Ground Improvement by Deep Vibratory Methods

Klaus Kirsch, Fabian Kirsch

• Publisher: CRC Press
• Year: 2016
• eBook ISBN: 9781315355092
• Edition: 2
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria civile

Vibro compaction and vibro stone columns are the two dynamic methods of soil improvement most commonly used worldwide. These methods have been developed over almost eighty years and are now of unrivalled importance as modern foundation measures. Vibro compaction works on granular soils by densification, and vibro stone columns are used to displace and reinforce fine-grained and cohesive soils by introducing inert material.

This second edition includes also a chapter on vibro concrete columns constructed with almost identical depth vibrators. These small diameter concrete piles are increasingly used as ground improvement methods for moderately loaded large spread foundations, although the original soil characteristics are only marginally improved.

This practical guide for professional geotechnical engineers and graduate students systematically covers the theoretical basis and design principles behind the methods, the equipment used during their execution, and state of the art procedures for quality assurance and data acquisition.

All the chapters are updated in line with recent developments and improvements in the methods and equipment. Fresh case studies from around the world illustrate the wide range of possible applications. The book concludes with variations to methods, evaluates the economic and environmental benefits of the methods, and gives contractual guidance.

The Open Access version of this book, available at http://www.taylorfrancis.com, has been made available under a Creative Commons Attribution-Non Commercial-No Derivatives 4.0 license

Chapter 1

An overview of deep soil improvement by vibratory methods
DOI: 10.1201/9781315372341-1

Realization of structures always makes use of the soil on which, in which, or with which they are built. Whenever engineers find that the natural conditions of the soil are inadequate for the envisaged work, they are faced with the following well-known alternatives of

1. Bypassing the unsuitable soil in choosing a deep foundation.

2. Removing the bad soil and replacing it with the appropriate soil.

3. Redesigning the structure for these conditions.

4. Improving these conditions to the necessary extent.

When the Committee on Placement and Improvement of Soils of the ASCE’s Geotechnical Engineering Division published its report Soil Improvement: History, Capabilities, and Outlook in 1978, it concluded that “it is likely that the importance of the fourth alternative will increase in the future” (p. 1). Indeed, in the decades since “the need for practical, efficient, economical, and environmentally acceptable means for improving unsuitable soils and sites” (p. 3) has increased.

Before selecting the appropriate soil improvement measures, it is necessary to determine the requirements, which follow from the ultimate and serviceability limit state of design. These are

• Increase of density and shear strength with a positive effect on stability problems.

• Reduction of compressibility with a positive effect on deformations.

• Reduction/increase of permeability to reduce water flow and/or to accelerate consolidation.

• Improvement of homogeneity to equalize deformation.

When leaving aside the method of exchanging the soil, ground improvement can be categorized into compaction and reinforcement methods. Table 1.1 shows a classification of the methods of ground improvement that are of practical relevance today.

Table 1.1 Ground improvement methods

Ground improvement methods
Compaction		Reinforcement
Static methods	Dynamic methods	Displacing effect	No displacing effect
			Mechanical introduction	Hydraulic introduction
• Preloading • Preloading with consolidation aid • Compaction grouting • Influencing groundwater	• Compaction by vibration: • Using depth vibrators • Using vibratory hammers • Impact compaction: • Drop weight • Explosion • Air pulse method	• Vibro stone columns • Vibro concrete columns • Sand compaction piles • Lime/cement stabilizing columns	• MIP^a method • CMI^b method • Permeation grouting • Freezing	• Jet grouting

Source: Sondermann, W. and Kirsch, K. Baugrundverbesserung. In Grundbautaschenbuch, 7. Auflage. Teil 2: Geotechnische Verfahren. Hrsg.: Witt, K.J., Ernst und Sohn, Berlin, Germany, 2009; Topolnicki, M., In situ soil mixing, in Moseley, M.P. and Kirsch, K. (eds), Ground Improvement, Spon Press, London, UK, 2004.

^a Mixed-in-place.

^b Cut-mix-inject.

This book deals with the important soil improvement methods that utilize the depth vibrator as the essential tool for their execution. Granular soils are compacted making use of the dynamic forces emanating from the depth vibrator when positioned in the ground. The reinforcing effect of stone columns, constructed with modified depth vibrators, in cohesive soils improves their load-carrying and shearing characteristics, and, as a modification of the vibro replacement technique, vibro concrete columns are constructed in a similar process by further modified depth vibrators to construct small diameter concrete piles in borderline soils with characteristics that cannot be improved anymore by the basic vibro techniques.

