations in Belgium, with focus on CPT - 14

50 years in the forefront of innovated CPT technology - 20

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GEOTECHNIEK SPECIAL CPT’18 - June 2018


D.A. de Lange Deltares, Delft, the Netherlands

Cone Penetration Testing in layered soils

J. Terwindt Deltares, Delft, the Netherlands

T.I. van der Linden Royal HaskoningDHV, Amersfoort, the Netherlands

Figure 1 - Sediment core with flaser bed sedimentation. Core diameter 65 mm

Introduction Cone penetration testing (CPT) is widely used to determine the geotechnical engineering properties of soils and delineate soil stratigraphy. The resolution of CPT in delineating stratigraphic layers is related to the size of the cone tip and the friction sleeve and the sample recording rate relative to the penetration rate. Where two different soil layers meet, a transition zone exists around the interface, since the measured resistance will be affected by both the under- and overlying layers. The dimensions of such a transition zone are a function of the stiffnesses of both layers but also of the cone tip size. For deposits containing intervals with multiple thin layers, the situation is even more complicated since the cone resistance may be affected by several surrounding layers. As a consequence the CPT interpretation within these intervals holds large uncertainty. For several applications a better understanding of CPT in thinly layered soils is desired. Examples are the estimation of the liquefaction potential of thin sand layers and the shear strength of thin soft layers. This study focuses on the cone resistance of sand layers in so called ‘flaser beds’. These are sedimentary bedding patterns created when sediment is deposited by intermittent flows, leading to alternating sand and clay layers. Such deposits typically exist in marine environments. Figure 1 shows an example of a sediment core containing such depositional features. The layers typically have a thickness of 5 mm to several centimeters. The effect of layer transitions on cone resistance has been investigated extensively by numerical as well as physical modelling (see

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GEOTECHNIEK SPECIAL CPT’18 - June 2018

De Lange et al (2018) for relevant papers). From the literature it appears that the size and the location of the transition zone depend on the ratio of the characteristic resistances in the stiff and soft soil layer. It also appears that to develop the full steady state cone resistance in a sand layer more penetration is needed than in a clay layer. Furthermore, it appears that the difference between the measured and the characteristic cone resistance in a “thin” sand layer, interbedded in soft layers, increases with increasing density index of the sand. Therefore, for multi-layer systems, it is expected that the cone resistance will be influenced by the layer thickness (relative to the cone diameter), the number of layers within the zone of influence and the characteristic cone resistances of the individual layers, which depend on, inter alia, the porosity and the stress level. Koppejan modelling Van der Linden et al. (2017) suggested to use the Dutch so called ‘Koppejan’-method for pile base capacity as an interpretation tool for CPTs in thinly layered soils. The study described in the present paper can be used to investigate the feasibility of such an approach by model experiments in well-controlled circumstances. The aim is to devise a correction method which will ascribe more realistic engineering properties to thin sand layers within thinly interlayered zones. The underlying reason to focus on sand layer properties is the interest in liquefaction properties of flaser beds in the presence of earth quakes. The measured cone resistance needs to be converted to a ‘clean sand’ resistance, hereafter referred to as ‘characteristic resistance’, in order to assist interpretation of the CPT values with existing correlations.


Abstract

Multi-layer systems consisting of multiple sequences of thin soil layers are typically deposited in marine environments. The geotechnical properties of the sand layers can be important, especially where it concerns sensitivity to liquefaction. Performing CPTs is a common method to determine geotechnical properties, but the interpretation of CPT within multi-layered soil zones holds large uncertainty. The Dutch

‘Koppejan’-method for determination of pile base resistances could be used to model the cone resistance in thinly inter-layered soils. Model experiments in well-controlled circumstances have been performed to investigate the feasibility of this approach to come up with correction factors to the measured CPT values.

Figure 2 - Schematization of the trajectories taken into account in the Dutch method

The Dutch Method, also called 4D-8D- or Koppejan-method, estimates the pile base capacity of a driven foundation pile by averaging the CPT values in a region between (max.) 4D below and 8D above the pile base, D being the pile diameter. The governing equation in this method reads:

Figure 3 - Schematized test set-up

the measured cone resistance in thinly interlayered zones: for H > D:

(2)

(1)

for H < D:

(3)

with qb;max the pile base capacity and qc the CPT value, where subscripts I, II and III refer to the trajectories for which a governing cone resistance has to be determined by some sort of averaging (different for I and II) (figure 2). The coefficients αp and βs are related to non-driven and non-cylindrical pile types and can be ignored for a CPT. For details the reader is referred to Van der Linden (2016), annex E.

where qb now represents the measured cone resistance, H the thickness of the layer the cone tip is located in, and D the penetrometer diameter. The characteristic cone resistance of the individual layers qc is used as input for the calculations.

The test set-up itself is a further development of the set-up discussed by Van der Linden et al. (2017). It consists of a cylindrical steel cell, 0.90 m inner diameter and 0.96 m high. The cell wall is lined with a rubber membrane and the space in between can be filled with a film of water (a geotextile was placed in between to ensure this). In this way the horizontal stress can be controlled. The horizontal stress applied is kept equal to 0.5 times the vertical stress. The vertical stress is applied by a flexible water-filled cushion which is placed on top of the soil model.

Physical model tests CPTs have been performed in artificially constructed deposits containing multiple soil layers. The test set-up consisted of a hydraulic plunger fixed on a reaction frame that was able to push a miniature cone into a cylindrical steel cell containing the artificially built-up soil deposits.

