Energy Geotechnics – Wuttke, Bauer & Sánchez (Eds)© 2016 Taylor & Francis Group, London, ISBN 978-1-138-03299-6

Innovative deep-mixing methods for oil & gas applications

G. SpagnoliDepartment of Maritime Technologies, BAUER Maschinen GmbH, Schrobenhausen, Germany

D. BellatoDepartment of International Projects and Services, BAUER Spezialtiefbau GmbH, Schrobenhausen, Germany

P. Doherty & G. MurphyGDG Geosolutions, Dublin, Ireland

ABSTRACT: Notwithstanding the increasing investments in the use of renewable sources of energy and naturalgas observed in recent years, oil still represents an important part of the global energy market. In particular,oil overall consumption and production are expected to continue growing, especially in emerging economies,though more slowly than in the past due to international policies aimed at slowing down climate changes. Plantsand infrastructures related to the oil industry are often inherently related to challenging projects, which, in mostof the cases, have to fulfill strict technical, environmental, and economical specifications. For instance, refinerysites located in sub-urban contexts have to face stringent environmental requirements both during productionand after their dismantling to ensure that no pollution spreads across the surrounding areas. On the other hand,the construction of offshore platforms in difficult geotechnical conditions presents issues, which can be solvedonly by means of innovative technical solutions. This paper presents two case histories in which deep-mixingtechniques were used to produce improved-soil elements for environmental and structural purposes related to theoil industry, i.e. a cut-off wall in an ex-refinery site and foundation piles for the support of offshore platforms incarbonate sands. Deep-mixing (DM) of soil is a well-established methodology in geotechnics introduced morethan 50 years ago in Japan. DM methods have been used so far in numerous applications both on-shore andoffshore projects. The use of DM is expected to become more and more popular in the future due to its economicand environmental advantages compared to other traditional construction techniques.

1 INTRODUCTION

Fixed platforms have been traditionally used to exploitoffshore hydrocarbon reserves in water depths of lessthan 200 m, although there are several examples offixed jackets installed in deeper water, e.g. Magnusplatform installed in 186 m water depth in the NorthSea (Clarke 1993), the Bullwinkle oil platform in theGulf of Mexico installed in 412 m water depth (Digreet al. 1989). Offshore fixed production platforms areglobally installed. In the period 2008–2012, the major-ity of the fixed platforms where installed in Asia andMiddle East. As a matter of fact, Asia’s demand forfixed platforms is increasing especially for develop-ments in Malaysia and China, with large proportion ofplatforms situated between 25 and 50 m water depth.Platforms installed in depths over 100 m are commonin South America and Africa. Regarding West Africa,Angola and Nigeria are the key countries driving themarket growth (Infield 2013). It is possible to observefrom Fig. 1 that the majority of fixed production plat-forms are pile-based. In fact, the most common type ofoffshore foundation is the open-end driven steel tubepile (Poulos 1988) with diameter ranging from 1m up

to 6 m (for offshore renewable energy) and lengths upto 300 m (Gerwick 2007). However, there are situa-tions where the pile installation by means of hammersis not possible, for instance in rock conditions, whereboulders and/or cobbles are encountered or in calcare-ous deposits.Therefore, drilled piles are normally usedin these conditions. George and Wood (1976) identifythree basic types of offshore drilled piles:

• Single-stage drilled piles, which can be formed bydrilling an oversized hole to the required penetra-tion, inserting a steel tube pipe and grouting theannulus between pile and soil;

• Primary driven pile, which forms the casing for theupper section of the insert pile. An oversized holeis then drilled to the required penetration below theprimary pile tip, and the annulus between the insertpile and soil and the primary pile is grouted;

• Belled pile.

