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Journal of Advanced Concrete Technology Vol. 19, 226-239, March 2021 / Copyright © 2021 Japan Concrete InstituteTechnical report

Action of Hydraulic Pressure on Portland Cement Mortars - Current
Understanding and Related Progress of the First-Ever In-Situ Deep
Sea Tests at a 3515 m Depth
Keisuke Takahashi1*, Yuichiro Kawabata2, Mari Kobayashi3, Shinpei Gotoh4, Shun Nomura5,
Takafumi Kasaya6 and Mitsuyasu Iwanami7
Received 15 February 2021, accepted 11 March 2021

doi:10.3151/jact.19.226

Abstract
In order to realize the utilization of cement-based materials in the special extreme environment, the deep sea, the authors
have launched a project targeted at creating a technology platform with in-situ methods and systems for monitoring andevaluating cement-based materials located at deep ocean bottom sites. The first in-situ test in the world with a view toinvestigating the time-dependence of the volumetric stability and microstructure of Portland cement mortar following its
long-term exposure to deep-sea conditions is currently underway at a 3515-m depth in the Nankai Trough. This paper
reviews previous studies about the influences of deep-sea hydraulic pressure on cement-based materials, verifies theaction of short-term hydraulic pressure using Portland cement mortars on a laboratory scale, and introduces the ongoingprogress of in-situ deep-sea tests. Results from laboratory tests indicate that dimensional changes were provoked by liquidwater infiltration and confinement while under short-term hydraulic pressure, however, time-dependent behavior understresses such as creep has not appeared. Weight gain, changes in pore-size distribution, compressive strength and bending
strength of the cement mortar were monitored after pressurization and depressurization processes.

1. Introduction
Defined as a marine environment at a water depth greaterthan 200 m (Danovaro et al. 2017), the deep sea is referred to as the last frontier and has attracted much research attention within the fields of natural science andresource development (Araki et al. 2017; Ramirez andBillet 2006; Sharma 2015; Miller et al. 2018). Carbon
dioxide capture and storage under the seafloor (Hume2018) and underwater structures in which the occupants
1

Chief researcher, Technical Development Center, Ube
Industries, Ltd., 1-6 Okinoyama Kogushi, Ube, 7558633,
Japan. *Corresponding Author,
E-mail: Keisuke.Takahashi@ube-ind.co.jp
2
Head of Frontier Technologies for Structures Group,
Structural Engineering Field, Port and Airport ResearchInstitute, National Institute of Maritime, Port andAviation Technology.
3
Researcher, Technical Development Center, Ube
Industries, Ltd.
4
Assistant professor, Department of Marine Electronics
and Mechanical Engineering, Tokyo University ofMarine Science and Technology.
5Technical researcher, Department of Mathematical
Science and Advanced Technology, Japan Agency for
Marine-Earth Science and Technology.
6Group leader, Research Institute for Marine Resources
Utilization Submarine Resources Research Center, Japan
Agency for Marine-Earth Science and Technology.
7
Professor, Department of Civil and Environmental
Engineering, Tokyo Institute of Technology.

live and work (Shimizu Corporation n.d.) could lead totechnological breakthroughs in utilizing marine areas.
Recently for advancement of the field of cutting-edge
physics, an undersea research infrastructure that willhouse the next generation neutrino telescopes KM3NeT
is being constructed in the Mediterranean Sea (Margiotta
2014; Le Breton 2019). In Japan, the construction of an
ocean bottom detector with the size of 10–50 kt at sea
depths of 1000 to 5000 m is being planned for imaging
the deep mantle structure by geoneutrino observation(Sakai et al. 2020). In order to realize these pioneeringand innovative projects, the establishment of infrastructures in deep-sea environments will be required, where
physicochemical conditions differ extremely from landand shallow-sea areas. Assuming these applications will
be developed, construction materials would be required
to maintain their robustness for a very long period (millennial scale).
Cement is widely used as a construction material due
to its versatility and its ability to resist destruction, wear
and environmental impacts. It has already been utilizedin shallow-sea areas at depths of ca. 20 m to constructembankments, piers and breakwaters. The durability of
cement-based materials in shallow-sea environments
have been investigated during the last 50 years, and thedeterioration process has been almost clarified (Mather
1964; Gjorv 1971; Metha 1990; Mohammed et al. 2003;
De Weerdt et al. 2014; Jakobsen et al. 2016). The durability of the cement-based materials under deep-sea
conditions, however, has hardly been reported, althoughthese materials could be applied in the future construction of infrastructures in deep-sea areas.
Very specific to deep seas are environmental condi-

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

tions such as high hydraulic pressure and constant low
temperature where carbonates become more soluble inseawater and pH is decreased slightly. Such specificenvironments have impacts on the durability of cement-based materials. The authors have reported thatsignificant deterioration was observed in a Portland cement mortar exposed 608 days to a seafloor environment
at a depth of 1680 m at Tarama Knoll and interpreted thata key influencing factor for the disintegration of themortar specimen could be extremely low temperature(Kobayashi et al. 2021). Although the effect of highhydraulic pressure has been rarely addressed in our previous paper, hydraulic pressure could be another key
influencing factor that can affect the durability of themortar under the deep-sea environment. There are somestudies focused on the influences of hydraulic pressure
on cement-based materials.
From the backgrounds above, action of hydraulicpressure on cement-based materials remains controversial. In Section 2, these works are reviewed, and current
understanding is shown. The distinction between underwater pressures such as hydrostatic and other changing pressures is not central to our study, hence we willrefer simply to hydraulic pressure in what follows. In
Section 3, we investigate the effects of short-term hydraulic pressure on Portland cement mortars during theinitial 7 days of submersion under 17 MPa (equivalent to
ca. 1700 m in depth) by measuring with X-ray microtomography scanning (X-μCT), and by measuring thestrain in the mortar specimen while under hydraulicpressure and the changes in material properties provokedby pressurization and depressurization. Disintegration of
the mortar specimen observed in our previous study(Kobayashi et al. 2021) is verified from the aspect ofhydraulic pressure on a laboratory scale.
Furthermore, in order to grasp the action of long-termand higher hydraulic pressure as well as the real behaviorof cement-based materials under actual deep-sea conditions, the authors have set up a project that targets thecreation of a technology platform of in-situ deep-sea
monitoring methods and the corresponding evaluation ofthese materials by using the on-site measuring apparatus(Section 4). Within the project, the in-situ test is currently
being performed at 3515 m depth in the Pacific Ocean’s
Nankai Trough in order to investigate the time-dependent
volumetric stability and microstructure-related durability
issues due to the long-term exposure to seabed. To thebest of the authors’ knowledge, this is the first-everin-situ deep-sea test on cement-based materials. The
specimen and measurement data that play an essential
role in this deep-sea research study will be retrieved in
2021. This paper introduces the progress of the in-situ
test and demonstrates the results of the related preliminary testing under 35 MPa of hydraulic pressure using a
hyperbaric chamber.