The employed cone penetrometers have a diameter of either 35.8 mm or 25.3 mm (corresponding to a cone face area of 10 cm2 and 5 cm2, respectively), depending on the test, and were manufactured by Fugro Leidschendam (the Netherlands). The penetrometers were pushed in by a hydraulic jacking unit at a rate of 4 mm/s with a measurement frequency of 4 Hz,

After simulation of a number of test results Van der Linden et al. (2017) propose different approaches for layer thicknesses greater and smaller than the cone diameter D. The following relations are proposed to approximate

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Figure 4 - Test set up (a) before mounting, top cushion at right (b) after three CPTs have been performed

resulting in a data point every 1 mm. It should be noted that this penetration rate differs from the standard rate applied in the field (20 mm/s). Furthermore, the cone resistance has not been corrected for water pressure since no pore pressure measurement was performed. The effects of both these aspects are considered to be minimal. The in situ vertical soil stress was monitored using a total stress transducer, fixed in the center of the model at 72 cm below surface level. Any change in volume of the soil model was monitored by measuring the volume change in the water supply of both the membrane and the cushion and any water dissipated through the bottom drain of the test setup. Figure 3

shows the schematized overview of the test setup, figure 4 the practical implementation in the geo-experiments hall of Deltares. The model preparation followed the methodology of Van der Linden et al. (2017), i.e. pluviation of dry sand in a partially water-filled cylindrical container. The density of the sand was controlled by controlling the water height (for low density samples) and periodically gently tamping the sand surface during pluviation (for medium density samples). Clay layers were placed after trimming pre-fabricated clay bricks to the required dimensions. Densification of the sand during the placement of the clay was minimized by temporarily lowering the phreatic level, so that capillary action temporarily could

Figure 5 - Test results of four 4 cm thick clay layers (position indicated in gray)

7

Tabel 1 - Experimental variables

penetrometer diameter D [mm]

25.3

35.8

density index ID [-]

0.30

0.60

Layer thickness H [mm]

20

40

80

Vertical stress Ď&#x192;v [kPa]

25

50

100

provide apparent cohesion to the sand. During the preparation the bulk density was monitored closely by measuring the sample height. Upon completion of the cone penetration tests the soil model was excavated and volume-mass density measurements were performed at various positions and depths in the model. More details of the applied soils and the procedure can be found in De Lange et al. (2018). It should be noted that the tested multi-layered samples contained sand and clay layers of equal thickness. This is certainly not always the case in the field. Therefore, the results cannot simply be extrapolated to all thinly inter-layered soils. Testing program The test program was a continuation of the program of Van der Linden et al. (2017). Different layer configurations, bulk density indices (ID), stress levels and cone diameters are applied in order to investigate the influence of these parameters. CPTs were performed on saturated layered soil deposits. The layered units of multiple clay and sand layers, having equal thicknesses, were sandwiched between two thicker

Figure 6 - Test results of six 2 cm thick clay layers (position indicated in gray)

GEOTECHNIEK SPECIAL CPTâ&#x20AC;&#x2122;18 - June 2018


sand layers. For each ID a uniform sand model was prepared in addition to the layered models in order to serve as a reference. Table 1 provides details about the variations applied (not all possible combinations are tested).

plotted in this graph. A good fit is obtained with the proposed method. Figure 9 shows the simulation of the tests performed at sample 3 with the normalized test

Figures 5 - 7 show results in two artificial soil samples: sample 2 and 3. Sample 2 contained a layered zone of 4 clay layers and 3 sand layers, each having a layer thickness of 4 cm, while sample 3 contained a layered zone of 6 clay layers and 5 sand layers, each having a layer thickness of 2 cm. An initial bulk density index of around 30% has been applied for both samples. Multiple 25 mm CPTs (2 or 3) were performed at the same soil model. First, the desired stress level has been applied and after reaching a sufficient degree of consolidation a CPT has been performed. Subsequent tests were performed at different locations and stress levels, leaving the previous cone(s) in place. The CPTs were performed at 300 mm from the container wall and with a distance between the CPT locations of 260 mm.

results. Although the simulation deviates from the measurements for the transition region between the layered zone and the upper and bottom sand layers, a good fit is obtained for the layered zone. Since this zone is the subject

Figure 7 - Normalized cone resistance of the four tests

Figure 5 shows the measured cone resistance in sample 2 for two stress levels (vertical stress 25 kPa and 50 kPa, respectively). The individual layers can be clearly distinguished. Also the effect of the applied stress level can be observed: higher cone resistances at higher stress levels. Figure 6 shows the measured cone resistance in sample 3 for the same stress levels. In this case, the individual thin layers can be hardly distinguished. The initial peak in cone resistance at about 5 cm depth is an artifact of a cylindrical tube which was in place to protect the cushion.

Figure 8 - Simulation of CPTs in sample 2

Figure 7 shows the cone resistance in sample 2 and 3 normalized for the applied stress level by equation (4) (based on Lunne et al., 1997) in which Ď&#x192;v is the applied vertical stress level. (4) Numerical modelling Simulations are made by calculating the tip resistance at each mm by the method described above. In order to get a more realistic simul-ation the moving average over a height of 27 mm (the cone height) has been calculated. The average of the measured cone resistance in the upper and bottom sand layer is used as characteristic cone resistance for the sand layers. For the clay layers a value of 0.035 is used, based on the test results. Figure 8 shows the simul-ation of the tests performed at sample 2. The normalized test results are also

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GEOTECHNIEK SPECIAL CPTâ&#x20AC;&#x2122;18 - June 2018


