博 士 （ 工 学 ） ソ ー チ ャ ン セ ン 学 位 論 文 題 名
Study on properties of very soft clays‑. with and without cement treatment‑ as fill materials
（埋立材料としての軟弱粘性土・未改良土およびセメント改良土・の特性に 関する研究）
学 位 論 文 内 容 の 要 旨
I.Study background and motivation
In Japan, the coastal development for industrial and residentiaf uses is one of the most important issues for economic growth. However, traditional reclamation materials such as dredged sand and crushed rock have hardly become available due to environmental concems. On the other hand, the amount of dredged soil produced from maintenance of navigation channels, sewages and construc‑
tions are so large that it requires appropnate measures to manage it. Because the available places for disposing dredged soiJ are limited, the most preferable solution is to recycle it as reclamation material.
However, natural properties of the dredged soil are unsuitable for such purposes, with high water con‑
tent, high compressibility, and low strength. Proper understanding of such soils' properties, both at natural and improved states, willlead to the supply of sustainable reclamation materials for the coastal and infrastructure development, imposing the minimum stress to the environment. Towards such fun‑
damental understanding, suitable experimental testing methods and frameworks for interpretation have been implemented. Generally, there are two ways of using dredged soils or very soft clays as fill mate‑
rial, i.e. directly applied to reclamation site or stabilized with solidifier such as cement. In the former method, the ground can be improved by vertical drain, vacuum consolidation, or preloading, for exam‑
ple. At the first stage of construction, cover material needed to be spread on the top of the soft ground then followed by installation of heavy machineries, which requires adequate strength. Therefore, the evaluation of underlying soil properties like the hardening stiffness and strength with time is very im‑
portant. For the latter method, dredged soils are commonly mixed with smaIJ percent of cement named as the Pipe Mixing or Super Geo‑Material methods, which are widely used in the reclamation projects, have high water coments and behave as a liquid at the early stage. This liquidity property allows it to be transferred through a pipe from a mixing plant to the construction site; the distance usually is as long as 2 km. In these techniques, a method for controlling and monitoring the quality of treated material is essential. If the materialis too stiff, the soil cannot be transported due to high resistance, while the beneficial properties such as evengual strength and stiffness are compromised by adding too much water. Both the water content and the cement ratio should therefore be carefully monitored and controlled so that they properly keep a balance between early‑stage fluidity and strength at the later stage. Thus, the precise determinaOon of stiffness and strength development during hardening process of the material would lead to a better understanding of its behavior.
2.Understanding very soft clays' properties and their evolution
The process of increasing in stiffness and strength of the soil with time under constant volume or water content is known as thixotropic hardening. This phenomenon has been recognized in geotech‑
nical engineering for decades. however, oniy destructive testing methods have been carried out and limited data have been reported by previous researchers. To overcome these drawbacks, nondeswc‑
tive bender element testing has been introduced in this study to evaluate the thixotropic hardening of shear modulus, comparing to thixotropic shear strength measured by laboratory vane test. In addi‑
tion, a wide range of water contents is used to vary properties of soil samples in order to investigate thixotropic effects under different levels. The consolidation test wich very low pressure is also per‑
formed in a cell equipped with bender efements. The low pressure consolidation tests provided the shear modulus values attained at End of Primary Consolidation (EOP) and during secondary consoli‑
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dation. These properties were compared with those from high pressure consolidation tests done by the past researchers, and it was confirmed that there is no inconsistency between them. The important find‑
ings from this study are: regardless of soil types, thixotropy effects are the most prominent at around liquid limit state and become less remarkable at lower and higher water contents; the shear modulus at liquid limit after 24 hours elapse is around 200 kPa. Mitchell (1960) has proposed the mechanism of thixotropy phenomena in terms of increment of the effective stress during the time, which is cre‑
ated particles rearrangement. Test results from this study cannot totally deny this explanation, but the increase in the stiffness due to the thixotropy is too large to be explained by suction.
