Investigation of strength and deformability of the deep-water clay base of the ferromanganese nodules collection unit

. The article considers methods, instruments and equipment for laboratory and field studies of bottom soils of the Clarion-Clipperton region of the Pacific Ocean. The characteristics of the physical and mechanical properties of the bottom sediments of the upper part of the first (cover) engineering and geological complex are presented. The features of deformation of the bottom soil under load are revealed. A general view is shown and the design parame-ters of a model sample of a deep-sea unit for collecting ferromanganese nodules are presented. A design scheme for transferring the load from the unit to the base has been compiled. The type of equation of state of the bottom soil for the condition of placing the unit on the bottom is chosen. The dimensions of the crawler track of the unit are determined. The prediction of the deformation of the water-saturated clay base of the aggregate was carried out using the methods of the theory of linear deformation of the soil.


Introduction
Deposits of ferromanganese nodules (FMN) are quite widespread in the deep-water areas of the ocean floor.The Clarion-Clipperton ore province, located in the northern equatorial part of the Pacific Ocean, is considered to be the largest in terms of Ni and Cu content and unique in Co and Mn [1].
The specificity of the conditions of FMN occurrence requires the use of special technical means for their extraction such as oceanic production complexes (OPC).The OPC structure includes a self-propelled collection unit (CU) designed to collect FMN on the ocean floor [2].The CU has to be designed in such a way that during operation there is no threat of destruction and deformation of the unacceptable size of its deep-water soil base.
At the stage of development work, a universal uninhabited underwater vehicle GK-5000 (Figure 1) was adopted as a CU model (prototype).The self-propelled crawler unit is designed to perform engineering and geological work on the seabed at depths up to 5,000 m [3].
In this regard, an urgent problem is the assessment of the conformity of the design parameters of the base with the dimensions of the CU crawler track.
The purpose of this study was to establish the mechanical parameters of the base for reliable placement of the CU model on the bottom soil.To achieve this goal, the following tasks were set: to investigate the geological structure, condition and physical and mechanical properties of bottom soils; to draw up a design scheme for the interaction of the CU with the base; to calculate the dimensions of the crawler track and the settlement of the CU base.The physical characteristics of the soil such as density ρ, moisture content w and particle density ρs were determined by cutting ring, drying, pyknometer and pressing methods; dry soil density ρd, porosity n and porosity coefficient e were calculated.To calculate the plasticity index IP and the liquidity index IL, the soil moisture at the liquid limit wL and plasticity limit wP was determined using the methods of balancing cone and rolling into bundles (GOST 5180-2015 <Soils.Methods for laboratory determination of physical characteristics=).All soil moisture characteristics were calculated adjusted for pore water mineralization.The ranges of physical characteristics of bottom soils are presented in Table 1.It has been established that in the active zone of interaction between the CU and the soil mass, predominantly weak and soft clayey and siliceous-clayey bottom sediments of the Holocene-Middle Miocene age of the first (cover) engineering-geological complex occur.The mineral composition of the clay fraction is represented by hydromica, smectite, and chlorite.The content of amorphous silica can reach 15-20%.Weakly compacted water sediments are characterized by a coarse aggregate structure and high dispersion.The most important distinguishing features of the bottom soil are high porosity, complete water saturation, absence of gases in free and dissolved form, and fluid consistency [4].

Soil mechanical properties
The characteristics of the strength and deformability of the bottom soil were determined by the methods of rotational shear (GOST 20276-2012 <Soils.Methods for field determination of the characteristics of strength and deformability=), compressive and triaxial compression (GOST 12248-2010 <Soils.Methods for laboratory determination of the characteristics of strength and deformability (corrected)=.Stamp tests in natural conditions were not carried out due to the lack of appropriate equipment.During the tests, the specific features of soft soil and the nature of the projected load acting on the base were taken into account such as CU movement speed v = 0.2 -1.6 m/s, specific soil pressure p = 7 kPa, water column height 4,000 -5,000 m.
The bottom soil was tested by the rotational shear method to determine the following characteristics: resistance to undrained shear cu = τmax, compressive compressionodometric deformation modulus Еoed and filtration consolidation coefficient cv, triaxial compression -transverse deformation coefficient ϑ.
Under the conditions of shipboard and stationary laboratories, these characteristics are determined by the results of testing samples in modified torsion shear apparatuses (TSA), filtration and compression machine (FCM), triaxial compression (BO-1), developed and manufactured at Saint Petersburg State University of Architecture and Civil Engineering in cooperation with VNIIOkengeologiya.
The TSA design (Figure 2) provides a cut of the bottom soil sample with a four-bladed impeller with the diameter of 3 cm, height 3 cm when it rotates at a given constant speed on a turntable; FCM performs compression of the sample in the working ring with a diameter of 7 cm, height 2 cm in small steps of specific pressure during water filtration through soil under the action of a hydraulic gradient I = 130; BO-1 provides sample expansion in a tensometric working ring with measurement of lateral (horizontal) soil pressure.
Samples were tested by an impeller according to the scheme of an unconsolidated fast shear without and with the application of a normal load; compression ones at loads from 1 to 25 kPa with pressure steps of 1 kPa; triaxial ones under two loading modes: load from 2.5 to 25 kPa with pressure steps of 2.5 kPa and a load of 12.5 kPa.The exposure of the pressure steps was 5 s.In the second mode, the load was applied instantly and held until the stabilization of the horizontal pressures in the sample [5][6][7].

