STATENS GEOTEKNISKA INSTITUT SWEDISH GEOTECHNICAL INSTITUTE

Investigations and load tests in silty soils Results from a series of investigations in silty soils in Sweden

ROLF LARSSON

LINKOPING 1997

~ STATENS GEOTEKNISKA INSTITUT SGI SWEDISH GEOTECHNICAL INSTITUTE v

Rapport No54Report

Investigations and load tests in silty soils Results from a series of investigations in silty soils in Sweden

ROLF LARSSON

This project was partly financed jointly by the Swedish Council for Building Research (BFR), Grant No. 930591-70 and the Swedish Geotechnical Institute.

LINKOPING 1997

Report

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Swedish Geotechnical Institute SE-581 93 Linkoping

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Roland Offset AB, Linkoping, July 1997

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Preface

'""rbis report deals with the results of a research project concerning investigations .l and evaluation of properties in silty soils. The investigations in this part of the

project have been preceded by a comprehensive literature survey reported in SGI Report 49 "Silt - Geotechnical Properties and their Determination". Thereafter, a series of investigations has been performed at different locations with silty soils and the results from different types of sounding tests have been compared to each other and to results from laboratory tests concerning properties and classification of the soils. In three locations, a number of in situ and laboratory tests have also been performed and compared to the results from large scale field loading tests.

The report contains comments and recommendations on the use of the various types of investigation methods together with special precautions that should be observed in silts because of the special properties and conditions encountered in this type of soil. It also contains comments and recommendations regarding commonly used methods for estimation of properties from the different types of test results and the calculation of bearing capacity of shallow foundations and settlements in silt. Most of these methods have been elaborated for use under normal conditions in sands and the findings in this report do not necessarily reflect their usefulness in these conditions.

The report is intended for geotechnical engineers, both designers and researchers, and others who are involved in geotechnical investigations and design in silty soils.

The research project has been financed jointly by the Swedish Council for Building Research (BFR), Grant No. 930591-0, and the Swedish Geotechnical Institute.

The author wishes to express his thanks to all those who have participated and cooperated in the project. Special thanks go to Per U5fling and his colleagues at the Swedish National Road Administration and to Mats Larsson and Lars-Goran Ivers at KM-Geokonsult AB for their invaluable help in locating and establishing the test fields in the Borlange area, and also to the owners, Mr. Lars-Ake Mattsson and

Investigations and load tets in silty soils 3

Vasakronan AB, for their kind permission to use the land. Special thanks also go to Anna-Lena Oberg and her colleagues at Chalmers University ofTechnology for their co-operation in the tests concerning the variability of the ground water situation in Vatthammar. Many other colleagues, both at the Swedish Geotechnical Institute and other institutions, have also been involved and contributed to the project to various extents. Finally, the successful execution of the project is largely a result of the skill and dedication of the staff at the Institute's division for Field and Measuring Techniques, often exerted under long working days and harsh climatic conditions.

Linkoping, December 1996

Rolf Larsson

SGI Report No 54 4

Readers guide to this report

Why this report is special

This book gives a broad review of the usefulness of different investigation and calculation methods when applied to silty soils. It focuses on the practical application and use of the most rational ways of solving the engineering problems encountered. It describes the possible sources of error when using different methods and also the possible hazards in design arising through the extensive influence of the ground water conditions, which during the lifetime of the structure may be very different from the conditions at the time of site investigation. The report also describes problems encountered during the execution of the test programme, which are very common also in practical foundation works but which are rarely reported.

The goal of this report

The goal of this report is to provide recommendations for the methods that should be used in investigations in silts, the special precautions that should be observed when using the methods and the way in which the results should be interpreted. The report also contains recommendations for the way in which bearing capacity and settlements for shallow foundations should be calculated in this type of soil.

Who should read this report and why

This report will give the reader an insight into the advantages and shortcomings of the investigation and calculation methods normally employed in silty soils.

The report will be useful for geotechnical engineers planning investigations for selecting the most appropriate methods, giving instructions on how they should be carried out, specifying what supplementary investigations and observations should be made and stating how the results should be interpreted. It is useful for field engineers understanding how the methods work in this type of soil, what precautions have to be taken and why it is important to follow different special

Investigations and load tets in silty soils 5

procedures. It also gives guidance to designers when selecting calculation methods, parameters and critical conditions during the design life. It gives an insight into the problems that may occur during execution of the foundation works and the precautions that need to be taken in that context.

Finally, the report gives clients for geotechnical investigations and design an insight into the relevance of different methods and procedures, which enables a better understanding of the quality of different procedures and why certain precautions have to be taken.

How this report is organised

The report starts with a summary setting out the main results and recommendations. It then continues with a detailed description of the investigations and tests performed and the results from these. The results of the investigations and the practical experiences and observations in connection with these are summarised in a special chapter, where also more detailed recommendations are given for the investigations that should be performed and the special precautions to be observed. A description of the various methods for calculation of settlements and bearing capacity of shallow foundation then follows. Finally, the results obtained with these methods are compared to the results obtained in the large scale loading tests in the field and more detailed recommendations are given for the methods and precautions that should be employed.

SGI Report No 54 6

Contents

PREFACE

READERS GUIDE TO THIS REPORT

SUMMARY 11 Effect of ground water conditions Site investigations Determination of shear strength Determination of compressibility Sampling and laboratory tests Calculation of settlements Calculation of bearing capacity

I INTRODUCTION ............................................................................................ I 7 Purpose and background of the investigation Scope of the investigation

2. INVESTIGATIONS 22

J. PRINCIPLE OF THE PLATE LOADING TESTS .................................................... 29

4. INVESTIGATIONS AND LOAD TESTS AT THE THREE MAIN TEST LOCATIONS .. 32 4.1 Mjardevi, Linkoping ............................................................................. 32

Test field First investigation Pressuremeter tests Raft foundation with pre-loading Dilatometer tests CPT tests and weight sounding tests

Investigations and load tets in silty soils 7

4.2 Vagverket, Borlange ............................................................................. 50 4.2.1 Test field ..................................................................................... 50 4.2.2 Investigations in the current porject ........................................... 52

CPT tests Standard CPT tests Seismic tests and excess pore pressure dissipation tests Pore pressure measurements Sampling and laboratory tests

Sampling Classification tests Oedometer tests Triaxial tests

Field vane tests - Dilatometer tests - Pressuremeter tests

4.2.3 Preparations for load tests and problems with ground water ..... 72 - First excavation

Ground water lowering 4.2.4 Plate load tests ............................................................................ 85

- Installation ofplates and instrumentation - Reaction system - Loading system - Measuring system - Loading procedure

4.2.5 Results of the load tests .............................................................. 93 0.5 x 0.5 metre plate 1 x 1 metre plate

- 2 x 2 metre plate Settlement distribution in the load tests

4.3 Vatthammar, Stora Tuna, Borlange .................................................... 110 4.3.1 Test field .................................