Quality assurance in Jet Grouting for a deep seated slab

1.1.11

Ground Improvement Techniques

Quality assurance in Jet Grouting for a deep seated slab in Amsterdam Dipl.-Ing. Thomas Passlick, Keller Grundbau GmbH, Bochum, Dr.-Ing. Karsten Doerendahl, Keller Grundbau GmbH, Maintal,

Presented by

Keller Grundbau GmbH Kaiserleistraße 44 D-63067 Offenbach Tel. +49 69 8051-0 Fax +49 69 8051 244

Piling and Deep Foundations - Conference, Amsterdam, 31.05.-02.06.2006

E-mail: [email protected]

www.KellerGrundbau.com

1.1.17

Technical paper 68-55 E 1.1.16

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Quality assurance in Jet Grouting for a deep seated slab in Amsterdam Dipl.-Ing. Thomas Passlick, Keller Grundbau GmbH, Bochum, Dr.-Ing. Karsten Doerendahl, Keller Grundbau GmbH, Maintal,

By means of further developed machines and techniques and with growing experiences in jet grouting, the challenge of this building method becomes more and more complex. To keep the concerning risks as low as possible, the requirements on the quality assurance continuously increase. The construction of a 37.0m deep seated slab for the Amsterdam metro station Vijzelgracht in marine clay represents such a complex task. This paper summarises the required measures of quality assurance. In particular a new hydraulic measuring device is introduced with which it was possible to detect the column diameter of 2.6m at a depth of 37.0m during the production process and with a very high accuracy. 1.

Introduction

The production of a deep seated slab using the jet grouting method surely is one of the most challenging fields of application for this method. Due to this the risks are very high. These risks are based on the method itself (e.g. as in most cases the treated soil cannot be controlled visually) and on the special site conditions influencing the production parameters. If columns have to be made at large depth and in a soil which is hard to erode, the quality assurance is very important in the phases of design and construction. It helps keeping the risks at a low level and thus makes some project possible to be realized. Last but not least the collapse of the Nicoll Highway building pit shows quite plainly the possible effect of disregarding the quality assurance, although this was not one of the main reasons. The measures of quality assurance described in this paper concern a special building project and do not have a general character. They intend to demonstrate the measures that are necessary to achieve and verify a column diameter of 2.60m at a depth of 37m in marine Eem clay.

Dipl.-Ing. Thomas Passlick, Dr.-Ing. Karsten Doerendahl, Keller Grundbau GmbH

2.

The jet grouting process (SOILCRETE®)

The jet grouting process is recognized as a cement soil stabilization. The soil around the borehole is eroded with the aid of air-shrouded high pressure cutting jets of water or cement suspension having a nozzle exit velocity ≥ 100m/sec. The eroded soil is rearranged and mixed with the cement suspension. The soil-cement mix is partly flushed out to the top of the borehole through the annular space between the jet grouting rods and the borehole. Different geometrical configurations of elements can be produced. The erosion distance of the jet varies according to the soil type, the kind of process and the jetting fluid and may reach up to 2.5m in case of using the standard methods. In contrast to the conventional ground stabilization methods jet grouting may be used for stabilisation and sealing of all kinds of soil ranging from loose sediments to clay. This also applies to non homogeneous soil formations and changing soil layers, including organic material. Three different jet grouting methods exist. The method to be used is determined by the prevailing soil conditions, the geometrical form and the required quality of the jet grouting elements. Single System (Soilcrete®–S) Single direct process operates with a grout jet of min. 100m/sec. exit velocity for simultaneous cutting and mixing of the soil without an air shroud. The Single System is used for small to medium sized jet grout columns. Double System (Soilcrete®–D) Double direct process operates with a grout jet of min. 100m/sec. exit velocity for simultaneous cutting and mixing of the soil. To increase the erosion capability and the range of the grout jet, air shrouding by means of shaped air jet nozzle is used. The Double System is mainly used for panel walls, underpinning and sealing slabs.

Fig. 1: Application limits for grouting techniques

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Quality assurance in Jet Grouting for a deep seated slab in Amsterdam

Triple System (Soilcrete®–T) Triple separation process erodes the soil with an air shrouded water jet of min. 100m/sec. exit velocity. Grout is injected simultaneously through an additional nozzle located below the water jet nozzle. The grout pump pressure ranges above 15bar. An alternative to this process operates without air shrouding of the water jet. The Triple System is used for underpinning works, cut off walls and sealing slabs. Since the 1990s some new improvements in jet grouting techniques were made. One of them is called “Superjet” and is based on the focus of the grout jet in order to reduce the loss of energy within the system. Thus larger columns with diameters up to 5m and more are possible. Those large diameters not only represent a technical improvement. They also increase the safety of deep seated slabs as mentioned later. With this technique a diameter of 2.6m in the Amsterdam marine Eem clay could be realized. 3.