Figure 9 - Simulation of CPTs in sample 3

of interest in this investigation, the proposed method can still be used. Conclusion Systematic experiments in well controlled circumstances are shown to contribute to the insight in geotechnical behavior of thinly inter-layered soil profiles. Further investigation is in progress to come up with a general correction methodology to CPTs for use in these soil profiles. Acknowledgments The authors would like to acknowledge the NAM (Nederlandse Aardolie Maatschappij B.V.) for financing the research and F.M. Schenkeveld and R. Zwaan for their invaluable assistance and input with regards to development of the test setup and model preparation. J.K. van Deen (Deltares) assisted in writing this paper, which is an abridged and rewritten version of De Lange et al. (2018). References - De Lange, D.A., Terwindt, J. and Van der Linden, T.I. (2018), CPT in thinly inter-layered soils, in: Proc CPTâ&#x20AC;&#x2122;18 - Lunne, T., Robertson, P.K. and Powell, J.J.M. (1997) Cone Penetration Testing in Geotechnical Practice, First edition, Blackie Academic & Professional, ISBN 0751403938. - Van der Linden, T.I. (2016) Influence of Mul-

tiple Thin Soft Layers on the Cone Resistance in Intermediate Soils, MSc thesis Delft Univ. of Technology, via https://repository.tudelft.nl/ - Van der Linden, T.I., De Lange, D.A., & Korff,

M. (2017) Cone Penetration Testing in Thinly Inter-Layered Soils, Geotechnical Engineering, Advance online publication. doi: 10.1680/ jgeen.17.00061

SPECIALIST IN THE FIELD OF GROUNDWATER From drainage and water storage to creating Room for the River. Your geotechnical specialist for review, design, construction and asset management. We advise and design with knowledge of subsoil and exploit the potential to use local soils. Fugro GeoServices B.V. Info@fugro.nl www.fugro.com


Pinpointing tie-back anchors using magnetic CPT investigation Rotterdam engineers have improved the art of the CPT based magnetic profiling technique. With the adaptations and improvements, it has become possible to reliably locate obstacles like steel anchor rods (tie-back anchors) In many current projects, the uncertain position of (steel) obstacles is a major cause of project delay (and cost increase) if the obstacles turn out to interfere with the planned foundation elements. Rotterdam engineers have upgraded the magnetic CPT instrument such that the magnetic distortion signals are obtained in the proper orientation to earth’s magnetic field orientation. As insiders know, the response of these sensors is very sensitive to tilt and orientation in

respect to the earth’s magnetic North. While in 2014 conducting a magnetic CPT site investigation to locate grout anchors for redevelopment of a quay, Rotterdam Engineers were encountered with difficult North orientation of the CPT tool. The CPT tool had no clear orientation indicators on the outside and as a result, proper North alignment in the field was difficult to achieve with this experience the manufacturer of the tool was requested to provide a physical reference for the sensor orientation to the outside of CPT tool. Currently, the magnetic CPT tool is provided with a visible orientation indicator that is aligned with the magnetic Y sensor in the cone. Because of the presence of a CPT truck (20

Dr. Ir. R. Spruit Geotechnical engineer

W. van Bommel Projectleader Geo-monitoring

tons of steel) and unknown metal anomalies influencing the local magnetic field, A proper magnetic outline cannot be made. A magnetic outline returning 0 nT at the X sensor will only be possible in a fully demagnetized area. In order to overcome this practical limitation, an optical tool is installed to the physical orientation indicator on the CPT tool. The optical tool can be used to rotate the CPT tool to a chosen orientation. In most cases, the geographic North is chosen as the reference orientation. As local deviation of earth’s magnetic field from the geographic North is known, proper corrections can be made. In early 2016 the upgraded technique was for first time used successfully at the Vogelenzang site in Rhenen. Rotterdam Engineers responded to a request from MOS Grondmechanica to assist in locating the steel anchors at this site. Due to extensive Figure 1 - Projectlocation.

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GEOTECHNIEK SPECIAL CPT’18 - June 2018


Abstract

Rotterdam engineers have improved the art of the CPT based magnetic profiling technique. With the adaptations and improvements, it has become possible to reliably locate obstacles like steel anchor rods (tie-back anchors). Rotterdam engineers have upgraded the magnetic CPT instrument such that the mag-

netic distortion signals are obtained in the proper orientation to earth’s magnetic field orientation. 3D modelling became more reliable and efficient due to an invention for the orientation of the magnetic North. The only distortion that still remains in the acquired data is now limited to the variable tilt angle of the cone over the CPT profile. An issues on which research is now being done to solve with a steerable CPT cone.

Figure 2 - Constructed alignment tool for CPT cone, aligned at the magnetic North

excavation and reshaping of the terrain the soil became instable. A sheetpiled wall with tie-back anchors was installed to improve slope stability. In the next construction phase the foundation piles for the housing project needed installation. The exact position of the tieback anchors was not documented. Due to the self-boring nature of the applied tie-back anchors, the uncertainty of the location was relatively high, making the areas for low risk pile installation very limited. For the majority of planned foundation piles, the chances of collision with the grout anchors were considered too high. Magnetic CPT plan Due to the specific behaviour of magnetic fields in relation to steel objects, a CPT plan for magnetic surveying requires a different layout than a geotechnical survey. In case of Vogelenzang, only the planned pile positions need verification of (absence of) obstacles. To fulfil this, the CPTs are positioned at 50 cm off the planned pile position parallel to the sheetpiled wall and perpendicular towards the possible anchor projection. This method eliminates the need to obtain a survey of the complete potential obstacle area and limits the number of CPT’s while returning enough information for the 3D modelling. 3D modelling The modelling of the site using the software of Potent has become more reliable and efficient due to the accurate north orientation of the acquired data. The only distortion that still remains in the acquired data is now limited to the variable tilt angle of the cone over the CPT profile. Nevertheless, the modelling is time consuming because the steel objects must be modelled and should be moved over 6 degrees of freedom to achieve the best fit between measured and simulated magnetic field response.