According to Poulos (1988), Gerwick (2007) andDoherty et al. (2016) offshore drilled piles are installedwith top drilling units with facilities for both direct andreverse circulation, with the latter normally used forrelatively large-diameter holes. Offshore piles resist

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Figure 1. Fixed platforms installed in the period 2008–2012(modified after Infield 2013).

compression and tension load, by transferring it alongits shaft and by end bearing on its tip (for compressionloads). However, calcareous soil deposits posed sev-eral problems regarding the pile capacity (e.g. Murff1987; Le Tirant and Nauroy, 1994; Doherty et al.2015). Calcareous (or carbonate) sands are foundmanly in the warm seas between latitudes 30◦N and30◦S in coastal areas of Australia, India, Saudi Arabia.However they are also frequent up to latitudes 50◦Nand 50◦S (Le Tirant and Nauroy 1994). It is very well-known that they show comparable friction angles tosilica sands at low confining stress, but at higher pres-sures, the material contracts, due to crushing of thecalcareous grains.

The tendency to contraction during shearing hastherefore serious implication for pile shaft resistance,which depends on the development of lateral stressalong the pile wall (e.g. Murff 1987; Colliat et al. 1999;Gerwick 2007; Spagnoli et al. 2015). The skin frictionof driven piles in carbonate sands (where the estima-tion of K and tan δ is uncertain) is given by (Le Tirantand Nauroy 1994):

where β varies from 0.05 to 0.2 (Dutt and Cheng 1984;Abbs et al. 1988). According to Randolph (1988) forlong piles, the very low β values suggest that it is notcorrect to relate the value of skin friction to the over-burden pressure. It seems more appropriate to think interms of absolute values of unit skin friction, which liein the range of 5 to 15 kPa. Drilled-and-grouted (D&G)piles overcome the difficulties associated with parti-cle crushing and compression at the pile interface andare frequently used for offshore foundations in thesedeposits (Lee and Poulos 1991). As a result, the radialeffective stress will remain close to the in situ hor-izontal stress, yielding significantly higher values of

shaft friction than for the driven counterparts. This hasresulted in the prominence of D&G piles in calcareoussand deposits (e.g. King et al. 1980; Gerwick 2007).However, D&G piles is a costly foundation solutionand therefore, despite the geotechnical properties ofcarbonate sands, driven piles with closed-ended pileshave been installed in the past for saving costs (e.g.De Mello et al. 1989) or grouted driven piles havebeen suggested, where cement grout after driving isinjected (Barthelemy et al. 1987). Barthelemy et al.(1987) tested a 762 mm diameter pile equipped withgrout pipes and instrumented with 36 strain gaugeswas driven into calcareous sands down to 24 m. Thefollowing paper presents and discuss the latest dataabout a novel mixed-in-place pile (MIDOS) alreadydescribed in Igoe et al. (2014); Spagnoli et al. (2014);Doherty et al. (2016). As the MIDOS is based on themixed-in-place technology, an onshore test with theCutter Soil Mixing (CSM) in an ex-refinery site is pre-sented. Laboratory tests of MIDOS pile in carbonatesands will also be briefly discussed.

2 THE DEEP MIXING METHOD

2.1 Introduction

The Deep Mixing Method (DMM) is an in-situ soiltreatment technology whereby binding materials areadded and blended with soils in order to improvetheir hydraulic and mechanical properties. Deep Mix-ing techniques, originally developed in Sweden andJapan during the 1960s and 1970s, are today well-established procedures in the geotechnical engineeringpractice of an increasing number of countries. The rea-sons for this success can mainly be due to the severalengineering purposes they serve as alternative, moreeconomic (e.g. Topolnicki 2004), and environmentalfriendly solutions with respect to the traditional meth-ods involved in ground improvement works and tothe constant technological development carried outon mixing rigs. The performance of deep mixingstructures depends significantly on the mixing pro-cess implemented at the site, which is much moreeffective when a homogeneous distribution and uni-form blending of the binding material with the soil isachieved (Mitchell 1981). First attempts to classify ina rigorous way deep mixing techniques were made byBruce (2000). This classification depends on severalfundamental operative features as follows:

• The method with which the binding material is intro-duced into the subsoil, namely in a Wet (pumped asa slurry) or a Dry (blown in pneumatically) form.

• The approach adopted to penetrate the groundand/or to blend and homogenize the chemical agentswith the soil: purely by Mechanical methods addingthe binder at relatively low pressures, or by a rotarymethod aided by Jet systems which injects fluidslurry at high pressure. The classification does notinclude jet-grouting, as it does not involve anymechanical mixing to improve the stabilized mass.