227

2. Research review: Influences of hydraulicpressure on cement-based materials
Hydraulic pressure can be a key factor in the deterioration of cement-based materials under deep-sea conditions since these materials as a porous medium withmicro to macro pores are subjected to hydraulic pressure
as well as liquid water infiltration. Several reports indicate that the pressure generates the isotropic confinement
and water infiltration, which decreases the compressive
strength and bending strength of concrete (Haynes andRail 1986; Craig et al. 1981; Pauli and Clapper 1967),
whereas confined concrete results in a strength increase(Mander et al. 1988). Fracture behaviors of the cement-based materials under high hydraulic pressure
could be dependent on pore volume, size, distribution
and tortuosity (Hori et al. 2015). Figure 1 illustrates the
rupture of the hardened Portland cement paste with awater-to-cement ratio of 0.60 under high hydraulicpressures. Once the pores in concrete are saturated with
infiltrated water, the internal pressure acting on the solid
skeleton is released. Spatiotemporal differential releaseof external and internal pressures at the infiltration
boundary could generate strain (Clayton 1998). The
fractured surface of columnar and cylindrical specimen
can be ellipsoidal in shape.
In 1971, the U.S. Naval Civil Engineering Laboratory
in California started to study hollow concrete spheres at
ocean depths ranging from 550 m to 1500 m. These
spheres had an outside diameter of 1.7 m and a wall
thickness of 0.1 m. The concrete mix was designed with awater-to-cement ratio of 0.40. They reported that thecompressive strength of cores drilled from the specimensthat were submerged for 10 years decreased about 15 %compared to the fog-cured specimens over this time
period, and specimens deeper than 1100 m were crushed
under the pressure (Haynes et al. 1976). The concrete
was saturated with seawater, as findings showed that the
specimens retrieved after 10 years contained 53 L ofseawater inside, which was 3 % of its interior volume(Rail and Wendt 1985). It should be noted that the
specimens for those compressive tests were affected bydepressurization during the salvage process. Microstructural damages, which could be generated by thewater infiltration during pressurization (Wang et al.
2016) and/or by the long-duration exposure to deep sea
conditions (Rail and Wendt 1985), might be emphasized
by the release of water confinement itself (Clayton 1998)
and/or the gasified air during depressurization associatedwith the retrieval of the specimens.
Clayton (1998) measured compressive strength andbending strength of concrete while under hydraulicpressure. The test apparatus consisted of a load cell
situated beneath the specimen inside a high-pressure
vessel. The concrete mix was designed with water-to-cement ratio of 0.55. In the case that pressure was
kept at 60 MPa for 20 h, compressive strength andbending strength while under hydraulic pressure de-K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

creased about 30% and 50% respectively, compared tothe unpressurized condition. Both compressive strengthand bending strength remained the same after the following depressurization from 60 MPa to atmosphericpressure (0.1 MPa). When the applied hydraulic pressure
was high enough, hydraulic confinement was dominantas the result and permanent damages occurred during therise and/or sustainment of the high hydraulic pressure.
Depressurization back to atmospheric pressure resulted
in no further significant damage. In contrast, in the casethat the applied pressure is not high, the bending strength
while under hydraulic pressure remained the same
compared to unpressurized condition even after 6 days ofpressurization at 30 MPa, but decreased about 25% after
the following depressurization to atmospheric pressure.
The damage in this case might be due to the differential
release of external and internal pressure as well as therelease of the confining stress itself.
An in-situ test was recently carried out in the Arctic
Sea at a 2500-m depth that monitored temperature andhumidity inside underwater housings made of ultra-highperformance concrete in order to determine alternative
materials to titanium, special stainless steel and aluminum for housings (Wilhelm and Curbach 2017; ROBEX
n.d.). On a laboratory scale, Wilhelm and Curbach (2017)
have described that the implosion behavior of concrete
housings can be simulated by empirical equations determined by Albertson (1973) modified with a nonlinear
material law for concretes, which was calibrated withresults from multiaxial strength tests (Hampel et al.
2009; Ritter and Curbach 2015).
The previously mentioned strength decrease and im

228

plosion behavior could be affected by in-situ dimensionalchanges, including creep, during long-term exposure to
hydraulic pressure as well as instantaneous elastoplasticstrain during the diving and pressurizing of the specimen.
In order to clarify the behavior of cement-based materials
under high hydraulic pressure, in-situ monitoring is required as such an approach has not been reported. In-situ
monitoring in deep seas will help to gain data and insights on a real behavior of cement-based materials underdeep-sea conditions, which includes pressurization/depressurization. The corresponding mechanisms
will also be elucidated.

3. Effects of short-term hydraulic pressure
on a laboratory scale – the first step forverifying mechanical damages on themortar specimens
As already reported in Kobayashi et al. (2021), specimens salvaged from the deep seafloor with a depth of
1680 m at Tarama Knoll were severely damaged asshown in Fig. 2. We have interpreted that decalcificationof portlandite and calcium-silicate hydrates (C-S-H) andthe formation of magnesium silicate hydrate, thaumasite
and ettringite could be accelerated due to extremely low
temperature and can result in the disintegration of themortar specimen. Although cement mortars are porous
medium with microstructure has risks of deformationand cracking under high hydraulic pressure, their behavior is yet poorly understood. Lab tests were therefore
performed to investigate the behavior of cement mortar
under high hydraulic pressure same with the exposure

Before pressurization
Sea water

SpecimenDuring pressurization
The area saturated with infiltrated water.
Internal pressure is already released.
The area that water
has not reached, whichis still confined.

