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Industrial buildings at Sobralinho, V.F. Xira

Geotechnical aspects on the behavior of the structures and access roads

André Sampaio1

1M.Sc. Student, Instituto Superior Técnico, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal;

andre.sampaio@ist.utl.pt

ABSTRACT: Soft soils have been subject of research for many years, the main focus being the hydro-

mechanical behavior and numerical modeling of these soils. A case-study located near the Tagus river

where industrial buildings are founded on soft soil is presented. The soil is characterized after a

specific site investigation, which comprised field and laboratory tests. A comparison is made between

the implemented foundation solution, the solution proposed by the designer and alternative solutions

typically used in geological scenarios comprising soft soils. A numerical analysis was undertaken

using a 2D finite element software to understand the performance of each solution, which were then

priced and the cost-benefit relations assessed. The implemented foundation for the buildings

comprised a piled raft whereas the alternative systems considered were forced drainage and

vibroreplacement stone columns.

KEY-WORDS: Soft soils, soil characterization, foundation solutions, 2D FE analysis

1. INTRODUCTION

The expansion of urban areas leads to the search for new locations for the construction of civil

engineering works, some of which are considered complex and challenging in terms of various

constraints and difficulties in the design of the project. The presence of soft soils in the foundation of

buildings and embankments is a problem that is increasingly emerging with the expansion of urban

areas. In these cases, building loads can generate large settlements which evolve over time, and may

prevent a shallow foundation solution. This therefore requires the search for a solution of deep

foundation or soil treatment to ensure an effective foundation solution of the structure and services.

In this context, a construction site located in Sobralinho, Vila Franca de Xira, was subject of study. The

development included the construction of eight industrial buildings on soft ground. A deep foundation

solution comprising piles was proposed for the buildings. The pavements of the access roads were

constituted of embankments, and light weight fill was proposed by the designer in some areas.

However, the actual construction on site of these embankments used traditional fill, bringing great

concerns about the serviceability of the buildings and access roads in the medium and long term.

The construction site is the basis of the present dissertation, its main goals being: the characterization

of the soft soil based on in situ and laboratory tests; the evaluation of the local conditions; and the

comparative analysis of the performance of the implemented and alternative foundation solutions in

terms of movements and costs. The alternative solutions comprise the solution proposed initially by

the designer, force drainage using geodrains and vibroreplacement stone columns. Each solution was

modelled with a 2D finite element software.

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2. CASE STUDY: INDUSTRIAL BUILDINGS IN SOBRALINHO

The case-study of this dissertation is a development of eight industrial two-storey buildings to be

mainly used as storage and office area. The site is located in Sobralinho, V.F.Xira and it lies

approximately 150m from the Tagus River (Fig. 1). The buildings are named “A” to “H” and each one

is about 40x60 m2 excepts for building “H” whose size is 20x60 m2. The structure of the buildings

comprises with prefabricated concrete beams and columns. Access roads surround the buildings.

Fig. 1 – Location and delimitation of the construction work site

2.1 Site investigation

The site is located in an area where Tagus River alluvial deposits exist. These are recent deposits

dating from the Holocene era and comprising silty clays, silts and organic clays. These formations are

underlain by Miocene-age formations consisting of silty-sandy clays, sandy silts and sands medium to

coarse grained. Made Ground comprising sandy material with scattered fragments of varied nature

was encountered overlying the alluvium deposits.

In order to characterize the formations present at the work site, the Client has completed a program of

site investigation works. The information available from this survey campaign consisted of nine

boreholes, carried out using drilling rigs, where SPT tests were performed; eleven static penetration

tests with pore pressure measurement (CPTU), together with dissipation tests; and seven oedometer

tests on MOSTAP soil samples taken at four different locations in plan. Given the local geology and

the high loads carried by the vertical elements of the building, a deep foundation solution was adopted

in order to ensure an appropriate transmission of the loads to the sandy substrate of Miocene age.

2.2 Foundation system for the industrial buildings

The proposed foundation solution of the industrial buildings consisted of deep foundations comprising

reinforced concrete piles with a nominal diameter of Ø600 mm or Ø800 mm for the foundation of the

structural columns. Additionally, unreinforced concrete piles with a nominal diameter of Ø500 mm

were adopted for the foundation of the groundfloor slabs from half of building "C" to building "H".

