ouer

lomb, Bra

Received in revised form 3 September 2014Accepted 12 September 2014Available online 3 April 2015

characterizes the soil deposit of a new Experimental Site in the DF via laboratory (standard character-

n cap

type of the common helical screw pile (well described by Clayton[6]). It has recently been introduced in construction sites in thiscity [7] where the soil reinforcement is done underneath bridgeor viaduct abutments. It can also be adopted for light foundations

atory and in siturate geotes the framThis mode

routine. With this tool, it was possible to carry out numerical sim-ulations in which the load curves, the stress and strain regimes, thefailure and working loads, and the load share from the systemscomponents (soil, pile, raft), among other variables, could beassessed. Some of them were directly compared to the experimen-tal instrumented data (as the load versus displacement curves),enabling conclusions on the mechanical behavior of the loadedsystems.

Corresponding author.E-mail address: cristhian-mendoza@unipiloto.edu.co (C.C. Mendoza).

1 Previously with Geotechnical Research Group at the University of Los Andes.

Computers and Geotechnics 67 (2015) 187203

Contents lists availab

Computers and

lsesystem.Alluvial Anker piles, as locally known in Braslia, are a modied

the incorporation of the soils cementing structure, was furtherdeveloped and incorporated into a numerical FEM (Abaqus)in the DF. Since it covers more than 80% of the districts surface, it isalso logical to study the behavior of deep foundation systems in asite with similar characteristics, specially for piled rafts where thesoil-raft contact do intervene in the mechanical performance of the

The site was thoroughly investigated via labortests in order to provide backbone data to calibmodels. The hypoplasticity was then adopted afor the constitutive model to be assessed herein.http://dx.doi.org/10.1016/j.compgeo.2014.09.0100266-352X/ 2015 Elsevier Ltd. All rights reserved.chnicaleworkl, withMidwest central area of the country, a at plateau with a common(tropical) soil deposit. Generally speaking, this region contains inits initial few meters a highly weathered, laterized and collapsibleclayey type soil, locally known as the Braslia porous clay. Researchtheses and past publications from the University of Braslia (UnB),as those from Araki [1], Cunha et al. [2], Cardoso [3], Mota [4] orAnjos [5] have already extensively studied this material and others

the tropical deposits in which it is currently being founded.Therefore, this paper focuses on the experimental behavior, on

the numerical simulation, and on the derived traditional variablesfor group and piled raft systems, constructed in the tropical soil ofthe DF with this pile type.

Real (large) scale load tests on several foundation systems con-structed in a new Experimental Site in this city were carried out.Keywords:Standard pile groupPiled raftHypoplasticityFinite element analysisLoad testTropical soilFoundation design

1. Introduction

The city of Brasilia, the Braziliaization, triaxial) and eld (standard penetration and at dilatometer) tests. It then moves to explain arecently adjusted (hypoplasticity) constitutive model that takes on consideration the inherent soils struc-ture to simulate the behavior of this typical geotechnical material. The model was calibrated via pointload test analyses and incorporated into a nite element methodology (FEM) routine internal to the tra-ditional Abaqus software. Real scale eld load tests on standard pile groups and piled rafts executed withthis pile type were carried out in the new site. FEM analyses were used to calibrate the model and toexpand the knowledge on the shearing mechanisms, generated stresses, displacement elds, load shar-ing, group efciency, and on the contribution of the supporting raft to the overall systems performance.Conclusions of practical and academic interest are given for this new type of foundation employed in theregion.

2015 Elsevier Ltd. All rights reserved.

ital, is situated in the

in similar structures or others (transmission towers, silos, etc.).Although it is not a new foundation technology, its usage, design,and mechanical behavior, still lacks a better understanding forArticle history:Received 7 July 2014

This paper presents and discusses the behavior of standard groups and piled rafts constructed with heli-cal screw piles founded in the typical soil of the Federal District of Brazil (DF). The paper initiallyResearch Paper

Mechanical and numerical behavior of grfounded in a tropical soil of the Midwest

C.C. Mendoza a,, R. Cunha b, A. Lizcano c,1aDepartment of Engineering, Civil Engineering, Pilot University of Colombia, Bogot, CobDepartment of Civil and Environmental Engineering, University of Braslia, Braslia, DFc SRK Consulting, Vancouver, Canada

a r t i c l e i n f o a b s t r a c t

journal homepage: www.eps of screw (type) pilesn Brazil

iazil

le at ScienceDirect

Geotechnics

vier .com/locate /compgeo

the systems.

Flat Marchetti dilatometer tests were carried out, in accordanceto the U.S.A. ASTM D6635-01 standard [11]. Unfortunately just one

ndsounding with this test was possible, as the blade got stuck ataround 8 m depth, and damaged the rods.

Fig. 4 presents the intermediate variables from the unique DMTcarried out (respectively the indexes for horizontal stress and3. Soil characteristics

3.1. In situ tests

SPT, SPTT and DMT tests were carried out in the site to geotech-nically characterize it and to provide an initial basis of modelparameters for subsequent analyses. Disturbed samples from theSPT thick-walled standard tube were also retrieved, and helpedin the visual & tactile assessment of the distinct soil layers. All testswere carried out in accordance to the Brazilian NBR6484 (2001)standard [10].

Fig. 3 presents both SPT and SPTT results, in terms of blowcounts and peak torque. It also describes the general division forthe layers at the site, in accordance to the following depths:

05 m: reddish, very soft to soft, laterized silty sand (Brasliaporous clay), with water level around 4.5 m;

58 m: brownish, medium to stiff, laterized sandy silt (Brasliaporous clay);

89 m: white, stiff to hard sandy silt (transition layer); 914 m: brownish, very stiff to hard silty clay (saprolite ofslate);

Deeper than 14 m: yellowish hard sandy silt (saprolite of slate).The paper discusses and concludes on aforementioned issuesthat are undoubtedly of interest for practical design engineers orresearchers in this area. It is based on a recently defended D.Sc.Thesis [8] of the University of Braslia.