Hydraulic pressure
Boundary of water infiltration
Fig. 1 Illustration of the rupture of the cement-based specimen due to water transport and confinement under high hydraulicpressure (left). Hardened Portland cement paste that was pressurized from 0.1 MPa to 60 MPa at a pressurizing rate of 1
MPa/min, and the 60 MPa was maintained for 60 min, then depressurized from 60 MPa to 0.1 MPa at a depressurizing rateof 1 MPa/min (right). The original shape of the hardened paste was 50 mm in diameter and 100 mm in height.

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K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

Table 1 Mix proportions of the experimental mortars (unit: g).
MortarCementSanda
PCE
MC
Defoamer
Tap water
A
500
500
4.0
2.4
1.0
300
B
500
500
0.5
1.0
0.1
300
a
Siliceous sand with a particle size of 0.5–1.0 mm was used for mortar A. Siliceous sand with a particle size of 0.1–2.0 mm, whichis standard sand produced by Japan Cement Association, was used for mortar B.

Table 2 Composition of Portland cement measured by X-ray fluorescence (unit: %). LOI: loss on ignition.
SiO₂
20.37

Al₂O₃
5.26

Fe₂O₃
2.67

CaO
65.31

MgO
1.19

SO₃
2.86

Na₂O
0.24

K₂O
0.28

LOI
1.00

Total
99.18

Table 3 Physical properties of mortars A and B. Flow value, air content, compressive strength and bending strength were
measured based on JIS R 5201:2015.
MortarA
B

Flow value (mm) Air content (%) Density (g/cm3)
295
5.0
2.0
268
5.0
1.9

condition at Tarama Knoll. In this paper the effects ofshort-term hydraulic pressure is explained. The action of
long-term hydraulic pressure is still under measurement
and will be published soon.
3.1 Materials and test specimens
The mortar mix A shown in Table 1 used for laboratory
experiments contains Portland cement (PC), siliceous
sand, polycarboxylic ether superplasticizer (PCE),
methyl cellulose thickener (MC) and defoamer. The
composition of PC, measured by X-ray fluorescenceaccording to the Japanese Industrial Standard (JIS R
5304), is shown in Table 2. The water-to-cement ratio
was 0.60 by reference to the upper limit of 0.65 for thismixing ratio in non-reinforced structures established by
Ports and Harbours Bureau, MLIT. et al. (2009). The dry
mortar mixes were pre-mixed and subsequently mixed
with water for 2 min at 700 rpm using a mechanicalmixer. Mortar specimens with the size of 40 mm × 40
mm × 160 mm were prepared for the measurement ofmaterials characterization before/after pressurization,strain measurement while under hydraulic pressure andCompressive strength (N/mm2)
38.4
34.0

Bending strength (N/mm2)4.45.8

X-μCT after pressurization/depressurization. Table 3
shows flow value, air content and density of the mortar A
immediately after mixing, and compressive and bending
strength of the mortar specimen cured for 28 days undersealed conditions. These testing and curing were performed at a constant temperature of 20ºC 65 %RH. Prior
to the measurements shown in Section 3.3, the mortar
specimens were cured for 28 days under sealed conditions at 20ºC. The variations of the temperature andhumidity were ±1ºC and ±5% RH, respectively.
3.2 High-pressure vessel and pressurization
protocolThe test apparatus for pressurization is shown in Fig. 3. A
high-pressure vessel was custom manufactured with an
inner diameter of 150 mm and an inner height of 250 mm.
The top lid can be fixed to the vessel by tightening bolts.
An underwater connector Seacon RMK-6-FS/MP (TEConnectivity Co.) was installed on the lid to connect thecables from the strain gauges and data logger.
The inner container of the high-pressure vessel was
filled with saturated lime water in order to avoid decal-

Fig. 2 Mortar specimens exposed to the deep seafloor with a depth of approximately 1,680 m at Tarama Knoll (left; thisphotograph was obtained from JAMSTEC) and the mortar specimens salvaged from Tarama Knoll (right).

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

cification of the mortar specimen. After inserting two
specimens and closing the vessel, the hydraulic pressure
was increased at a rate of 0.5 MPa/min to 17 MPa for 35
min using a hand pump. The pressure was then kept
constant at 17 MPa for a period of 7 days at a maximum.
Depressurization was performed by opening the pressure
reducing valve manually, which meant the depressurization rate was not controlled. Applied maximum pressure
value of 17 MPa and pressurization rate of 0.5 MPa/min
simulated the real operation using a deep-submergence
research vehicle Shinkai 6500 (JAMSTEC document
catalog YK17-17).
3.3 Measurements
Materials characterization was carried out before andafter hydraulic pressurization/depressurization. Also, thedeformation as well as liquid water infiltration throughpores of the cement mortar while under hydraulic pressure were investigated. Every test was repeated morethan twice under the same conditions to confirm thereproducibility of measurements.
(1) Strain while under hydraulic pressure
For the specimens dedicated to strain measurement, astrain gauge PML-60 (Tokyo Measuring Instruments
Laboratory Co, Ltd.) with the size of 13 mm × 5 mm ×
125 mm was embedded in the mortar specimen. Leadwire from the specimens was covered with waterproofing
materials and connected to the data logger via the underwater connector from the top lid. Strain was measured
continually during the 7 days while applying the hydraulic pressure of 17 MPa.
(2) Pore-size distribution, weight change, compressive strength and bending strength
Differences in materials characterization such as thepore-size distribution, weight, compressive strength andData logger

bending strength of the mortar specimens between, before and after periods of hydraulic pressurization were
investigated. The mortar specimens were pressurized for
periods of 1 day and 7 days. The pore-size distribution
was determined via the mercury intrusion porosimetry(MIP) method using AutoPore IV 9500 (Micromeritics
Instrument Corporation) porosimeter. The contact angle
was set at 140º. The mortar specimens for MIP were cut
into 5-mm cubes and the hydration was stopped withisopropanol. Weight change of the specimens before andafter pressurization was given as a percentage. Compressive strength and bending strength were measured
based on JIS 5201 R: 2015.
(3) X-ray microtomography scanning (X-μCT)
X-μCT was conducted in order to investigate damages on
the mortar specimen resulting from short-term water
infiltration. The model ScanXmate-D200RSS900
X-μCT (Comscantecno Co., Ltd.) with a maximum
voltage of 115 keV and X-ray tube current of 110 μA wasused. The transmitted X-rays were detected by a 418 mm
by 418 mm flat panel with a resolution of 1504 pixels by
1504 pixels. A radiograph was acquired every 0.3° for a
total of 1200 images. The three-dimensional voxel size
was 52 μm × 52 μm × 52 μm. Consequently, 1504 slices
were generated in each of the vertical and horizontal
directions. The durations of pressurization were 30 min,1 day and 7 days. Tap water was used for this test. Aftereach predetermined duration, the specimen was depressurized and taken out of the vessel. Subsequently,
three-dimensional images of X-μCT scans were taken.
3.4 Results and discussions(1) Deformation and water infiltration of mortar
during short-term pressurization
Figure 4 depicts an example of three-dimensional images of X-μCT scans before and after pressurization for 1

