37

Foundations in urban areas. Codes and standards

V. M. Ulitsky State Transport University, Saint Petersburg, Russia A.G. Shashkin, C.G. Shaskin, M.B. Lisyuk NPO Georeconstruction-Fundamentproject, Petersburg, Russia

ABSTRACT: The report presents the main stages of design of structures in urban areas, which should be re-flected in modern codes. Some examples include preliminary assessment and historical analysis of a geotech-nical situation, soil-structure interaction analysis, determination of foundation settlement, and evaluation of a technological impact on preservation of existing buildings.

Keywords: foundations, soil-structure interaction, settlement, codes, urban environments

1 INTRODUCTION

In congested urban areas all geotechnical design so-lutions and parameters are quite often limited more strictly by the issues of preservation of adjacent buildings, important structures, historical monu-ments, etc., rather than new buildings and structures under design.

Thus the main principle for a geotechnical engi-neer designing foundations of buildings in urban ar-eas is first of all not to cause any harm to the histori-cal landmarks in the neighborhood.

Therefore a serviceability limit state design de-sign of foundations in urban areas takes on special significance. One of the features of this approach is that the buildings adjacent to the construction site must be also analysed in terms of additional settle-ments, differential settlements, tilt, etc.

Consequently it is very important to use soil-structure interaction approach in geotechnical de-sign. In fact, joint analysis of new and existing buildings is a very efficient tool in providing safe and cost-efficient design in urban areas.

Many researches analysed and compared different codes for various geotechnical problems, including design of pile and shallow foundations, soil-structure interaction aspects, formulation of safety factors, etc. (e.g. Bauduin et al, 2003, Fadeev et al, 2006, 2007, Frank, 2006, 2006a).

Based on this research and practical experience of

the authors of the report it can be suggested that modern codes and standards would include the fol-lowing items:

- Preliminary assessment of a geotechnical prob-lem;

- Requirements for site investigations and soils characterisation and tests for urban areas, including environmental investigation;

- Main requirements for project; - Components of geotechnical supervision of the

project, which comprise a historical analysis of a current geotechnical problem, analysis of develop-ment (reconstruction) problem, and requirements for geotechnical calculations;

- Taking into account risk factors while using dif-ferent geotechnologies;

- Requirements for monitoring to accompany the works, including quality assurance, as well as moni-toring of important structures after their construction (Powderham, 1996, 2003). Some of these items will be considered below using examples from geotechnical practice in St. Peters-burg.

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2 PRELIMINARY ASSESSMENT OF A GEOTECHNICAL PROBLEM

Preliminary assessment of a geotechnical problem should be based on:

- Information on purposes of a new develop-ment/reconstruction object;

- General plan of the object; - Plan of underground networks and communica-

tions and their significance; - Major construction design solutions (including

underground spaces); - Approximate values of foundation loads; - Archive data on engineering and geological

condition of the construction site. As a part of the preliminary assessment of a geo-

technical problem the following works are carried out:

- Preliminary assessment of condition of the adja-cent buildings on the basis of visual examination;

- Determination of the geotechnical category of the new development/reconstruction object;

- Assignment of investigation and examination works in accordance with a certain geotechnical category;

- Establishing all possible options of substructure construction or, if necessary, options of foundation strengthening/underpinning of the building under re-construction;

- Approximate cost estimation of the substruc-tures construction and/or foundation strengthen-ing/underpinning works;

- Approximate cost estimation of all geotechnical works.

Two examples of the preliminary assessment of a geotechnical problem are given below dealing with the projects of land reclamation and a high-rise building construction.

2.1 Land reclamation The first example deals with the preliminary assess-ment of a geotechnical problem on Vasilievsky is-land in St. Petersburg. The concept of forming a new territory to the west of the seashore of the island has been worked out recently. In accordance with the concept the area of the territory is 4.5 square kilome-tres. Its elevation will be 3.2 meters above the sea level.

The purpose of the research was to estimate on the first stage the compaction methods of the natural subsoil and the reclaimed dredged soils, and on the second stage – to estimate the foundation options for the buildings of the sea port terminal (fig. 1, 2).