3.Understanding stabilization process of very high water content clay with cement For constmction projects required to be completed in short time, the dredged soils have been com‑
monly treated with cement and used as reclamation material. Although the properties of cement‑
treated soil have been extensively studied by previous researchers, their interests lie in the materials produced by the Deep Mixing Method, which treats the in situ soil whose water content is relatively low. This method produces much stronger material by generous amount of cement and the strength in this method is assessed at relatively longer curing time (longer than one week). As for cement‑treated soil as previously mentioned, the transponability is interesting and may be goyerned by several factors, such as shear strength, stiffness and viscosity. This thesis is focusing on the two mechanical properties:
the shear strength and the stiffness dur:ing the hardening process of cement‑treated soil. In the case of initially slurry‑like material, conventional tools, such as triaxial or unconfined compression apparatus, are less useful, and new techniques are needed to be devised. In this study, stiffness was determined by bender element test, while shear strengths were measured by laboratory vane test and fall cone test.
A more suitable testing method for measuring cement‑treated soil strength at initial hardening process is proposed. The study revealed the followings: changes in cement‑treated soil stiffness at very early stage are easily observed using bender element tests. The minimum shear wave velocity detectable from this study is about 2.8 m/s, corresponding to a shear modulus of about 12 kPa; the shear strength measured by the vane and the fall cone tests are similar, however, the fall cone test is more convenient because the time required for the test is considerably shorter than that for the vane test; a correlation between the shear modulus (G) and shear strength (s) was observed under wide ranges from very early stages to high strength, and was formulated as approximately G = 300 s.
4.Conclusions
These tests results are very useful to create reclamation projects, especially to construct artificial tidal land for bird sanctuary and to establish a new theory for the consolidation of the ground filling by dredged soils with extremely high water content as well as to understand the effects of ageing on consolidation properties of namral soils. In addition, the obtained results are helpful in precisely estimating the volume change of very soft clay deposits because the thixotropic effects resist against the process of self‑weight consolidation.
The relationship between shear modulus and shear strength of cement‑treated soilis also applicable to that of very soft clays under the thixotropic hardening and of natural clays investigated by in situ testing from various parts of the world. That means their mechanisms are similar. This correlation is practically useful for predicting material's strength, which would normally require destructive mea‑
