PORR ProjectsWorld of PORR 160/2012

Klaus Meinhard

The Foundation Engineering Division of PORR Bau GmbHInfrastruktur has held an Austrian patent for Tempjet since2009. The European patent will be granted in 2012. Theinnovative Tempjet method allows fast and efficientdetermination of the diameter and cement content of jet-grouted columns – even at great depths. It is no longernecessary to excavate test columns and conduct time-consuming examinations. PORR has also been sellinglicences for the application of the Tempjet method for aboutone year now, and some renowned international companieshave already purchased such a licence. The Tempjet methodhas already been successfully used for quality control in jet-grouting works in Austria, Germany, Hungary, Poland, theUK, Greece, Denmark and Switzerland – either as a serviceprovided by PORR or by licence holders themselves. Acompany based in Cleveland, Ohio which offers qualityassurance services on the global market has also expressedits interest in PORR's Tempjet method.

General informationThe thermal simulation model described in this articleprovides a new method for determining the diameter ofcolumns which are produced by jet grouting, a methodfrequently used in specialised civil engineering. The model isbased on a numerical simulation of temperature changes inhydrating jet-grouted columns and of heat transfer to thesurrounding soil. The properties of the grout used in themethod, the thermal characteristics of the jet-groutingsuspension and the physical and thermal characteristics ofthe native soil constitute the input parameters. A comparisonof the temperature changes at the centre of the column asmeasured at the construction site and of the temperaturecurve that results from the numerical modelling processprovides an indication of the diameter and cement content ofthe column. The key economic advantage of this solution isthat it is no longer necessary to drill additional holes or toexcavate test columns to measure their diameters.Regardless of the method used for producing the columns(single, double or triple fluid jet grouting), excellent results canbe achieved with various grouts and in a wide range ofdifferent soils. The aim of the method is to allow continuousquality assurance on site throughout the entire course of thejet-grouting works.

State of the artThe most reliable way of determining the properties of a jet-grouted column until now has been to produce test columns,which are left to set for at least two days and are thenexcavated (see fig. 1). The European Standard EN 12716,which applies to jet-grouting procedures, also provides forusing this method. A major drawback of

these quality control measurements is that, apart from beingrather time-consuming, they only provide very selectiveinformation about the properties of the jet-grouted elementsas obtained from the measurement points. If jet-groutingworks are carried out at greater depths, an excavation of testcolumns is often neither technically feasible nor economicallydefensible.

Examples of measuring methods which are currently used byspecialised civil engineering companies for determining thediameter of jet-grouted columns include the use of a probe ora mechanical measuring device which folds like an umbrella,erosion measurements at pre-installed level indicators,examinations of the slurry flowing back out at the surface,sound level measurements using hydrophones andultrasound measurements. The main constraints that limit theapplicability of these methods are the characteristics of thenative soil, the geometrical properties of the elements andeconomic considerations.

Fig. 1: Excavated test columnsImage: PORR

Theoretical backgroundA method for determining the diameter of jet-grouted columnsand the material properties of the grout used for jet groutingwas developed in a research project which was carried out byPORR Technobau und Umwelt AG in cooperation with the

TEMPJET: Continuous Quality Control and QualityAssuranceThermal modelling software to determine the diameter of jet-grouted columns

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PORR ProjectsWorld of PORR 160/2012

Institute for Mechanics of Materials and Structures at ViennaUniversity of Technology and completed in 2007. Thismethod combines in situ temperature measurements with asimulation model that describes the exothermic settingreactions of cement-bound materials.

The temperature changes measured at the construction sitegenerally show an initial increase of the column's temperaturedue to the exothermic setting reaction taking place during thehydration of the cement. After 10 to 60 hours, a peaktemperature is reached. The progress of the subsequenttemperature decrease is mainly influenced by the propertiesof the native soil and the dimensions of the column.

Numerical parameter studies have shown that the cementcontent of a column mainly influences the temperature rise atthe start of the setting process (see fig. 2a: Parameter study –temperature changes at the centre of jet-grouted columnshaving a constant diameter of 150 cm, their cement contentvarying from 100 to 500 kg/m3) and that an increaseddiameter results in a later peaking and a slower decrease ofthe temperature (see fig. 2b: Parameter study – temperaturechanges at the centre of jet-grouted columns having aconstant cement content of 300 kg/m3, their diametersvarying from 80 to 240 cm). These effects can also beobserved in in situ temperature measurements.

Fig. 2aImage: PORR

Fig. 2bImage: PORR

Due to these significant differences, a comparison oftemperature changes as measured at the construction siteand the results of numerical calculations provides anindication of the cement content and the diameter of thecolumns.

The quality of the results, i.e. the properties of the jet-groutedelement that are identified by the comparison, very muchdepends on the description of the cement hydration processand of the thermal properties of the soil and the groutsuspension on which the model is based.

Measuring methodThe temperature changes in the columns are determined by ameasuring system consisting of a data logger and atemperature probe.