The land reclamation is intended with hydraulic filling of sands with compaction. Required volume of sand is about 30.000 m3. The depth of water area is up to 6.5 m.

Soil profile to the depth of 50 m features a 5.3 m fill stratum near the coast. Below the fill there are

marine deposits of sand of 0 to 6.3 m thick and silt of 0 to 14.3 m thick. They are underlain by lacus-trine-glacial soils, represented by clayey soils up to 21 m thick. Below them there are moraine clayey soils up to 23 m thick. The roof of these soils changes from the depth of 25 m in the northern part of the territory to 8 m in the southern part. From the depth of 24 to 49 meters there are bedrock protero-zoic clays.

The reclaimed soil is medium and coarse-grained sand with density of 15 до 17 кN/м3 and deforma-tion modulus E=20-25 MPa for freshly reclaimed soil, and E=30-35 MPa one month after the reclama-tion.

According to the classical theory of one-dimensional consolidation the calculated settlements of the area are from 1 to 50 cm, and eighty-percent degree of consolidation is reached over the period of 0.7 to 13 years. This period depends on thickness of deformable soils and the depth of the water area. Development of the territory is intended in two years after filling. Therefore the vertical drains were rec-ommended.

In the framework of a preliminary assessment of this geotechnical problem the following works have been completed:

1. Estimation of the predicted time of consolida-tion of the natural subsoil and freshly reclaimed soils

2. Assessment of the efficiency of the proposed methods of reclamation, including:

- Expected consolidation time; - Degree of soil improvement; - Possibility of the use of reclaimed soils as a

subsoil for new buildings; - Possible time schedule of the beginning of the

constructional works on the reclaimed territory. 3. Estimation of different types of foundations for

new buildings including a possibility of soil im-provement.

4. Cost analysis of the proposed foundations. 2.2 Project of a high-rise building Another example is the preliminary assessment of a high-rise building on soft highly compressible clayey soils. The proposed location of the high-rise buildings was the western part of Vasilievsky island. As discussed in the previous section, the soil profile in this area is characterized by the thick stratum of soft highly compressible soils.

39

Fig. 1. Preliminary assessment of a geotechnical problem. Plan of land reclamation in western part of St. Petersburg

Fig. 2. Contours of settlements of central part of the reclaimed area (m). 3D analysis

The height of the proposed buildings was about

300 m. The main concern of the architects and de-signers was that a heavy high-rise building may de-velop big pressures on the subsoil and this might lead to the inadmissible settlements and tilting of the building.

For assessment of the geotechnical problem soil properties have been taken from the available ar-chive data. The architectural design solution with all important structural elements has been introduced into the design scheme, and all loads from the build-ing to subsoil have been assessed (fig. 3). The main conclusion of a preliminary geotechnical assessment was the following: to construct this building it is necessary to provide a distribution of pressure to big areas of subsoil, otherwise big non-uniform settle-ments might be expected.

(a)

(b)

Fig. 3. Preliminary assessment of a geotechnical problem: (a) Proposed solution on forming a spatial stiffness of a high-rise building; (b) Settlements of the subsoil (m)

In this case the simplest option for stiffness increase is to build cross bearing walls within the stylobate as well as the underground structure. However it would restrict employment opportunities of underground area (which could be used for parking-lots, stores etc.). This problem could be tackled by distribution of stiffness at different levels of the building. The design scheme envisaged creation of rigid walls

Central mono-lith core of stiff-ness

Lateral monolith cores of stiffness

Internal mono-lith walls of under-ground structureand stylobate

Internal mono-lith walls of 4-11 floors

40

above the stylobate, in high-rise part of the building (within 5-8 lower floors), as well as within the sty-lobate and underground structure, i.e. beyond the high-rise part (fig. 3).

Such a solution was very important for the archi-tects and designers of the building. Besides, a part of this work on a preliminary assessment of a high-rise building would include a technical prescription for site investigation and soil characterisation.

3 SOIL CHARACTERISATION AND SITE INVESTIGATIONS AND IN URBAN AREAS

Soil characterisation should be made with account of a preliminary assessment of geotechnical problem. This assessment for important projects can include a task for the detaled site investigations.

