Unusual building settlement damage due to horizontal soil movements

S. Hov1 Atkins, Stockholm, Sweden

ABSTRACT

Soft clays have a tendency to show great deformations over time when loaded due to consolidation of excess pore water pressureand creep. Normally, this results in vertical settlements, which are often taken care of in geotechnical design work by placing theload on piled foundations. It is also well known that loading from for example embankments can cause horizontal deformations. This however, is not always taken into account in geotechnical design work. This paper discusses a case study of unusual soil deformation causing damage to a large industrial complex in Sweden, founded on precast concrete piles in a soft quick clayformation. A vertical crack, roughly 5 cm in width, had formed most likely due to horizontal soil deformations. It was thoughtthat this could increase the bending moments in the piles putting the foundation safety at risk. A ground investigation wasperformed using triaxial and oedometer laboratory tests and subsequent modelling using the finite element method. Thehorizontal soil movements could successfully be modelled and the risk of pile buckling was assessed. An unexpected effect ofpotential underpinning works, to cease the crack development, was found to have a possible reverse effect on the foundationsafety. Keywords: clay, laboratory test, creep, deformation, settlement, numerical analysis, foundation.

1 Geotechnical engineer, Atkins Sverige AB, Stockholm, Sweden, solve.hov@atkinsglobal.com.

1 INTRODUCTION

The work presented in this paper is the result of an investigation attempting to assess the cause of, and potential measures for, a building damage on an industrial complex. The damage consists of a vertical crack between two buildings in the Stockholm metropolitan area.

At an early stage, it was concluded that horizontal soil movements mainly in an underlying soft clay formation caused the crack. It is well known that horizontal deformations take place when clay is unevenly loaded. A ground investigation was performed in order to be able to model the soil deformations. The foundation safety was then assessed.

Part of this project has earlier been described by [1]. New measurements and analysis have since then been made.

2 DESCRIPTION OF PROJECT

The industrial complex consists of a one-storey warehouse building approximately 133 x 225 metres, and a three-storey office building 15 x 18 metres, which is semi-attached to the warehouse building, see Figure 1. The buildings are constructed of prefabricated concrete elements and have crawl spaces, and are both founded on precast reinforced 235 x 235 mm concrete piles. The warehouse building is founded on firm ground at its southern part.

Figure 1. Plan view.

Figure 2. Photographs showing the vertical crack.

The industrial complex was constructed in

1998 and an outward movement of the office building was noticed shortly after. The movement formed a vertical crack between the two buildings, and was measured to be 3 to 5 cm in width 10 years after construction. The crack formation can be seen in Figure 2.

Also, large settlement of the ground surrounding the buildings has occurred. The ground has the last 10 years been levelled twice by the placement of 10 to 15 cm each time.

In an attempt to cease the movements, the office building was underpinned in 2005 with two concrete blocks founded on raking piles. These blocks were constructed close to the northern facade of the office building. This attempt however, largely failed because of small vertical loads on the raking piles making them ineffective.

3 GROUND CONDITIONS

As a part of the ground investigation, field and laboratory works was performed consisting of cone penetration tests, disturbed and undisturbed sampling with subsequent soil description, fall-cone tests, incremental oedometer and triaxial tests. Survey measurements and ground water measurements were also made. Oedometer tests were performed on clay samples to investigate the clay deformation properties and creep characteristics. Triaxial tests were performed to investigate stiffness and shear strength both in vertical and horizontal directions.

The industrial complex is situated on an extensive sedimentary area, consisting of glacial and postglacial clays, with surrounding rock outcrops and end till formations.

From an originally nearly horizontal ground surface, fill was placed around and below the buildings. The fill height is around 2 m around the office building and around 1 m north of the office building, see Figure 1 and 3. The fill consists of sand, gravel and dry clay crust. The soils beneath this fill consist of 1 to 1.5 m clay crust over softer clay, which extends to a total depth of approximately 8 to 9 metres. A thin layer of organic clay was found between the clay crust and the inorganic clay formation.

The ground water level in the till formation underlying the clay, varies between 1.2 and 1.5 metres depth below the ground surface.

Figure 3. Cross-section through the office building.