2. Experimental Site

All the experiments are related to a new Experimental Sitelocated in Solotrat Ltds headquarters in the DF, in the outskirtsof the city of Braslia. Fig. 1 graphically depicts the location ofthe city within the national (Midwest), regional (DF) and local (dis-trict) context. Approximate coordinates of the site are 154805900(S)and 475705800(W), with a mean elevation of 1084 m above sealevel.

Within this site several standard penetration tests with (SPTT)and without (SPT) torque measurements were carried out, togetherwith Marchetti Dilatometer tests (DMT), and load tests on founda-tion systems (isolated-I, standard groups-PG and piled rafts-PR),within a particular arrangement depicted in Fig. 2. Undisturbed soilblocks were also retrieved from a trench excavated in the site (seethis same gure).

As previously noted, the main difference between the loadedsystems was the contact (PR), or not (PG), of the top raft withthe supercial soil during tests. In the particular conditions ofthe former case, it was strictly followed the general denition ofJanda et al. [9] for PR systems.

As one can nally note in Fig. 2, systems of 16 piles (PG and PR)were tested with distinct internal arrays for some cases, whichdemanded a multitude of reaction piles (also depicted) all around

188 C.C. Mendoza et al. / Computers amaterial). From this data one concludes that the material behavesas normally consolidated silty sand up to around 5 m and as anoverconsolidated sandy silt from 5 to 8 m.3.2. Laboratory tests

Laboratory tests were performed not only to complement theassessment of parameters of this new site, but also to evaluatethe performance (and calibrate) a new rheological model for thesoil. All tests were done with an undisturbed block sampleretrieved at around 3 m deep in the site, inside a trench (seeFig. 2) excavated for this purpose.

Characterization tests were composed of sieving and sedi-mentation analyses, plus Atteberg limits. Based on that, the samplewas classied as CH by the Unied Classication system, with aplastic Index of 12% and a natural unit weight around 15 kN/m3.

Ten triaxial tests in totalwere also performed, someof themwithdistinct (shearing) velocities, others with relaxation of stresses,some with distinct stress path trajectories and one test with anunstructured sample. All tests were done in saturated conditions.

Four samples were initially submitted to an anisotropic consol-idation in the triaxial chamber, with stress ratios (g = deviatoric/mean stress q/p0) respectively equal to 0.50.0, 0.3, and 0.5.Fig. 5 presents such results.

The rst consolidation was performed with a stress ratio ofg = 0.3 (Fig. 6) to a constant vertical strain velocity, and the pointwhere relaxation begins, and nishes, has a mean effective stressof 335 kPa. After that, consolidation continued till a p0 of 535 kPa,being unloaded to 5 kPa afterwards. The sample was then loadedagain to a p0 of 580 kPa. In addition, a test with this same stresspath was made in a sample without structure (Fig. 6). Two othersamples were then anisotropically consolidated from the initialunload stage, with g equals to 0.00.5 and 0.5, as noticed in thissame Fig. 5.

The second consolidation was performed with a stress ratio ofg = 0.5 to a mean effective stress (p0) of 140 kPa, under a constantvertical strain velocity, being followed by an unloading to the iso-tropic state (g = 0.0). Subsequently, in the same sample, consol-idation was performed in isotropic conditions (g = 0.0) to a p0 of590 kPa. Fig. 7 shows the consolidation curve for the sample at gequals to 0.50.0, while Fig. 8 a similar one to an g respectivelyequal to 0.5.

From Fig. 7 it is observed the change of preconsolidation stresswith the change of the stress trajectory. Note nevertheless thatboth two trajectories do return to the same consolidation line.

In Fig. 8 a stress ratio of 0.5 was adopted, together with variablevelocities of vertical strain. The test was performed with a defor-mation rate of 0.01 mm/min, being suddenly changed to0.001 mm/min afterwards. This variation was done at two distincteffective stress levels to check the response to velocity rate effects.The results are also consistent with the work presented byTatsuoka et al. [12], but on a smaller scale.

Six extra triaxial tests were also made, under drained andundrained shearing conditions. Such results were used to obtainboth the critical state parameters and the deformability moduliof the supercial soil. Fig. 9 presents the stress paths in the q x p0

environment.The three samples sheared under undrained conditions (with p0

of 110, 200 and 300 kPa) had a constant vertical deformation rateof 0.05 mm/min. Results in this gure show that the paths reachthe critical state line and continue through it, leading to a gain ofshearing resistance with strain. This is not a typical behavior forclays, as illustrated by Roscoe et al. [13].

Moreover, it was also observed that the higher were the conn-ing pressures the higher were the obtained peak deviator stressesat unitary strain (Fig. 10b). A typical behavior in soil mechanics,according to Whitlow [14].

Geotechnics 67 (2015) 187203The other three samples sheared under drained conditions(equally with p0 of 110, 200 and 300 kPa) were tested with distinctvalues of deformation rate, as shown in Fig. 10a. Again, to check on

Fig. 1. Approximate location of Solotrats site (Google Maps and Earth and Arcview).

Fig. 2. Location details of systems and in situ tests.

C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 187203 189

Fig. 3. Stratigraphy and

Fig. 4. DMT main results.

Fig. 5. Consolidation paths in triaxial compression at distinct stress ratios.

190 C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 187203the soils response to velocity rate effects. Results in Fig. 9 alsoshow that the paths reached the critical state line. Its noticeablethat shear velocity has no inuence on test results, indicating thatthis soil has virtually no viscous effect.

An important aspect is the presence of cementation, similar tothose reported by Lagioia and Nova [15] for cemented soils.Basically one notices that, to a mean effective stress of 110 kPa,the cementing inuence in the soil stiffness corresponds to 7% ofthe axial strain. To 200 kPa of stress, the inuence reduces to

SPT-SPTT results.24% of the axial strain, and to 300 kPa the corresponding inu-ence of the cementation decreases to values as lower as 1.5% ofthe axial strain (Fig. 10a). This is an important aspect since thesevalues are similar to instrumented deformations of typical geo-technical structures.

