Quality Control and Assurance of the Deep Mixing Method

Masaki Kitazume

• Publisher: CRC Press
• Year: 2022
• ISBN:
  9781000537994
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria edile

The deep mixing (DM) method developed in Japan and Sweden in the 1970s has gained popularity worldwide. The DM-improved ground is a composite system comprising stiff stabilized soil and unstabilized soft soil, which necessitates geotechnical engineers to fully understand the interaction of stabilized and unstabilized soils and the engineering characteristics of in-situ stabilized soil. The success of the DM project cannot be achieved by the well-determined geotechnical design alone but is guaranteed only when the quality and geometric layout envisaged in the design is realized in the field with an acceptable level of accuracy. The process design, production with careful quality control and quality assurance are the key issues in the DM project. This book is intended to provide the state of the art and practice of quality control and assurance on deep mixing in detail based on the experience and research efforts accumulated in the past 50 years.

Chapter 1 Overview of deep mixing method and scope of the book

DOI: 10.1201/9781003223054-1

1.1 Definition of soft ground

It becomes difficult to locate a new infrastructure on a stiff and hard ground in urban areas throughout the world. Renovation or retrofit of old infrastructures should often be carried out in the close proximity of the existing structures. Good-quality ground for constructions is becoming a precious resource to be left for the next generation. Due to these reasons and environmental restrictions on the public works, ground improvement is becoming an essential part of the infrastructure development projects both in the developed and developing countries. This situation is especially pronounced in Japan, where many construction projects must locate on soft alluvial clay grounds, artificial lands reclaimed with soft dredged soils, highly organic soils and so on. These ground conditions would pose serious problems of large ground settlement and/or instability of structures. Apart from clayey or highly organic soils, loose sand deposits under water table would cause a serious problem of liquefaction under seismic condition. When these problems are anticipated not to assure the performance and function of superstructure, the ground is called a ‘soft ground’ and needs to be improved and reinforced. Required performance and function of the ground are, however, different for different structures. It is not appropriate to define a ‘soft ground’ by its geotechnical characteristics alone but by incorporating the size, type, function and importance of superstructure and construction period. Only if the type of superstructure is specified, it is possible to define the ‘soft ground.’ Table 1.1 provides rough idea of the ‘soft ground’ for several types of structure in terms of water content, unconfined compressive strength, SPT N-value, ground thickness and bearing capacity (The Japanese Society of Soil Mechanics and Foundation Engineering, 1986).

Table 1.1 Definition of soft ground for several types of structure (The Japanese Society of Soil Mechanics and Foundation Engineering, 1986).

	Highway			Railway		Building	Fill dam
	Water content (%)	UCS, qu (kN/m^2)	SPT N-value	SPT N-value	Thickness (m)	Bearing capacity (kN/m2)	SPT N-value
Organic soil	>100	<50	<4	0	>2	<100	<20
Clayey soil	>50	<50	<4	2	>5	<100	-
Sandy soil	>30	≒ 0	<10	4	>10	-	-

If superstructure to be constructed would be unstable under given conditions of external loads and of original ground, or if expected deformation during and/or after construction would exceed an allowable value from the viewpoint of expected function of superstructure, necessary countermeasures must be undertaken. The following four approaches can be applied: (a) changing type of superstructure and/or type of its foundation, (b) replacing soft soil by better quality soil, (c) improving properties of soft soil, and (d) introducing reinforcing material into soft soil. ‘Ground improvement’ covers (b), (c) and (d) above, and can be defined as any countermeasures given to soft soil in order to attain the successful performance of superstructure if otherwise unattainable. The ground improvement techniques can be classified, based on their working principles, into replacement, densification, consolidation/dewatering, grouting, admixture stabilization, thermal stabilization, reinforcement and miscellaneous. These techniques have been introduced to or originally developed in Japan during the past decades.

The deep mixing method, one of the admixture stabilization techniques, was developed in Japan and put into practice in the middle of the 1970s. Since then, the wet and dry methods have been applied to many improvement purposes and a lot of research studies and case histories have been accumulated (Kitazume and Terashi, 2013).

1.2 Outline of admixture stabilization

1.2.1 Basic mechanism

Admixture stabilization is a technique of mixing chemical binder with soil to improve the consistency, strength, deformation characteristics and permeability of soil. When, for example, cement absorbs the pore water in the soil, cement mineral, 3CaO.SiO₂, for example, reacts with water in the following way to produce cement hydration products, Equation (1.1).

$$2\left(3\text{CaO}\cdot {\text{SiO}}_2\right)+6{\text{H}}_2\text{O}=3\text{CaO}\cdot 2{\text{SiO}}_2\cdot 3{\text{H}}_2\text{O}+3\text{Ca}{\left(\text{OH}\right)}_2$$(1.1)

During the cement hydration, calcium hydroxide, Ca(OH)₂, is released. The cement hydration product has high strength, which increases as it ages, while calcium hydroxide contributes to the pozzolanic reaction. The improvement becomes possible by the ion exchange at the surface of clay minerals, bonding of soil particles and/or filling of void spaces by chemical reaction products. Although a variety of chemical binders have been developed and used for the admixture stabilization, the most frequently used binders nowadays are lime and cement due to their availability and cost. The mechanisms of the lime and cement stabilizations were studied in the 1960s by the highway engineers in relation to the improvement of base and sub-base materials for road construction (Ingles and Metcalf, 1972). The physical and engineering properties of lime and cement-stabilized soil have been studied extensively since then. The rather complicated mechanism of cement stabilization is simplified and schematically shown in Figure 1.1 for the chemical reactions between clay, pore water, cement and slag (Saitoh et al., 1985).

Figure 1.1 Chemical reactions between clay, cement, slag and water (Saitoh et al., 1985).

1.2.2 Type of admixture techniques

Many types of admixture stabilization techniques have been developed in Japan, which can be classified into the in-situ mixing and the ex-situ mixing, as shown in Table 1.2 (after Coastal Development Institute of Technology, 2008). The in-situ mixing techniques are developed to improve the physical and mechanical properties of original soil for assuring the successful performance of superstructure on a ground, where original soil is stabilized with the chemical binder in-situ by means of mechanical mixing and/or high-pressure injection mixing. The in-situ mixing techniques can be subdivided into surface and shallow depth stabilization, mid-depth stabilization, and deep depth stabilization principally depending upon the depth and purpose of improvement. The ex-situ mixing techniques have been developed to enhance the beneficial use of dredged soils, inappropriate soils and construction surplus soils. These techniques are intended to provide additional characteristics to an original soil, such as better liquefaction resistance, smaller density, smaller volume compressibility or high strength. In the ex-situ mixing techniques, the soils are once excavated or collected, mixed with the...