Shear creep parameters of simulative soil for deep-sea sediment

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1 J. Cent. South Univ. (014) 1: DOI: /s Shear creep parameters of simulative soil for deep-sea sediment MA Wen-bo( 马雯波 ), RAO Qiu-hua( 饶秋华 ), LI Peng( 李鹏 ), GUO Shuai-cheng( 郭帅成 ), FENG Kang( 冯康 ) School of Civil Engineering, Central South University, Changsha , China Central South University Press and Springer-Verlag Berlin Heidelberg 014 Abstract: Based on mineral component and in-situ vane shear strength of deep-sea sediment, four kinds of simulative soils were prepared by mixing different bentonites with water in order to find the best simulative soil for the deep-sea sediment collected from the Pacific C-C area. Shear creep characteristics of the simulative soil were studied by shear creep test and shear creep parameters were determined by Burgers creep model. Research results show that the shear creep curves of the simulative soil can be divided into transient creep, unstable creep and stable creep, where the unstable creep stage is very short due to its high water content. The shear creep parameters increase with compressive stress and change slightly or fluctuate to approach a constant value with shear stress, and thus average creep parameters under the same compressive stress are used as the creep parameters of the simulative soil. Traction of the deep-sea mining machine walking at a constant velocity can be calculated by the shear creep constitutive equation of the deep-sea simulative soil, which provides a theoretical basis for safe operation and optimal design of the deep-sea mining machine. Key words: shear creep parameter; simulative soil; deep-sea sediment; shear creep test; Burgers model 1 Introduction With the increasing depletion of land resources, exploitation and utilization of seabed mineral resources have become a hot issue worldwide [1 3]. In China, research of deep-sea mining technology is relatively late and currently it is focused on deep-sea mining system after investigation of deep-sea resource environment. The deep-sea mining system usually includes mining machine, hose, buffer, rigid pipe and mining vessel [4 5], where the mining machine is crawler traveling mechanism and its traction is decided by shear resistance of the deep-sea sediment. Since the deep-sea sediment of larger void ratio and higher water content [6 8] has lower shear strength and more obvious rheological properties than ordinary land-soil, the crawler-type mining machine easily slips on the sea-bed during mining process. Therefore, study of shear rheological properties of the deep-sea sediment is vital to the stability and safety of the deep-sea mining system. Due to the difficulty in obtaining the deep-sea sediment in situ, a kind of reasonable simulative soil on land is needed to substitute the deep-sea sediment for a large number of experimental studies. At present, although a few literatures have been on shear mechanical properties of the deep-sea sediment, such as the shear strength [9 10] and shear mechanical of terramechanics [11 13], it is short of studying shear rheological properties of the deep-sea sediment. Based on the real deep-sea sediment collected from in Pacific C-C area [14 15] by our group, the best simulative soil is prepared and its shear creep characteristics are studied by the direct shear creep test in order to determine its shear creep parameters. Shear resistance of the simulative soil related to traction of the deep-sea mining machine can be obtained, which is important to provide guidance for optimal design of the deep-sea mining machine. Experimental.1 Preparation of simulative soils Since the deep-sea sediment is mainly composed of a lot of montmorillonite and a little of quartz, feldspar, mica, chlorite and serpentine [14], four kinds of different bentonites were selected as raw materials (namely B1 B4) and their mineral components (mainly consist of montmorillonite) and particle size were tested by X-ray diffractometer and laser-diffraction particle-size analyzer, as shown in Table 1 and Table, respectively. It is seen that all of the bentonites have close mineral component and particle size to the deep-sea sediment and they can be used as the raw materials of simulative soils. Four kinds of simulative soils (namely S1 S4) were prepared by mixing the four kinds of bentonites with a Foundation item: Project(517451) supported by the National Natural Science Foundation of China Received date: ; Accepted date: Corresponding author: RAO Qiu-hua, Professor PhD; Tel: ; raoqh@csu.edu.cn