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The pile capacity was assumed to be mainly due to end-bearing capacity with minimal contribution of

the shaft resistance. As such, the length of the piles was determined by the depth of bearing stratum

(NSPT > 60), with the piles penetrating three diameters deep into it. A grid of foundation beams

connecting the piles caps was designed to resist to bending moments transmitted by the structural

columns. The ground floors of the buildings are constructed on a load transfer platform which is

founded on concrete piles as shown in Fig. 2 and as explained in Sampaio (2014).

Fig. 2 – Cross-section of the adopted foundation system for the buildings

2.3 Foundation solution for the access roads

The initial idealized foundation system for the access routes comprised two type of embankments, one

made of lightweight aggregate fill material and the other of current granular fill material (typical unit

weight of 9 kN/m3 and 20 kN/m3 respectively).

Since the embankment loading is directly transmitted to the foundation soil, large settlements are

expected due to immediate and consolidation settlements that occur in the alluvial deposits.

The use of lightweight fill for the embankment was initially recommended for areas where the

predicted settlements extended the settlements considered admissible by the client. This was would

accur from half of the building "C" to the building "H". The cost of lightweight fill is however significantly

greater that standard fill. Thus, in an attempt to reduce the construction cost, this solution was

discarded by the client, who selected the use the typical granular material for all the access roads.

Fig.3 presents a cross-section of the embankment solution as implemented on site.

Fig. 3 – Cross-section of the foundation embankments for the access roads

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3. GEOTECHNICAL CARACTERIZATION

3.1 Ground Profile and Soil Classification

From the analysis and classification of samples recovered from the boreholes, it was possible to

describe the geology of the site, which is summarized in Table 1.

Table 1 – Geological reconnaissance of the study area

EPOCH FORMATION LITHOLOGY

Present Made Ground Fill of sandy to silty nature, reddish-brown, with disperse fragments of varied nature.

Deposits of silty to sandy nature, dark brown, with roots on top.

Holocene Alluvium

Reddish-brown and greyish sandy CLAY, with local interlayers of organic material

and disperse coarse flint GRAVEL.

Dark grey clayey MUD. Dark grey muddy CLAY.

Miocene Areolas de

Cabo Ruivo

Brown to red silty and sandy CLAY. Orange sandy SILT.

Orange medium to coarse SAND with coarse subrounded flint GRAVEL.

In order to collect more information about the ground profile and soil types, data from the CPTU test

were also used. Some of the derived values from this test are the friction ratio, Rf, the corrected cone

resistance, qt, and the excess pore pressure, ∆u. Ground profiling is one of the main applications of

this test and for this purpose the classification suggested by Robertson (1990, updated in 2010) was

adopted. The author proposed a chart (Fig. 4) that relates the normalized cone resistance, Qtn, to the

normalized friction ratio, Fr, creating in this domain (Qtn - Fr), areas indicating different soil behavior

types (SBTn), which are described in Table 2. An example of the output of this analysis made in CPTU

8 is presented in Fig. 5. This process was repeated for the total number of CPTU tests.

Colours were assigned to each SBT zone to facilitate the visualization and interpretation of profiles.

3.2 Soil Unit Weight

The unit weight was estimated based on CPTU results by applying the correlation (Eq. 1) suggested

by Robertson (2010), which made possible a continuous study of this property on the foundation soil.

236.1pqlog36.0Rlog27.0 atfw (Eq. 1)

pa = atmospheric pressure = 100 kPa

3.3 Stress History

The preconsolidation stress, ’p, was determined in 3 oedometer tests using the method proposed by

ASTM D2435. The in-situ effective overburden stress, ’v0, is estimated with the information of the 2

closest CPTU tests (unit weight and excess of pore pressure). Knowing ’p and ’v0 the

Overconsolidation Ratio, OCR, is determined dividing the first by the latter. The results are

summarized in Table 3. The three samples are typically organic clays. The CPTU tests also allowed

an estimate of OCR with depth as explained in Sampaio (2014). The method used proved to be very

accurate when comparing the estimates with the ones from the oedometer tests results on close

locations. Fig. 6 presents the estimates resultant from both tests.