As soon as there is more than one tie-back anchors showing there footprint in the image the modelling becomes challenging to fit to the signals. Since CPT survey positions are chosen next to pile, the model obtains the most valid information over the planned pile position. If the tie-back anchors have deviated from their planned trajectory, this will be clearly visible in the image of the acquired data for the planned pile position. In another project the survey of the foundation of the St. Sebastiaan Bridge in Delft has been evaluated. The foundation level of the existing prefabricated concrete piles was uncertain making verification of the pile toe position necessary.

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GEOTECHNIEK SPECIAL CPT’18 - June 2018

Prefabricated concrete piles contain steel reinforcement which is normally enough to detect the pile toe. Even though the CPT cone was carefully aligned with the North, almost none of the conducted CPT’s detect the pile toe. At this location the survey possibilities were limited due to available space (only a small CPT rigs could access the site). At the same time underground pipes and cables were limiting the start positions of the CPT’s as well. During CPT testing, the cone deviated from the intended vertical profile. As a result, the actual distance between the CPT trajectory and the piles was larger than intended, making pile detection more difficult and less certain. This illustrates the need for CPT’s that allow for adjustment of the penetration


Figure 4 - The model in 3D

Figure 3 - The analysis from magnetic distortion to the 3D model

old pilesheetwall

vertical collision

exsample excavation core diameter

projected anchors

angle to be able to compensate for unwanted deviation of the cone. Because in most complex situation the inverse function does sometimes returns a valid result after manual locating the anchor by monkey testing.

sandlayor at-10m

Presentation of the 3D model As soon as object are situated below the surface it becomes difficult to explain the wher abouts to the client and contractor. In complex situations like the project Julianaplein in the Hague Central station a digital 3D modal can offer a solution. By using a hololens, the 3D model can be physically projected onto and in the saubsurface of the project location. The Hololens projects the hidden objects like a overlay at the building site. The hololens of Windows works wireless and gives the opportunity to view the situation from every potion in the building site. It is also possible to scale to a room size model. The 3D model itself is a DXF export of Potent. Steeringcone As noted, currently the reliability of the magnetic CPT method is limited by the uncontrollable inclination of the CPT cone during its way through the soil. A magnetic CPT profile has ideally a straight vertical profile since the sensitivity is limited to a few meters in diameter. Due to the lack of a steering mechanism on a CPT cone, deviation from the vertical position of the normal (non-magnetic) CPT profile is accepted as long as the deviation is not exceeding 14 degrees tilt angle. This means that a deviation of a few meters over a typical length of 25 m is tolerated for most geotechnical surveys. In case of a magnetic survey, such deviation can lead to the unwanted blind spots in the surveyed area. To improve the CPT survey technique, Rotterdam Engineers have developed a prototype for a steering tool. The basic principle

Figure 5 - The Hololens presentation of the 3D model at the actual building site Julianaplein the Hague.

has been inspired by HDD (horizontal directional drilling) techniques and has already been conceived in 2013. Only recently the mechanism and the manufacturing has been successfully developed into a working prototype. In the near future, the prototype steering tool will be tested on site. The Steeringcone will enable to keep the tilt angle within a few degrees and the horizontal profile de-

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GEOTECHNIEK SPECIAL CPTâ&#x20AC;&#x2122;18 - June 2018

viation within a few centimetres. Also during inclined CPT testing, keeping to the planned inclination angle will become possible. The Steeringcone also improves the CPT quality for geotechnical purposes and will probably reduce the risk of CPT rod and cone failure as this chance of failure is coupled to the deviation angle.


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Setting up of a system of standardprocedures for geotechnical investigations in Belgium, with focus on CPT

Introduction In a price driven market the quality of geotechnical investigations is uneven. The Belgian member society BGGG-GBMS of ISSMGE, therefore decided to install a TaskForce (TF) that would tackle the problem. As CPT are the primary tests performd in Belgium (first CPT equipment used in Belgium in 1939 ) the drawing up of a standard procedure for CPT jobs was set out as a primary goal of the TF. The members of the TF are representative for the actors concerned: the academic world, industry, geotechnical contractors, engineering offices, governmental agencies, research centres and companies performing CPT. Over the course of the work sessions of the TF,

enhanced insight led to the decision of drawing up not one but a set of several standard procedures for geotechnical investigations : • General Provisions [1], referring to the general principles for planning, execution and reporting of geotechnical investigations, including informative annexes with recommendations for the extent and spacing of investigations. • Cone penetration tests (CPT) – part 1 [2], referring to planning, execution and reporting of geotechnical investigations consisting of CPT. • Cone penetration tests (CPT) – part 2 [3], referring to advice concerning geotechnical design on the basis of CPT. • Borings and sampling - to be drafted. • Laboratory testing - to be drafted. For CPT assignments 2 documents were

Gauthier Van Alboom Chairman of Taskforce on Quality of geotechnical investigations, of BSSMG

drawn up, in order to make a distinction between planning, execution and reporting on the one hand and general qualitative and/or quantitative advice for geotechnical design on the other hand. This was felt necessary as in the Belgium context a CPT report almost always contains this kind of advice, given “free of charge”. The guidelines in these standard procedures are intended as well for the client (contractors, engineering offices, architects) as for companies that perform the relevant tests. They can be downloaded from the website http://www.bggg-gbms.be Belgium being a mainly bilingual country, documents are available in Dutch and French (fig 1).

Tabel 1 - Types of assignment for geotechnical investigations Type of assignment

Scope of assignment

Content of report

Required qualification

G1

Execution of geotechnical investigation + reporting of test results

Test results and directly derived values (e.g. friction ratio from CPT)

Geotechnician*

G2

G3

G4

Execution of geotechnical investigation + reporting of test results + deskstudy + evaluation of test results in terms of need for further testing.

