Land subsidence and ground sinking are as remarkable and extremely severe phenomena as the gigantic tsunami that struck Tohoku and Kanto in Japan after the 2011 Off the Pacific Coast of Tohoku Earthquake (hereinafter the “Tohoku Earthquake”). Moreover, the mechanisms of both events are extremely complex because several factors induced land subsidence and ground sinking from tectonic, geologic, and geotechnical aspects, exerting overlapping influences that persist even now, four years after the earthquake.

Roughly speaking, this land subsidence and ground sinking is classifiable into three categories: (i) land sinking along coastal areas because of tectonic movements (Imakiire and Koarai, 2012, Yasuhara et al., 2012;, (ii) settlement of sandy deposits followed by liquefaction, and (iii) long-term post-earthquake recompression settlement in soft clays caused by the dissipation of excess pore pressure. Land sinking described in category (i) induces inundation of wide coastal areas, particularly during severe storm-surges and strong typhoons. However, land subsidence in the latter two categories (ii) and (iii) occurs in inland areas, strongly influencing local residents. A difference is apparent between the mechanisms of settlement of both types in that settlement included in category (ii) is related to saturated sandy deposits, whereas settlement in category (iii) is associated with saturated clayey deposits, which cause them to take a long time to cease. Based on differences among the causes and mechanisms related to the three kinds of land subsidence described above, suitable countermeasures against subsidence should be taken, corresponding to their different characteristics.

This paper presents two case histories of post-earthquake settlement of clay deposits from among the three categories of ground sinking and land subsidence described above because normally such settlement has been overlooked in numerous historical earthquakes.

Generally speaking, liquefaction-induced settlement described in the previous section ceases in a very short time. As described later, post-earthquake long-term settlement is caused mainly by consolidation of clayey soils situated under sand deposits simply because the hydraulic conductivity of clay layers is much lower than that of sand deposits. For that reason, a long time is necessary for the dissipation of excess pore pressure generated during earthquakes (Yasuhara et al., 2001; Yasuhara and Matsuda, 2002; Matsuda et al., 2014).

Figure 1 depicts the variations of settlement at bench marks after 1974, when the total amount of settlement was greater than 200 mm up to 2010. Settlement measuring locations are benchmarks located in the eastern part of Sendai on the Quaternary alluvial lower plain of unconsolidated layers of around 50 m. It is apparent from Fig. 1 that a sudden increase of settlement occurred after the 1978 Miyagioki Earthquake. It then progressed at a higher rate of settlement than before the earthquake. The sudden increase of settlement in 1978 resulted from the collapse of clay soil particle structures: an immediate effect of the earthquake action.

Acceleration of post-earthquake settlement as shown in Fig. 1 originated from the superimposition of land subsidence before the earthquake and consolidation settlement caused by the dissipation of excess pore pressure generated during the earthquake.

Figure 2 depicts an example of variations of settlement over time before and after the Tohoku Earthquake in 2011. The tendency presented in Fig. 2 is almost identical as that presented previously in Fig. 1.

Kazama (2014) reported that post-earthquake settlement of clay deposits are caused by collapse of clay soil particle structures immediately after the earthquake and the recovery of collapsed structures to the new state of clay structures This mechanism for post-earthquake settlement of clay soils is understood from Fig. 3, which portrays the progress of settlement in the form of void ratio, e vs. logarithmic of vertical stress, log p. To validate the proposed concept for estimating post-earthquake settlement, undrained cyclic triaxial tests followed by drainage were conducted by Kazama using undisturbed and remolded clay specimens taken from the site where settlement had taken place. Subsequently, the results from undrained cyclic triaxial tests followed by drainage under the conditions of 0.4 for cyclic stress ratio and 0.2 Hz. for cyclic frequency are presented for comparison in Fig. 3 along with results from odometer tests on the undisturbed and remolded specimens of the same clay as that used in cyclic triaxial tests.

Void ratios after termination of undrained cyclic triaxial tests followed by drainage are shown on the vertical line corresponding to the consolidation pressure with e-logp′ curves in Fig. 3. Three dotted circles are shown after each undrained loading followed by drainage, which simulates the circumstances under which three earthquakes struck separately after some intervals.

If initial void ratios under a current consolidation pressure pi′ for undisturbed and remolded conditions corresponds to cases before and after the earthquakes are assumed as ei,u and ef,r, respectively, then post-earthquake settlement Spost is given as Spost=ei,u-ef,r1+ei,uH where H stands for the clay layer height. If one assumes 2.15 and 1.85 for ei,u and ef,r, respectively, and 10 m for the clay layer height, then one obtains 47.6 cm for post-earthquake settlement Spost.

Another methodology can be used to estimate the post-earthquake settlement of clay layers proposed by Yasuhara et al. (1991, 2001). It is given as Spost=0.225CcHlog⁡(11-ucyp′c), where Cc stands for the compression index of clay soil, H signifies the clay layer height, ucy represents excess pore pressure generated by the earthquake, and pc′ denotes the consolidation or overburden pressure. When we take values from Fig. xx for Cc, H, ucy and pc′ as 1.06, 5.0 m, 0.5 pc′ and 110 kN m-2, then we obtain 33.9 cm for Spost. It is assumed here for calculation using Eq. (2) that the normalized excess pore pressure in the clay layer, ucy/pc′ is 0.5. Therefore, the amount of settlement will be less than this value, 05, when ucy/p′c is smaller than 0.5. The amount of settlement of 33.9 cm obtained using Eq. (2) is less than the 47.6 cm obtained using Eq. (1).

To prevent the effects of river dykes on post-earthquake settlement of residences, the following countermeasures were undertaken (see Fig. 7). i.

Remove 2.2 m of dykes to reduce dyke self-weight.

As might be readily apparent from the results depicted in Figs. 5 and 6, the following behavior was observed after carrying out the countermeasures stated above. i.

Remaining excess pore pressure on grounds both outside and inside the dykes became less than that before partial removal of dykes.

As a summary of the tendencies stated above, probably the countermeasures adopted herein have been effective to date for reducing post-earthquake settlement of residents near the river dykes. However, monitoring of settlements should be conducted because the influence of groundwater abstraction on continuous settlement of residential areas remains uncertain.

The paper presents an outline of the present situation of ground sinking, land subsidence, and settlement induced by tectonic movements on a global scale and ground movements on a local scale induced by the Tohoku Earthquake in 2011. Particularly, the paper describes a specific examination of post-earthquake settlement of clay layers observed in some locations in the Tohoku and Kanto regions. This paper describes two case histories related to this category of settlement: One in Miyagi for a methodology for predicting settlement and another in Ibaraki for effective countermeasures for settlement. Investigation of the two case histories suggests that careful attention should be devoted to the fact that such settlement is time-dependent and that monitoring of settlement should therefore continue for a long period. These phenomena show different tendencies from those of two kinds of settlement originating from tectonic movement, followed by liquefaction.