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Figure 2. General classification of deep mixing methods(modified after Topolnicki 2004).

• The vertical position of the mixing tools along thedrill shaft: the incorporation of the binder and itshomogenization with the soil can be carried out atthe End (or within a distance comparable with thecharacteristic size/diameter of the tool) or along asignificant portion of the Shaft.

In practice, not all the combinations provided by thisflowchart are available for ground improvement appli-cations since wet slurry, jetted shaft mixing (WJS) anddry binder, rotary, shaft mixing (DRS) do not exist, andno jetting introducing dry binder (DJS or DJE) havebeen already developed. Furthermore, recent tech-nologies based on driven mechanisms different fromthose involving rotary systems cannot be correctlyidentified using this scheme.

In order to overcome such limitations, Topolnicki(2004) elaborated a more general classification whichallows distinguishing between different in-situ soilmixing treatments (Fig. 2). The basic indicators usedto operate this distinction are the same consideredby Bruce (2000), even if the mixing principles arehere extended to include recent advancements in deepmixing machinery.

2.2 Cutter Soil Mixing (CSM)

The Cutter Soil Mixing (CSM) technique is a wetmechanical mixing technique developed in 2003 thatdiffers significantly from conventional DMMs, since itmakes use of two sets of cutting wheels rotating aroundhorizontal axis (Fig. 3A) to produce rectangular pan-els of improved soil rather than one or more verticalrotating shafts creating circular stabilized columns.Therefore, CSM can be considered a WME technique.

The counter-rotating mixing wheels and the cuttingteeth push the soil agglomerates through narrow gapsbetween vertical steel plates, named “shear plates”(Fig. 3B). This produces a sort of forced mixing actionwhich enhance incorporation and homogenization ofthe binder into the soil.

Figure 3. CSM technique: (A) cement slurry flowing out thenozzle located between the cutting wheels; (B) CSM mixingwheels and shear plates.

The CSM unit can be mounted on a guided Kellybar or a wire rope suspended cutter frame equippedwith special steering devices. Depths up to 60m havebeen successfully reached using wire rope-suspendedrigs provided with four sets of mixing wheels. Pan-els having a width ranging from 2.4 m to 2.8 m and athickness between 0.55 and 1.5 m can be created.

2.3 The MIDOS technology

The MIDOS pile is a hollow steel pile, which is loweredinto the soil while at the same time a mixing tool, whichis advanced inside the hollow pipe, mixes the in situsoil with cement slurry (Fig. 4). Hence, a mixed soilcement body is created.

A test pile has been installed in the Bauer’s field testfacility in Bavaria (Germany) in a silica sand depositwith the following dimensions: the steel pile had anexternal diameter of 1.5 m with a wall thickness of15 mm.The steel has at its lower end a mixing chamberwith an outer diameter of 1.9 m. Holes in the steel pileand in the mixing chamber allow the soil-cement-mixto communicate between inside and outside of the pile.The steel pile with the mixing chamber remains in thesoil, surrounded by a soil-cement-mixture. The pilelength was 17 m and it was installed in less than 3hours (see Igoe et al. 2014).

Because the MIDOS reached a static uplift capacityof about 9 MN (Igoe et al. 2014) and a static bear-ing capacity mobilized by the dynamic pressure ofthe load test of 15.4 MN, it was decided to extend theinvestigation to calcareous sands, which as previouslydescribed, present foundation issues.

3 CASE HISTORY FOR THE ONSHOREMIXED-IN-PLACE TECHNOLOGY: LEUNAEX-REFINERY

3.1 Introduction

Between August and September 2005, a cut-off wallcomposed of CSM panels was built in an ex-refinery

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Figure 4. MIDOS pile tested in Germany in 2013 in silicasand.

site in Leuna (Germany) in order to limit the ground-water contamination in the surrounding area. Thewall was 450 m long (6400 m2) and 203 panels werenecessary for its completion.