• Test results and directly derived values • Results of consulted relevant sources of information (geotechnical, geological and hydrogeological data) • Recommendations for further testing

Global geotechnical investigation assignment • Deskstudy • Planning and execution of investigation • Reporting test results • Global evaluation of test results • Determination of baseline parameters **

• Test results and directly derived values • Results of consulted relevant sources of information (geotechnical, geological and hydrogeological data) • Recommendations for further testing • Global evaluation of investigation • Baseline parameters for geotechnical design

Geotechnical control tests/monitoring: • Planning and execution of control tests • Planning and execution of monitoring

• Results control tests • Results monitoring

Geotechnician*

Geotechnical expert*

Geotechnician*

* With respect to the required qualification of the person in charge of the geotechnical survey a distinction is made between geotechnician and geotechnical expert (see textbox) ** Baseline parameters are lower bound values of geotechnical characteristics, embedded in a safe approach of the design (see further)

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GEOTECHNIEK SPECIAL CPT’18 - June 2018


Abstract

In order to assure a better and uniform quality for geotechnical investigations in Belgium the Belgian member society of ISSMGE decided to draw up a set of several standard procedures for geotechnical investigations, and a TaskForce to tackle the problem was created. As CPT are the primary tests in nearly any soil survey in Belgium the drawing up of a standard procedure for CPT jobs was set out as a primary goal of the TF. This paper describes the general approach of assignments for geotechni-

cal investigation and design (the General Provisions document), and the 2 CPT documents referring to respectively the planning, execution and reporting of CPT-jobs, and the general qualitative and/or quantitative advice for geotechnical design based on CPT results.

Figure 1 - Covers op standard procedures Cone penetration tests part 1 and 2. Registration of geotechnician/geotechnical expert by BGGG-GBMS and BSIGRM (Belgian Society for Engineering Geology and Rock Mechanics) Within the framework of the standard procedures for geotechnical investigation, the need to provide the required qualification for the author of the report (in most cases also the person in charge of the survey) was recognised. Therefore the implementation of a registration system for geotechnicians and geotechnical experts was initiated. • A geotechnician is responsible for drawing up and reporting of the geotechnical investigation, differentiated for different type of tests (CPT, sampling, geophysiscal tests, laboratory tests …) • A geotechnical expert is responsible for evaluation of the geotechnical investigation, and for geotechnical consultancy and design, differentiated for the geotechnical category of the construction concerned.

Tabel 2 - Types of assignment for geotechnical design Type of assignment

Scope of assignment

D1

Advice regarding geotechnical design; this type of assignment is further differentiated into 4 subcategories, with respect to whether qualitative or quantitative advice is given, and also with respect to the geotechnical category (in accordance with EC7) of the considered project.

D2

Full geotechnical design of the construction.

D3

Geotechnical support during construction.

D4

Implementation and follow-up of geotechnical monitoring.

General Provisions document The General provisions document defines the different types of assisgnments for geotechnical investigations (G1 to G4). The scope of the assignement, the content of the report and the required qualification for the person in charge of the survey are summarized in table 1. For the proposal of different types of assignments the French approach of “Missions d'ingénierie géotechnique” [4] was an inspiration source.

A document with the criteria and the procedure for granting a registration as geotechnician or geotechnical expert by the BGGG-GBMS and BSIGRM is being drafted. Start of implementation is expected in 2019.

The General Provisions document also defines different types of assignments for geotechnical design. The main topics are summarized in table 2.

•  consult relevant sources of information (geotechnical, geological and hydrogeological data, history of the site); • chose the type of test in accordance with the predominant failure mechanism(s); •  chose the number of tests in accordance with the homogeneity/heterogeneity of the subsoil and the risk level of the project.

As stated the general provisions also give guidance for the extent of a geotechnical survey (type and number of tests) for different type of constructions. In order to set out the extent of the investigation one should: •  define the geotechnical category of the construction project;

The type of test to be executed (CPT, sampling, laboratory tests …) is determined on the basis of scores attributed to the risk level of the construction (e.g. depth of excavation), the soil profile (homogeneous/heterogeneous, presence of soft clays, peat), distance to constructions sensitive to settlement …

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GEOTECHNIEK SPECIAL CPT’18 - June 2018


Figure 2 - Screenshot DOV website with locations op CPT and borings (orange and green dots).

o  presence of backfill material, waste materials; o presence of foundation remnants; o presence of contaminated soils.

• Specific data concerning the extent of the CPT survey: o number of tests; o location of tests (plan with X,Y-coordinates in general coordinate system and absolute elevation Z); o depth to be reached (or required thrust capacity op CPT-equipment); o CPT-type (electrical, piezocone, mechanical) and application class; o use of friction reducer (imposed or not); Information to be obtained by CPT-company prior to execution of the CPT: • the location of public utility pipes in the vicinity of the test location; • for assignments type G2 and G3 the CPT-company shall consult relevant sources of information (geotechnical, geological and hydrogeological data, history of the site). • a site reconnaissance giving an indication of practical problems to be expected during the survey (this is however optional, depending on the assignment given by the client).

For the number of tests recommended values are given , depending on the nature of the geotechnical problem, geology and soil conditions.

and/or solicitation and limited risk level and construction projects of medium to great size and with medium risk level.

This recommendations are intended to give guidance, but the engineering judgment and competence of the geotechnical expert remain of paramount importance.

For the former, requirements are less strict (e.g. mechanical CPT are also accepted while for the latter only electrical CPT may be used). This is done so as to make no unrealistic requirements for CPT-jobs for simple projects like dwellings.