Generally, soil investigations in urban conditions should include a study of archive documents, bore-holes drilling and taking samples, static probing, laboratory and in-situ research of soils and under-ground waters, and making a forecast of geological condition changes with account of environmental conditions (Dashko et al, 2002).

Foundation and subsoils investigation in general includes: archive material study, boreholes drillings for carrying out foundations measurement and con-dition survey, taking samples, static and dynamic probing, laboratory researches of subsoils, ground waters and foundation materials, piles examination to determine their preservation, length, integrity and bearing capacity (including if necessary test of piles in pile rafts).

Recent advances in soil characterisation are de-scribed in reports of Jaksa et al (2005), Schnaid (2005), Wolsky & Lipiński (2006).

Site investigations should provide enough infor-mation to determine parameters of modern non-linear models of soils for soil-structure interaction calculations. This requirement should be reflected in codes. The models applied in calculations should be not too complicated or oversimplified (Van Impe, 2004) and use of a few well defined and physically meaningful parameters. Here we refer to the soft soil model developed by the authors (fig. 4). This model uses the data from the oedometric and triaxial CU and CD tests (fig. 5).

This model has been used in soil-structure inter-action calculations performed for many historical and new buildings in St. Petersburg (see Sections 4, 5, 7).

4 HISTORICAL ANALYSIS OF A

GEOTECHNICAL PROBLEM

Historical analysis of a geotechnical problem is very important for projects dealing with preservation

Fig. 4. Principle of elasto-plastic model build-up through ap-proximation of τ-γ and p-εv dependencies. 1 – contours of equal volumetric deformations, 2 – contours of equal shear deforma-tions at loading stage, 3 – contours of equal shear deformations at unloading-reloading stage

p (kPa)

-0.2

-0.18

-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

00 50 100 150 200

(a)

p (kg/cm2)

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

00 1 2 3 4 5 6 7

(b)

Fig 5. Selection of parameters from the traxial (a) and oe-dometric (b) tests: 1 – experimental data; 2 – test simulation using the model

or reconstruction of historical monuments and their foundations (Ulitsky et al, 2003).

Such analysis should include the following steps: - Analysis of the actual stress - strain conditions

of subsoil of preserved buildings, and, if necessary, of adjacent buildings;

- Estimation of the influence of present vibration background on settlement development;

- Estimation of ongoing settlements of buildings (under own weight and outside factors), that is de-

2

1εz

2

1

εz

41

fined through calculations or observations the loca-tion of geodetic marks and gauges;

- Estimation of the allowable additional settle-ment of the existing buildings during reconstruction works or new development.

For important projects it is also necessary to per-form the following works:

- Historical analysis of foundation behaviour of preserved/reconstructed buildings and buildings ad-jacent to reconstructing object or to new develop-ment together with substructure behaviour of the ex-isting buildings;

- Calculations of the total assumed deforma-tions and percentage of different causes in settlement development of the existing buildings.

(a)

(b) Fig. 6. (a) The building of the Admiralty, (b) historical founda-tion under the Admiralty tower

An example of a historical analysis is the Admiralty building in central St. Petersburg (fig. 6a). The building has well pronounced cracks. The purpose of the historical analysis in this case was to find out the reason of these cracks development to make decision about further strengthening or preservation of the monument.

The Admiralty tower was constructed in 1734 by I. Korobov. In 1811-1823 the Admiralty was recon-structed by A. Zakharov, who enlarged the tower and made some structural rearrangements.

The tower has a rigid structure. It rests on stone foundations supported by wooden piles (fig 6b). The subsoil under the tower is loaded more than the ad-jacent lower wings of the building. Therefore the tower suffered bigger settlement than the adjacent wings. The cracks appeared in the wings near the lo-cation of windows.

The subsoil of the Admiralty is comprised of fine-grained saturated sand of medium density and soft clayey sands. A thorough survey of the founda-tion has been made. The stone foundations were in-spected by a mini TV camera lowered down the sur-vey holes predrilled through the stone foundations. It was found out that the average percentage of voids in the foundation body was in the range of 1 to 10%. General condition of the foundations was found to be satisfactory, except for one part of the foundation under a transverse walls of the tower.