In addition to the cut-off wall, a retaining wall wasproduced for a separate test purpose.

3.2 Geotechnical characterization of the site

A comprehensive geotechnical investigation wasaccomplished in advance to the CSM wall construction.

It included several boreholes and dynamic penetrationtests. Furthermore, laboratory tests for the determina-tion of grain size distribution, permeability and triaxialunconsolidated and undrained strength were carriedout on soil samples retrieved from the boreholes. Someof these samples were also classified by means ofAtterberg limits and chemical analyses. Inside eachborehole a piezometer was installed to monitor thegroundwater level and the quality of the water beforeand after treatment.

The resulting grain size distribution curves aredepicted in Fig. 5. Each curve in the graph is individu-ated by an alphanumeric string reporting the boreholenumber, the sample number, and the depth at whichthe sample was taken. The curves show the great het-erogeneity characterizing the subsoil at the site ofLeuna.

Triaxial undrained tests were performed on undis-turbed cohesive samples collected from the site. Theundrained shear strength varied from 120 kPa to247 kPa, denoting the high overconsolidation of theclay deposits.

The classification of these soils based on theCasagrande plasticity chart is shown in Fig. 6. Thefinest fraction could be classified as inorganic siltyclay of medium plasticity (CL). Moreover, hydraulicconductivities of the order of magnitude of 10−10 to10−11 m/s were measured on soil specimens represen-tative of the fine deposits of Leuna.

Close to each borehole, a dynamic penetration testwas executed in order to obtain a more effective cor-relation between test results and visual inspection. Anexample is presented in Fig. 7, in which the DP resultsare compared with the information contained in thecorresponding borehole report.

From the in-situ and laboratory investigation cam-paign the following geotechnical profile was derived.

The first layer consisted of loose to medium-densesandy fillings, with a variable thickness of 0.0–2.2 mfrom the ground level (up to 2.64 m close to B5413).

The second level, containing a larger fine frac-tion with respect to the first, was composed of Loess,glacial marl, or a less weathered gravel terrace depend-ing on the considered site area. The cohesive part hada soft to hard consistency, while the granular materialwas characterized by a loose to medium-dense state.Some organic formations of soft to stiff consistencywere encountered between 2.2–6.1 m from the surface.

Sand and gravel of the alluvial terrace, prevalentlyat a loose to medium dense state, formed the thirdlevel. In some DP tests, NDP was over 60 blows per10 cm. These high values were probably related to thepresence of boulders or cobbles. The thickness of thislayer was variable throughout the jobsite area and wasbetween 6.1 m and 10 m below the ground level. Onthe north (B5407 to B5409 and B5500), the surfacequaternary deposits described so far were found to liedirectly on the bedrock.

In the remaining area, a tertiary layer was locatedin between. The tertiary deposits incorporated asequence of non-carbonate sands, silts, and clays with

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Figure 5. Grain size distribution curves obtained from thesoil samples collected at the site.

Figure 6. Classification of the natural subsoil at the jobsiteof Leuna by means of Casagrande chart.

a thickness ranging from 13.5 m to 41.0 m. The cohe-sive formations had a stiff to solid consistency, whichled, in some cases, the DP tests to refusal. The tertiarysands were denoted by a medium dense state increas-ing in density with depth. In some boreholes, localorganic formations of lignite were identified.

Finally, the weathered and disaggregated sandstonebedrock with a stiff to semi-solid consistency wasreached at a variable depth, which in the North-westernarea of the jobsite was approximately comprisedbetween 5.0 m and 7.0 m.

Figure 7. Dynamic penetration test results obtained fromthe preliminary ground investigation at the jobsite of Leuna –Borehole B5418.

Table 1. Mix design at Leuna – Cut-off and structural wall.

Parameter Cut-off wall Structural wall

Cement (CEM III) (kg) 373 709∗Water (l/m3) 858 682Bentonite (kg) 40 20w/c (water/cement) 2.3 0.96w/b (water/bentonite) 21.45 34.1

∗: initially set equal to 909 kg.

