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ピーク強度(τ_f)は垂直応力(σ)の関数であるばかりでなく過圧密比(n_P)の関数でもある。すなわち次式が成り立つ。τ_f=b+T・σ^n・n_P上式においてnpを先行圧密荷重(P_0)で置き換えると, τ_f=b+T・σ^・P_0ここにb, T, nは土性によって決まる定数である。これはP_0の一次関数であると同時に, P_0を一定にした場合のτ_f∿σ関係すなわち過圧密領域のセン断強度特性を示す。したがって, 強度定数を求めるためには, 問題にしている過圧密領域のτ_f∿σ関係の湾曲部を直線で置き換えて限定包絡線とする。このように, 任意の粘土に対しある限られた範囲のτ_f∿n_P関係を実験的に求めることによってその定数が決まるので, P_0が変化した場合のτ_f∿σの特性と強度定数を推定できる。, The shear strength line of an overconsolidated clay has a curvature when the shear strengths are determined by undrained tests and plotted against total normal stresses (σ). The degree of the curvature increases regularly with increasing preconsolidation stress (P_0) and overconsolidation ratio (n_p). In this paper the mathematical relationships that exist between the shear strength envelopes and P_0 and n_p are developed. The results are summarized as follows : 1. The peak strength (τf) may be expressed in terms of σ and n_p : ^τf=b^^^- + T. σ^n. n_p where b^^^-, T and n are constants dependent on the degree of overconsolidation and a given soil. 2. Using P_0 as a replacement for n_p, the following expression is obtained : τf=b^^^- + T. σ^. P_0 It shows not only the fact that the value of τf is proportional to the value of P_0,but also the relationship between τf and σ for P_0=constant. 3. The constants b^^^-, T and n are therefore determined from τf∿n_p relationship in the limited stress range for a given soil, and then the shear strength curves for various preconsolidation stress can be illustrated.

実験結果から次の点が明らかになった。1)ピーク以後のτ∿D曲線の形状は解析的に次式で表わされる。[numerical formula]ここに, b, cは過圧密比や土性によって決まる定数, τ_f, D_fはピーク強度とそれに対応する変位を示す。2)過圧密状態におけるτ∿Dの挙動は試料の持つ応力履歴によって大きく影響される。すなわち過圧密の程度が高い土ほど, 固結化が進み, 強度の増加とピーク後の強度低下が顕著である。3)ピーク強度と変位の比(τ_f/D_f), ブリトル指数(I_B), 初期変形係数(E_i)等のτ∿D曲線の形状係数と過圧密比とは相関関係が極めて高い。, It is a well-known fact that if an overconsolidated clay is strained beyond failure, its strenght will decrease and finally reach a certain constant value, the ULTIMATE STRENGTH (it is referred to especially as the RESIDUAL STRENGTH under slow drained shear test), which will remain constant for further straining. For the overconsolidated plastic clays the drop in shear strength after failure is very significant. In order to investigate the relation between stress-displacement behaviour after passing the peak i. e. rate of progressive reduction in strength from the peak to the ULTIMATE (or RESIDUAL) condition and degree of overconsolidation, a series of direct shear test has been conducted under each different conditions of overconsolidation ratio (the ratio of preconsolidation stress to effective normal stress, n_p=P_0/σ) respectively for P_0=constant, σ=constant and n_p=constant. From the test results the following points were clarified. 1. The general shape of the stress-displacement curve (τ∿D curve) of the postpeak can be expressed analytically by [numerical formula] where b and c are constant on the overconsolidation ratio and the materials. 2. The stress-displacement behaviour of the state of overconsolidation is greatly dependent on the stress history of the sample-the higher the overconsolidation ratio, the stiffer the soil. 3. The shape factors of the stress-displacement curve, initial tangent modulus (Ei), ratio of stress to displacement at failure (τf/Df) and brittle index (I_B), seem to be well expressed as a function of the overconsolidation ratio.

ピーク強度(τ_f)は垂直応力(σ)の関数であるばかりでなく過圧密比(n_P)の関数でもある。すなわち次式が成り立つ。τ_f=b+T・σ^n・n_P上式においてnpを先行圧密荷重(P_0)で置き換えると, τ_f=b+T・σ^・P_0ここにb, T, nは土性によって決まる定数である。これはP_0の一次関数であると同時に, P_0を一定にした場合のτ_f∿σ関係すなわち過圧密領域のセン断強度特性を示す。したがって, 強度定数を求めるためには, 問題にしている過圧密領域のτ_f∿σ関係の湾曲部を直線で置き換えて限定包絡線とする。このように, 任意の粘土に対しある限られた範囲のτ_f∿n_P関係を実験的に求めることによってその定数が決まるので, P_0が変化した場合のτ_f∿σの特性と強度定数を推定できる。, The shear strength line of an overconsolidated clay has a curvature when the shear strengths are determined by undrained tests and plotted against total normal stresses (σ). The degree of the curvature increases regularly with increasing preconsolidation stress (P_0) and overconsolidation ratio (n_p). In this paper the mathematical relationships that exist between the shear strength envelopes and P_0 and n_p are developed. The results are summarized as follows : 1. The peak strength (τf) may be expressed in terms of σ and n_p : ^τf=b^^^- + T. σ^n. n_p where b^^^-, T and n are constants dependent on the degree of overconsolidation and a given soil. 2. Using P_0 as a replacement for n_p, the following expression is obtained : τf=b^^^- + T. σ^. P_0 It shows not only the fact that the value of τf is proportional to the value of P_0,but also the relationship between τf and σ for P_0=constant. 3. The constants b^^^-, T and n are therefore determined from τf∿n_p relationship in the limited stress range for a given soil, and then the shear strength curves for various preconsolidation stress can be illustrated.