3.3. Soil parameters

Using the interpreted data from in situ tests via well known(empirical) equations proposed by Skempton [16], Meyerhof [17],Clayton [18], Marchetti [19] and Lacasse and Lunne [20], togetherwith previously shown lab. (triaxial) data, it was possible to deriveestimates for strength and deformation parameters at each soilstrata.

Fig. 6. Curve of consolidation for triaxial compression with g = 0.3.

Experimental Site, side by side in a layout surrounded by reaction

had this contact with the top block (which needed to be con-structed to load it), being also considered as a PR system. PG sys-tems were simulated by excavating a gap underneath the raftbefore the tests.

For the purpose of this paper only the tests loaded in the verti-cal direction will be presented, although laterally loaded tests werealso performed (see Mendoza 2013 [8]).

4.1. Alluvial Anker pile

As previously stated, Alluvial Anker piles were constructed, andformed the basis of the foundation type in the loaded systems.

C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 187203 191piles that has facilitated the load tests.The tests started on December 2010 (isolated pile) and nished

on June 2011 (6 piles PR). For each system (or no. of piles), the testswith the soil in contact with the raft (PR) were always carried outrst, followed by those without contact (PG), done in the same pre-viously tested system.

One should note however that by the fact that PG systems werecarried out at the same previously tested ones (PR), and that anexcavation process took place from one series of tests to the others,that some inherent input error may be included in the results,given loadingunloading effects. Nevertheless, it is believed thatthe errors may be of small magnitude to hinder the tendencies ofthe correct results.

Fig. 11 depicts the general characteristics of the tested systems.This geotechnical interpretation set was complemented by pub-lished results from the traditional UnB Experimental Site, whichhas a similar soil prole (see Araki [1], Mota [4] and Anjos [5]).

Table 1 presents the original derived parameters, valid in thecontext of a Mohr Coulomb rheological model, as initially inter-preted for the Solotrat site. Laboratory results have also conrmedthat shearing velocity is not a key aspect of the problem, hence,constitutive models without the viscous effect can be undoubtedlyused without detriment to the analyses.

In Fig. 6 the effect of the structure of the soil is related to the dif-ferences between the consolidation line of the unstructured sam-ple and the corresponding one of the structured sample. Similareffect is observed in Fig. 9, where cohesion is noted (in the spaceq p0) basically given by the structure of the soil.

4. Pile load tests

Several foundation systems were constructed in the

Fig. 7. Curve of consolidation for triaxial compression with g = 0.50.0.The triangular shaped system with 3 piles was solely testedwith the contact soil with raft (hence a PR). The single pile also

Fig. 8. Curve of consolidation for triaxial compression with g = 0.5.Figs. 1214 show the general aspects of this particular founda-tion, as well as a typical drilling hydraulic machine.

The piles were either executed with a nominal diameter of17 cm and 12 m in length (reaction piles), or 13 cm dia. and 8 min length (tested piles).

The piles are done by continuous drilling with simultaneousinjection of a coolant uid, which can be water or a watercementmixture (the latter was adopted herein).

The uid is injected through a rigid hollow steel tube with anenlarged base (cone shaped cutting edge. See Fig. 12). The tubeitself forms the structural element of the pile, and is not withdrawnafter soil excavation. It is totally immersed, and surrounded, by thepressurized watercement uid injected during self-drilling andpost drilling stages. Once cured, this uid forms the corrugatedshaft of the pile.

Fig. 15 schematically depicts the distinct execution stages ofthis pile type, from the assemblage in the drilling machine to thenal injection stage.

Perhaps one of the main advantages of the Alluvial Anker is theexecution time. Given the fact that it practically has in the samestage both soil excavation and piles shaft reinforcement and execu-tion, it can be indeed done in a fast manner.

Figs. 16 and 17 bring the average execution times for each ofaforementioned diameters, during drilling and subsequent post-drilling (injection) phases.

As one can notice, for the (optimum) conditions of theExperimental Site, and taking on account that no major drawbackshappened during eld operations (i.e., Murphys Law did not applyin this case), an average execution time of 15 min. was accom-plished per pile. This relates to the total time considering all execu-tion phases.

4.2. Load test procedures

All tests were done in accordance to the Brazilian NBR 12131(2006) [21] standard for slow maintained pile load tests.

Therefore, they consisted of equal increments by no more than20% of the piles workload, followed by the load stabilization for atFig. 9. Stress path in drained and undrained conditions for normally consolidatedsoil.

for (

ndFig. 10. Stress and strain curves from192 C.C. Mendoza et al. / Computers aleast 30 min, during which displacement readings (1, 2, 4, 8, 15 and30 min) are taken. The testing load is raised to a nal value equalsto two times the predicted workload of the pile, when the system isnally unloaded.

For the load tests a hydraulic jack of 2000 kN capacity wasadopted, together with a load cell of 1 kN of internal resolution.Displacements were measured by four analogical dial gauges withresolution of 0.01 mm each, installed all around the base plate ofthe jack.

Fig. 18 shows the arrangement of the testing elements on top ofthe supercial rigid raft, while Fig. 19 a typical set up with reactionframes and reaction piles.

4.3. Testing results

In total, twelve load tests were carried out in this site, beingseven related to PR systems and 5 to PG ones. Figs. 20 and 21respectively show the obtained results for both systems.

Both gures contains a linear load x settlement relationshipthat represents the conventional failure load criterion, in accor-dance to the Brazilian NBR 6122 (2010) [22] standard. It was

Table 1Elasto-plastic (Mohr Coulomb) parameters interpreted for Solotrats ExperimentalSite.