The clay was classified as having very low to extremely low undrained shear strength, cu, varying between 9 and 19 kPa with clear strength anisotropy. Natural water contents varied between 60 and 70 % with liquid limits far below, around 45 to 60 %. Hence, the clay is highly sensitive, with St = 30 – 55, and is partly defined as quick clay. Further description of the clay is given by [1].

Quick clays are highly sensitive to mechanical disturbance, which reduces the bearing capacity substantially, and at a completely remoulded condition the clay behaves like a thick liquid. Such clays are common in the Scandinavian countries.

It is well known by geotechnical engineers and researchers that embankments can cause horizontal movements. This can be explained by basic soil behaviour and particularly by the rotation of principal stresses and the direction of plastic strains. Different loading conditions, such as an embankment on horizontal ground, will cause the major principal stress, σ1, to rotate. Figure 4 schematically illustrates this. This is the case for all uneven loading conditions.

As the direction of plastic strains coincide with the direction of the major principal effective stress, the plastic strain, in this case primary consolidation and secondary creep deformations, will also have a horizontal component. This is particularly noticeable in soft clays with low K0 values.

Figure 4. Schematic illustration of principal stresses rotation

in soft clay beneath an embankment.

4 GEOTECHNICAL MODELLING

Modelling was performed using the finite element programme Plaxis, and the ‘soft soil creep model’. The model, which is based on the Modified Cam clay concept, takes into consideration the soils consolidation properties, creep characteristics, and shear strength. A full description is given by [2].

The major advantage of this model compared to others is that it captures the basic soil behaviour of soft clays, particularly the pronounced creep strains. The soft soil creep model is however, very sensitive to changes in over-consolidation ratio, OCR, and changes in OCR highly affects the calculated strains. In this case this could be dealt with, as both settlements and horizontal movements largely were known.

As a result, the model could be ‘calibrated’ with respect to this. The purpose of the modelling was firstly to investigate if the movements were to continue for much longer, and if so, what the magnitudes would be. The model was set up so that soil deformations were calculated as a result of the fill load. The buildings were not modelled, since the end-bearing concrete piles take all structural loads. The geometric model was approximately as shown in Figure 3.

Input parameters to the model consists of a Mohr-Coulomb failure criterion c’ and φ’, which was interpreted from triaxial tests. The deformation parameters are defined by λ* and κ* which describes the consolidation and swelling properties respectively, and µ* which defined the creep behaviour in a strain versus ln time plot. These parameters were interpreted from incremental oedometer tests.

The oedometer load increments were held constant roughly one week to be able to get a reliable interpretation of the creep strain. Pore water pressure measurements were also made in the oedometer. Figure 5 shows an example of µ* interpretation.

Figure 5. Example of an oedometer load increment and

interpretation of the creep parameter µ*. The oedometer interpretation of λ* and κ*

were very close to the empirical correlations with the plastic index as given in [2].

The calculations confirm the initial assumption that the deformations caused by the fill are to a large extent horizontal. Figure 6 shows the horizontal displacement beneath the office building. Because the piles are casted in the pile caps, the movement at the top was

assumed to correspond with the building movement, i.e. just above 3 cm after 10 years. This was predicted to increase with another 2 cm the next 20 years.

Figure 6. Horizontal displacement after 10 and 30 years of movements in Section A, located as shown in Figure 3. lk =

buckling length and δ = deflection, as described below.

Figure 7. Measurements of, and predicted (calculated),

building movement.

5 MEASUREMENTS OF MOVEMENTS

The crack expansion has been measured since the ground investigation and modelling work was completed. Measurements are done using two permanent steel rulers drilled into the warehouse building on each side of the office building.

Figure 7 shows these readings. The movement back and forth is due to variations in temperature causing the concrete foundation to expand and shrink. Despite this effect, an apparent total outward movement can clearly be seen. The predicted continued movement, i.e. approximately 1 mm per year, is also shown in the figure, with relatively good agreement.

6 RISK OF BUCKLING OF PILES

It is assumed that the piles follow the surrounding soil deformations, as experience indicate in similar conditions, [3]. A few case histories in the Stockholm area also show that there is a risk of buckling of slender concrete piles when subjected to a forced deflection. This is due to additional structural bending moments.

As the modelled horizontal movements seem to be realistic, e.g. Figure 7, the calculated deformation pattern in Figure 6 is used to measure the forced deflection of the piles due to the surrounding soil deformations.