The project

As part of the Noord-Zuid lijn building project Max Bögl got the assignment to build the three metro stations Vijzelgracht, Ceintuurbaan and Rokin in a cut-and-cover method. For the deep seated slab of the Vijzelgracht metro station Max Bögl instructed a joint venture “KSS” consisting of KELLER Funderingstechnieken B. V., STUMP Funderingstechniek B. V. and SMET F&C. The original design concept included small diameter columns produced from under the cover. Due to the Keller Grundbau experience, the KSS proposal could change the diameter to 2.6m to be produced from surface level. This led to a significant reduction of production time and costs. For the continuation of the public transport the metro station has to be built in two phases. The first phase was done by KSS on an area of 15m x 250m with three large drilling rigs, each of them with a height of 33m. All machines and equipment necessary for the production with the three rigs had to be placed on this area. Therefore a skilful logistic work was needed. The soil layers at the Vijzelgracht metro station are shown in Fig. 2. The Jet grouting slab is located in the marine Eem clay layer, characterized by VAN DER STOEL (2001) as follows: Marine Eem clay, BS class CL, grey-green, limitedly silty to limitedly sandy, γdry = 13.1 kN/m³, γwet = 17.9 kN/m³, n = 0.5, e = 0.99, w = 36%, wL = 41%, wP = 24%, Ip = 17% The marine Eem clay was formed through sedimentation in the sea. It has a loose structure with a high water content. The mechanical properties of the clay depend on its load history. For jet grouting work in the marine Eem clay the following aspects have to be considered: • The clay can better be cut by a jet of water than by a jet of grout. • It may lead to a high viscosity of the waste slurry, which can cause problems especially at large depth. • As the soil is always part of the column, the clay reduces its strength and its stiffness. • In order to maintain both a high strength and large cutting energy, it is recommended to separate the working phases “jetting” and “grouting”.

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Fill Aanvulling Tidal deposits Holoceen

(clay, peat)

1st sand layer

1e zandlaag Intermediate Alleröd

layer

2e 2ndzandlaag sand layer

eemklei Eemclay tussenzandlaag Intermediate sand

layer

glaciale Glacial klei clay

rd

3 zandlaag sand layer 3e Fig. 2: Soil layers at the Vijzelgracht metro station Due to the fact that the column grid was designed by the client, the quality requirements were very detailed. • All production parameters (as time, depth, lift rate, speed of rotation, drilling pressure, grout pressure, grout flow, air pressure and air flow) had to be recorded online during the production process, analyzed, verified and handed over within the next production day. • The required strength should be larger than 5.5N/mm² and the required stiffness larger than 2500N/mm². • The deviation of the starting point for the drilling had to be smaller than 0.05m. • The original deviation of the drilling itself had to be smaller than 0.5%, checked at 5% of the columns. • 1% of the slab had to be tested by corings, which led to 13 corings with 39 samples. • The diameter of every 50th column had to be verified by measurements. 4.

The measures of quality assurance

In order to keep the disturbances on the production process on a low level (and consequently the costs) and to accomplish the high quality requirements, a consistent concept for the quality assurance was necessary. This included a test field with several test columns to find out the right jetting parameters as well as a requirement for an electronic on-board

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monitoring equipment to record, interpret and visualize the relevant parameters during the production process. In addition, a measuring system was necessary to verify the position and the diameter of the jet grout column directly after the production phase to guarantee a permanent constancy of fulfilling the quality requirements. To point out the importance of the topic, a short look on the collapse of the Nicoll Highway on April 2005 should be done, without giving a complete explanation of the main reasons. The following facts clearly indicate that the level of quality assurance was definitely too low for such a huge building project: There was neither electronic on-board monitoring equipment nor a continuous proof of the achieved column diameter except for the rare coring done at every 1000 linear meters of jet grouting. A long time before the construction failed, the measured deflections of the diaphragm wall showed that the compressive strain in the treated soil have far exceeded the design parameters. This should clearly alert us to the importance of the quality assurance during the design and production of both types of slabs, the watertight one and the static one. This is valid particularly for jetting at large depth and in cohesive soils. 4.1

Field of test columns

In the area of the Rokin metro station several test columns at a depth of 32m were made to verify the assumed parameters for the jet grouting and to verify the column diameter respectively. As a result the required drilling deviation had to be changed from 0.5% to 1%. Due to the geology and the obstacles in the ground a deviation of 0.5% could economically not be realized. Furthermore the standard jet grouting methods (Double System) did not show the same results at the Rokin metro station as in the PIP test columns mentioned by VAN DER STOEL (2001). To reach the same diameters much higher energy would have been needed. With the Superjet method diameters > 2.60m could have been produced. Additionally these test columns showed that the new measuring device used by KELLER was able to clearly identify all the produced column diameters. 4.2