Parameter Symbols Unit Value

First layerFriction angle / 29Elasticity modulus E MPa 9Cohesion c kPa 14Poissons ratio l 0.35Second layerFriction angle / 35Elasticity modulus E MPa 38Cohesion c kPa 20Poissons ratio l 0.29Third layerFriction angle / 39Elasticity modulus E MPa 60Cohesion c kPa 50Poissons ratio l 0.27Fourth layerFriction angle / 35Elasticity modulus E MPa 43Cohesion c kPa 28Poissons ratio l 0.29therefore based on this criterion that the ultimate load of each sys-tem was dened, in the intersection between the standard line andthe experimental load test result.

In some few cases where such intersection did not occur, due toan insufcient displacement of the tested pile, the results had to beextrapolated by the Van der Veen (1953) technique [23] (red line inFigs. 20 and 21).

Fig. 22 shows the failure loads estimated by aforementionedmethodology, for both foundation systems. It is clearly noticeablethat PR systems do have a reasonable increase in load given thecontact of the raft with the supercial soil. The average increasewas in the range of 18% of the conventional failure load of thePG systems, which is not negligible.

5. Constitutive models used in FEM

The selected constitutive models to the nite element sim-ulations were, respectively, the hypoplastic with structure, thestandard elastoplastic one and the simple elastic model.

5.1. Hypoplastic with structure model

a) CID triaxial tests and (b) CIU ones.

Geotechnics 67 (2015) 187203The rst tested model was the hypoplastic with structure. Thismodel was adopted in the rst layer of the strata (see Table 1),where the soft Braslia porous clay prevails. It was further incorpo-rated into the nite element simulations, yet to be presented.

In the present paper the proposal made by Masn [24] was cho-sen given the simplicity required to implement it into the code, aswell as recorded good accuracy. However, a brief discussion of thesoils structure and the constituent models with structure is pre-sented next.

5.1.1. Introduction in the structured soils and constitutive modelsThe structure models were developed by research of Burland

[25], Leroueil and Vaughan [26], Adachi, et al. [27],Anagnostopoulos, et al. [28], Cuccovillo and Coop [29], Cotecchiaand Chandler [30] among others. It has shown the difference inthe behavior of reconstituted and natural soils, explained such dif-ferences as the lack of structure (arrangement of particles andbonds between particles) associated to natural soils. Based on that,some researchers tried to develop constitutive formulations thatcould take on consideration the structures effect. Among them,one can name Gens and Nova [31], Vatsala, et al. [32], Liu andCarter [33], Masn [24], Yan and Li [34]. The majority of for-mulations change the shape and size of the state boundary surface

nd GC.C. Mendoza et al. / Computers a(SBS) of the soil by two state variables (in functions of the stressstate): the rst is sensitivity (s) and second is the shift of the SBStowards the tensile stresses zone (natural cohesion). It means thatthe stress tensor (r) of the model is the tensor without structure(rReconstituted), besides of the stress tensor for the soil structure(rStructure) (Eq. 1), with a parallel coupling. This proposal has alreadybeen made by Baudet and Wu [35] and Vatsala et al. [32], wherethe stress tensor for soil structure (rStructure) is a simple linear elas-tic relation which disappears with the increasing stress.

_r _rReconstituted _rStructure 1

5.1.2. Hypoplastic with structure modelThe theoretical framework of hypoplasticity was developed by

Kolymbas [36] and dened with a continuous tangential stiffnessof the strain rate [37]. Afterwards Kolymbas performs the

Fig. 11. Arrangement, location and character

Fig. 12. Self-drilling steel tube and open tip-Alluvial Anker piles base and body.eotechnics 67 (2015) 187203 193formulation of its hypoplastic model and since then there havebeen several modications as presented by Wu [38],Wolffersdorff [39] and Niemunis [37] among others. Previous

istics of the tested foundation systems.

Fig. 13. Adopted drilling machine.

models were proposed for granular soils, nevertheless, there havebeen extensions to represent the behavior of ne soils as proposedby Niemunis [37] and Masn [24] to natural soils (with structure).

The modication in the hypoplastic with structure was theincorporation of a structure degradation law by means of the pro-posal made by Baudet and Stallebras [40]. The proposal consists inthe incorporation of a larger size swept-out-memory (SOM) sur-face (this is a close approximation of the SBS), by alteringHvorslevs equivalent stress by a scalar (s) value (in a constant vol-ume section through SOM), as illustrated in Fig. 23.

The modication done by Masn [24] basically adds 3 newparameters (s0; k;A). The rst (s0) is the initial value of the statevariable of the structure factor or sensitivity (s), shown in Eq. (3)(law of degradation). The other factors in the equation are the(sf ) factor, which is the limit to a stable state with a value of 1(Fig. 23); the (k) factor, which is a parameter that controls thedegradation of the structure; k which is the slope of the virgin iso-tropic compressibility line, in a double natural logarithm chart; (d)(Eq. (4)) which is a damage strain that depends on the volumetricand shear strain rates; and the (A) factor which controls the impor-tance of the shear strain, with values in the range 0 < A < 0:5. Acomplete mathematical formulation of the model is given inAppendix A.

The model is represented by Eq. A.1, where Tis an objective

stress rate, D is the Eulers stretching tensor, L and N are fourth

Fig. 14. Finished top of Alluvial Anker pile.

194 C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 187203Fig. 15. Schematic phases of the Alluvial Anker pile execution (after Barbosa [7]).

between the undrained bulk and the shear moduli. The (/c) angle

Gauss point. This program follows the same methodology ofmaterials in the Abaqus software. It was written in Fortran code,and inputs increases of strains and returns stress increments.

Given the previously mentioned (Section 3) anisotropic consol-idations, CID and CIU triaxial data, it was possible in this stage todirectly compare numerical and experimental results. Figs. 2426show such comparisons, illustrating a reasonably good agreementof all trajectories, in the spaces e x p0; q x p0 and q x a. Table 2species the derived model parameters, from aforementionedcalibrations.