The additional bending moments cause the axial bearing capacity of the piles to decrease. It was therefore decided to make an assessment on the risk of buckling of the piles beneath the office building.

Calculation of the risk of buckling was performed according to the recommendations given by the Swedish Commission on Pile Research, [4]. The method takes into consideration the structural strength of the pile material and the strength and resistance of the surrounding soil. The calculation method was slightly modified to account for the forced deflection caused by the horizontal soil movements. The design recommendations are based on partial coefficients. For calculation purposes, the total deflection of a pile is assumed

to consist of three components within the pile’s buckling length lk as shown in Figure 8.

Figure 8. Deflection of a pile (δ0 = initial geometric

curvature, y0 = deflection due to axial loading, δs = forced deflection due to soil deformations, Pk = buckling load).

Firstly, the pile has an initial geometric

curvature δ0 due to manufacturing and installation factors. This is assumed to be lk/300 for concrete piles according to Swedish practice.

Secondly, an additional deflection, y0, is due to axial loading of the pile. This deflection is calculated as a function of the initial deflection and axial load, see equation inserted in Figure 8.

Finally, a forced deflection caused by horizontal soil movement δs is interpreted over the buckling length after 10 and 30 years, respectively, see Figure 6.

All deflections are assumed to be sinus-shaped over the buckling length in an elastic medium. Bending moments from the initial curvature and the additional deflection due to axial load is calculated as:

kPP

PM−

=1

5.0 δ (1)

where P = axial load, Pk = buckling load. The buckling load depends on the surrounding soil stiffness and structural properties of the pile.

The bending moments due to the forced deflection from horizontal movements, δs, are calculated using the differential equation of the elastic deflection curve:

IE

Mlx

l dkks ==

ππδδ sin'' 2

2 (2)

The resistance from the surrounding soil is accounted for by a sub-grade reaction modulus, which defines the horizontal stress along the pile as a function of the additional deflection, y0, up to a limit value. This limiting value is normally reached after approximately 2 to 4 cm of deflection, after which a reduced modulus is assumed. The modulus is derived by empirical relationships with the clays undrained shear strength and by passive triaxial tests results.

The bearing capacity of the pile is defined as either the axial structural pile capacity neglecting the surrounding soil, or the moment capacity of the pile accounting for surrounding soil resistance, whichever is the lowest. The moment capacity will decrease with time as the horizontal soil movements continue.

Results of the pile bearing capacity calculations are shown in Figure 9.

Figure 9. Pile bearing capacity of the existing pile beneath the

office building as a function of time and horizontal soil movements.

7 DISCUSSION AND CONCLUSIONS

The calculations show that if the movements are allowed to continue, the ultimate state bearing capacity will exceed the load at ultimate state, i.e. there is no risk of pile buckling in the near future. If however, the office building were underpinned, there would be a risk of buckling of the existing piles. This as the horizontal soil deformations would continue whilst the top of the piles would be in a fixed position as shown in Figure 6, increasing the pile deflection even further. It was thus recommended that any such works must be accompanied by removing the fill so that the horizontal soil movements are ceased.

The project is an example of the difficulties involved with soft ground geotechnical engineering, and how this can be handled by using appropriate soil models which are calibrated to real measurements.

An unexpected result of potential underpinning works was found, having a possible reverse effect on the foundation safety, as it would increase the risk of buckling of existing piles. In the quick clay formation, this could have a devastating consequence.

ACKNOWLEDGEMENT

The Author is grateful to Rolf Rosén, Golder Associates AB, and Thomas Linder, Carpri AB, for their support and valuable comments during the project.

REFERENCES

[1] S. Hov & R. Rosén, Ett exempel på beräkning av horisontella jordrörelser och pålars reducerade lastkapacitet (In Swedish), Bygg & Teknik, No. 1, 2009.

[2] R. B. J. Brinkgreve, W. Broere & D. Waterman, Plaxis 2D – Version 8, ISBN 90-808079-6-6, 2004.

[3] A. Fredriksson (2008), personal communication. [4] A. Fredriksson, P.E. Bengtsson & Å. Bengtsson,

Beräkning av dimensionerande lastkapacitet för slagna pålar med hänsyn till pålmaterial och omgivande jord (In Swedish), Rapport 84a, Commission on Pile Reserach, 1995.