Since the development of vibro compaction during the 1930s and vibro replacement stone columns in the 1970s, they have become the most frequently used methods of soil improvement worldwide because of their unrivalled versatility and wide range of application.

As we will see, deep vibratory sand compaction is a simple concept, and, therefore, design and quality control of compaction in cohesionless soils have remained almost entirely empirical. The development of predictive design methods based on fundamental soil dynamics was probably inhibited by the simplicity of in-situ penetration testing for settlement and bearing capacity calculations. The study of the effect of resonance developing during compaction in granular soils surrounding the vibrator and improved mechanical and electronic controls of the compaction process have, only relatively recently, opened opportunities for significant advances in this field.

The introduction of coarse backfill during vibro compaction and the resulting formation of a granular or stone column was a logical and almost natural development when unexpectedly cohesive or noncompactable soils were encountered. The composite of the stone column and surrounding soil stimulated theoretical studies on settlements and shear resistance by applying standard soil mechanics principles.

When soils needing improvement of their characteristics contain layers of organic material or are too soft to allow the safe formation of a stone column, with a modified depth vibrator columns can be built in the ground using dry or liquid concrete as backfill material, which, after curing of the cementitious material, act like small diameter concrete piles, frequently with enhanced soil characteristics below the toe of the pile and alongside its shaft.

Each of the systems, vibro compaction, vibro stone columns, or vibro concrete columns, has its characteristics and method of execution, and even machine types are different for the two systems of ground improvement, as are design principles, field testing, and quality control.

When in motion, depth vibrators send out horizontal vibrations, and are all excellent boring machines in loose sandy and soft cohesive soils.

The horizontal motion emanating from the depth vibrator being positioned in the ground is the distinctive characteristic that differentiates this method from all the other methods, which utilize vertical vibrations. These methods, which are in general less effective for the compaction of granular soils and which cannot be used for the improvement of fine-grained cohesive soils, are generally not recognized as true vibro compaction methods and hence are not discussed here in great detail.

Depth vibrators are normally suspended like a pendulum from a standard or special crane for vertical penetration into the ground. The vibrator sinks by the desired depth, sometimes assisted by water or air flushing, additional weight when necessary, or even by downward thrust developed by special cranes with vertical leaders. In granular soils, the surrounding sand is compacted in stages during withdrawal, and in cohesive soils imported backfill is employed to form a stiffening column.

The choice of technique follows from the soil and groundwater conditions given in site investigation reports. As will be explained later, the grain size distribution diagram of the soil to be improved is a valuable tool for this choice. Sand and gravel with negligibly low plasticity and cohesion can be compacted by the vibrations emanating from the depth vibrator (vibro compaction), while with an increasing content of fines vibrations are dampened rendering the method ineffective. Experience shows that the limit of vibro compaction is reached with a silt content of more than approximately 10%. Clay particles at even smaller percentages (1%–2%) cause a similar effect. In these cases, soil characteristics can only be improved by adding granular material during the process of forming compacted stone columns (vibro replacement stone columns).

Following a historic overview of the development of these soil improvement methods, two chapters deal with the improvement of granular soils by the vibro compaction method (Chapter 3) and of fine-grained cohesive soils by the vibro replacement stone column method (Chapter 4). The chapters include detailed descriptions of the equipment used and the specific physical processes controlling the two methods and provide state-of-the-art design principles, including methods to assess and mitigate seismic risk when applying either improvement method. Quality control procedures, the evaluation of suitable soils, and practical method limitations together with recent case histories complete these chapters. Chapter 5 deals with method variations, related processes and, especially, with vibro concrete columns where the stone backfil...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface to the Second Edition
• Preface and Acknowledgments to the First Edition
• Acknowledgments to the Second Edition
• Authors
• 1 An overview of deep soil improvement by vibratory methods
• 2 A history of vibratory deep compaction
• 3 Vibro compaction of granular soils
• 4 Improvement of fine-grained and cohesive soils by vibro replacement stone columns
• 5 Method variations and related processes
• 6 Environmental considerations
• 7 Contractual matters
• References
• Index