The rst tested model was the hypoplastic with structure. Thismodel was adopted in the rst layer of the strata (see Table 1),where the soft Braslia porous clay prevails. It was furtherincorporated into the nite element simulations, yet to bepresented.

5.2. Elasto-plastic model

Fig. 16. Average time spent for drilling phase.

C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 187203 195is analogous to the (M) parameter, i.e., the slope of the critical stateline on the CC model.and second order [24] invariants. The model is written as a nonlin-ear increasing function of time to correlate stresses and strains.

T L : D NkDk 2

_s j

ks sf _d 3

_d _2v

A1 A _

2s

r4

The other ve variables of the model can be obtained from anatural or a reconstituted sample, tested in a triaxial isotropic con-solidated chamber.

The variables (k;j;N) are similar to those in the Cam-Clay (CC)model, and can be assessed in a double logarithm chart. The vari-able (r) is obtained from undrained triaxial tests as the ratioThe hypoplastic model with structure was then implemented,and validated, with the incremental driver program [41]. This isthe program to implement constitutive models to the level of a

Fig. 17. Average time spent for injection phase.The second model tested herein was a simple, standard, elasto-plastic model that responds to the known Mohr Coulomb failurecriteria. This model has been implemented for the nite elementsimulation. Due to fact that this model has only four parametersand due to the fact that all of them have a physical explanation,it has a great popularity in the geotechnical practices [42]. Givenits simplicity (and lack of data from a deeper prole), this modelwas adopted in the remaining 3 layers of the strata (see parametersin Table 1). It was also further incorporated into nite element sim-ulations, yet to be presented.

In the elastic range, the relationship between stress and straintensor is linear, with two parameters: Youngs modulus (E) andPoissons ratio (l). This behavior is valid until the stress-pathreaches the yield envelope, at which time plastic deformationstarts. The yield envelope is of a Mohr Coulomb type and thereare with it two soil parameters (/ = friction angle of the soil andc = cohesion) associated.

5.3. Elastic model

The third model is an elastic model. This is a very simple modelwhere the relationship between the stress and strain tensor is lin-ear by means of a elastic modulus, which is function of the Youngsmodulus (E) and Poissons ratio (l) [43]. This model is used in FEManalyses of pile foundations under assumptions that the piled raftis innitely rigid in comparison with the soil (therefore the soildeforms rst and to a larger extent).Fig. 18. Details of measurement system and rigid raft.

Given aforementioned aspects of the geometry, the next stagewas the generation of the 3D mesh. In order to reach a nal, opti-mum, condition in terms of simulation time, stability and qualityof response, several elements were tested in terms of type, size,distribution and number (see Mendoza 2013).

Hence, C3D8 (continuous and 8 nodes), C3D8R (continuous, 8nodes, and reduced integration) and C3D8P (continuous, 8 nodeswith pore pressure measurement) were respectively selected forthe dry soil/pile elements and the saturated soil. Fig. 28 showsthe geometry for the 1-pile (PG and PR) cases, and delineates con-tour conditions that were similarly adopted for all analyses.

Once the geometry was created, loads in the model wereapplied in three sequential stages, as follows: Geostatic initialoverburden stresses; individual pile excavations (via elementextraction); and system loading (via constant strain rate till a totalvertical displacement of 15 cm).

The latter stage was carried out in drained mode, once raft ele-

was noticed that, in general, the stress bulb (inuence of up to

196 C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 1872036. Finite element analyses

Finite element (FEM) analyses of all foundation systems werecarried out in order to verify the geotechnical parameters andadopted layering (from previous sections), and the suitability ofthe rheological models to properly simulate the physical phenom-ena. Besides, the FEM analyses were used to further calibrate theparameters (allowing slightly value changes) in order to use thistechnique to predict testing scenarios in a subsequent future stage(which, by the way, is not covered here given space limitations).

The initial simulation steps and the nal calibration have alsoexpanded the knowledge on the shearing mechanisms, generatedstresses, displacement elds, load share, pile efciency, and onthe contribution of the supporting raft to the overall systems per-formance to be presented in this section.

6.1. FEM environment

Abaqus environment was used to enable the 3D analyses of thesystems. In all cases, boundary effects were avoided by placing thecenter of the raft 30 (individual pile) diameters away from lateral

Fig. 19. Typical arrangement of the static load test.frontiers. Likewise, a distance of 1=2 pile length was left betweenpile tips and the lower end. Fig. 27 schematically depicts thegeometry for the 6-pile (PG and PR) cases.

Fig. 20. Load test results for PR systems.10% of applied stresses on raft) and the displacement bulb (like-wise for raft displacement) can stretch vertically and horizontallyaround the raft. A practical average number would be 4 times theshorter dimension of the raft in the vertical direction, and 2 timesin the horizontal one.ments were inserted into analyses (with gap, PG, or without, PR, tosupercial soil). The water level and a naturally consolidated K0were also considered for the soil layers.

Fig. 29 depicts the initial conditions adopted throughout theanalyses.

6.2. FEM simulations

Comparisons between the numerical predictions after initialadjustments and calibrations, and experimental data, are pre-sented in Figs. 30 and 31 in terms of the overall load versus settle-ment curve.

The results clearly show that the FEM simulation was able tograsp the overall physical behavior obtained in the eld, at leastin terms of the generated external displacements. Assuming thatthis outcome sufces to assure a reasonable understanding onthe other aspects of the shearing mechanism, some extra (numeri-cal) results will be presented and discussed next.

Fig. 32 for instance depicts the vertical displacements and stres-ses generated in the surrounding soil of the 5 pile PR system.Although not shown here (but presented in Mendoza 2013 [8]) thisresult is somehow similar to those from the other PR systems. ItFig. 21. Load test results for PG systems.

nd GFig. 22. Ultimate (failure) load results.

C.C. Mendoza et al. / Computers aBesides, the bulbs also extend vertically from the tip of the pilesto a dimension of around 23 times the piles diameter.

