TC214 Soft Soils

Background and Scopes

Soft soil engineering is often regarded as a classical geotechnical field. There are also some engineers / researchers who ridicule as “do you still study soft soils?” Looking at the current situation of land use in the world, most of good lands have already been developed, and frontiers of land development today are often in coastal areas, offshore land reclamation sites, or undeveloped wetlands, etc. In those areas where soft soil is thickly deposited, construction projects are facing geotechnical challenges. The soft ground is not only a soft clay deposit with high plasticity having a high water content and a high void ratio but also various kinds of soils such as highly sensitive soil, organic soil, peaty soil showing very high compressibility with remaining plant fibers, etc. Liquefiable soil due to earthquake is also a target in soft soil engineering. Peat is a very difficult soil in evaluation of its physical and mechanical properties. For soft soil deposits, installation of vertical drains with preloading is often used to accelerate the consolidation; however, we have to pay attention to continuity of the vertical drains and stability of preload fill. On the other hand, there is an advanced technology which can be applied to soft soil deposit, such as vacuum consolidation in which we don’t need to consider the continuity of PVD and the stability under vacuum loading. Some kinds of problematic soils such as sensitive soil for which it is very difficult to evaluate its intact characteristics, organic soil for which it is very difficult to apply cement stabilization technology, and volcanic ash for which it contains crushable soil particles, will be subject to soft soil engineering. Accordingly, from engineering sense, this TC is named as Foundation Engineering for Difficult Soft Soil Conditions.

As the design method has been changing from specification based design to performance based design on the basis of reliability based design, main subjects of soft soil engineering has also changed from “fail or not-fail problem” to deformation analysis. Advancement in construction technology makes it possible to enlarge structure dimensions, and then some stiff soil layers are recently considered relatively soft, even though those layers had been regarded as stiff. For example, the subsoil layers of the Kansai International Airport in deeper portion are Pleistocene clay layers, which are generally very stiff and often used as a bearing layer; however, those Pleistocene clay layers are regarded as relatively soft in this case because of very large incremental consolidation pressure. Actually, they observed residual settlement more than 3 m, and currently, annual settlement rate is approximately 60 mm/year.

Various types of soil are subjected in soft soil engineering, and variety of challenging geotechnologies in the field of ground investigation, laboratory testing, ground improvement such as acceleration of consolidation and solidification, prediction calculation in performance based design, etc. The followings are proposed subjects in TC214’s future activities.

Soil types:

-Marine clay (non-glacial)
-Marine clay (glacial)
-Lacustrine clay
-Silty clay/clayey silt
-Peat
-Dredged soil (treatment/ reclamation)
-Sandy soil for reclamation (liquefaction)
-Volcanic ash/ pumice sand filling (liquefaction due to looseness)

Projects:

-Man-made island (e.g. Kansai International Airport) [Possible collaboration with TC217: Land reclamation]
-Embankment (road and railway) [Possible collaboration with TC202: Transportation]
-Dyke/Levee [Possible collaboration with TC201: Dykes and Levees]
-Waste soil treatment
-Hydraulic cutoff (barrier) for waste reclamation site [Possible collaboration with TC215: Geo-Environmental]
-Foundations (mainly deep foundations) in soft soils
-Deep excavations in soft soils (e.g. West Link Gothenburg)
-Earth fills (liquefaction of loose pumice sands, e.g. 2018 Hokkaido Earthquake)

Phenomenon/Issues:

-Consolidation settlement (primary and secondary)
    Long-term consolidation (man-made island), Differential settlement (embankment), ground subsidence (dropping of groundwater level)
    With/without ground improvement
    Degradation/ deterioration in strength and stiffness (sensitive clays)
    Strain-rate effects on the stiffness and the strength

-Stability
    Seawall/ Embankment
    Staged construction (with vertical drains)
    Ground improvement (if necessary)

-Interaction between soil and foundation
    Piled foundation under vertical/horizontal soil movement

-Treatment
    Dewatering/ Solidification (cement, lime)

-Behaviour under moderate temperature changes (accelerated creep)

-Installation and soil disturbance effects (smear)

-Microstructural effects (pore-size distribution/ aggregation/ macro- and micro-porosity)

-Liquefaction of loose soil.