The structural survey of the building made it pos-sible to identify all cracks in the walls. It was dis-covered that many cracks appeared in the bearing walls of the tower.

To find out the reasons of the cracks development in the tower a 3D soil-structure interaction analysis has been performed. All findings made during the geotechnical and structural survey of the building have been taken incorporated into the design scheme.

Initial construction and consequent reconstruction of the building have been modelled. First the defor-mations of old Korobov’s Admiralty tower (1732) have been assessed, and then the modifications of the design scheme have been made with account of the added walls (1816) and additional loads (fig. 7). Thus, the real construction history had been simu-lated.

The contours of settlements accumulated after the reconstruction of the building are shown on fig. 7. The total estimated settlements have a value of about 20 cm. It should be noted that the soil model used in the analysis can take into account a long-term creep of the Admiralty’s subsoil.

Brick wall

Granite stabs

Sanf fill Saturated sand

Granite cocle

Ground floor

Sewer Water pipe

Foundation made of limestone slabs

m BC m BC)

m BC)

m BC)

Medium sand

42

Fig. 7. Contours of computed of settlements of the Admiralty (cross section along the symmetry axis)

The performed analysis has made it possible to iden-tify the reasons of the cracks development in the structural elements of the building. The main crack appeared in the tower wall adjacent to the lower wing. The reason of this crack development is the non-uniform settlement of the building. The devel-opment of settlements may still go on, which can be explained by a long-term creep of the subsoil. The non-uniform settlement leads to the generation of shear stresses in the walls with their maximum val-ues up to 235 kPa (fig. 8a).

(a)

(b)

Fig. 8. Calculations of the structure of the building: (a) con-tours of the shear stresses in the masonry (kPa), caused by the differential settlement of the tower and the lower wing; (b) zones of the possible development of the cracks caused by shear deformations of the transverse wall

The shear stresses cause the development of the cracks in the tower walls (fig 8b). There is a good correspondence between the soil-structure interac-tion calculation results and the observed behaviour of the building (fig. 9).

Thus, the conducted historical analysis of this geotechnical problem taking into account joint be-haviour of the Admiralty’ structure and subsoil as well as the construction history helped to identify the reasons of the deformation of this famous monument in the central Saint Petersburg.

Fig. 9. Development of cracks in the Admiralty tower

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5 DESIGN OF FOUNDATIONS USING SOIL-STRUCTURE INTERACTION APPROACH

A lot of research has been done recently in design of foundations with account of the modern codes (Bauduin, et al, 2003, 2005, Frank, 2006, 2006a, Ilichev and Fadeev, 2003, Fadeev, 2006, 2007, and others).

Many design codes either recommend or directly prescribe application of soil-structure interaction calculations (Frank, 2006, 2006a). To fulfill these requirements it is important to employ the most up-to-date methods of SSI analysis.

Local geotechnical codes in St. Petersburg require using soil-structure interaction analysis under the following conditions:

- Uneven compressibility of soil underneath foundation footing with underlying natural subsoil and underneath skin friction piles;

- Undermining/underexcavation in locations where subsoil is likely to change its properties ow-ing to underground construction underneath already constructed or envisaged objects (metro, large di-ameter sewer collectors, underground passages, etc);

- Development of additional settlements caused by ongoing or envisaged new development in the vi-cinity, i.e. in the area of envisaged influence and possible risk (TSN 50-302-2004).

The main advantage of modern SSI calculations is to combine the latest achievements of soil me-chanics and superstructure calculations. This combi-nation can be provided only through FEM joint modeling of 3D subsoil bulk and superstructure.

Now SSI calculations are successfully used in analysis of high-rise building, underground struc-tures, tunnels, bridges, railways, etc (see, for exam-ple, Brandl, 2006, Katzenbach et al, 2006, Lee et al., 2005, Mroeh et al, 2002, Shashkin, 2006, Ulitsky et al, 2006, Zdravkovic, 2005).