The main results from the numerical analyses in terms of thedirect comparison, and individual assessment, of the distinct PGand PR systems are given next.

6.3. Main results from FEM analyses

6.3.1. Pile efciencyEfciency factor (g) was calculated in accordance to the def-

inition expressed in Eq. (5). It is basically a relationship betweenthe ultimate capacity of the group over the ultimate capacity of asingle pile similar to those in the group, without inclusion of anyeffect of the raft. Group efciency (Ge) on the other hand was cal-culated in accordance to Eq. (6). This variable expresses therelationship of the average (pile) load in the group divided by theload of a (similar) single pile at the same vertical displacement ofthe group. Both equations are solely valid for the PG systems.

PPG gXnpi1

PP 5

where PPG = ultimate load capacity of the group; g = efciency fac-tor; np = number of piles; and PP = ultimate capacity of a similar sin-gle pile.

Fig. 23. Isotropic compression behavior of natural and reconstituted soil (afterMasn [24]).eotechnics 67 (2015) 187203 197Ge Pwrknp

Psng6

where Ge = group efciency; Pwrk = working load of PG system(equal to ultimate/1.5); np = number of piles; and Psng = load of asimilar single pile at same group displacement.

Given the fact that the single pile had a contact of the raft withthe soil (being considered as a PR), its experimental value could not

Fig. 24. Comparison to triaxial results: (a) Compression test with g = 0.00.5. (b)Compression test with g = 0.3. (c) Compression test with g = 0.5.

Fig. 25. Comparison of stress paths for CID and CIU triaxial tests.

nd198 C.C. Mendoza et al. / Computers abe employed in equations (Eqs. (5) and (6)). Hence, Table 3 solelypresents the results from the numerical simulations.

An average efciency factor of 0.97, i.e., approximately one, wasobtained indicating that with the given geometric disposition ofthe systems (pile to pile distances), there were almost zero detri-mental effects given by the superposition of individual stress anddisplacement (pile) bulbs. It also means that pile group failurerather than block failure did happen (taking on account nomencla-ture given by Mandolini et al. [44]), thus conrming the pseudo-independent behavior from each of the piles of the group.

Moreover, an average group efciency of 92% was obtained,indicating that under similar displacements, a pile within thegroup had a slight smaller load than the equivalent one of a similarsingle pile. It points out to a small, but existing, interactionbetween the piles of the group.

Fig. 26. Left: Comparison of stress paths in drained co

Table 2Model parameters with simulations.

j k N /c r A s k

0.0022 0.060 2.13 31 0.35 0.4 1.5 2.5

Fig. 27. Boundaries for analyses of 6-pile systems.nditions; Right: Similar for undrained conditions.

Geotechnics 67 (2015) 1872036.3.2. Load shareLoad share between each element of the PR foundation system,

i.e., piles and raft, was also derived with the numerical simulations.These individual loads were then divided by the systems ultimateloads (capacity) and by their working loads (ultimate/2.0), asrespectively presented in Figs. 33 and 34.

In both cases similar tendencies were noticed. That means, thehigher is the number of piles in the system, the lower is the (per-centage) load share taken by each pile individually. For instance,note that for the single 1pile-PR, the pile contributed with morethan 80% of both ultimate and working loads of the system,whereas for the 6piles-PR the individual contribution hasdecreased to values lower than 20%.

Besides, in terms of the load absorbed by the raft and by thegroup itself (sum of each piles contribution), it is also clearly seenthat the higher is the number of piles, the higher will also be theimportance of the raft to the overall systems capacity (with theexception of the larger 6piles-PR system). For instance, note againthat for the single 1pile-PR, the relative weight of the raft to theoverall (ultimate and working) capacity is very low compared tothe piles importance. Nevertheless, although still small, as onemove towards higher (pile) number PR systems, the relative con-tribution and weight of the raft slightly increases.

Fig. 28. Mesh for analyses of 1-pile systems.

nd GC.C. Mendoza et al. / Computers a6.4. Main experimental results

6.4.1. Rafts performanceA direct comparison between PG and PR systems yields a

quantitative (and indirect) measurement of the performance ofthe raft to the overall systems behavior, or, in other words, howmuch the system improves by having a close contact between theraft and the supercial soil.

Thus, a practical result gathered from previous numerical analy-ses would be the average value of the raft contribution (in percent-age) to either ultimate or working loads of the systems. In bothcases, and for all systems, an average raft load of 12% was calcu-lated with such analyses.

Perhaps this performance is related to the (poor) supercialcharacteristics of the porous Braslia clay. Indeed, Janda et al. [9]have already noticed similar behavior from numerical analyses ofCFA (continuous ight auger) pile-PR systems founded in theUnB Experimental site. In this particular case, the raft had the abil-ity to increase the bearing capacity by only 15% for the simulatedsystems.

Fig. 29. Initial condition

Fig. 30. Comparison of results for PR systems.eotechnics 67 (2015) 187203 199Another way of checking this performance is given by the bear-ing capacity coefcient fPR, as dened by Eq. 6.

fPR PPRPGP

7

where fPR = capacity coefcient; PPR = load capacity of the PR sys-tem; and PGP = similar capacity of the PG system.

According to Mandolini et al. [44] fPR may be assumed as a mea-sure of the increase of bearing capacity due to raft-soil contact. Itwas calculated and presented in Table 4 with experimental, ratherthan numerical, results.

Results from Table 4 clearly indicate the small, but benecialeffect of the raft (in average 18%, as noticed for Fig. 22 too).Besides, it agrees with Mandolini et al. [44] accounts that such fac-tor should decrease with an increasing No. of piles.

Also according to these authors, there is a critical spacing ratio(scrit/d) for a PR system above which the failure changes from blockfailure to a pile group one. This latter case is related to an almost(pseudo) independent pile behavior, that fails without much ofinteraction with adjacent piles, or with the systems components

s of FEM analyses.

Fig. 31. Comparison of results for PG systems.

(a) Vertical displacementsgenerated in the simulation.