Hand pumpUnderwater connector

230

Top lid

Lead
Saturated
lime water
Strain gauge

250 mmSpecimen

150 φmmFig. 3 Apparatus for the strain measurement of hardened specimens while under hydraulic pressure of 17 MPa.

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

day. The histogram of the brightness values obtained
from the three-dimensional images as a function of thepressurization periods is shown in Fig. 5. The brightness
values were averaged with 1504 slices. The gray level ofeach pixel represents a density value corresponding to
the linear attenuation coefficient of the element contained in that pixel. There were two main peaks. The
lower gray level (darker color: ca. 60–100) corresponds
to pores. The peak with the higher gray level (around
110–180) is attributed to cement hydrates such as C-S-H.
The peaks with the lower gray level became smallerwhen pressurized, which indicates that the pores werepartially or fully saturated with liquid water with increasing periods of pressurization. After 30 min ofpressurization, some pores close to the specimen surface
could be saturated. Gray level at around 60–100 after 1
day was almost identical to that after 7 days, suggesting

231

that almost all the pores were saturated with water after 1
day of pressurization. It should be noted that damage by
short-term water infiltration was not observed in CT
images. In fact, after depressurization, no crack was
visible on the surface of the mortar specimen.
Figure 6 represents the strain while under 17 MPa of
hydraulic pressure as a function of time. Contraction(compression) was considered to be a negative value.
Applied hydraulic pressure was increased from atmospheric pressure to 17 MPa for the first 35 min. The strain
decreased (specimen contracted) approximately linearly
until the applied pressure reached 17 MPa, at which point
the strain then increased (specimen was restoring to theoriginal shape) gradually and returned to almost zero
within 4 h, which can be provoked by water saturation of
empty pores in the specimen. The strain remained about
zero until 7 days elapsed, which indicated that creep had

20 mm

20 mm

Fig. 4 Three-dimensional images of X-μCT scans of the mortar specimen before pressurization (left) and after pressurization for 1 day (right). Lower density point, e.g. empty pores that look darker in the image, have a smaller brightness value.
Higher density point, e.g., water saturated pores and cement that look lighter in the image, have a higher brightness value.
Before pressurization
After pressurization for 30 min
After pressurization for 1 day
After pressurization for 7 days

60

100 110180

0255
Fig. 5. Histogram of the brightness values for the three-dimensional images of each specimen. Brightness values wereaveraged with 1504 slices. The gray level at ca. 60–100 corresponds to pores, and ca. 110–180 is attributed to cement
hydrates.

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not occurred during this time period. Although the timingof the measurement is different, the result of the strain
measurement is quite consistent with that of X-μCT (Figs.
4 and 5).
These results indicate that damages and dimensionalchanges at submillimeter to submicron scales as wasreported in our previous study (Kobayashi et al. 2021) do
not appear due to the action of short-term hydraulicpressure.
In addition, these results may be simply explained by
the classic theory in the soil mechanics; the principle ofeffective stress by Terzaghi (1923). The effective stress
σ’ is related to total stress σ and the pore water pressure u,
as shown below.
σ = σ’ + u(1)(2) Pore volume, weight change, compressive
strength and bending strength
Figure 7 represents incremental pore volume and cumulative pore volume of the mortar specimens before
and after the hydraulic pressurization at 17 MPa. After
pressurization for 1 day and subsequent depressurization,
the pore volume of >70 nm in diameter increased slightly
and the peak position of 40 nm in diameter before pres-

100

100

0

0

-100

-100

Strain (×10 -6 )Strain (×10-6 )Here, compression stress is assumed to be positive.
From the macroscopic mechanics point of view, local
effective stress acting on the skeleton σ’ by the external
water pressure σ is gradually increased before the liquidwater infiltrates the mortar, which is a porous media, thenreleased when liquid water is totally infiltrated into themortar. At that time, the pore water is connected to theexternal water in the vessel, and the pore water pressure
u is equal to the external pressure σ, which means σ’ = 0
and no volumetric strain generates. In this experiment,

the mortar with a water-to-cement ratio of 0.6 was used,
so the microstructure was not relatively dense. Therefore,
the liquid water easily infiltrated, leading to stress relaxation on the skeleton.
From the standpoint of this principle, the mortar
specimen size may play a role: once water infiltration isdelayed for large-scale specimens or dense specimens,
the effective stress of the skeleton of any area that is not
filled with liquid water should resist against the external
total stress, possibly leading to damage development. On
the contrary, the area where the liquid water has not
infiltrated is under isotropic confinement, which increases the strength. Therefore, damages due to hydraulicpressure would strongly depend on the balance between
the hydraulic pressure for stress development of theskeleton and the water infiltration for stress release of theskeleton.

-200
-300
-400
-500
-600

-200
-300
-400
-500

0

10

20

30

40

50

-600

60

1

10

100

Elapsed time (min)Elapsed time (h)Fig. 6 Strain of the mortar specimen while under 17 MPa of hydraulic pressure as a function of time. (left: initial 60 min, right:
from 1 h to 7 days).
0.20

Before pressurization
After pressurization for 1 day
After pressurization for 7 days

0.35
0.30
0.25
0.20
0.15
0.10
0.05

Before pressurization
Cumulative pore volume (cc/g)

Pore volume (dV/dlogd;cc/g)

0.40

After pressurization for 1 day

0.16

After pressurization for 7 days
0.12

0.08

0.04

0.00

0.00
1

10

100

Diameter (nm)

1000

10000

1

10

100

Diameter (nm)

1000

10000

Fig. 7 Incremental pore volume (left) and cumulative pore volume (right) of the mortar specimens before and after thehydraulic pressurization at 17 MPa. dV/dlogd: dividing the differential pore volume (dV) by the logarithmic differential pore
diameter (dlogd).