2 J. Cent. South Univ. (014) 1: certain percentage of water in order to have the same vane shear strength as the deep-sea sediment in situ [15]. Table 3 lists their tested values of physical and mechanical parameters, including wet density, water content, liquid limit, plastic limit, cohesion and internal friction angle by cutting-ring method, drying method, liquid and plastic limit method and quick direct shear method. It is seen that S3 simulative soil has the closest physical and mechanical parameters to the deepsea sediment and becomes the best substitutive for the deep-sea sediment.. Test arrangement Since the best simulative soil S3 (C=6. kpa, φ=1.7 ) is in a plastic state (its water content between liquid limit and plastic limit), it could be sampled for direct-shear creep test under constant compressive stress (σ) and shear stress (τ). The shear creep specimen was standard cylinder with circular area A 0 of m and height h of 0.0 m. As shown in Fig. 1, direct-shear creep apparatus under stress control was used to measure shear displacement time curve under constant compressive stress (σ) and shear stress (τ). According to designed ground stress σ 0 (σ 0 =5 kpa) of the mining machine [13], constant compressive stresses σ (σ 0 <σ<σ C, σ C is compressive strength) were selected as 5, 10, 15, 0, 5 and 30 kpa (6 groups), where the shear strengths τ 0 of the S3 simulative soil (τ 0 =C+σtanφ) were 6.4, 6.5, 6.7, 6.8, 6.9, and 7.1 kpa, respectively. Therefore, the constant shear stresses for shear creep test (τ<τ 0 ) were set as 1,, 3, 4, 5, and 6 kpa (6 groups). Under a specific constant σ and τ (total 36 groups), shear displacement S was automatically recorded by NS- WY0 displacement sensor every second (high precision) until shear displacement rate (ds/dt) was stable. 3 Results and analyses 3.1 Shear creep curve Figure shows shear creep curves of the S3 simulative soil under different constant compressive and shear stresses. It can be seen that all of the curves are divided as three stages: transient creep stage (transient deformation at t=0), unstable creep stage (creep rate is decreased) and stable creep stage (creep rate is unchanged), which is similar to ordinary land soils in creep stages but different in creep deformation mechanisms. Under the constant compressive stress for direct shear creep, transient creep is caused by non-elastic Table 1 Mineral components of simulative soils Soil sample w(montmorillonite)/% w(quartz)/% w(calcite)/% w(kaolinite)/% w(anatase)/% w(feldspar)/% B1 bentonite B bentonite B3 bentonite B4 bentonite Table Particle size ranges and compositions of bentonites and deep-sea sediment Type of soil samples Particle size range/μm Sand particle content/% Silt particle content/% Clay particle content/% B1 bentonite B bentonite B3 bentonite B4 bentonite Deep-sea sediment Table 3 Physical and mechanical parameters of simulative soils and deep-sea sediment Types of soil samples Wet density, ρ/(g cm 3 ) Water content, ω/% Liquid limit, ω L /% Plastic limit, ω p /% Cohesion, C/kPa Internal friction angle, φ/ S1 simulative soil S simulative soil S3 simulative soil S4 simulative soil Deep-sea sediment

4684 Fig. 1 Direct-shear creep apparatus J. Cent. South Univ.

The transient shear creep displacement S 0 is non-linearly increased with the shear stress (Fig.

As time increases, the water films will irregularly deform to fail (called softening effect), which results in relative displacements and

3 4684 Fig. 1 Direct-shear creep apparatus J. Cent. South Univ. (014) 1: deformations of many water films solidified on surfaces of clay particles. The transient shear creep displacement S 0 is non-linearly increased with the shear stress (Fig. 3), differently from the ordinary land soils where the transient creep stage is mainly caused by elastic deformations of the clay particles. As time increases, the water films will irregularly deform to fail (called softening effect), which results in relative displacements and rearranged tight connecting of the clay particles (called hardening effect), i.e., unstable creep stage. The unstable creep rate is decreased since the softening rate is decreased while the hardening rate is Fig. Tested shear creep curves under constant compressive stress: (a) σ=5 kpa; (b) σ=10 kpa; (c) σ=15 kpa; (d) σ=0 kpa; (e) σ= 5 kpa; (f) σ=30 kpa

