抄録
地質年代の新しい第四紀層,とくに更新世中期以降の粘性土地山は,変形性が大きく,トンネル掘削による大きな変形と地表面沈下を伴うことがある.約900mにわたる粘性土地山でのトンネルの掘削を通じて,トンネルの変形と地表面沈下が,粘性土の強度特性に強く規制され,同時に強度特性がボーリング調査の標準貫入試験(N値)と相関があることからN値が現場での変位予測一計測管理手法として適用性の高いことを確認した.また同地層を掘削中,第四紀層としてはまれな水平圧縮により形成されたと考えられる顕著な摺曲とこれに伴う80mに達する右横ずれを伴う断層(せん断帯)を通過した.当該地質では地層の変形により細かい亀裂が発達し,結果としてトンネルの変形と地表面沈下が増大しており,この区間で変位計測結果から推定されるトンネルと地山の変形特性は上記のN値や一軸圧縮強度特性のみでは評価できないことが明らかとなった.岩盤ボーリングで用いるRQDを評価に加えると変形係数は,RQDの減少とともに低下していることから,N値とあわせてRQDで亀裂の多い地山とトンネルの変形特性を予測することが可能であることを示唆した.
Cohesive soil of Quaternary age, in particular of middle Pleistocene or later age, may be due to significant deformation and surface settlement in response to tunneling. In the excavation of a tunnel of about 900m long through this type of soil, tunnel deformation and surface settlement were strongly controlled by the soil strength properties, which were correlated with the N-value of the standard penetration test obtained during test drilling. This confirms that these soil properties can be used for the prediction of deformation and the control of measurement at site. During the excavation, a prominent fold zone, presumably caused by horizontal compression, and an associated shear zone involving a right-lateral strike-slip fault movement of 80m were encountered. This kind of observation is rare in Quaternary formations. In the studied soil, fine cracks developed by deformation further increased deformation and surface settlement. As a result, the deformation properties of the soil could not be predicted only from the N-value and strength properties. The modulus of deformation decreased with rock quality designation (RQD), a parameter used in core boring. This implies that the deformation properties of cracked soil can be predicted by using RQD together with the N-value.