The choice of production parameters

As mentioned above, several jet grouting methods exist, each one having its own advantages and disadvantages. It is the client´s and the experienced contractor’s responsibility to choose the right method that represents the most economic alternative of low risk. On the one hand the mechanical properties of the treated soil (e.g. homogeneity, strength, stiffness) depend on the jet grout method and its parameters, on the other hand this depends on the local soil conditions, as the soil is always a component of the jet grouted mass. In view of the jet grouting slab in Amsterdam, this means a reduction of the strength due to the marine clay unless the production process will be separated into two phases. The first is cutting the clay with a grout of low cement content and afterwards filling the volume with a grout of high cement content. This can be done in one step using the triple system or in two sequenced steps using the single or double system. At the metro station Vijzelgracht the clay was first cut with a modified double system (Superjet) in two sequenced phases. This guaranteed the required combination of a large column diameter with a high strength in the treated soil. 6


Basically there are several different machine parameters needed to achieve a certain hydraulic energy, so several combinations of parameters are possible to produce the same hydraulic energy. Finding out the right combination of parameters requires a lot of experience. Field tests should be made to verify these parameters. At the metro station Vijzelgracht an on-board electronic monitoring equipment was used to record, interpret and visualize the following production parameters: time, depth, lift rate, speed of rotation, drilling pressure, grout pressure, grout flow, air pressure and air flow. Due to a clearly arranged visualization a prompt evaluation could be done for each single column. So the flexibility was given to be responsive to discrepancies and thus to adjust the production process. Jet grouting is defined by cutting the soil with a very high pressure. This assumes that a permanent flow of the grout-soil mixture (spoil) has to be monitored to prevent heaves or settlements of the adjacent buildings. The viscosity of the spoil again depends on the chosen production method and the soil conditions, so that this marks another important parameter to be attended. As mentioned above, the marine clay poses a challenge at this point. Due to the application of the Superjet method and the right combination of production parameters column with a diameter of 2.60m at a depth of 37m could be realized. The results of several corings indicated that the required values for the strength and stiffness had been achieved.

Fig. 3: Visualization of the production parameters

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4.3 The column grid and the drilling alignment tolerance The general difficulty in designing a column grid is to find the right column distance, neither too large nor too small. When the column distance is too large, gaps between the columns may appear. When the column distance is too small, the drilling for the column may end in a neighbour column which has already been made. So this jetting leads to unsuccessful work without a complete column or no column being formed. The size of the columns therefore not only influences the total number of columns, but it strongly influences the risks of gaps and shadows. The larger the diameter of the column the smaller are these risks. Consequently the chosen diameter of 2.60m represents the most economical and reliable solution.

Fig. 4: Part of the column grit The column grid required a maximum drilling alignment tolerance of 1.0% of the depth. To fulfill this requirement, the drilling method, the drilling speed and the stiffness of the rods had to be adapted to the soil conditions. Finally, a medium deviation of 0.6% had been achieved. 4.4

Identification of the column diameter

One of the most important and demanding challenges within the quality assurance for jet grouting is the evidence of the correct column diameter. It is not only the degree of difficulty which increases exponentially with the depth of the column, but also the amount of time and costs, as most of the time an elaborated measuring method leads to a decrease of productivity. The oldest and most reliable method to determine the column diameter is the production and excavation of test columns. Obviously this is only economical for small depths, because this method needs a lot of time so that it can only be used in advance of the site and not during the production process. 8


