تاثیر شیمی آب حفره ای بر روی رفتار هیدرومکانیکی رس دریایی نرم لیانیونگانگ Effect of pore water chemistry on the hydro-mechanical behaviour of Lianyungang soft marine clay

In the east and the south of China, quaternary sediments, such as marine soft clays, are widely deposited. These sediments are quite common in the area around Lianyungang city (LYG), Jiangsu Province, in relation to the soft marine clay deposited in the north of the Yellow sea (see Fig. 1). Note that the clays in LYG area were deposited in the sea regression environment from about 4800 to 6000 years ago. Roughly 1000 years ago, the ancient Yellow River (or Huanghe) changed its runway and entered the Yellow Sea in the south of the LYG region, bringing large amount of sediment from the loess plateau in the northwest of China. These works were confirmed by geologists and geographers (see for instance Xue et al., 2003). Recently, many new structures, such as highway, railways and buildings, have been constructed in this region, explaining the particular interest of many researchers for this clay — the mechanical behaviour of the LYG marine clay was intensively studied in the last decades. Liu et al. (2008) conducted a number of in-situ piezo-cone penetration (CPTu) tests and verified the existing methods of determining soil parameters by CPTu. Deng (2005) performed in-situ vane tests and compared the values of shear strength before and after remoulding. They concluded that the sensitivity value of the LYG marine clay is approximately 6–۸٫ Hong et al. (2010) analysed the compression behaviour of the reconstituted LYG marine clay in the plane of log (1 + e) and log σ′v and concluded that the intrinsic remoulding pressure is a function of the ratio of initial water content to liquid limit (LL). Although a number of studies on the LYG marine clay have been conducted, the effect of pore water salinity has not been investigated, even though the pore water composition can be quite different for different coastal lines and can change significantly by land desalination. Indeed, the soil close to the seas or the salterns has been inevitably eroded over time by fresh water, leading to desalination, and in turn changing the soil behaviour and causing hazards to infrastructure such as highways and embankments. Several authors have reported the significant effect of pore-water chemistry on the hydro-mechanical behaviour of soils. Chen and Anadarajah (1998), Sridharan and Prakash (1999) and Kaya et al. (2006) studied the pore water effect on the sedimentation of dispersions and slurries with NaCl, CaCl2, AlCl3 and some organic fluids, and observed that the sedimentation behaviour and the final volume depend on the ion valence, concentration, pH value and dielectric constant of the fluid. They tried to describe the mechanism using parameters such as the dielectric constant and Zeta potential, based on the diffuse double layer theory. Kaya and Fang (2000), Sridharan et al. (2002), Di Maio et al. (2004), Gajo and Maines (2007) and Yukselen-Aksoy et al. (2008) analysed the effects of cation valence, ion concentration, pH of pore water and clay minerals on the Atterberg’s limits (LL and plastic limit: PL) based on the diffuse double layer theory. They noted that when the LL of clays is lower than 110%, the salinity effect on the Atterberg’s limits is not significant. Gajo and Maines (2007) observed that the LL of a Na-bentonite from Italy firstly increases slightly from 436% to about 500% with the increase of ion concentration, then decreases sharply to approximately 100% with further increase of ion concentration, the threshold ion concentration being 0.1 mol/dm3 . Abdullah et al. (1997), Sridharan and Prakash (1999), Ören and Kaya (2003), Di Maio et al. (2004), Abdullah et al. (1997), and Gajo and Maines (2007) studied the pore water salinity effect on the compressibility behaviour of clays and compacted soils. Ören and Kaya (2003) observed that the compressibility of homoionised clays treated with Na+, Ca2+ and Al3+ solutions differs from each other, and the compression index order is: Al-clay N Ca-clay N Na-clay. Abdullah et al. (1997) observed that highly plastic Jordan clays with the major clay minerals as illite and a mixed layer of illite–smectite, treated without or with Ca2+, K+ and Na+ solutions have different maximum dry densities. In all cases, drastic decreases in swelling potential and ratio of the secondary consolidation coefficient to the compression index were identified. Gajo and Maines (2007) studied the pore water chemistry effects on the shear strength of remoulded and compacted soils and observed that with deionised water, the residual friction angle is the smallest; it increases with the concentration of HCl or NaOH. Smiles (2008) analysed the pore water effect on hydraulic conductivity and proposed a relationship between hydraulic conductivity and water chemical potential. The studies mentioned above dealt with either remoulded soils or compacted soils or slurries. Some authors investigated natural stiff soils. Wakim (2005) placed natural clay samples in a controlled salinity environment to study the salinity effect. Deng et al. (2011a) performed oedometer tests on natural Boom Clay with deionised and site waters. In their tests, the completion of the pore water replacement was not veri- fied and was thus uncertain. To the authors’ knowledge, the pore water chemistry effect has not been studied for soft marine clays. In this study, the in situ CPTu and borehole investigation were performed at two sites, i.e. Site 1 and Site 2 (see Fig. 1). Site 1 is located in a saltern 50 m far from the coastline of the Yellow Sea and involves clay at 4.91% pore water salinity. Site 2 is 30 km far from the coastline of the Yellow Sea and involves clay at 0.56% pore water salinity. In the laboratory, a special oedometer was used to study the effect of salinity on the volume change behaviour of the clay from Site 1. The mercury intrusion porosimetry (MIP) was applied for the microstructure observation. Moreover, the effects of pore water chemistry on the Atterberg’s limits were examined for further verification.