4 J. Cent. South Univ. (014) 1: Fig. 3 Transient shear creep displacement carrying with compressive and shear stresses increased. Compared with the ordinary land soils (ω<50%), the S3 simulative soil has much shorter unstable creep stage. That is because it has much higher water content (ω=165.6%) and easily reaches dynamic balance between the softening and hardening effects (i.e., stable creep stage). Obviously, the larger the shear stress τ, the stronger the softening effect (mainly cause creep) and thus the larger the stable creep rate (ds/dt). When the shear stress τ is small (τ kpa), the softening effect is quite weak (without creep deformation approximately) and the stable creep rate tends to zero. Table 4 lists the stable creep rates of the simulative soil in details. Table 4 Stable creep rate ds/dt of simulative soil Creep rate/(10 5 mm s 1 ) σ/kpa τ=1 kpa τ= kpa τ=3 kpa τ=4 kpa τ=5 kpa τ=6 kpa Average It is also found that both the transient creep displacement (Fig. 3) and the stable creep rate (Table 4) are decreased as the constant compressive stress σ is increased because of tighter connection of the clay particles and smaller deformation of the water films under the larger σ. 3. Shear creep parameter 3..1 Creep model Burgers rheological model is adopted to describe creep characteristics of the simulative soil (Fig. 4), where Fig. 4 Burgers rheological model shear stress τ and shear strain γ must be rewritten by shear force Q and shear displacement S for test results of S t curve. Constitutive equations of spring and dashpot model are rewritten by Q K S Q S for spring for dashpot where Q, S and S are shear force, and shear displacement shear displacement, respectively; K and β are shear elastic and viscous parameters proportional to shear modulus G and viscosity η, respectively. Constitutive equations of Maxwell body (H V) and Kelvin body (H V) become S S M K D 1 K1 1 Q D QK K D M where D= / t is first-order of differential operator. According to the series rule Q Q S S M M Q S K K (1) () (3) the rheological constitutive equation of Burgers model can be deduced 1 K1 K 1 K1 Q D Q DQ K1K K1K 1 1DS D j (4) K 1 Therefore, the shear creep constitutive equation is obtained when Q (Q=τA 0 ) is constant as follows. 1 t 1 t K / S t A0 1 e (5) K1 1 K

5 Creep parameter calculation Figure 5 shows fitted shear creep curves under constant compressive stress, which is obtained by fitting tested shear creep data of the simulative soils with Eq. (5). Accordingly, creep parameters of the simulative soil under different and τ can be calculated as listed in Table 5, where the minimum and maximum correlation coefficients are and 0.99, indicating that Burgers rheological model can be used for better describing creep characteristics of the simulative soil. Figure 6 shows shear creep parameters of K 1, K, β 1 and β varying with σ and τ. Clearly, the larger the σ J. Cent. South Univ. (014) 1: under the same τ is, the smaller the shear creep deformation is and thus the larger the shear creep parameters are (ability to resist deformation). In addition, the shear creep parameters are fluctuated with τ when the constant σ is larger (σ>15 kpa), while slightly changed with τ when the constant σ is smaller (σ<15 kpa), since larger σ would result in local crushing and inhomogenous shear surface of the simulative soils. Considering the fluctuated shear creep parameters finally tend to certain stable values, average creep parameters under the same σ can be used as the creep parameters of the simulative soil (Table 5). Fig. 5 Fitted shear creep curves under constant compressive stress: (a) σ=5 kpa; (b) σ=10 kpa; (c) σ=15 kpa; (d) σ=0 kpa; (e) σ= 5 kpa; (f) σ=30 kpa

6 J. Cent. South Univ. (014) 1: Table 5 Shear creep parameters Compressive stress, Shear stress, σ/kpa τ/kpa K 1 /(kn m 1 ) K /(kn m 1 ) β 1 /(N s m 1 ) β /(N s m 1 ) Correlation coefficient, R Average Average Average Average Average Average