3.3 Mix design, production data, and results

The cut-off CSM panels had a maximum depth of 16 mbelow ground level and a thickness of 0.64 m. Theconstruction was performed following the one-phasesystem and the back-step procedure (fresh-in-hard),with the realization of 2.8 m wide primary panels and2.0 m wide secondary panels, i.e. with an overcut of0.4 m at each side.

As far as the cut-off wall production concerns, themix parameters provided in Tab. 1 were adopted at thesite. In order to introduce into the ground an amountof cement corresponding to 152 kg/m3 of natural soil,a flow rate of 858 l/m3 was assumed. The cement usedwas a CEM III 32.5 N NW/HS (85% slag and 15%OPC). The retaining wall, on the contrary, was cre-ated with a higher amount of cement to ensure theachievement of a predefined strength level (Tab. 1).

The same flow rate of 858 l/m3 was adopted to intro-duce into the ground an amount of cement correspond-ing to about 550 kg/m3 of natural soil. Furthermore,the same binder as in the cut-off wall was used.

Hydraulic conductivity values of the order of mag-nitude of 10−9 to 10−11 m/s were obtained frompermeability tests performed on cut-off wall sam-ples. Unconfined compressive strength (UCS) valuesranging from 0.5 MPa to 6 MPa were measured onsamples collected from the structural wall.

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4 THE MIDOS TECHNOLOGY FOR OFFSHOREPILED FOUNDATION APPLICATIONS

4.1 Laboratory tests in calcareous and silica sands

To investigate the feasibility of using MIDOS in cal-careous sands, a laboratory study was required toassess both the geotechnical performance of the pileand the structural behavior of the Mixed-in-Place(MIP) grout, which forms the pile body. Dohertyet al. (2015; 2016) showed that the silica sand tested(Blassington sand) was chosen as a control or refer-ence sample. This was deemed particularly importantbecause the MIDOS system has already been shownto be an effective pile installation technique in sil-ica deposits, with the load carrying capacity shownto be in-line with predicted values for conventionalD&G piles. As a result, direct comparison of the silicaand calcareous sand behavior was deemed essentialfor this study. The testing regime focused on both thepotential geotechnical and structural failure modes. Acarbonate sand from Dog’s Bay (west coast Ireland)was used. Portland CEM II cement was used through-out the testing regime and a water/cement (w/c) ratioof 0.4 was adopted for the tests as suggested by ISO19902 for fixed steel offshore structures. Differentcement-to-sand ratios (c/s) were used for the tests,i.e. 15-25-35%. These values were chosen to simu-late a high installation (15% c/s) and low installationtime (35% c/s), where the cement would have moretime to be mixed with the soil. The 25% c/s is a com-promise value between the low and high installationtime. A series of tensile tests and UCS tests were per-formed at 3 days, 7 days and 28 days and across therange of c/s ratios noted above. The shear rate for thetensile test was 50 N/s whereas for the compressiontest was 0.5 MPa/s (Doherty et al. 2015). The resultsdemonstrated identical trends in all cases, regardless ofc/s ratio and curing time. This is particularly evidentin Fig. 8, which directly compares the compressionand tension tests of grout samples with the same test-ing time and c/s values (Doherty et al. 2015). Overthe entire range of parameters considered, the groutsamples demonstrated comparable behavior, suggest-ing that the structural performance of the MIDOS pilebody should be similar in both calcareous and silicadeposits.

4.2 Finite element analysis

Spagnoli et al. (2015) showed MIDOS pile modelswere developed with Plaxis 3D to analyze a range ofpile length to diameter ratios in silica and calcareoussand deposits. In order to investigate the behavior ofMIDOS piles in calcareous sand, suitable soil parame-ters were calibrated in order to reflect the load bearingbehavior of this type of soil more precisely.

The range of model dimensions and the corre-sponding model numbers are presented in Table 2. Sixgeometry models were analyzed for each soil type,to cover the range of diameter/length combinationspresented.

Figure 8. Comparison of UCS for grouted samples of sil-ica and carbonate sand (top); comparison of tensile forcefor grouted samples of silica and carbonate sand (below)(modified after Doherty et al. 2015).