Commercially available temperature measuring systemswhich allow simultaneous measurements to be taken atseveral temperature measuring cables and automaticallyrecord the results are used as data loggers. PORR hasdeveloped and produced GSM measuring devices for onlinemonitoring to enable automatic recording of measurementresults at construction sites (see fig. 3: Measuring system,consisting of a data logger including a GSM module forremote monitoring, a spare battery and connected thermosensors).

Fig. 3Image: PORR

The temperature probe consists of a PVC sleeve of the typethat is commonly used for electric installations in buildingconstruction. The sleeve has a diameter of approximately 14mm, and up to six measurement cables can be inserted into it.Temperature probes are assembled as needed, dependingon the conditions at the construction site, including the depthof the soil layers to be examined and the total length of thecolumn (see fig. 4: Temperature probe with internaltemperature measuring cables).

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PORR ProjectsWorld of PORR 160/2012

Fig. 4Image: PORR

In the future, use might be made of a measuring cableprotected by worldwide patent which allows digitalmeasurement of temperatures throughout the jet-groutedcolumn, e.g. at distances of approx. 30 cm. This cable wasfirst tested in March 2012.

Use at construction sitesWith the thermal modelling solution, it is no longer necessaryto produce special test columns to determine the columndiameter. The temperature can be measured in any column ofthe building structure. It is, however, important to adjust thelength of the respective column if necessary, as it shouldextend above and below each temperature sensor by a lengthcorresponding to the obtained diameter, in order to preventthe temperature changes from being influenced from theupper or lower edge of the column.

It is important to make sure that the sensors are positioned atthe exact centre of the column and at the desired depthimmediately after grouting. They are usually inserted via thedrill rod of the drilling tool, allowing for the simultaneouspositioning of several sensors at different depths.

The positioning of the temperature probe is illustrated in detailin the following figures.

Fig. 5a: The drill rod is drilled into the soil until it reaches thelower edge of the jet-grouted column.Fig. 5b: The rod is broken open, and the temperature probeis inserted.Fig. 5c: The temperature probe is inserted into the jet-grouted column until it reaches its lower edge.Fig. 5d: The drill rod is re-assembled in order to be pulledout; the probe remains in the drilled hole.Fig. 5e: The drill rod has been completely pulled out; theprobe remains in the soil.Fig. 5f: Data logger connected to the probe for conductingtemperature measurements (this photo shows the testing ofnewly developed measuring devices from the

Cleveland-based company PDI, conducted at the constructionsite of the Palais Fürth building project in Vienna.)

Fig. 5aImage: PORR

Fig. 5bImage: PORR

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PORR ProjectsWorld of PORR 160/2012

Fig. 5cImage: PORR

Fig. 5dImage: PORR

Fig. 5eImage: PORR

Fig. 5fImage: PORR

After the drill rod has been pulled out, the data logger isconnected to the temperature sensors, and data recordingstarts. Although the recorded data can be checked at any

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PORR ProjectsWorld of PORR 160/2012

time, the presented method can only be applied after thehydration temperature has peaked (approximately after 15hours for columns with a diameter of about 1.0 m, after 60hours for columns with a diameter of about 2.5 m).

Once the data have been read from the data logger, eachindividual curve derived from the recorded measurements(see fig. 6: Read-out temperature file) is assigned to thedepth at which the corresponding temperature sensor wasinstalled. Temperature changes in the column are thennumerically simulated for each depth. In addition to the inputparameters, including a specification of the grout, soilparameters (raw unit weight, thermal parameters), etc., asearch range is defined for the simulation by setting minimumand maximum values for the possible column diameter andcement content. The numerically calculated temperaturecurves for a large number of pairs of values (diameter andcement content) within this range are then compared to theactual temperature curve measured at the construction site.The pair of values that gives the best match between thecalculated curve and the curve measured in situ describes thepredicted properties of the jet-grouted column at therespective depth (see fig. 7: Simulation result showingmatching temperature curves – blue: curve measured in situ;red: numerical simulation).

Fig. 6Image: PORR

Fig. 7Image: PORR

Limitations of useThe presented method is based on the correct recording oftemperature changes in the course of hydration and of theheat transfer to the surrounding soil. If the hydration heat ofthe applied grout is low (e.g. grouts with a blast-furnace slagor rock flour content of > 80%), the distinct temperature riseat the centre of the jet-grouted column that is required for thesimulation will not occur. In this case, the process cannot bemodelled. Groundwater streams and neighbouring columnsthat are hydrating at the same time also adversely affect theprecise determination of column diameters by means ofthermal modelling, as they may prevent a steady, rotationallysymmetrical dissipation of heat from the jet-grouted column.

SummaryIn recent years, thermal modelling to determine the diameterof jet-grouted columns has been applied at numerousconstruction sites, yielding a high correspondence betweennumerically predicted diameters and the actual diameters ofexcavated test columns. This method also allowsdetermination of the cement content of a jet-grouted column.Due to the low additional costs, its application has provedvery efficient for quality assurance during jet-grouting works,always bearing in mind the limitations described above.

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