5.1. Main effects of SSI calculations The main effect of soil-structure interaction behav-iour can be expressed in terms of a redistribution of stresses in subsoil and forces in superstructure. Dif-ferential settlement can change, compared to calcu-lations with no account of superstructure stiffness, and structural elements can acquire new forces due to differential settlements. As a result the values of calculated differential settlements face considerable modification compared to traditional calculations. Some details of the main effects of coupled calcula-tion of soil and superstructures with various founda-tions are given below. Slab foundations As is well known the elasticity theory problem fea-tures values of infinite stresses under rigid plate edges. Since real material cannot sustain infinite

stresses, the contact pressure graph would have a saddle shape.

This uneven graph of pressures should definitely reflect in stresses of the superstructure. As a rule, ri-gidity of foundation slabs is lower than that of walls. Thus differential settlements predominantly will be carried by longitudinal and transversal bearing walls of the superstructure.

Therefore SSI calculations results are very impor-tant to superstructure designers because a pro-nounced increase of forces can be taken into account in design of the superstructure. If structures are made of cast-in-place reinforced concrete the zones of stresses concentration must be additionally rein-forced. In case of brick buildings increase of stresses in walls may be inadmissible.

Therefore it’s necessary either to envisage foun-dation providing reduction of total and differential settlements (for instance, piles) or to create a rigid structure in the lower part of the building which would carry the additional forces.

Pile foundations Certain soil-structure interaction effects relevant for raft foundation behaviour can also characterize pile foundation behaviour. The main effect of SSI ap-proach is stress redistribution in the structural ele-ments and foundation, which results in increase of forces in corner piles and subsequent unloading of central piles (Katzenbach and Reul, 1997). With long piles and close pile grid distance this effect would act within the entire pile field.

Generally, the codes should insist that in com-bined analyses of subsoil-foundation-superstructure complex it is necessary to take into consideration probable deviation from theoretical of characteristics of building, subsoil and thickness values of strata. The most effective in calculations is such influence that may be generated by adverse combinations of those characteristics.

It is necessary to conduct several calculation se-ries of a system with various calculation characteris-tics of its constituents with the objective of estab-lishing optimum foundation solution. This will allow to reduce risk from new development and recon-struction, for example, on soft soils where time vari-able properties may be present.

An example of a pile foundation behavior of a 16-storey building located on soft soils in St. Petersburg is given below (fig. 10). In this problem the model-ing of soil was carried out by application of viscoe-lastoplastic model (Shashkin, 2006). Dependencies between settlement in time and load increments were built-up. In this problem redistribution of pile forces (fig. 11) takes place during a relatively small period of time (about 10 years). It can be seen that differ-ence in pile forces increases in time. This phenome-non is linked to differential settlement development.

44

Other examples of SSI calculations are shown in the sections 2, 4, and 7 (see fig. 2, 3, 7-9, and 17).

6 INFLUENCE OF GEOTECHNOLOGIES ON ADJACENT BUILDINGS IN URBAN AREAS

When considering the issue of neighbouring build-ings preservation or strengthening the following rule must applied: means of protection must be adequate to provisional impact.

Usually it is not too difficult to establish impact associated with loading or unloading of subsoil, while the influence rendered by technological fac-tors is much more complicated to define. It is rec-ommended to attempt calculation of impact gener-ated by various piling technologies onto subsoil, to established the importance of relevant technological factors, as well as the dimensions of impact areas, for example, of pile jacking (oscillating), pile driv-ing and vibration techniques of piling and sheet pil-ing in relation to existing structures have been also established (Poulos, 2003, Ulitsky, 2003). It is espe-cially important for urban areas with marine soft soils which often are structurally unstable media in-clined to remolding at externally generated impact

which procedure is accompanied by reduction of their mechanical properties, such as bearing capacity and strength and increase of their compressibility.

As can be seen from analysis of deformed build-ings in central Saint Petersburg, (fig. 11), the portion of dilapidation brought about by works implementa-tion drawbacks of adjacent construction is 39%.