Standard procedures for CPT – part 1 The standard procedure for CPT – part 1 gives guidance for planning, execution and reporting of CPT soil investigations. Responsibilities are clearly assigned, either to the client (or its representative) or to the CPT-company. In addition an order form resp. a standard specification format for a CPTassignment is given. In the standard procedures distinction is made between construction projects with limited size

Planning phase Information to be submitted by client prior to execution of the CPT: • Accessibility (use of CPT-truck, track-truck or light weight CPT system) and attainability of the test site. • History of the site, relevant for the execution of the CPT such as: o presence of underground utility pipes on private domain;

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GEOTECHNIEK SPECIAL CPT’18 - June 2018

Execution phase CPT are to be executed and test results reported in accordance with ISO-standards EN ISO 22476-1 (electrical and piezocone CPT) and EN ISO 22476-12 (mechanical CPT). Further following information is to be reported: • cone type; •   reference readings of the measured parameters, before and after test;
 • time registration during test; • calibration data of cones and sensors (to be provided if requested by client); CPT-results in digital form are to be added to the report. Standard procedure for CPT – part 2 The standard procedure for CPT – part 2 gives guidance for general qualitative and/or quantitative advice for geotechnical design on the basis of the CPT test results. CPT assignments type G2 and G3 can be combined with assignments type D1 (Geotechnical advice regarding geotechnical design); this type of assignment is further differentiated


into 4 subcategories, with respect to whether qualitative or quantitative advice is given, and also with respect to the geotechnical category (in accordance with Eurocode7) of the considered project). The report of a type D1 assignment for ge-

otechnical design, shall anyway clearly be separated from the report of the corresponding assignment of geotechnical investigations type G2 or G3. Essential topics to be dealt with in assignments type G2 are:

Figure 3 - Site plan with CPT locations of the actual soil investigation and available test locations.

• desk study prior to the planning and execution of the CPT; the aim is to specify adequately the number and location of tests, taking advantage of available test data, knowledge of the geology of the project site, relevant historical data (e.g. presence of gullies due to major breaching of dikes, filled in ancient canals and ditches …); in Belgium geotechnical data (CPT results, boring logs, water level measurements …) can be downloaded from the respective sites Database of the subsoil of Flanders (DOV) for Flanders and Géoportal de la Wallonie for Wallonia; fig 2 shows a screenshot from the DOV website. • reporting of test results as stated in the CPT-1 document •  evaluation of CPT test results, compared and matched with existing test data in the vicinity and geology of the site. •  A synthesis of the soil investigation program; • Proposal for extra CPT (type, number, location and depth) and/or other tests if deemed necessary; this is substantiated with analysis of the executed tests and available information (homogeneity/heterogeneity of the soil profile, any anomalies, disruptive elements (e.g. filled in ditches and canals, gullies …) Essential topics to be dealt with in assignments type G3 are: •  Desk study prior to the planning and execution of the CPT, according to requirements type G2

Figure 4 - Linear ground profile with 2 homogeneous zones and representative CPT profiles, CPT1 and CPT2

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GEOTECHNIEK SPECIAL CPT’18 - June 2018


Tabel 3 - Table from Belgian National Annex to EC7 qc (MPa)

Rf (%)

γk above GWL ** (kN/m³)

γk beneath GWL** (kN/m³)

ϕ’k (°)

c’k (kPa)

cu,k (kPa)

Soil type

compacity/ Secondary soil type consistency

Gravel*

-

medium dense dense

10 – 20 > 20

<1

18 19

20 21

35 40

0 0

-

Silty or clayey

medium dense dense

10 – 20 > 20

1-2

19 20

21 22

32 37

0 0

-

-

loose medium dense dense very dense

2-4 4-10 10-15 > 15

<1

16 17 18 18

27 30 32 35

0 0 0 0

Silty or clayey

loose medium dense dense very dense

2-4 4-10 10-15 > 15

1-2

16 17 18 19

18 19 20 20

25 27 30 32

0 0 0 0

-

-

soft medium stiff stiff very stiff

0.4-1 1-2 2-4 >4

2-4

17 18 19 20

17 18 19 20

22 22 22 22

0 2 4 8

10 25 50 100

sandy

soft medium stiff stiff very stiff

0.4-1 1-2 2-4 >4

1-3

17 18 19 20

17 18 19 20

25 25 25 25

0 2 4 8

10 25 50 100

-

soft medium stiff stiff very stiff

0.4-1 1-2 2-4 >4

3-6

16 17 18 19

16 17 18 19

20 20 20 20

2 4 8 15

20 50 100 200

sandy

soft medium stiff stiff very stiff

0.4-1 1-2 2-4 >4

2-5

16 17 18 19

16 17 18 19

22 22 22 22

2 4 8 15

20 50 100 200

medium stiff stiff very stiff

0.2-0.5 0.5-1 >1

>6

10 12 14

10 12 14

15 15 15

2 5 10

10 20 40

sand

loam

clay

peat

18 19 20 20

-

* For natural gravel deposit; for backfilled gravel γ’k = 35° should be adopted. For temporary constructions a limited cohesion can be adopted, provided substantial motivation and arrangements for control. ** GWL = freatic groundwater level

•  reporting of test results according to requirements type G1 • synthesis of soil investigation and proposal for extra tests according to requirements type G2 • determination of baseline parameters for geotechnical design; Baseline parameters are lower bound values of geotechnical characteristics, embedded in a safe approach of the design. The choice of these parameters take into account possible limitations (type and number of tests) of the

soil investigation, and possibly whether or not adequate monitoring was provided. These baseline parameters are therefore to be considered as guide values. They can be maintained or adjusted when a complete design study is carried out. Shear strength characteristics on the basis of CPT values, as given in the table of the Belgian National Annex of Eurocode 7, are an example of such baseline parameters (table 3).

ing to values for density and shearing resistance parameters, that may be considered as characteristic values.