7 4688 J. Cent. South Univ. (014) 1: Fig. 6 Shear creep parameters carrying with compressive and shear stresses under different σ: (a) K 1 τ; (b) K τ; (c) β 1 τ; (d) β τ When the deep-sea mining machine of dead weight (designed ground stress σ) walks at constant velocity (v) on the deep seabed, its traction is mainly provided by shear resistance of the deep-sea sediment, which can be calculated by the shear creep constitutive equation as follows: 1 vt (6) A 1 t 1 0 t K / 1 e K K 4 Conclusions 1 1 1) Based on mineral components and in-situ vane shear strength of the deep-sea sediment, four kinds of bentonites were selected as raw materials to mix with a certain percentage of water for preparing the simulative soil for deep-sea sediment. The best simulative soil should have the closest physical and mechanical parameters to the deep-sea sediment, including mineral component, particle size, wet density, water content, liquid limit, plastic limit cohesion and internal friction angle. ) Shear creep curve of the deep-sea simulative soil can be divided into three stages: transient creep, unstable creep and stable creep, where the unstable creep stage is short. Since the deep-sea simulative soil (ω=165.6%) has more water films than the ordinary land soil (ω<50%), irregular deformation of water films easily occurs. The softening rate is decreased, while the hardening rate is increased, which rapidly results in dynamic balance of the softening and hardening effects (stable creep stage). 3) Burgers creep model can better describe the shear creep characteristics of the deep-sea simulative soil. The shear creep parameters are increased with the compressive stress (τ is constant) and changed slightly ( 15 kpa) or fluctuated to approach a constant value (>15 kpa) with the shear stress τ. Average creep parameters under the same σ could be used as the creep parameters of the simulative soil. 4) Traction of the deep-sea mining machine walking at a constant velocity can be calculated by the shear creep constitutive equation of the deep-sea simulative soil, which provides a theroretical basis for safe operation and optimal design of the deep-sea mining machine. References [1] HONG S Y, KIM J H, CHO S K. Numerical and experimental study on hydrodynamic interaction of side-by-side moored multiple vessels [J]. Ocean Engineering, 005, 3(7): [] DAI Yu, LIU Shao-jun. Theoretical design and dynamic simulation of new mining paths of tracked miner on deep seafloor [J]. Journal of

8 J. Cent. South Univ. (014) 1: Central South University, 013, 0(4): [3] TANG Xiao-yan, LIU Shao-jun, WANG Gang. Modeling of heave compensation system for deep-ocean mining and its simulation of fuzzy logical control [J]. Journal of Central South University (Science and Technology), 008, 39(1): (in Chinese) [4] RAO Qiu-hua, WANG Zhi, LIU Shao-jun, FANG Min. Interaction of fluid-solid coupled flexible hose and mining machine in deep-ocean mining system [C]// Proceedings of the Eighth (009) ISOPE Ocean Mining Symposium. Chennai, India, 009: [5] XU Hai-liang, Yin Ping-wei, XU Shao-jun, YANG Fang-qiong. Pump-lockage ore transportation system for deep sea flexible mining system [J]. Journal of Central South University of Technology, 008, 15(4): [6] INDERBITZEN A L. Deep-sea sediments: Physical and mechanical properties [M]. New York: Plenum Press, 1974: [7] SONG Lian-qing. Geotechnical properties of oceanic polymetallic nodule sediments [J]. Acta Oceanologica Sinica, 1999, 1(6): (in Chinese) [8] ZENG Le-ping, HUANG Ju-fang, QIU Guan-zhou, CHU Feng-you, CHEN Dan, TONG Jian-bin, LUO Xue-gang. Isolation and identification of Rhodosporidium diobovatum DS-005 from deepsea sediment of eastern pacific ocean [J]. Journal of Central South University of Technology, 009, 16(6): [9] BRANDES H G. Geotechnical characteristics of the deep-sea sediments from the North Atlantic and North Pacific oceans [J]. Ocean Engineering, 011, 38(7): [10] CHOI J S, HONG S, CHI S B, LEE H B, PARK C K, KIM H W, YEU T K, LEE T H. Probability distribution for the shear strength of seafloor sediment in the KR5 area for the development of manganese nodule miner [J]. Ocean Engineering, 011, 38(17): [11] LI Li, LI Shu-lin. Simulation and mechanical characteristics of terramechanics of surface soil on deep-sea bed [J]. Engineering Mechanics, 010, 7(11): (in Chinese) [1] SCHULTE E, HANDSCHUH R, SCHWARZ W. Transferability of soil mechanical parameters to traction potential calculation of a tracked vehicle [C]// Proceedings of the Eighth (003) ISOPE Ocean Mining Symposium. Tsukuba, Japan, 003: [13] WANG Jiang-yin, Cao Wen-gui, Zhai You-cheng. Experimental study of interaction between deep-sea sediments and tracks [J]. Rock and Soil Mechanics, 008, 3(s): (in Chinese) [14] MA Wen-bo, RAO Qiu-hua, WU Hong-yun. Macroscopic properties and microstructure analyses of soft seabed soil in deep-sea [J]. Rock and Soil Mechanics, 014, 35(6): (in Chinese) [15] WU Hong-yun, CHEN Xin-ming, GAO Yu-qing, HE Jiang-san, LIU Shao-jun. In-situ shearing strength and penetration resistance testing of soft seabed sediments in western mining area [J]. Journal of Central South University (Science and Technology), 010, 41(5): (in Chinese) (Edited by FANG Jing-hua)

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