R1.14%

R2.21%R3.1

2%

R1.37%

R1.217%

R3.339%

R3.217% R2.1

13%

Figure 11. Causes for damage to existing buildings during ad-jacent construction in St. Petersburg: R1.1 – deformation causes related to mistakes in site investiga-tion/condition surveying; R1.2 - deformation causes related to faulty design; R1.3 – deformation causes related to faulty works implementation; R2.1 - deformation causes related to faulty maintenance of building; R2.2 - deformation causes re-lated to faulty maintenance of adjacent area; R3.1 – prospect-ing/condition surveying drawbacks of adjacent construction; R3.2 – design drawbacks of adjacent construction; R3.3 – works implementation drawbacks of adjacent construction.

Below we list an example of technological impact to an adjacent buildings. In 1998 one of the world’s leading geotechnical companies was carrying out continuous flight auger (CFA) bored piling in central St. Petersburg. Resulting from impossibility to cre-ate proper drilling conditions, whereat one turn of the auger would correspond to downward advance of same auger by one flight, remoulding of subsoil of adjacent building was brought about.

Boreholes with geometrical volume of 8 m3 would consume 12 m3 of concrete, sometimes amounting to 24 m3, and in two cases to as much as 50 m3. Resulting from large scale piling the adjacent multi-storey building constructed in 1905 and lo-cated at 20 m from the excavation had started devel-oping deformations which exceeded 30 mm by the beginning of 1999 (fig. 12).

The works were suspended owing to economical reasons and everyone had a possibility to observe 'pure' after effect of such piling. Currently the build-ing has settled by more than 90 mm. Such nature of deformations corresponds quite well to our forecast provided before the works have started. Ground probing conducted prior to commencement of the piling, during the piling, and following site suspen-sion produced interesting results. Following comple-tion of piles construction the soils considerably re-duced their cone resistance properties (fig. 13).

(a)

0

100

200

300

400

500

600

0 20 40 60 80 100 120

1234567

(b)

Fig. 10. Pile forces redistribution: (а) layout of piles in piled raft; (b) change of pile forces in time (Shashkin, 2006).

Time, years

Forces, kN

45

Ligovsky, 44 (Pertsov House) s=f(t) chart (side wall)

35,5

47

58

6467

3133,5

24,5

0

510152025

3035404550

55606570

19.0

2.19

9819

.03.

1998

19.0

4.19

9819

.05.

1998

19.0

6.19

9819

.07.

1998

19.0

8.19

9819

.09.

1998

19.1

0.19

9819

.11.

1998

19.1

2.19

9819

.01.

1999

19.0

2.19

9919

.03.

1999

19.0

4.19

9919

.05.

1999

19.0

6.19

9919

.07.

1999

19.0

8.19

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.09.

1999

19.1

0.19

9919

.11.

1999

19.1

2.19

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.01.

2000

19.0

2.20

0019

.03.

2000

19.0

4.20

0019

.05.

2000

19.0

6.20

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.07.

2000

19.0

8.20

0019

.09.

2000

19.1

0.20

0019

.11.

2000

19.1

2.20

0019

.01.

2001

19.0

2.20

0119

.03.

2001

19.0

4.20

0119

.05.

2001

19.0

6.20

0119

.07.

2001

19.0

8.20

0119

.09.

2001

19.1

0.20

0119

.11.

2001

19.1

2.20

0119

.01.

2002

19.0

2.20

0219

.03.

2002

19.0

4.20

0219

.05.

2002

19.0

6.20

0219

.07.

2002

19.0

8.20

0219

.09.

2002

19.1

0.20

0219

.11.

2002

19.1

2.20

0219

.01.

2003

19.0

2.20

0319

.03.

2003

Data

Осадка,

мм

/ Se

ttlem

ent,

mm

23 24 25 26

Points19.02.98 26.03.03

Fig. 12. Settlement development in time resulting from CFA piling (Ulitsky, 2003)

Fig. 13. Change of cone penetration resistance of soils: a) prior to commencement of piling; b) during piling; c) following completion of piling (Ulitsky, 2003)

Date

46

Therefore domestic geotechnical codes of St. Peters-burg, published after this case, hold as prerequisite for the designer to opt for such foundation solution of the new or reconstructed building and such corre-sponding construction technology which would be certain to result in no precarious situations with the existing buildings.