surement, based on stiffness that can be easily obtained by field nondestructive seismic survey, such as the surface wave exploration.
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IE 駐 鹸露 豁戯 懿
学位論文審査の要旨 主 査 教 授 田中 洋 行 副 査 教 授 杉山 隆 文 副査 准教授 石川達也
学 位 論 文 題 名
Study on properties of very soft clays − ‑with and without cement treatment ―― as fill materials
（ 埋 立 材料 と して の 軟 弱粘 性 土・ 未 改 良土 お よび セ メ ント 改 良 土． の 特性 に 関 する 研 究）
我 が園では園土が狭隘 で平地が少ないため，沿岸域の利川は経済発展のために重要である．しかし なが ら，環境の観点から 従来の代表的な埋立材料である砂や砕岩の採取が難しくなっている一方で，
航路 浚渫の維持，下水道 の処理，地下建設をどによって大量の土砂が発生しており、適切な処理が緊 急の 課題となっている， これらの問題に対する解決策として，上記によって生じた土砂を埋立材とし て再 利用する考え方があ る，しかしをがら，これらの材料，特に航路浚渫によって発生した土砂は従 来の 埋立材料と比べて， 高圧縮性で低強度であるため，そのまま使川すると地盤工学上種々の問題を 引き 起こす．本研究は， 高含水比の粘性土を対象として，その有効利用を図るために．基本的を地盤 工学 的特性を明らかにす るものである．
航 路浚渫土砂の現在用 いられている利用方法は，そのまま埋め立てて，その後バーチカルドレーン によ って改良する場合と ，埋立前にあらかじめセメントを入れて固化する方法とがある．後者のセメ ント を用いる工法では， 生成される処理土 の単位体積重量が 砂や砕岩を用いる通 常の埋土より小さ いの で，原地盤に対する 沈下低減や地下構造物に作用する土圧を大幅に低減できる，このセメント処 理 土は ，管中混合処理エ 法やスーパージオ マテリアル(SGM)工法として ，中部空港や羽田空 港の建 設に 大規模に用いられた ，本研究では，高含水比の土の基本的特性を調べるとともに．管中混合処理 工法 を対象とした混合初 期のセメン卜処理 土の特性について 調べた．
土 試料を一貝練り返し ，含水比を一定の状態で放置すると，試料の強度あるいは剛性率が時間とと もに 増加する現象は「シ キソトロピー」と呼ばれ，古くから多くの研究者によって研究されてきた．
しか しながら，その詳細 は未だ不明を点が多い．本研究では，最近地盤工学の分野で注目を浴びてい るべ ンダーエレメント試 験によって，シキソトロピーによる剛性率の増加を計測した，この試験方法 の利点は，非破壊試験をので，同じ供試体（同じ試験条件）で何度もせん断波速度を計測できるので．
経過 時間だけに限定して ，その効果を評価できる，ベンダーエレメント試験に加えて．ベーンせん断 試験 による強度の測定も 行い，G=300Su(こ こに，G拡 剛性率，Suは非排水せん断強さ）なる関係を見 い出 した，さらに，シキ ソトロピーのメカニズムを明らかにするために，低荷重(2〜20kPa)の下での 圧密 によるGの 増加を計測した， 過剰間隙水圧が消散 した後にも，二次圧密（あるいはクリープと呼 ばれ ている）によって剛 性が増加することが知られているが。本研究によってもその増カロが認めら れ た． この条件では間隙 が時間とともに滅 少するので，この影 響を受けてGの増加割合はシ キソト ロピーの場合より大きぃことが予想されたが，丶実験結果はその逆という興味ある事実が判明した，ま た， シキソトロピーは土 粒子の配列の変化 によって生じた有 効応カの増加による ものとの考え方が
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有カであるが，本研究によって，この考え方以上のGの増加が認められた．自然堆積した粘性土の間 隙比は．原位置の有効応カで室内で圧密した供試体より大きいことが知られている．自然堆積した地 盤fま，場合によっては何千年も載荷されているので，二次圧密などの影響を顕著に受けているので，
短期間で試験が行われる室内で得られる供言式体より間隙比は小さいはずである，この矛盾点は地盤 工 学 の分 野 で 長 い 間討 議 さ れ た 課題 で あ る が ， 今回 の 試 験結果 が新た な解 釈が期 待され る，
管中 混合処 理工 法やSGM工法 では， 材料土 とセメントがプラント船で混合され，パイプによって 打設 地点ま で運搬 される ．そ の距離 は2km以上にも及ぶ．この輸送期間中に高い強度が発現すると パイ プ輸送 に障害 が生じ．逆にセメント量が少をすぎると，所定の養生後にも強度が得られをくを る，したがって，混合直後のカ学特性は重愛であるが，この禰の研究は殆どなされていなぃ，本研究 では ，ベン ダーエ レメン トに よるGの増 加と，ベーンおよびフオールコーンによるせん断強度を求 めて．養生時間に対してどのように増カロするかを調べた，その結果，未改良土と同様にセメント処理 土に おいて もG=300Suをる 関係が 確認さ れた ．また ，養生 時間が150分までは，対象土や配合条件
（含水比，セメント量）によらず，Gの増加害I合は一定であるという，実務上有意義を関係が得られた，
これを嬰するに著者は．±唯立材暈として地雛工学上不適切であるとされてきた航路浚渫土をti効 に活用するために，種々の条件（対象土，含水比，養生日数，セメント処理・未処理）でその特性を明 らかにし．地鶴工学の発展に寄与するところ大なるものがある．
よ っ て 著 者 は ， 北 海 道 大 学 博 士 （ 工 学 ） の 学 位 を 授 与 され る 資 格 あ る もの と 認 め る ．
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