Fig. 32. Soil displacements and stresses aro

Table 3Efciency factors from numerical simulations from PG systems.

System Ultimate g GeLoad [kN] [] []

1pile-PG 419 2piles-PG 850 1.01 913piles-PG 1100 0.88 974piles-PG 1800 1.07 885piles-PG 1900 0.90 906piles-PG 2520 1.00 93

Fig. 33. Load share between components of the PR system, in relation to theultimate loads.

200 C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 187203(raft and soil around). Moreover, PR systems that fail as a pile grouptend to have fPR greater than one.

Taking on account results from Table 4, and the fact that spac-ing ratios above 4 are noticed for the systems of Fig. 11, one canconclude that they indeed failed as pile group ones (which by theway has already been noted in the section of load share).

6.4.2. Displacements at capacity loadCunha and Sales [45] report a eld investigation on the behavior

of piled raft foundations in the UnB Experimental Site, where four

(b) Vertical stress generated in thesimulation.

und 5 pile PR system and along depth.

Fig. 34. Load share between components of the PR system, in relation to theworking loads.

nd GPR systems were loaded at distinct conditions of soils water con-tent and individual geometries. The settlements attained duringthe tests ranged between 20 and 45 mm, and in any case, thePR systems did not reach a settlement larger than 3% of B (systemsbreadth) at the maximum load.

Using the available data for the experimental PR systems, it wasalso possible to construct Table 5. This table presents the values ofload (PPR) and displacement (D) at ultimate conditions (in accor-dance to the Brazilian conventional failure load criterion). It alsobrings the breadth of each of the systems and the relationship D/B.

As noticed, the systems reached an average settlement around2.6% of B, the shortest rafts dimension. This value agrees withaforementioned results for similar soil conditions. Similarly asother (previously given) numbers in this paper, this relationshipcan be adopted as a practical design number in a rst roughassessment.

7. Conclusions

Table 4Experimental bearing capacity coefcients using data from both PG and PR systems.

System Ultimate load fPRPPR [kN] PPG [kN] []

2piles-PG 1000 650 1.533piles-PG 1200 1100 1.094piles-PG 2000 1780 1.125piles-PG 2190 1950 1.126piles-PG 2700 2520 1.07

Table 5Experimental results using data from the PR systems.

System Ultimate values BreadthPPR [kN] D [mm] B [mm] D/B [%]

2piles-PG 480 6.8 500 1.362piles-PG 1000 10.0 350 2.853piles-PG 1200 10.8 350 3.084piles-PG 2000 14.8 580 2.555piles-PG 2190 15.9 580 2.746piles-PG 2700 19.1 580 3.29

C.C. Mendoza et al. / Computers aThis paper focused on the experimental and numerical behaviorof standard groups and piled rafts constructed with helical screwpiles (a novel feature in the region), founded in the typical soil ofthe Federal District of Brazil. This is a particular tropical and later-ized soil, which characteristics that can be somehow found in otherdeposits of the Midwest region of this country.

The paper investigated and characterized a new ExperimentalSite, presenting an overview of the main geotechnical parametersfor a simple elasto-plastic model via laboratory and in situ tests.Specic point load (lab) tests were coupled to numerical FEManalyses to calibrate a new (modied) hypoplastic model thatcan incorporate the soils structure. This model was further adoptedinto numerical simulations to include some of the complex fea-tures of the supercial porous clay strata of the site.

The calibrated numerical tool aimed the expansion of theknowledge on the behavior of the tested foundation systems, interms of traditional (piled raft) variables, design considerations,and overall (shearing and displacement) mechanisms. Practicaland academic conclusions of real added value for professionals ofthe studied region or elsewhere are given, as follows:

1. Hipoplasticity with the modications proposed in the presentpaper has proved to grasp reasonably well the main, complex,geotechnical characteristics of the supercial tropical soil ofthe Federal District of Brazil. This rheological model candenitively be used into numerical simulations as those pre-sented herein, to acquire knowledge on the approximate behav-ior of common engineering structures founded on thisparticular strata.

2. For (pile group) systems under similar conditions as those stud-ied herein, the average efciency factor is close to unity, indi-cating that detrimental effects given by the superposition ofindividual stress and displacement bulbs are negligible. It alsomeans, and conrms, that individual pile failures, rather thanblock failures, are the main shearing mechanisms that takesplace underneath the systems during soil plastication.

3. Besides, at identical displacement levels, a pile within the (pilegroup) system has a slight smaller load than the equivalent oneof a similar single pile. This feature leads to a conclusion that,although small, there is indeed some interaction between thepiles of the group.

4. For (piled raft) systems under similar conditions as those stud-ied herein, the region of inuence (stress and strain bulbs)around the raft can stretch to around 4 times the rafts breadthin the vertical direction, and 2 times in the horizontal one. Thisbulb also extends downwards below the pile tips, within a zoneof around 23 times the piles diameter.

5. Besides, for such (piled raft) systems, there is a load sharebetween the elements that compose the system, i.e., raft, pilesand surrounding soil. The contribution of the raft to the totalload is not high, but nevertheless not insignicant. It has beenshown that the raft was able to absorb a value in the range of12% of the total (ultimate or working) load. As one movetowards systems with higher number of piles, hence with largerraft dimensions, the relative importance of the raft to the totalsystems capacity slightly increases, as it also decreases the per-centage of load share taken by each pile individually.

6. Finally, it is clearly noticeable that (piled raft) systems do have areasonable increase in load given the contact of the raft with thesupercial soil. It has been shown an average increase in therange of 18% of the conventional failure load of standard (pilegroup) systems, which is by no means negligible. Moreover, atsuch ultimate conditions, it has also been shown that (piledraft) systems do not displace more than around 3% of the raftsbreadth in the vertical direction.

7. Helical screw piles have shown to be feasible to be employed inthe region under certain construction characteristics (viaducts,soil reinforcement, small structures, and so on), where the fastspeed of execution (15 min) and eld behavior (slender frictionpiles for compression or tension loads), add a striking competi-tiveness to this pile when compared to other solutions.