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

233

Table 4 Weight change, compressive strength and bending strength of the mortar specimens before and after the hydraulicpressurization at 17 MPa. Weight gain was considered a positive value. Measured data of 3 specimens were averaged.
Before pressurization
After pressurization for 1day
After pressurization for 7 days

Weight change (%)
–
3.2
3.3

surization was shifted to 20 nm in diameter after pressurization for 1 day. The loss of pore connectivity and thecompressive damages on pores could be considered as apore closure. The pores closed following 1 day of pressurization and subsequently reopened after 7 days ofpressurization. These changes in pore-size distribution
could be caused by the combination of water infiltrationand confinement that could lead to the initial closing of
the pores and the subsequent opening due to the differential release of external and internal pressures, as wellas the release of the confining stress itself. As saturated
lime solution was used in this measurement, pore opening cannot be attributed to decalcification of portlandite
and C-S-H (Phung et al. 2016; Bossa et al. 2015). Further
investigation will be necessary to clarify how this microstructural changes by the short-term hydraulic pressure affect the long-term durability and dimensional
stability.
Table 4 shows weight change, compressive strengthand bending strength of the mortar specimens before andafter the hydraulic pressurization at 17 MPa. Water absorption of the mortar specimens increased the weightafter 1 day of pressurization. The prolonged pressurization period did not result in changes in weight gain.
Water uptake to the specimens can be cross-checked in
Figs. 4 and 5, where water saturation in pores did not
differ in the histogram of X-μCT between 1 day and 7
days. Compressive strength and bending strength after
the pressurization for 1 day and 7 days were slightly
decreased compared to before pressurization. The decrease in strength could be provoked by the water infiltration and confinement. Visible damages and cracks
were not observed on the specimen after pressurizationfor 1 day and 7 days.

4. Challenging project: in-situ exposure
test at 3515 m depth in Nankai TroughAs mentioned in Section 3, damages by the short-term
hydraulic pressure can not be observed at submillimeterto submicron scales but some microstructural changes
were shown, which might lead to durability concern.
Authors have already started investigations to elucidate
the effects of long-term and higher hydraulic pressure
both at a laboratory scale and actual deep-sea field. A
related progress of our challenging project is introducedin this section.
With the view toward deep-sea infrastructure construction, the authors have set up a project targeted creating a technology platform for the use of in-situ moni-Compressive strength (N/mm2)
36.0
35.4
33.1

Bending strength (N/mm2)
4.6
4.3
4.4

toring methods at deep ocean bottom sites and the corresponding evaluation of cement-based materials by thein-situ measurement apparatus. Our first in-situ trial
measures the time dependence of volumetric stability andmicrostructure of a hardened cement mortar as a result of
long-term exposure to the 3515-m deep seabed environment at Nankai Trough in the Pacific Ocean. Prior to
the in-situ exposure test at Nankai Trough, the pressurethat acted on the large mortar specimens and the strain
were measured using a hyperbaric chamber.
Due to the limitation of experimenting opportunity,each measurement in this section was conducted oncewith one mortar specimen.
4.1 Project details(1) Specimen
The mortar mix B shown in Table 1 was used to prepare
the cylindrical larger scale specimens. The water-to-cement ratio was 0.60 as well in 3.1. The dry
mortar mix was pre-mixed and subsequently mixed withwater for 3 min at 1100 rpm using a hand-held mechanical mixer. Then, the mortar was poured into a cylindrical mold with a 350-mm diameter base and a
400-mm height. This mold size allowed for four strain
gauges PML-60 and three pressure gauges PWF-50MPB(Tokyo Measuring Instruments Laboratory Co, Ltd.) tobe embedded as shown in Fig. 8. The head size of thepressure gauge was 14.8 mm in diameter and 51 mm inlength. Strain gauges were also embedded midway
lengthwise in the specimen: two centered along its axis
perpendicular to each other and flanked also by one at
100 mm and one at 40 mm from the curved side surfaceof the specimen (positions described as 1/2r and 1/4r
respectively). Pressure gauges were also embedded
widthwise 80 mm from each base of the specimen, situated contiguous to the center, and at 1/2r and 1/4r. All
cables were covered with waterproofing materials. The
surface of the upper and lower bases of the cylindricalmortar specimen were coated with a water-impervious
epoxy resin in order to monitor the effects of water infiltration from side surface.
Table 3 shows flow value, air content and density ofthe mortar B immediately after mixing, and compressive
and bending strength of the mortar specimen cured for 28
days under sealed conditions. Both testing and curing
were performed at a constant temperature of 20ºC 65%
RH. Prior to the in-situ subsea exposure, the mortar
specimen was cured for 28 days under the sealed condition at 20ºC. Variation of the temperature and humidity
was ±1ºC and ±5 % RH, respectively.