7 CALCULATIONS OF FOUNDATION SETTLEMENTS

The choice of the foundation type and the technol-ogy of its implementation in urban areas can be of-ten described in terms of the following expression:

0

uad

n

i

iad SS ≤∑

=

where Sad is the value of additional settlement, rela-tive settlement differential or tilt of structures under reconstruction and/or building adjacent thereto brought about by i-th factor, which is influence of either static loading (unloading) of subsoil or method of works implementation;

n – number of such factors; Sad u is a maximum permissible additional settle-

ment, relative settlement differential or tilt of struc-tures and/or adjacent buildings. This value should be defined with account of soil-structure interaction calculations (Ulitsky, 2003).

The values of Sad can be expressed in the follow-ing form:

Sad = loadadS + w

adS + techadS

where load

adS is caused by excavation and by weight of a new building; w

adS is due to change of hydro-geological conditions on construction site, e.g. ground water lowering and consequent consolidation and mechanical suffosion of silty soils during ground water lowering; tech

adS is caused by the appli-cation of the constructional and technologies on and in the vicinity of the construction site, e.g. tunneling, pile installation, etc. (e.g. Achmus & Kaiser, 2003, Franzuis et al, 2006).

All mentioned components are divided into 3 groups: static, hydrodynamic, and technological. Evaluation of these settlement components is a com-plicated engineering task and cannot be done by simple engineering methods which are prescribed in many codes. Accurate evaluation of these values can be performed only by using modern non-linear soil models. For example, evaluation of the component

loadadS requires application of viscoelastoplastic ap-

proach as well as solving consolidation problems. Evaluation of the component w

adS can be made by solving seepage problems.

Meanwhile, codes and standards still prescribe using so-called simplified engineering methods of

settlement calculations, which are based on the lin-ear theory of elasticity (see for example, formula D.1 of Eurocode 7).

The main parameter of these simple methods is Young’s modulus. In reality, these methods can es-timate only an order of the expected settlements be-cause of the limitations of their initial assumptions (Goldstein, 1979). It can be said that generally for soils Young’s modulus is a very approximate pa-rameter providing a certain reliability of foundations settlement calculations for ordinary buildings for which a big experience of construction is accumu-lated everywhere.

Nowadays geotechnical engineers have to solve very complicated tasks in urban areas including de-sign of high-rise buildings with big loads on subsoil, calculations of deflections of retaining structures, etc. For these types of problems use of such parame-ters as Young’s modulus may lead to big errors in assessment of settlements.

Even if we consider a simple example of a sepa-rate foundation on elastic media and calculate the deformation of the subsoil, using the value of Pois-son’s ratio, e.g., of 0.3, we shall see that more than 70 percent of the foundation settlement is caused by the deformations of the form change (shear), and only 30 percent – by compression.

In engineering practice we often see quite oppo-site situation – determination of Young’s modulus is made on the basis of compression (e.g. oedometric, triaxial) tests, and deformations of the subsoil form changes (i.e. shear deformations) and their develop-ment in time are often neglected in the analysis of settlements.

It is reasonable that both the volumetric and shear strains should be included in the settlement analysis, including a long-term prediction of settlements de-velopment. In this case the principle of settlement calculation could be quite simple and can be de-scribed in the following way:

- Subsoil deformations are divided into volumet-ric and shear strains;

- Soil settlement due to consolidation is deter-mined on the basis of either oedometric or triaxial tests;

- Soil settlement due to shear strains is deter-mined on the basis of triaxial CD or CU tests;

- When calculating settlements due to consolida-tion it is necessary to take into account their devel-opment in time by solving the problem of filtration under the relevant pore pressure values.

- When calculating settlements in time, it is nec-essary to consider a delay of shear strains develop-ment in time, caused by a soil viscosity.

The proposed approach based on using standard soil tests and generally does not require performance of any special tests. The success of application of this method depends mainly on the sufficient scope soil testing of soil samples of good quality.

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To validate workability of the proposed method set-tlements of 13 existing buildings in St. Petersburg have been analysed.