Table 2. Silica sand soil model properties (modified afterSpagnoli et al. 2015)

Diameter Pilechamber length

Model no Soil (m) (m)

1 Silica sand 2 202 2 403 2 604 3 205 3 406 3 607 Calcareous sand 2 208 2 409 2 6010 3 2011 3 4012 3 60

In tension the shaft resistance is the primary resis-tance component. The radial effective stress at failureis highly dependent on the initial horizontal effectivestress (at rest condition) σ ′

rc, as the MIDOS pile aimsto maintain a constant horizontal lateral stress dur-ing installation the simplistic “wished-in-place” modelconstruction should provide a reasonable initial instil-lation stress condition – compared to that of drivenpiles where the driving stresses can be impossible toaccurately model. The soil models used in this studyare assumed to be normally consolidated and the lateralearth pressure coefficients are automatically deter-mined by Plaxis as Konc = 1 − sinϕ′. In practice the

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initial soil stresses would be assessed and inputteddirectly to Plaxis using the results of in-situ geotech-nical testing such as CPT, to simulate the specificconditions encountered at a real site. As a simplifiedmethod of allowing for some strength reduction a “softsoil” layer was modelled underneath the chamber to adepth equal to the chamber diameter. The soft soil wasgiven stiffness parameters equal to half of the regularsand parameters and a friction angle equal to the con-stant volume friction angle of the soil in the respectivemodel.This has only been considered in the silica sand,as the soil beneath the chamber in calcareous sand ismodelled using the crushed parameters. The silica andcalcareous sand models were calibrated using triaxialtests. For the calcareous sands, triaxial tests conductedat varying effective confining stresses (100 kPa and1000 kPa) found that sand stiffness deteriorated signif-icantly at higher confining stresses, subsequent grainsize distribution on those samples showed clear evi-dence of particle degradation during the 1000 kPatriaxial tests. Due to the limitations of the hardeningsoil model, the soil degradation and stress could not becalibrated into a single soil model. Therefore two setsof soil properties were adopted and the soil proper-ties were optimized using triaxial tests on uncrushedand crushed samples. A limit stress of 400 kPa wasselected as the pressure at which the sand will beginto deteriorate. Once this stress was reached, the soilproperties were changed to that of the crushed sand tomodel the stress softening behavior of the calcareoussand. This process required the model to be first runusing the intact soil only. The resulting soil stresseswere then examined and the depth of soil where thestresses exceed the 400 kPa determined. The soil bodybelow this depth was assumed to be crushed, whileabove this depth, the non-crushed soil model was used.Therefore, in the next stage of analysis, a mixed soilmodel was generated, where the soil properties atdepths below the identified depth were modified torepresent the behavior of crushed soil, and the modelwas re-run. This methodology was found to provide amore realistic response than adopting a fully crushedor non-crushed approach (Spagnoli et al. 2015). Com-parison of the tension and compression performanceof the various pile geometries showed some scatterin the results obtained, however in general the sim-ulations highlight similar performance between thepiles in silica and calcareous sand (Spagnoli et al.2015).

The results of numerical analysis of the piles insilica sand were also compared with those predictedusing the API method, see API (2014), and a CPT-based approach, see Igoe et al. (2014), assumingthree different values for the average qc of the seabed(see Fig. 9). The comparison shows that the Plaxismodels predict significantly lower capacities than theanalytical predictions. It should be noted that FiniteElement Modelling is not particularly suited to largedeformation analysis and convergence issues may stopthe model before the ultimate limit state capacity, aspredicted in the analytical methods, is reached.

Figure 9. Numerical vs. analytical capacity assessment ofthe piles in silica sand for the models no. 4, 5 and 6.

5 CONCLUSIONS

Offshore driven piles are the most common foundationtype. However, drilled-and-grouted piles, althoughtime consuming, are used where driven piles reachtheir limits. The Deep Mixing Method (DMM) isan in-situ soil treatment technology whereby bindingmaterials are added and blended with soils in orderto improve their hydraulic and mechanical properties.This paper analyzed two case histories, where this tech-nology successfully was used for an onshore and for apotential offshore application.

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