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

(2) Ongoing in-situ exposure test and measurement apparatus
Our results represent that contraction occurs initially inthe mortar specimen due to hydraulic confinement, andwater infiltration could release the internal pressure andrestore the specimen to its original shape, however, theeffects of long-term hydraulic pressure have not beenclarified. The in-situ subsea test being performed at 3515
m depth in Nankai Trough will reveal the creep behavior
and the corresponding microstructure changes of themortar specimen as well as synergistic effects between
hydraulic pressure and cold seawater attack.
Figure 9 depicts the apparatus designed for in-situ
measurements of strain and pressure on the seabed. The
data logger TDS–150 (Tokyo Measuring Instruments
Laboratory Co., Ltd.) was stored in a pressure resistanthousing made of aluminum alloy. The inner vessel of thispressure resistant housing was 120 mm in diameter and467.5 mm in length. Electric insulating polyimide film
was pasted on the inner surface of the vessel in order toavoid energization due to the contact of the substrates tothe inner surface caused by vibration during the diving.
Strain and pressure gauges embedded in the mortar
specimen were connected to the data logger via the underwater connector Subconn Circular BH25F and IL25M(MacArtney Co.) penetrating the lid of the pressure resistant housing.
The mortar specimen and the apparatus designed forin-situ measurements were set on the seabed at deep sea
research site at 3515 m depth by the remotely operated
vehicle ROV Kaiko MK–IV during the KR 20-07 cruise.
Figure 10 shows the ROV Kaiko MK-IV and its basket
where the mortar specimen and the apparatus were
loaded. The deep sea experimental field is located at thenorthern edge of Nankai Trough off Sagami Bay in Japan,
N34°1.65’ E138°31.50’ as pointed out in Fig. 11. The
temperature, salinity and dissolved oxygen of seawater atthe experimental site were constant at 1.5ºC, 35 PSU and3.1 mL/L, respectively. Operations were monitored by
the camcorder equipped onboard the ROV. Figure 12
depicts the mortar specimen and the apparatus during thediving operation to the research field. The diving speedof Kaiko MK–IV was ca. 50 m/min, which was equivalent to a pressurization rate of 0.5 MPa/min. The installation operation of the mortar specimen and the apparatus
onto a frame at the deep sea research site by using amanipulator of the ROV Kaiko MK-IV is shown in Figs.
13 and 14. The in-situ test began on August 25th in 2020,
and the specimen and apparatus will be salvaged in 2021.

Side view

Epoxy resin coat

Pressure gauge

80

40

234

Direction of water infiltration
40
Pressure
gauge

Strain gauge
100

175

400

350

Strain gauge

X

X

Detection point
1/4r

Center

1/2r

Detection point

80
350

100
Top view(cross section of X–X)

Fig. 8 Schematic of the embedded strain (red) and pressure gauges (blue) in the cylindrical mortar specimen (unit: mm).

Fig. 9 Apparatus designed for in-situ deep sea measurements of strain and pressure. Data logger TDS–150 was stored ina pressure resistant housing made of aluminum alloy.

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

(3) Testing results under 35 MPa of hydraulicpressure using a hyperbaric chamber
A preliminary testing was conducted to measure thepressure exerted on the mortar specimen and strain while
under 35 MPa of hydraulic pressure using a hyperbaric
chamber possessed by Japan Agency for Marine-Earth
Science and Technology (JAMSTEC), which is related tothe ongoing in-situ exposure test. The effective dimension of the inner vessel of the hyperbaric chamber was
600 mm in diameter and 1600 mm in height. The inner
vessel was filled with tap water. Both pressurization anddepressurization rates were auto-controlled at 0.6
MPa/min, which simulates diving and salvaging speedsof the remotely operated vehicle (ROV) Kaiko MK-IV
used for the in-situ exposure test. The pressure was in-235

creased from 0.1 MPa to 35 MPa and then kept constantat 35 MPa for 3 h, followed by a decrease to 0.1 MPa.
Strain gauges and pressure gauges embedded in themortar specimen were connected to a data logger via an
underwater connectors Subconn Circular BH25F and
IL25M, penetrating the top lid of the hyperbaric chamber.
Strain and pressure were measured continuously during
the 5 h from the start of applying hydraulic pressure. The
measured strain and pressure values were corrected forthe length and cross-sectional area of the extended lead.
Figure 15 shows the changes in strain and pressure in
the mortar specimen while under hydraulic pressureusing the hyperbaric chamber. Strain decreased (thespecimen contracted) linearly due to water confinement
as the applied hydraulic pressure increased at a constantFig. 10 The ROV Kaiko MK–IV (left) and mortar specimen and the apparatus designed for in-situ measurements loaded in
the basket of the ROV Kaiko MK-IV (right) during the KR 20-07 cruise.

Fig. 12 Mortar specimen and apparatus for in-situ measurement during the dive to the deep sea experimental
field location at Nankai Trough.

Fig. 11 Nautical chart of northern edge of Nankai Troughoff Sagami Bay in Japan (within red frame), and installation point N34°1.65’ E138°31.50 (blue point).

Fig.13 Operation using a manipulator of the ROV Kaiko
MK–IV to install the mortar specimen and the apparatus
onto the frame at the deep sea test field on the ocean
floor.

236

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

Fig. 14 Completed installation of the mortar specimen andthe apparatus on the frame of the deep sea test field.

which are closer to the side surface of the specimen.
Strain values measured with the gauges embedded
lengthwise in the specimen had settled at ca. –700 μ after
approximately 4 h elapsed.
A sharp “bump”, a sudden decrease, appeared in thepressure curve of 1/2r after ca. 3 h elapsed. This bump
could indicate the generation of negative pressure due topronounced pore opening and/or cracking near the detection point of the pressure gauge. When water flowed
into that point where the negative pressure acted, theonce-dropped pressure could return to the pressure
reached before the pore opening and/or crack initiation.
Another sharp bump, however, a sudden increase, wasobserved in the pressure curve of the center after ca. 4 helapsed. Although the reason for this abrupt increase has
not been identified, it noticeably coincided with the startof depressurization process.
Figure 16 depicts a photo of the specimen taken after
the depressurization process. Cracks appeared along the

Fig. 16 A photo of the mortar specimen taken after the
depressurization process shows cracks (marked with a
red arrow) appeared along the curved side of cylindricalsurface nearing the upper and lower circular bases.