Although the range of the numerical analyses of the presentpaper was limited in scope and dimension, generalized conclusionshave been drawn, and knowledge was undoubtedly generated. Theprovided information can of course be referenced as an initialguideline in the design of similar foundation systems for theregion, or perhaps in others that equal conditions apply. It alsoserves as a rough insight in the complex soil-structure problemthat is related to particular foundation systems constructed inregions of structured, laterized and tropical soil deposits.

Acknowledgements

This study was made possible through an existing joint techni-cal co-operation research program from the Pilot and Los AndesUniversities in Colombia and the University of Braslia in Brazil,where students and professors from both institutions were able

eotechnics 67 (2015) 187203 201to correspond and interact.The authors also thank the Brazilian sponsorship organizations

CNPq and CAPES for all related support in this and in all other

[24] Masn D. Hypoplastic models for ne-grained soils. Thesis of doctor ofphilosophy; Charles University; Prague, Czech Republic; 2006.

ndstudies carried out by the second author, either in terms of per-sonal research grants, or via sabbatical and student scholarships.

One of such scholarships allowed the rst author to pursue hisDoctorate in Brazil, strengthening the cooperation links betweenthis country and his homeland.

The rst author thanks to the project Study of the mechanicalbehavior of bases for pavements constructed with soilcementmixes for the nancial support.

Appendix A

The equations needed to implement the model in a UMAT (usedmaterial) for Abaqus program are given below:

The basic equation is shown in Eq. (A.1).

T f sL : D f sf dNkDk A:1

Dening (T) as the change rate of the Cauchy stress tensor in

time, (D) as the rate of change of strain in time, the fourth-ordertensor (L) (Eq. (A.2)) and the second-order tensor (N) (Eq. (A.9)).

L 1bT : bT c1F2I c2a2bT bT

A:2

(L) is a constitutive fourth order tensor which is function of the

stress tensor (bT) (the Cauchy stress tensor (T) divided by the tracetensor) and the criterion of the critical state of MatsuokaNakai(F) (Eq. (A.3)), a) (Eq. (A.4)), as well as scalars of the factor (c1)(Eq. (A.5 and (c2) (Eq. (A.6)), and nally (I) is a fourth order tensorunit.

F 18tan2 w 2 tan

2 w

22

ptanw cos 3h

s 122

p tanw A:3

a 3

p3 sin/c22

psin/c

A:4

Factors (c1) and (c2) relate to the material compression law in(a) (Eq. (A.7 and (r) is a constant of the ratio between bulk modulusand the undrained shear modulus. Also, it is already taken intoaccount the inuence of the structure factor (Si) (Eq. (A.8)).

c1 23 a2 2a

3

pa

9rSi

!A:5

c2 1 1 c1 3a2 A:6

a 1ln 2

lnk jSik jSi

3 a2a

3

p

A:7

Si s s sf s A:8

Tensor (m) (Eq. (A.10)) and function (Y) (Eq. (A.11)) can be usedto obtain the tensor (N) (Eq. (A.9)) with the materials ow rule.Function (Y) (Eq. (A.11)) relates the critical stress with the stresstensor invariants function.

N L : Y mkmk

A:9

m a

FbT bT bT

36bT : bT 1aF

2 bT : bT !" #

A:10

Y 3

pa

3 a2 1 !

I1I2 9I31 sin2uc8I3 sin

2uc

" #A:11

To complete the components of the equation there are the scalar

202 C.C. Mendoza et al. / Computers afactors (f s) (Eq. (A.12 and (f d) (Eq. (A.13)) representing picnotropyand barotropy factors of the material. They are affected by the soilsstructure (f sr) factor (factor unstructured model f s) multiplied by a[25] Burland JB. On the compressibility and shear strength of natural clays.Geotechnique 1990;40:32978. http://dx.doi.org/10.1680/geot.1990.40.3.329.

[26] Leroueil S, Vaughan PR. The general and congruent effects of structure innatural soils and weak rocks. Geotechnique 1990;40:46788. http://dx.doi.org/10.1680/geot.1990.40.3.467.

[27] Adachi T, Oka F, Hirata T, Hashimoto T, Pradhan T, Nagaya J, et al. Triaxial andtorsional hollow cylinder tests of sensitive natural clay and an elasto-viscoplastic constitutive model. In: Proc Xth European conference on soilmechanics and foundation engineering, vol. 1; 1991. p. 36.

[28] Anagnostopoulos AG, Kalteziotis N, Tsiambaos GK, Kavvadas M. Geotechnicalproperties of the Corinth Canal marls. Geotech Geol Eng 1991;9:126. http://(Si) factor. In the (f d) factor the Hvorslev stress is multiplied by ascalar (s) with the addition of the structure.

f s SitrTk

3 a2 2aa3

p 1A:12

f d 2pspe

aA:13

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C.C. Mendoza et al. / Computers and Geotechnics 67 (2015) 187203 203

Mechanical and numerical behavior of groups of screw (type) piles founded in a tropical soil of the Midwestern Brazil1 Introduction2 Experimental Site3 Soil characteristics3.1 In situ tests3.2 Laboratory tests3.3 Soil parameters

4 Pile load tests4.1 Alluvial Anker pile4.2 Load test procedures4.3 Testing results

5 Constitutive models used in FEM5.1 Hypoplastic with structure model5.1.1 Introduction in the structured soils and constitutive models5.1.2 Hypoplastic with structure model

5.2 Elasto-plastic model5.3 Elastic model

6 Finite element analyses6.1 FEM environment6.2 FEM simulations6.3 Main results from FEM analyses6.3.1 Pile efficiency6.3.2 Load share

6.4 Main experimental results6.4.1 Rafts performance6.4.2 Displacements at capacity load

7 ConclusionsAcknowledgementsAppendix A References