Quantitative criteria for the selection and stabilisation of soils for rammed earth wall construction A thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy in the Faculty of the Built Environment University of New South Wales by Van Stephan Burroughs 2001 ABSTRACT Modern building procedures and requirements demand that the selection and stabilisation of soils for the purposes of rammed earth construction be better quantified. This study examines the relationships between soil properties, stabiliser treatments, and stabilised strength and density for 111 soil samples taken from sites in New South Wales (Australia), and develops new quantitative criteria for soil assessment, selection, and stabilisation. Laboratory measurements of soil particle size distribution, plasticity, and shrinkage were made for each soil. Various quantities from 0-6 % of lime, cement, and asphalt were added to the soil samples, and the resulting 230 specimens were compacted, and cured for 28 days. Determinations were made of the optimum moisture content, maximum dry density, and compressive strength of the stabilised material. The samples showed stabilised strengths ranging from 1.0-5.4 MPa, with a mean of 2.62 MPa, and densities from 1.44-2.21 t/m3, with a mean of 1.86 t/m3. The results show that over 90 % of the variation in stabilised strength and density of the samples is due to variation in soil properties, with differences in stabiliser type or stabiliser quantity being relatively minor. The most important soil properties explaining stabilised strength are linear shrinkage and plasticity index. These properties have been used to categorise the soils into three groups on the basis of their suitability for stabilisation as measured against a compressive strength criterion of 2 M Pa. Favourable soils have shrinkages of < 7.1 % and plasticities of < 16 %, and 90 % of these samples passed the 2 MPa criterion. Satisfactory soils have shrinkages of 7.1-13.0 % and plasticities of 16-30 %, and 65 % of these samples had strengths in excess of 2 MPa. Unfavourable soils have shrinkages of > 13 % and plasticities of > 30 %, and only 10 % of these samples exceeded the 2 M Pa value. Soils in the favourable and satisfactory categories can be further discriminated using textural information. On that basis, all soils classified as favourable, and those classified as satisfactory and which also have sand contents < 60 %, are recommended as being suitable for stabilisation. Soils not fulfilling these criteria are unlikely to be successfully stabilised and should be rejected. These results stress the importance of selecting a soil favourably predisposed to stabilisation. Field techniques to search for such soils could be refined on the basis of the new soil criteria presented. Use of the criteria should also minimise unnecessary laboratory testing of the density and strength of soils that subsequently prove unsuitable for stabilisation. A flow chart is presented to guide practitioners through the different stages of soil testing, assessment, and rammed earth stabilisation. i ACKNOWLEDGMENTS I wish to acknowledge the assistance of the following people: • Dr. Bill Lawson, Faculty of the Built Environment, University of New South Wales, for guidance, encouragement, understanding and supervision of the project. Without him none of this would have been possible. • Dr. Steve Lawrence and Mr. Greg Worthing CSIRO, North Ryde, Division of Building, Construction and Engineering for their support and use of the library and giving me access to M r. George M iddleton’s walls and research documentation. • Mr. Roger Rose, School of Construction, Canberra Institute of Technology; Bruce Campus; and Mr. John Nichols, soils laboratory technician, for the use of the testing laboratory, machinery, instrumentation and technical assistance. • The Building & Construction Industry Training Advisory Board of ACT and NSW who gave positive support in the form of a financial grant to complete my research. • To all the support and encouragement from my professional colleagues in South Africa: Mr. Stefan Kohler, Project Management; Professor Rodney Harber, University of Natal Durban; Mr. Mike Bolton, Building Technology CSIRO; Dr. Graham Grieve, Cement and Concrete Institute; and The Forum for Earth Building Technology Committee. • Mr. John Morgan, structural/civil engineer, who gave me support and advice on soil testing. • The University of South Dakota, Brookings, South Dakota, USA, for access to the library achieves research section on rammed earth construction conducted by Dr. Ralph Patty. • To all my Australian, New Zealand and International Colleagues who corresponded with me and engaged in dialogue in reference to technical problems involved in rammed earth wall construction. • To the Australian International Aid Agency for giving me employment, which in turn allowed me the opportunity to use my research and to transfer improved earth technology to the people of South Africa. • To my wife, Jane, for all the hours of transcribing, proof reading, and patience given to me in my quest. ii TABLE OF CONTENTS ABSTRACT..........................................................................................................................I ACKNOWLEDGMENTS..................................................................................................... II TABLE OF CONTENTS.....................................................................................................III TABLE OF FIGURES........................................................................................................ IX TABLE OF TABLES.........................................................................................................XII CHAPTER 1: INTRODUCTION........................................................................................... 1 1.1 BROAD THEME OF STUDY ................................................................................................ 1 1.2 EARTH WALL CONSTRUCTION BASICS................................................................................. 1 1.2.1 Methods of building with earth.................................................................................. 2 1.2.2 Characteristics of rammed earth buildings................................................................... 3 1.3 STABILISATION OF SOIL FOR EARTH WALL CONSTRUCTION...................................................... 7 1.3.1 Introduction........................................................................................................... 7 1.3.2 Soil suitability for stabilisation.................................................................................. 8 1.3.3 Stabilisation treatments and effects on soils................................................................. 9 1.3.4 Stabilisation outcomes and guidelines........................................................................12 1.4 RATIONALE.................................................................................................................13 1.4.1 History of rammed earth construction ........................................................................13 1.4.2 Need for better understanding of soil-stabiliser relationships..........................................14 1.4.3 Need for better guidelines .......................................................................................16 1.5 RESEARCH OBJECTIVES .................................................................................................17 1.6 STRUCTURE OF THE THESIS.............................................................................................18 CHAPTER 2: BACKGROUND AND PREVIOUS WORK.......................................................20 2.1 HISTORY OF THE SCIENCE OF SOIL STABILISATION FOR EARTH WALL CONSTRUCTION...................20 2.1.1 Introduction..........................................................................................................20 2.1.2 The work of Ralph Patty (1930s)..............................................................................21 2.1.3 The work of George Middleton (1940s) .....................................................................23 2.1.4 Soil stabilisation research and literature 1950 onwards..................................................25 2.2. SOIL PROPERTIES IMPORTANT FOR RAMMED EARTH STABILISATION.........................................26 2.2.1 Introduction..........................................................................................................26 2.2.2 Textural properties of soil .......................................................................................27 2.2.3 Plasticity..............................................................................................................29 iii 2.2.4 Shrinkage.............................................................................................................31 2.2.5 Compactibility......................................................................................................32 2.2.6 Mechanical properties of soil ...................................................................................32 2.3 SOIL CLASSIFICATION FOR ENGINEERING PURPOSES..............................................................34 2.4 INTRODUCTION TO SOIL STABILISATION.............................................................................36 2.4.1 Soil stabilisation and its objectives............................................................................36