0

40

-200

35

-400

30

Strain (μ)

-600

25

-800

20

-1000

15

-1200

10

-1400
-1600

5

-1800

0

-2000

Pressure (MPa)rate of 0.6 MPa/min. At 1/4r, pressure was detected 10
min after the start and became equivalent to the applied
hydraulic pressure after 45 min. The pressure at 1/2r was
generated at 40 min after the start and became equivalent
to the applied hydraulic pressure at ca. 3 h. The pressure
at the center was generated after 3 h from the start but themeasurement was ended before the pressure reached thepoint equivalent to the applied hydraulic pressure. These
results could be interpreted as follows: once liquid waterreached approximately the detection point of the embedded pressure gauge, the pressure value started toincrease. The pressure value became equivalent to theapplied hydraulic pressure when the gauge’s detection
point was saturated with infiltrated liquid water (stressrelaxation on the skeleton).
When the applied hydraulic pressure reached 35 MPa
at 1 h after pressurization, approximately –1600 to –1800
μ of strain was observed at each part of the specimen.
After 1 h from the start, the strain increased (the specimen was restoring to the original shape) at the center aswell as at 1/2r and 1/4r. Although the pores at the centerand 1/2r could not have been fully saturated with water,some internal pressure could be released due to water
saturation of empty pores closer to the curved side surface of the specimen. Compared to the center, the increase of strain became pronounced at 1/4r and 1/2r,

-5
0

1

Strain at center_horizontal
Strain at 1/4r
Pressure at 1/2r

2

3
4
Elapsed time (hours)
Strain at center_vertical
Applied hydraulic pressure
Pressure at 1/4r

5

6Strain at 1/2r
Pressure at center

Fig. 15 A preliminary test to simulate the deep-sea exposure. Changes in strain and pressure act on the mortar specimen
while under hydraulic pressure using the medium-sized hyperbaric chamber.

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

237

Epoxy resin coat

Hydraulicconfinement

This area has not beensaturated with
infiltrated liquid water

Cracksgenerate along
the dashed line

Water saturated area
where internal pressure
was released

Mortar specimen

Fig. 17. An image of crack generation due to the balance between the release of internal pressure in the specimen and3-axis hydraulic confinement of the specimen.

curved side surface near the surfaces of the upper andlower bases of the specimen and could possibly extend
inside the specimen as illustrated in Fig. 17. As liquidwater could infiltrate only from the side surfaces, portions of the water saturated area, where internal stress on
the solid skeleton was released, could not be balancedwith hydraulic confinement (external stress), and therefore, cracks were generated diagonally from the upperand lower base surfaces toward the curved side surface.
This damage on the specimen could be indicating the size
effect as mentioned in 3.4 (1).

5. Conclusion and outlook
Our quest to utilize cement-based materials in the special
extreme environment, the deep sea, has just begun.
Considerable research and development that will focus
on material designs, structure designs and construction
methods will be necessary to build and maintain underwater deep-sea infrastructures. Our first trials focused on
the action of hydraulic pressure at a laboratory scalebrought the following observations:(1) Initial damages by short-term water infiltration andconfinement were less visible by X-µCT,(2) Mortar specimen contracted due to the initial water
confinement, then appeared to return to the original
shape within a few hours due to the water saturationin empty pores that leads to the release of internal
stress on the skeleton. However, time dependence
such as creep behavior neither appeared during thefirst 7 days of the laboratory test using small-scalespecimen, nor the first 4 h of the test using thelarge-scale specimen in the hyperbaric chamber,(3) Through pressurization and depressurization processes, weight gain, changes in pore-size distribution
and slight decrease in strength were observed.
These findings indicate that the action of short-term
hydraulic pressure could not be a direct factor deterio-

rating the mortar specimens as reported in our previous
study – significant disintegration of the mortar specimens exposed to deep-sea environment at Tarama Knoll.
Microstructural changes by the short-term pressurization might affect the long-term durability and volumetricstability. Therefore, further investigations will be necessary to elucidate the effects of long-term and higher
hydraulic pressure.
The first-ever in-situ test in deep-sea environmentswith a view to investigating the time dependence ofchanges in volume fraction, microstructure and synergistic effects between hydraulic pressure and cold seawater attack due to the long-term exposure to deep ocean
bottom conditions is currently being performed at a3515-m depth in the Nankai Trough. Results of thein-situ measurement and the corresponding discussion
will be published soon.
Acknowledgements
We would like to thank Professor Toshiro Yamanaka andAssociate Professor Hiroko Makita at Tokyo Universityof Marine Science and Technology for useful discussions.
We thank Dr. Daiki Takano at Port and Airport ResearchInstitute for performing the image processing of X-μCT.
References
Albertsen, N. D., (1973). “Influence of compressive
strength and wall thickness on behavior of concrete
cylindrical hulls under hydrostatic loading (TR R-790).”
Technical Report of Civil Engineering Laboratory,
Port Hueneme, CA: Naval Construction Battalion
Center.
Araki, E., Saffer, M., Kopf, A. J., Wallace, L. M., Kimura,T., Machida, Y., Ide, S., Davis, E. and IODP Expedition
365 shipboard scientists., (2017). “Recurring andtriggered slow-slip events near the trench at the
Nankai Trough subduction megathrust.” Science, 356,
1157-1160.

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

Bossa, N., Chaurand, P., Vicente, J., Borschneck, D.,
Levard, C., Aguerre-Chariol, O. and Rose, J., (2015).
“Micro- and nano-X-ray computed tomography: A
step forward in the characterization of the pore networkof a leached cement paste.” Cement and Concrete
Research, 67, 138-147.
Clayton, N., (1998). “Effect of water pressure on concrete
strength.” In: O. Gjorv, O., Sakai, K. and Banthia, N.,
Eds., The 2nd International Conference on Concrete
under Severe Conditions, Tromsø, 978-987.
Craig, H., Broecker, W. S. and Spencer, D., (1981).
“GEOSECS Pacific expedition.” Vol. 4, Sections andprofiles, U.S. Government Printing Office.
Danovaro, R., Corinaldesi, C., Dell’Anno, A. andSnelgrove, P. V. R., (2017). “The deep-sea underglobal change.” Current Biology, 27(11), R461-R465.
De Weerdt, K., Justnes, H. and Geiker, M. R., (2014).
“Changes in the phase assemblage of concrete exposed
to sea water.” Cement and Concrete Composites, 47,
53-63.
Gjorv, O. E., (1971). “Long-time durability of concretein seawater.” ACI Journal, 68(10), 60-67.
Hampel, E., Speck, K., Scheerer, S., Ritter, R. andCurbach, M., (2009). “High performance concrete
under biaxial and triaxial loads.” ASCE Journal ofEngineering Mechanics, 135(11), 1274-1280.
Haynes, H. H., Highberg, R. S. and Nordby, B. A., (1976).
“Seawater absorption and compressive strength of
concrete at ocean depths (N-1436).” Technical note ofNaval Civil Engineering Laboratory, Port Hueneme,
California: Naval Construction Batallion Center.
Haynes, H. H. and Rail R. D., (1986). “Handbook fordesign of undersea, pressure-resistant concrete
structures.” Port Hueneme, California: Naval Civil
Engineering Laboratory.
Hori, T., Chijiwa, N. and Iwanami, M., (2015). “A
fundamental research on fracture behavior of concrete
under high water pressure.” Journal of Japan Societyof Civil Engineers, Series B3, 71(2), I-179-I-184.
Hume, D., (2018). “Ocean storage of CO2.” The Maritime
Executive [online], Available from:<https://www.maritime-executive.com/index.php/features/ocean-storage-of-co2> [Accessed 11 Feb 2021].
Jakobsen U. H., De Weerdt, K. and Geiker, M. R., (2016).
“Elemental zonation in marine concrete.” Cement andConcrete Research, 85, 12-27.
JAMSTEC, (n.d.). “Medium sized hyperbaric
chambers.” [online], Available from:<http://www.jamstec.go.jp/e/about/equipment/yokosu
ka/chugata_kouatsu_jikken.html> [accessed 11 Feb
2021].
JAMTEC, (2017). “Yokosuka cruise report (YK17-17).”
Available from:<http://www.godac.jamstec.go.jp/catalog/doc_catalog/metadataDisp/YK17-17_all> [accessed 11 Feb 2021].
Kobayashi, M., Takahashi, K. and Kawabata, Y., (2021).
“Physicochemical properties of the Portland cement-basedmortar exposed to deep seafloor conditions at a depth