For all calculations the following rules have been applied:

- The boundaries of the calculation profile have been chosen to avoid their influence on the calcula-tion results. The lower boundary of the calculation profile has been selected below the stratum of hard clays with low compressibility.

- 3-D calculation profile includes the soil profile, foundations and superstructure of the building with account of its rigidity.

- To evaluate the development of settlements in time numerical calculations have been made in step-by-step mode with account of the construction his-tory.

One of the examples of calculations is given be-low. The building was constructed in St. Petersburg on Vasilievsky Island in 1969-1970 (fig. 14)

The building was designed for construction in complicated site conditions and adjusted for large and non-uniform foundation settlements. Buildings of this type comprise of number of standard sections. The plan dimensions are 30х13 m, with total length from 60 to 210 m (fig. 15). The building features transverse bearing walls with constant spacing of 6 m, and one interrupted lengthwise dividing-wall, the length of which is 18 m. Bearing walls, dividing-walls and intermediate floor slabs form a rigid spa-tial structure.

Fig. 14. General view of the residential building

Strip foundations 4 m wide are located perpendicular to the transverse bearing walls. The foundations were lined with a sand cushion, under which there is a layer of medium sand with deformation modulus 14.0 MPa. Pressure throughout the site reached 130 kPa. Soil profile is characterized by the soft highly compressible clays underlain by dense clayey sands at the depth of 28 m (fig. 16).

Calculated and measured settlements are shown in Table 1 and in fig. 17.

Table 1. Calculated and measured settlements of the building

Measured SNIiP 2.02.01-83

SSI analysis

Average settlement 600 320 590

Fig. 15. Plan of a panel building of BC-12 type

Fig. 16. Soil profile for the building: 1a - fill, 1,2 - medium sands, 3- plastic clayey sand with inclusions of organic materi-als, 5 – soft varved sandy clay, 6 – silty varved clay, 7 –plastic sandy clay with gravel, 8 – hard clayey sand, 9 – hard cambrian clay

From all 13 case histories the following conclusions can be made (see also the paper by Ulitsky, Van Impe, et al, submitted to the XIV ECSMGE):

Engineering methods of settlement calculation, traditionally ‘prescribed’ in codes and based on the use of the Young’s modulus may give wrong estima-tion of settlement of buildings on soft clays.

The consolidation model for soft clays predicts a slower development of settlements than observed in-situ. It should be also taken into account that the co-efficients of permeability Kf, obtained from the labo-ratory tests are usually much higher than in-situ val-ues of Kf due to high water pressure gradients in

12

3

4

5

6

7

8

9

48

laboratory. Hence, consolidation theory alone cannot predict actual development of strains in time for soft clays.

-700

-600

-500

-400

-300

-200

-100

00 1000 2000 3000 4000 5000

Оса

дка,

мм

Fig. 17. Comparison of calculations and the monitoring data

Development of settlements in soft soils is mainly caused by the viscoplastic deformations of soil form change (shear deformations). This effect can explain a long-term development of settlements in time for soft soils in-situ with very low decrease of pore pressure (see, for example, Larsson, 1981, Leroueil, 2006).

8 CONCLUSIVE REMARKS

Participation of geotechnical experts must be re-flected in constructional codes in all stages of the design and construction of buildings and structures in urban areas, from the preliminary assessment of a problem till the monitoring of erected buildings.

It is essential to use soil-structure interaction ap-proach in design of foundations of important struc-tures in urban areas. SSI approach is very efficient at all stages of design of buildings. It is a very power-ful tool in prediction of behavior of designed build-ings and structures and their influence on adjacent historical buildings.

Settlements of buildings in urban areas must be assessed with account of SSI and real stress-strain state in the subsoil, accounting for development of volumetric and shear strains of soil in time.

Choice of geotechnologies and foundation type should be influenced by the limitation of the addi-tional allowable settlements both for the building under design and, not less importantly, for the adja-cent structures.

Therefore codes should insist on the interactive design and the observational method to validate the accuracy of design solutions and to have a possibil-ity of design modifications during construction in urban areas to provide safety of buildings.

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