Another popular way to identify the column diameter is coring. If the starting point and the deviation of the drilling are measured and the exact position of the column is known, the minimum diameter can be determined. Additionally, a sample out of the column can be taken and tested for strength and stiffness. As the direction and the amount of the deviation can hardly be influenced (neither of the column drilling nor of the core drilling), the accuracy of this method is not always satisfying. Since the result of one core drilling is either “The diameter is bigger than…” or “The diameter is not bigger than…”, several core drillings are needed if the exact diameter is required. The deeper the columns are the lower the accuracy gets. Depending on the depth of the columns and the number of core drillings per column, a certain risk still remains. For hydrophone measurements based on the same method of drilling one or several extra holes close to the column are necessary to identify its diameter. However, this is done before the jetting of the column. If the grout jet then hits a gauge the minimum diameter is given. If another gauge is not hit by the grout jet a range of diameters can be determined. Like the core drillings, the accuracy depends on the depth and the deviation of the column drilling and the measurement drillings. For example at a depth of 37.0m and with a drilling deviation of 1% the additional distance between the column drilling and one measurement drilling (with a distance A at surface level) can diverge in a range from (A + 0.74m) to (A – 0.74m). So, to get a nearly exact value for the diameter a lot of measurement drillings are needed. Another way to identify the diameter of a jet grout column is to use geophysical techniques. Here the diameter of the column should be determined using the differences in conductivity or resistance of the jet grouted mass and the soil. As the jet grouted mass always consists of a certain amount of soil, the complexity of these methods is to clearly define the edge of the column. So the accuracy varies. Unfortunately the development of most of these methods is not complete, so that they cannot be used in practice at large depth and during the production process. The identification of the column edge with a mechanical device is also a well-known method. The benefit of this method is the high accuracy and that it usually obstructs the production process less than the other methods. So far this measuring type could best be used for small and medium sized columns (max. approx. 2.0m) close to the surface level (max. approx. 8.0m). On the building site for the metro station Amsterdam Vijzelgracht a new developed mechanical measuring device has successfully been used for the first time at a depth of 37.0m to identify a column diameter of 2.6m. This device was developed by Keller Grundbau and differs from other devices in a patented hydraulic operation of the two arms: Directly after completion of a column the measuring device is attached to the borehole drill rod and in the retracted position lowered to the bottom of the borehole. On reaching the final depth, the measuring arms are swung out into the fluid column. A hydraulic line through the drill rod connects the measuring cylinder to a further hydraulic cylinder located at surface level. The hydraulic cylinder on the surface operated by a hand pump in such a way that the piston rods of the measuring arm will be extended. A significant increase in hydraulic pressure indicates that the piston rods of the measuring arms have reached the column wall. A scale on the hydraulic cylinder located at surface level shows the total extended movement of the measuring arm and thereby the diameter of the column. The maximum diameter to measure with this device is 4.2m at any depth. 9


Fig. 5: New hydraulic measuring device Since the tool has only got two instead of three arms the direction of the measurement can easily be determined. Therefore, it is also possible to measure secondary columns (next to a column already jetted) and columns next to existing structures (like diaphragm walls etc.) with this device. With the use of this new developed device on the building site for the metro station Amsterdam Vijzelgracht a medium accuracy of the column diameter of ca. 1% was reached with a minimum disturbance of the production process. The required column diameter of 2.6m was confirmed in nearly all the measurements. In summary, it may be said that due to this new technique one of the uncertainties during the construction of this metro station could successfully be erased. In general the costs for the quality assurance may take a significant part of the total jet grouting costs, but the resulting costs of a defective quality might be much higher. Last but not least the client should be conscious of that.

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Literature Essler, R., Yoshida, H., 2004. Jet grouting. Chapter 5 in Ground Improvenemt, 2nd edition, edited by Moseley, M. P., Kirsch, K., London, New York, pp. 160 – 196 Kluckert, K. D., 1996. 20 Jahre HDI in Deutschland – Von den Fehlerquellen über die Schäden zur Qualitätssicherung. Proceedings of the Baugrundtagung, 1996, Berlin, Deutsche Gesellschaft für Geotechnik (DGGT), pp. 235 – 258 Magnus, R., Ming, L. J., Ing, T. C., 2004. Report on the incident at the MRT cicle line worksite that led to the collapse of the Nicoll Highway on 20 April 2004. ministry for Manpower, Singapore Raabe, E. W., Glitsch, W., 2005. Integrität von Düsenstrahlkörpern – Bewertungskonzept für Planung, Ausführung und Kontrollen im Tunnelbau. Proceedings of the 4th Geotechnik-Tag, February 2005, Munich, Lehrstuhl und Prüfamt für Grundbau, Bodenmechanik und Felsmechanik der Technischen Universität München, No 37, pp. 105 – 124 Sondermann, W., Pandrea, P., Dörendahl, K., 2005. Düsenstrahlarbeiten in extremen Tiefen in marinen Tonen bei hohen Festigkeitsanforderungen und Einpassung an bestehende Bauelemente. Proceedings of the 1st Hans Lorenz Symposium, Oktober 2005, Berlin Trunk, U., Breitsprecher, G., 2004. Qualitätssicherung bei der Herstellung von Soilcrete®-Sohlen. Proceedings of the 11th Braunschweiger Deponie- und Dichtwandseminar, March 2004, Braunschweig, Mitteilung des Instituts für Grundbau und Bodenmechanik der Technischen Universität Braunschweig, No 74, pp.249 – 270 Van der Stoel, A. E. C., 2001. Grouting for pile foundation improvement. DUP Science, Delft University Press, Delft, The Netherlands

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Quality assurance in Jet Grouting for a deep seated slab

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