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Fig. 4 – Normalized CPT Soil Behavior Type (SBTn) chart,

Qtn – Fr (Robertson, 2010)

Fig. 5 – SBTn profile obtained from CPTU 8

Table 2 – SBTn description

Zone Soil Behavior type

1 Sensitive, fine grained

2 Organic soils – clay

3 Clays – silty clay to clay

4 Silt mixtures – clayey silt to silty clay

5 Sand mixtures – silty sand to sandy silt

6 Sands – clean sand to silty sand

7 Gravelly sand to dense sand

8 Very stiff sand to clayey sand*

9 Very stiff, fine grained*

*Heavily overconsolidated or cemented

Table 3 - ’p, ’v0 e OCR values estimated from oedometer tests

Test Depth (m) ’p (kPa) ’v0 (kPa) OCR

E1 A2 3.5 91.2 41.3 2.2

E2 A3 7.5 158.5 61.8 2.6

E3 A3 8.2 151.4 68.1 2.2

3.4 Shear strength

3.4.1 Undrained shear strength, Cu

The data of CPTU tests - corrected cone resistance, qt, and excess pore pressure, u - was used to

evaluate the undrained shear strength of the soil, through the application of Eq. 3 and Eq. 4, solutions

suggested by Robertson & Cabal (2012). Fig. 7 shows the results of both solutions on CPTU 8. It is

important to note that Eq. 4 make good estimates when applied to very soft soils but it is of limited use

at this site due to the sand layers on the alluvium deposits which reduce the pore pressures and

therefore affects the estimates of Cu. Information about Nkt and NΔu can be found in Sampaio (2014).

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kt

vtu

N

qC

(Eq. 2 )

uu

N

uC

(Eq. 3 )

Eq. 3

Eq. 4

Fig. 6 – OCR estimates from CPTU and oedometer

tests results

Fig. 7 – Comparison of Cu estimates given by Eq. 3 and

Eq. 4 for CPTU 8

3.4.2 Friction angle

Typical ranges of critical friction angles were defined for the foundation soils: 32º to 35º for the

Miocene-age silty to sandy soils and 18º to 28º for Alluvium deposits and Miocene-age clayey soils.

3.5 Deformability and compressiblity

The Young’s moduli, E, and the constrained modulus, M, were estimated using two different

correlations with CPTU test results proposed by (Robertson and Cabal, 2012) and defined in Eq.5 and

6, for “Young, uncemented predominantly silica sands” and “Fine grained soils and organic soils”, respectively.

0vtE q'E (Eq. 4 )

0vtM qM (Eq. 5 )

The compressibility of the soft alluvial soils was also assessed through the oedometer test. The

parameters are summarized in Table 4. More information can be found in Sampaio (2014).

Table 4 – Compressibility and consolidation parameters from oedometer tests

Oedometer Test Cc [-] Cs [-] C e0 Cc/(1+e0) Cv (m2/s) kv (m/s)

E1 A1 3.4E-08 1.0E-09

E1 A2 0.64 0.13 0.02 0.84 0.35 4.5E-08 3.4E-10

E2 A2 1.5E-07 2.7E-09

E1 A3 1.4E-07 3.5E-09

E2 A3 1.42 0.28 0.02 1.51 0.57 1.2E-07 1.6E-09

E3 A3 1.34 0.27 0.02 0.74 0.77 6.8E-08 7.3E-10

E1 A4 1.9E-07 5.1E-09

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5 6 7 8

Dep

th (

m)

OCR

CPTU 8

CPTU 9

Oedometertests

0

2

4

6

8

10

12

14

16

0 100 200 300

Dep

th (

m)

Cu (kPa)

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3.6 Consolidation and hydraulic conductivity

The soil consolidation parameters, Ch and Cv, and hydraulic conductivity, k, were estimated from the

dissipation tests conducted in three of the CPTU tests and from all of the oedometer test. The method

suggested by Burns and Mayne (1998) for the determination of the parameter Ch was used. The

application of this method resulted Table 5. More information can be found in Sampaio (2014).

