Discussed herein are formulae for designing landslide-restraint piles with the horizontal subgrade reactions in consideration. With the intensity of the subgrade reaction p assumed to be lineally proportional to the horizontal displacement of pile axis y, that is,
p=k_h⋅δ⋅y
where δ is the pile diameter and k_h is the coefficient of horizontal subgrade reaction, there are still three different approaches: the first one assumes the force of landsliding H as acting only along the slip surface and the pile length as being infinite both upwards and downwards from the slip surface; the second one assumes, with the loading condition as the same, the pile length as being infinite downwards only; and the third one assumes H as distributing all along the pile above the slip surface while the root part of the pile being infinite.
Observations in large-scale shearing tests in situ and excavations of yielded piles in situ present us three markedly different curves of pile deflection as illustrated in Fig. 5. Any of piling formulae are to explain the differences between these three.
Fig. 1 shows a case in which the pile was so constructed as to yield only to the shearing force but actually ran to rupture by bending moment. Landsliderestraint piles are recommended to be due tested against the rupture by bending moment.
§1 presents the general solution to the elastically supported elastic pile under distributed load; §2.1 the solution particular for the case with additional lateral force H₀ and bending moment M₀ at the head; §2.2 that for the free-head pile and §2.3 that for the pile only under H₀ and M₀. §2.4 deals with pile of finite lengths both above and below the slip surface. As is seen in Fig. 6 or in Fig. 11 the point of y2=0 is likely to deepen far below the slip surface with the value βh decreased: the finite-length-pile formula presented in §2.4 will be warranted. Fig. 10 is the chart for easy calculation of M_max and based on exps. (11), (6), (7) and (8), the parameter being n, the fourth root of the ratio of k_h values. Fig. 11 gives values for calculating resultant subgrade reaction and necessary length of pile below the slip surface. The meanings of symbols are:
EI: rigidity of pile, δ: diameter of pile
k_h: coefficient of horizontal subgrade reaction
y: displacement of pile axis,
E_s=k_h·δ·y and β=4√E_s/4EI: for the landsliding mass,
E_s, and β for the part below the slip surface
n=β/η, h: depth to the slip surface
K=η/E_s H: resultant force of landsliding
η: load conversion factor
=2H/h² for hydrostatically distributed load
i: deflection angle, M: bending moment, S: shear
ε, ν and ν: dimensionless factors of deflection, bending moment and shear, respectively.
ρ and λ₀: dimensionless factors of rusultant subgrade reaction and root length of pile, respectively.
Note: Suffix 0, 1 and 2 or head bar denote pile head, sliding mass and the root part of the pile, respectively. Numbers with asterisk refer to those in the literature.

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地すべり等軟弱地盤を取り扱う場合に, 粘性土の構成要素である間隙水が外力に対してどう応答するのかが, 常に重要な問題となる。従来の仮説に頼るのではなく, より詳細に実験的に解明するために, 小型の間隙水圧計を直接粘性土中に挿入し測定して, レオロジー的に考察してみた。その結果, 間隙水圧自体が弾性的な応答と塑性的な応答から成り, また含水比の変化に伴い粘塑弾性的に応答形態が変化することが解った。

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1m深地温測定調査によって地すべり活動に直接大きな影響を及ぼす惧れのある地下水流脈の存在位置を推定し, その結果に基づいて四基の集水井が施工された。その施工効果は地すべり地内に掘削された19本の試錐孔を利用した約5年間にわたる地下水位観測によって検討された。それによると, 地下水流脈沿いの試錐孔の水位が目立って低下したことが示された。また流脈外に掘削された集水井内には殆んど地下水の湧出は認められず順調に工事は進み, 井内から流脈内に横孔集水試錐を施工してはじめて多量の水の湧出が認められた。一方流脈内に施工された集水井は掘削当初から湧水・土の押し出しに悩まされたという。

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