Investigation:

[Possible collaboration with TC101: Laboratory testing, and TC102: In-Situ Testing]
-Sampling and laboratory test
    UC, Triaxial CIU, CAU, CK₀U (compression and extension)
-Sounding
    CPT, RI-CPT, vane
-Exploration
    PS-logging/ Seismic cone
-Monitoring
    Settlement/ compression of sublayers, excess pore water pressure, density

Prediction:

-Conventional theory
-Numerical analysis [Possible collaboration with TC103: Numerical Methods in Geomechanics]
-Physical modelling [Possible collaboration with TC104: Physical Modelling]
-Field observations
-Empirical method
    Curve fitting

Ground improvement technology:

[Possible collaboration with TC211: Ground improvement]
-For deposits
    Preloading/ surcharge loading
    Replacement
        Sand compaction piles (SCP: high replacement ratio, low replacement ratio)
    Replacement/ Consolidation/ Ddensification
        Vertical drains
    Prefabricated vertical drains (PVD) with preload/vacuum, Sand drains (SD) with preload
    Solidification (chemical treatment)
        Deep mixing method (DMM) with cement/lime
-For slurry
    Solidification
        Cement treated soil (pneumatic mixing method, airfoam treated soil)
    Dewatering (filter press)
    Electro-osmosis

Chair:          Prof. Yoichi Watabe (Hokkaido University, Japan)

Vice-Chair:  Prof. Minna Karstunen (Chalmers University of Technology, Sweden)

Secretary:   Prof. Suched Likitlersuang (Chulalongkorn University, Thailand)

Main theme:

Prediction methods from laboratory and site investigation to modelling and design in soft soil engineering

Terms of reference

ISSMGE-TC214 on Soft Soil Engineering ”Foundation Engineering for Difficult Soft Soil Conditions” will focus on the following three broad topics under the main theme “Prediction methods from laboratory and site investigation to modelling and design in soft soil engineering”:

1. Physical and mechanical properties – There are variety of soft soils with different characteristics in the world. Interpretation of fundamental properties of those soils and understanding the difference of each soil are necessary. This issue includes field investigation, laboratory testing, and constitutive modeling (or interpretation of laboratory test results).
2. Prediction of soil behavior – To meet performance based design, soil deformation has to be analytically (or empirically) predicted. However, because of difficulties in site investigation and laboratory testing due to softness, there are still some issues to be filled in this research field to achieve performance based design.
3. Ground improvement / soil treatment – Ground improvement is one of the best solutions to use a soft soil deposit as a foundation ground. Also, soil solidification by mixing with a binder (cement, lime, blast furnace slag, etc) is recently preferably used to shorten construction period instead of conventional technology such as vertical drains; however, organic contents often adversely affect solidification.

The main aim of the TC214 is to promote a greater interaction among practitioners (engineers) and academicians engaged in soft soil engineering related so as to achieve greater cooperation and facilitate the exchange of information, experience, knowledge, and new ideas in these two areas. In the next four years (2020-2024), TC214 would endeavor to organize at least two small to medium size workshops during ISSMGE conferences: (i) the 20th ICSMGE at Sydney, Australia, in 2022, and (ii) one of regional conferences under auspices of ISSMGE in 2023. In each workshop, the three themes listed above will be discussed. One principal aim of these workshops is to ensure publication of excellent case histories and associate construction experience, as well as technical proposals to overcome soft ground problems for future projects.

In addition, TC214 will propose to organize an international symposium (new IS-series on Soft Soil Engineering with 150 to 300 participants) on soft soil engineering. The first symposium will be held in 2023. This proposal will be discussed more later, but these future activities depend on the infection status of the new coronavirus COVID-19.