Table 5 – Ch and kh values determined from the dissipation tests

Test Depth (m) Ch (m2/s) M (MPa) kh (m/s)

CPTU 2 8.20 1.70E-07 48.5 3.44E-11

CPTU 5 5.99 2.50E-08 50.9 4.81E-12

CPTU 5 7.20 1.20E-06 85.1 1.38E-10

CPTU 6 3.86 2.00E-07 1.8 1.10E-09

CPTU 6 8.32 3.70E-08 36.7 9.88E-12

CPTU 6 10.95 1.75E-07 24.3 7.07E-11

The coefficient of consolidation, Cv, and the respective coefficient of permeability, kv, were also

calculated using Eq. 7 to Eq. 9 knowing t90 (time necessary to reach 90% of the dissipation of excess

pore pressures) for each loading stage. The mean values for all loading stages are shown in Table 4.

90

290

vt

hTC

(Eq. 6 ) vwv mCk (Eq. 7 )

'

e

e1

1m

0v

(Eq. 8 )

3.7 Design Ground Profile: Geotechnical zones and parameters

The aforementioned parameters assess were used to define geotechnical zones where soils had

similar hydro-mechanical behavior, but not necessarily the same geological lithology. Tables 6 and 7

show the geotechnical zones considered (ZG4 to ZG0) and their relevant geotechnical parameters.

Table 6 – Description of each geotechnical zone considered

GEOTECHNICAL ZONE ZG4 ZG3 ZG2 ZG1 ZG0

TYPICAL DESCRIPTION Muddy clay

Made ground and overconsolidated

muddy clay

Very stiff silty to sandy clay

Hard silty to sandy clay

Dense sand and silty sand

Table 7 – Geotechnical parameters of the soil

Parameter ZG4 ZG3 ZG2 ZG1 ZG0

Unit weight, (kN/m3) 14 - 16 16 - 18 18 - 20 19 - 20 20 - 22

Vertical hydraulic conductivity, (m/s) kv 2.2x10-9 2.2x10-9 1.2x10-8 1.2x10-8 5.0x10-6

Young’s moduli, (MPa) E’ - - 25 - 40 40 - 80 > 80

Poisson coefficient 0.30 0.30 0.30 0.30 0.30

Undrained shear strength, (kPa) Cu 10 - 20 40 - 120 120 - 200 > 200 20

Friction angle, (º) 24 24 24 26 34

Compression index Cc 1.14 1.14 - - -

Recompression index Cs 0.23 0.23 - - -

Creep index C 0.02 0.02 - - -

Initial void ratio e0 1.03 1.03 - - -

Overconsolidation ratio OCR 1.00 3.00 - - -

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4. NUMERIC MODELLING

4.1 Method of Analysis

A 2D finite element software was used to model different foundation solutions. The aim was to predict

the settlements resulting from the construction of the buildings and the embankments in the most

critical areas. The model used the geotechnical zones and parameters described in Section 3.7, to

which were assigned different constitutive models. The soft-soil-creep model was used for the very

soft materials (i.e. ZG4 and ZG3) since it takes into account most of the aspects of the complex

stress-strain behavior of these soils. The Mohr-Coulomb model was chosen for the remaining

materials since these would not make a strong influence on the global behavior of the solution.

4.2 Foundations solutions

The following solutions were studied: (1) the “as constructed” solution; (2) the one proposed by the

designer that uses lightweight fill embankments; (3) a soil treatment solution by forced drainage using

geodrains; and (4) a soil treatment solution with vibroreplacement stone columns. The models created

for each foundation solution are shown in Fig. 8 to Fig. 12, together with some relevant results. A

performance analysis is made in section 4.3 for each solution.