Representative qc-values for each considered layer are entered as input in the table, lead-

Providing the ANB-table has a double finality: •  on the one hand these conservative

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GEOTECHNIEK SPECIAL CPT’18 - June 2018

Values for deformation characteristics can also be derived from CPT-values, but one should be aware of the fact that these are only approximate, and do not take into account the stress dependency of deformation characteristics.


assumptions are in accordance with the provisions of Eurocode 7, as a limited geotechnical investigation (with no laboratory testing) corresponding to a higher level of uncertainty, should result in safer/ more conservative values for geotechnical characteristics • on the other hand the geotechnical expert can propose better geotechnical characteristics if substantially justified; if not the values proposed in the ANB table prevail. Assignments D1 for geotechnical advice a) Assignment type D1.1 General qualitative advice concerning foundation concepts, their feasibility, main concerns (e.g. presence of layers sensible to settlement), necessity of groundwater lowering, … without however going into detail and without calculations or quantitative substantiation. This qualitative advice is relative to the considered construction project (e.g. number of floors of building, underground construction levels …), and specific features of the project. One of the objectives of the implementation of the CPT standard procedures was to do away with passe-partout design advice and calculations, that were meant to cover a range of hypothetic foundation systems. b) Assignment type D1.2 This type of assignment results in an evaluation of the CPT results and in a qualitative advice (as described in D1.1), along with exploratory calculations in support of the qualitative advice. The evaluation of the CPT test programme takes into account the results of the CPT test programme and also the results of available test data, knowledge of the geology of the project site, and relevant historical data, as reported in the desk study. Fig 3 shows a site plan with locations of new and already available CPT. Based on these data the project site is divided in one or more geotechnically homogeneous zones, each zone represented by a corresponding representative CPT profile. To do so one or more linear ground profiles may be drawn up, resulting in the above mentioned geotechnical zoning. Fig 4 shows an example of linear ground profile, with representative CPT for corresponding homogeneous zones and proposed soil layering.

For each soil layer appropriate geotechnical baseline parameters are attributed, as stated above. Subsequently calculations for bearing capacity (shallow or deep foundations) and settlement analysis are made for each representative soil profile. This evaluation can also result in a detailed proposal for additional tests (as well CPT or other in situ and laboratory tests). Conclusions By setting up a system of standard procedures for geotechnical investigation the BGGG-GBMS wants to assure a good and uniform quality for these surveys in Belgium. The CPT-documents part 1 and 2 (together with the general provisions document) are an important first step towards this goal. CPT are after all, for nearly eighty years, the primary tests performed in any geotechnical investigation (if not inappropriate due to hard soil or rock conditions). A framework for quality assurance in geotechnical investigations has been drawn up, with clearly assigning responsibilities to parties concerned. The standard procedures are to be referred to in the Belgian National Annex of Eurocode 7, and by doing so enforced for use in professional practice. The corner stone of this project is the registration procedure for the persons responsible for the geotechnical surveys and their evaluation, as there are the geotechnician or the geotechnical expert. A document with the criteria and the procedure for granting a registration as geotechnician or geotechnical expert by the BGGG-GBMS and BSIGRM is being drafted. Start of implementation is expected in 2019. The author wishes to thank the Board of BGGG-GBMS and the members of the Taskforce for their contribution and support to accomplish this task. References [1]  Standaard procedures voor geotechnisch onderzoek: algemene bepalingen. (also available in French) http://www.bggg-gbms.be [2]  Standaard procedures voor geotechnisch onderzoek: sonderingen – Deel 1: Planning, uitvoering en rapportering. (also available

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GEOTECHNIEK SPECIAL CPT’18 - June 2018

in French) - http://www.bggg-gbms.be [3  Standaard procedures voor geotechnisch onderzoek: sonderingen – Deel 2: Geotechnisch advies bij het ontwerp. (also available in French) - http://www.bggg-gbms.be [4]  French standard NF P 94-500 Missions d'ingénierie géotechnique [5]  Databank Ondergrond Vlaanderen-DOV https://www.dov.vlaanderen.be [6]  Géoportal de la Wallonie http://geoportail.wallonie.be/home.html


50 years in the forefront of innovated CPT technology

In 1968 Arie Pieter van den Berg founded A.P. van den Berg in Heerenveen which focused on the design and production of machines and equipment for Cone Penetration Testing (CPT). This by origin Dutch soil investigation method was still in its infancy and Arie van den Berg assumed the role of ambassador for this technology. In a relatively short time, he contributed to a worldwide acceptance of the method and the associated CPT technology and managed to build a strong international reputation for his company. In 2018, the year that the company celebrates its 50th anniversary, this reputation still stands. The drive for innovation, the high product quality and in-house engineering and production are main success factors behind their leading market position. The introduction of the very compact and fully automated SingleTwist™-system, attests the focus on innovation once again.

All it takes to put together an automated CPT system, i.e. one that performs a full CPT and a subsequent complete reel-in of the string on a single operator command, is a Folder, a Twister and a Sprocket (a wheel to connect the two). This very compact SingleTwist™ system can be combined with virtually any A.P. van den Berg pusher, including onshore COSON penetrometers and ROSON seabed systems. Needless to say that, since the ST-rods come fitted with an Icone cable, users enjoy the full bandwidth of A.P. van den Berg’s digital bus and become compatible with all Icone sizes and all available click-on modules: Seismic, Conductivity, Magneto & Vane. COSON-ST: fast & hands-free CPT machine for onshore soil investigations The COSON-ST builds on the unrivalled track record of the Icone technologies and large range of CPT rigs including the Track-Truck®, an A.P. van den Berg invention. Through the years each design is continuously improved, i.a. resulting in developments that enhance the operator’s working conditions. One example is the COSON double acting penetro-

ST-rods: the (un)folding CPT string The basis of the SingleTwist™-system consists of the (un)folding ST-rods. These rods can be in one of just two states, twisted or untwisted, and it takes very little to transform them from one state to another. Untwisted, the ST-rods behave a bit like a pearl necklace and can be easily folded onto a reel. Once twisted, the rods form a solid CPT string with a push/ pull/buckle performance equal to a string of standard CPT rods. Because the rods have a self-seeking bayonet thread they close effortlessly and require just a final, short, single twist to become firmly interlocked, hence the name SingleTwist-rods. Having rods with this patented design, the transformation process during CPT pushing as well as pulling is like a natural flow and done almost entirely mechanically, making it very robust indeed.