Fig. 2. Torsion shear apparatus.
To compare the results of measurements of strength characteristics, soil sections were made with field impellers with a diameter of 6.0 and 7.5 cm, a height of 10 cm, respectively, in a box sampler, in the depth range of 20-160 cm immediately after lifting the sample on board the vessel, and insitu at depths up to 0.45 m from the bottom surface from the UGI-4M deep-sea research facility [8,9].The ranges of strength and deformation characteristics of bottom soils are presented in Table 2. Comparison of soil strength parameters measured in natural occurrence and on board a ship shows their discrepancy within 28-34%.The gradient of soil strength increase within the investigated depth of 0-4.0 m is 2 kPa.This trend is confirmed by the results of static sounding of bottom sediments from an underwater installation.ав?5Aи5, p >а FABEиF5?LAая 45фBD@ация, Graphs of the results of compression tests show that bottom soils have low structural strength as compression of samples in the odometer begins at the first stage of pressure pstr = 1 kPa.In the pressure range of 1-13 kPa, the process of soil compaction corresponds to the law of compression compaction (Figure 3).
Statistical processing of the results of determining the physical and mechanical characteristics of bottom soils (GOST 20522-2012 <Soils.Methods for statistical processing of test results=) of the upper part of the section (up to 0.5 m thick) underlying the FMN deposits within the Eastern part of the Exploration area, allows us to consider it as a single engineering and geological element (EGE) with characteristics for calculating the CU base for the second group of limit states (Table 3).

Calculation model of the base
The design scheme of the <base -CU= system should take into account the most significant factors that determine the stress-strain state of the base.These factors include CU design features (weight, movement speed, overall dimensions of the support and propulsion system and the FMN collection system), CU design situations (setting on the bottom, storm or technical sediment, movement along the bottom, taking into account slopes of the bottom surface, separation from the bottom, etc.), calculation models of the mechanical behavior of the soil, taking into account the mobile and dynamic nature of the load application, physical and mechanical characteristics of soils and the nature of their stratification, the degree of disturbance of the natural structure of the soil during interaction with the CU.
In the course of research, it was found that the bottom soil is an ideal two-phase system (soil mass) that can be deformed due to a decrease in volume, accompanied by squeezing water from the pores of the soil (Figure 3).These features of the soil are prerequisites for choosing the solution to the problem of linear filtration consolidation proposed by Terzaghi for predicting the settlement of the CU base.The limit of applicability of this model can be the average pressure along the base of the crawler track of the CU, until which the relationship between settlement and pressure is close to linear.Such a pressure can be found using Puzyrevsky's solution for the initial critical load.

Calculation of the size of the ground bed and CU settlement
Let us consider the design situation, when the CU model after descent from the vessel is installed on the surface of the bottom soil.We assume that the local load from the weight of the CU is applied to the base almost instantly.The distribution of pressure p along the ground bed of the crawler track is assumed to be uniformly distributed by intensity: where Gsb is the weight of the CU, taking into account the weighing effect of water, Gsb = 45 kN; l is the length; b is the width of the crawler track in contact with the base; l = 1.925 m; b = 0.4 m.The design scheme for transferring the load to the base is shown in Figure 4.
where Eu is the shear resistance of bottom sediments, Eu = 6 kPa.
The pressure from the CU model to the base is 1.6 times higher than the initial critical load.Failure to comply with the condition p ≤ ptilt.begincan lead to significant errors in settlement calculations.
Let us limit the pressure on the base by the condition p = Dtilt.begin.Then the required area of the soil bed of the crawler track will be equal to: To determine the maximum final settlement of the base smax under the center of the soil bed of the CU crawler track, let us use the scheme of a linearly deformable half-space and the equivalent layer method [10].
We find the thickness of the equivalent soil layer according to the formula: where Aω0 is the coefficient of the equivalent soil layer for the maximum settlement of the flexible foundation, determined according to Table 3 at where Eoed the odometric modulus of bottom soil deformation, Eoed = 350 kPa (Table 3).
To predict the consolidation time t, we find the coefficient of soil consolidation where k is the filtration coefficient, k = 0.17 • 10 -2 m/day (Table 3).
Then the time corresponding to the degree of base consolidation U0 = 0.95 at N = 2.8 [11] is determined from the expression:

Conclusions
Bottom sediments are an ideal two-phase system (ground mass) capable of being deformed either due to squeezing water out of the pores, or due to the flow of the soil, which is reflected in the formation of upwater swells.With a rapid application of the load (the mode of setting the CU to the bottom) for the forecast stresses and deformations in the soil halfspace, it is possible to use the solutions of the theory of linear soil deformation.The limit of applicability of the theory can be the average pressure on the CU bed, equal to 18.84 kPa.
It is recommended to increase the dimensions of the crawler track of the CU model in accordance with the presented calculation.The predicted settlement of the CU model when setting on the bottom is 10 cm.

Fig. 4
Fig. 4 Scheme of transferring the load to the base along the bed of the GK-5000 crawler track: side view (a); the same in front (b).Assuming for bottom sediments in a state of incomplete consolidation,  = 0 and the deepening of the CU into the base d = 0 we determine the initial critical load: ÿþ .Ā ýÿĀ = E u = 3,14 • 6 = 18.84 kPa,(2)

Table 1 .
Physical characteristics of bottom soil.

Table 2 .
Mechanical characteristics of the bottom soil.

Table 3 .
Calculated mechanical parameters of the soil.The CU base, composed of clay soil, is characterized by high compressibility and low strength.