Fig. 8 – Model of Solution 1 (current granular embankment fill) and Solution 2 (lightweight embankment fill)

Fig. 9 – Deformed mesh (Solution 1, left). Evolution of displacements with time (Solutions 1 and 2, right)

Fig. 10 – Model of Solution 3 - forced drainage using geodrains

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Fig. 11 – Displacements of the buildings G and H at groundlevel (Solution 3)

Fig. 12 – Model of Solution 4 - Vibroreplacement stone columns (left). Deformed mesh (right)

4.3 Performance analysis

The displacements calculated for each case are: the maximum settlement in the buildings after 15

years, spav; the maximum relative settlement on the buildings, dspav; the maximum settlement on the

access roads after 15 years, sarr; the maximum differential settlement between the roads and the

buildings, smáx; and the maximum relative rotation inside the building, máx. The comparison of these

parameters for each solution is made in Table 9. The numerical analysis shows that embankments

made of current granular material fill (Solution 1) induce settlements up to 1000mm. Thereby the “as

built solution” is expected produce differential settlements between the roads and buildings up to

1000mm in the long-term. This study also showed that Solution 2 (embankments of lightweight fill) can

reduce the settlements almost in half proving a better performance than the implemented solution.

Table 8 – Estimated displacements for each foundation solution

Performance Parameter

(1) Implemented (2) Lightweight embankment

(3) Forced drainage

(4) Stone columns

Building G spavG * (mm) 6 5 143 -

Building H spavH * (mm) 10 8 158 52

Building G dspavG * (mm) 3 2 79 -

Building H dspavH * (mm) 6 5 46 12

Access Road sarr (mm) 978 564 20 2

Building + Road smáx (mm) 968 556 138 5

Building G/H máx * 1/1000 1/1700 1/160 1/950

Solution 3 shows settlements on the access roads much lower than the previous ones. However, a

building damage assessment should be undertaken given the high smáx and máx values. Solution 4

appears to provide the best overall performance since the estimated movements of the ground floor of

the building and settlements of the roads in the long-term are not expected to cause serious concerns.

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4.4 Pricing

The pricing of each solution was made for the area covered by the numerical models. The costs were

based on Portuguese market prices and are presented in Table 10. Detailed information about these

prices can be found in Sampaio (2014).

Table 9 – Estimated costs of each solution for the studied area

(1) Implemented (2) Lightweight embankment (3) Forced drainage (4) Stone columns

600,000 € 622,000 € 734,000 € 2,800,000 €

5. CONCLUSIONS AND FURTHER INVESTIGATIONS

The numerical analysis show that, for the implemented solution, building and infrastructure damage is

expected to occur over the years due to high settlements predicted (up to 1000mm), making inevitable

the need of future repairing. The solution proposed by the designer (solution 2) showed lower

settlements (560 mm), but still too high to avoid repairing in at least 15 years. The cost of this solution

is about 4% higher than the one implemented.

Forced drainage appears to provide a low-stiffness foundation system to the buildings, with

unacceptable relative settlements and rotations that may cause the ultimate limit state being reached

(máx close to 1/150). However, if it were possible to implement a sufficiently rigid foundation for the

buildings, this solution could behave better than the solutions 1 and 2, since it benefits from a

significant reduction in the differential settlements between the buildings and roads.

Solution 4 stands out for its better performance. It is the only solution where both total and differential

settlements across the area of the building H are acceptable, ensuring no need for repairs in the

future. However, the cost of this solution is considerably greater than any other solution analyzed.

Future studies could comprise further analysis of the alternative solutions. For the solution of forced

drainage consideration should be given to the design of a cost-effective foundation system for the

buildings able to prevent excessive differential settlements. For the solution of soil treatment by

vibroreplacement stone columns, further analysis and optimization of the layout of the stone columns

could be undertaken, aiming to reduce the cost as much as possible. Other different foundation

solutions could also be analyzed in the future or a combination of the several already studied.

The monitoring of the settlements of the access routes and the costs associate with maintaining the

serviceability of these would provide useful data for future projects, and allow alternative solutions to

be compared to the as constructed solution.

6. REFERENCES

Robertson, P. K., & Cabal, K. L. (2012). Guide to Cone Penetration Testing for Geotechnical

Engineering (5th Edition ed.). California: Gregg Drilling & Testing, Inc.

Sampaio, A. (2014). Industrial buildings at Sobralinho, V.F. Xira - Geotechnical aspects on the

behaviour of the structures and access roads. MSc. Dissertation. Instituto Superior Técnico, University

of Lisbon, Lisbon. (in Portuguese)