20

GEOTECHNIEK SPECIAL CPT’18 - June 2018


meter for continuous pushing in combination with the automatic rod screwing device, that will fasten the rods automatically once the rod is placed in this device. This combination reduces handling requirements for the operator. Hence, less physical strain. The remaining challenge has been to create a fast CPT system that does not require any manpower at all, has a compact design to fit a CPT cabin and is compatible with all Icone click-on modules. The COSON-ST is the answer. By integrating the patented SingleTwist™system into a CPT cabin with the COSON penetrometer, a fast CPT machine for hands-free operations is created. Several folder sizes are available holding up to 70 m of CPT string. The COSON-ST does not require any manpower during the push/pull cycle of a CPT. The operator only has to provide the start/stop signals. The pushing process is continuous, generally resulting in shorter pushing times, increased penetration depths and no dissipation effects associated with a single clamp’s discontinuous pushing process. The pulling process with the COSON-ST is also 20% faster than with a single clamp system, so the total CPT cycle time is reduced considerably.

ROSON-ST: up to 50 m CPT with the compact & easy to handle seabed CPT system The ROSON-ST builds on the unrivalled track record of the existing ROSON and Icone technologies. The digital Icone has been around for almost 15 years and the ROSON technology has proven itself for robustness, reliability and CPT quality over the last 35 years. The remaining challenge has been to eliminate the need to support the CPT string, particularly with ever increasing CPT depths. This external string support makes a ROSON system heavier, more difficult to handle and time consuming to set up. Compact solutions available today introduce shortcomings of their own, concerning achievable penetration depth, data reliability, cone sizes to choose from and increased wear and tear. The ROSON-ST, the integration of the patented SingleTwist™system into the proven ROSON system, is the answer to these issues. Through the compact design, the ROSON-ST is easy to handle from most vessels. It can be deployed for projects from shallow to ultradeep water. Both a 1,500 m version as well as a deep water version for water depths up to 4,000 m are available. The ROSON-ST does not

21

GEOTECHNIEK SPECIAL CPT’18 - June 2018

require any exterior CPT string support, assuring fast deployment and high productivity. The use of the patented ST-rods ensures the straight push that may be expected from a CPT system. It is suitable for 50 m penetration. The standard configuration with a 10 tons pushing force will be more than enough for the majority of your projects, but a 20 tons version is also available. “Live” experience of the unique system During CPT’18 on 21 and 22 June in Delft (the Netherlands) A.P. van den Berg will be demonstrating the COSON-ST, so everyone is cordially invited for a “live” experience of this unique system. Interested attendees of CPT’18 can simply visit A.P. van den Berg’s exhibition space (both inside and outside). Those who are not attending the conference, but are interested in a demo, can make an appointment by contacting. Johan de Lange (j.delange@apvandenberg.nl) or Eddy Kuiper (e.kuiper@apvandenberg.nl).


Abstract

Download all back issues of Geotechniek

The Dutch company MI-Partners and the Technical University of Delft are two partners in the European Consortium NeTTUN. This consortium consisting of 21 partners has the goal to significantly improve tunnel boring. MI-Partners and the TU Delft will develop a system that generates a map of the soil in front of the boring head. Using this map the tunnel boring process can be made more robust and safer.

Table 1

Surface vibrator

TBM vibrator

Use Environment Dimensions Positioning

Stand-alone Atmospheric; open air ‘Unlimited’ Manual

Typical mass

Baseplate: 200 kg Reaction mass: 1000 kg

In TBM >> 5 bar; > 50°C; dirt Limited by TBM dimensions Automatically; retraction during excavation Baseplate: 50 kg Reaction mass: 80 kg

sensors and vibrator into the image map of the ground.

Read all recent articles online Figuur 3 - Schematic picture of the vibrator and sensors mounted on the TBM. The force wave is sent by the vibrator and its reflections are sensed in various ways depending on the soil structure.

thermore all obstacles that are present in the ground should not damage the vibrator and the sensors. Hereto, predictive modelling is used, where the behaviour of the system under these various circumstances is modelled e.g. in Finite Element models.

The biggest challenge is to combine the precision equipment and sensitive measurement devices into the harsh environment of a tunnel boring Promote your company, product or expertise machine. The vibrator and the sensors should have to work accurately under a wide range of environAt the end of this year a stand-alone prototype mental properties. The local temperature and TBM vibrator will be finished which will be tested pressure can change over a wide range, and furin the field during 2014. At the end of 2016 the

system should be fully integrated onto the TBM which will lead to a safer way of making tunnels.

Acknowledgements This research is part of the NeTTUN project, which receives funding from the European Commission’s Seventh Framework Programme for Research, Technological Development and Demonstration (FP7 2007-2013) under Grant Agreement 280712. www.nettun.org 

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Geotechniek juni 2018 - CPT special  

Onafhankelijk vakblad voor het geotechnische werkveld - Geotechniek CPT'18 Special

Geotechniek juni 2018 - CPT special  

Onafhankelijk vakblad voor het geotechnische werkveld - Geotechniek CPT'18 Special

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