238

of 1680 m.” Cement and Concrete Research, 142,
106335.
Le Breton, R., (2019). “KM3NeT: Next-generation
neutrino telescope in the Mediterranean Sea.” Nuclear
Instruments and Methods in Physics Research Section
A: Accelerators, Spectrometers, Detectors andAssociated Equipment, 936, 204-207.
Mather, B., (1964). “Effects of sea water on concrete.”
In: Highway Research Board -Highway Research
Record, 33-42.
Margiotta, A., (2014). “Status of the KM3NeT.” Journalof Instrumentation, 9(4), C04020.
Mander, J. B., Priestley M. J. N. and Park, R., (1988).
“Theoretical stress-strain model for confined concrete.”
Journal of Structural Engineering, 114(8), 1804-1826.
Mehta, P. K., (1990). “Concrete in the Marine
Environment.” New York: CRC Press.
Miller, K. A., Thompson, K. F., Johnston, P. and Santillo,D., (2018). “An overview of seabed mining including
the current state of development, environmental impacts,
and knowledge gaps.” Frontiers in Marine Science, 4,418.
Mohammed, T. U., Hamada, H. and Yamaji, T., (2003).
“Marine durability of 30-year old concrete made withdifferent cements.” Journal of Advanced Concrete
Technology, 1(1), 63-75.
Pauli, D. C. and Clapper, G. P., eds., (1967). “Project
SEALAB report – an experimental 45-day undersea
saturation dive at 205 feet – SEALAB II project group(ONR report, ACR-124).” Technical Report of Officeof Naval Research Department of the Navy,
Washington DC: Office of Naval Research.
Phung, Q. T., Maes, N., Jacques, D., De Schutter, G. andYe, G., (2016). “Investigation of the changes in
microstructure and transport properties of leachedcement pastes accounting for mix composition.”
Cement and Concrete Research, 79, 217-234.
Ports and Harbours Bureau, National Institute for Landand Infrastructure Management, and Port and Airport
Research Institute, MLIT., (2009). “Technical standards
and commentaries for port and harbor facilities inJapan (Part II, Chapter 11).” Tokyo: The Overseas
Coastal Area Development Institute of Japan,
338-341.
Rail, R. D., Wendt, R. L. and Naval Civil Engineering
Laboratory, (1985). “Long-term deep-ocean test of
concrete spherical structures – Results after 13 years(NCEL Technical Report R–915).” Port Hueneme,
California: Naval Civil Engineering Laboratory, 65.
Ramirez, L. E. and Billet, D. S. M., (2006). “Deep-sea
ecosystems: pristine biodiversity reservoir andtechnological challenges.” In: C. M. Duarte, Ed. The
Exploration of marine biodiversity, scientific andtechnological challenges. Bilbao, Spain: Fundacion
BBVA, 63-92, 154pp.
Ritter, R. and Curbach, M., (2015). “Material behavior ofultra-high-strength concrete under multiaxial stressstates.” ACI Material Journal, 112, 5, 641-651.

K. Takahashi, Y. Kawabata, M. Kobayashi, S. Gotoh, S. Nomura, et al. / Journal of Advanced Concrete Technology Vol. 19, 226-239, 2021

ROBEX, (n.d.). “ROBEX - Robotic Exploration of Extreme
Environments.” Available from:<https://www.robex-allianz.de> [Accessed 11 Feb
2021].
Sakai, T., Inoue, K., Watanabe, H., Mcdonough, W. F.,
Ueki, K., Abe, N., Sakurai, N., Kyo, M., Araki, E.,
Kasaya, T. and Yoshida, H., (2020). “Study of ocean
bottom detector for observation of geo-neutrinos frommantle.” In: Japan Geoscience Union – American
Geophysical Union Joint Meeting 2020, U20-P01.
Sharma, R., (2015). “Environmental issues of deep-sea
mining.” Procedia Earth and Planetary Science, 11,204-211.
Shimizu Corporation, (n.d.) “Ocean spiral – Deep sea
future city concept.” Available from:<https://www.shimz.co.jp/en/topics/dream/content01/

239> [Accessed 11 Feb 2021]
Terzaghi, K., (1923). “Die Berechnung derDurchlässigkeitsziffer des Tones aus dem Verlauf derhydrodynamischen Spannungserscheinungen.” Akademie
der Wissenschaften in Wien, Sitzungsberichte,
Mathematisch-naturwissenschaftliche Klasse, Abteilung,
II a. 132, 105-124. (in German)
Wang, Q., Liu, Y. and Peng, G., (2016). “Effect of waterpressure on mechanical behaviour of concrete underdynamic compression state.” Construction and Building
Materials, 125, 501-509.
Wilhelm, S. and Curbach, M., (2017). “Experimental andnonlinear numerical analysis of underwater housingsfor the deep sea, made of ultra-high performance
concrete.” Structural Concrete, 18, 216-224.