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An integrated geotechnical–geophysical
investigation of soft clay at a coastal site in the
Mekong Delta for oil and gas infrastructure
development
Pham Huy Giao, N.T. Dung, and P.V. Long
Abstract: An adequate site investigation of soft clays is important for construction of oil and gas facilities on coastal areas
in the Mekong Delta. This paper presents an integrated geotechnical–geophysical study of the soft clay deposit at the Ca
Mau site, located on the west coast of southern Vietnam. The geotechnical data were analyzed to provide a detailed char-
acterization of the subsoil profile and soil parameters that can be used in the design works. It is also the first time the elec-
tric imaging procedure has been successfully applied in the Mekong Delta to assist the local geotechnical engineers in site
investigation of a very soft clay deposit.
Key words: oil and gas facility, Mekong Delta, soft clay, geotechnical characteristics, electric imaging.
Re
´sume
´:Une e
´tude de site ade
´quate dans les argiles molles est importante pour la construction d’installations pour l’ex-
ploitation de l’huile et du gaz dans les zones co
ˆtie
`res du delta du Mekong. Cet article pre
´sente une e
´tude inte
´gre
´edege
´o-
technique et ge
´ophysique du de
´po
ˆt d’argile molle sur le site de Ca-Mau localise
´sur la co
ˆte ouest du Vie
ˆt-nam du Sud. Les
donne
´es ge
´otechniques ont e
´te
´analyse
´es pour fournir une caracte
´risation de
´taille
´e du profil du sous-sol et des parame
`tres
de sol qui peuvent e
ˆtre utilise
´s dans les travaux de conception. C’est aussi la premie
`re fois que la proce
´dure d’imagerie
e
´lectrique a e
´te
´mise en application avec succe
`s dans le delta du Mekong pour assister les inge
´nieurs ge
´otechniciens locaux
dans l’e
´tude du site d’un de
´po
ˆt d’argile tre
`s molle.
Mots-cle
´s:installation d’huile et de gaz, delta du Mekong, argile molle, caracte
´ristiques ge
´otechniques, imagerie e
´lec-
trique.
[Traduit par la Re
´daction]
Introduction
With more and more petroleum facilities being developed
on the soft clay grounds of the Mekong River Delta (MRD)
in southern Vietnam, a poor site investigation could result in
damage to newly constructed facilities, a major concern to
administrators, construction managers, and engineers in the
oil and gas sector. Deficiencies in site investigation have
mostly been due to (i) a rapid development of oil and gas
facilities, resulting in limited soil investigation and soil im-
provement; (ii) inadequate or outdated laboratory and in situ
geotechnical testing capability; and (iii) a lack of conform-
ance with the geotechnical standards and engineering prac-
tice codes.
This paper has the following objectives: (i) introduce the
Mekong soft clay and emphasize the need for a systematical
study prior to engineering and energy infrastructure develop-
ment in coastal areas, (ii) review and analyze geotechnical
data at a typical coastal site at Ca Mau in the Mekong Delta,
and (iii) show the application of a near-surface geophysical
technique (electric imaging (EI)) in mapping the soft clay
deposit at the study site.
Holocene evolution of the Mekong River
Delta and the distribution of soft clays
The Mekong River is one of the largest rivers in the
world, flowing from the Tibetan Plateau to the South China
Sea through the Indochina Peninsula and forming a delta at
its mouth (Ta et al. 2002). The MRD is located from 8830’N
to 11800’N latitude and from 104830’E to 107800’E longi-
tude (Fig. 1); it has a total area of about 49 520 km2,of
which 79% is in southern Vietnam. The Mekong Delta is a
low-level plain, criss-crossed by a maze of canals and rivers.
It has low elevations, commonly from 0 to 4 m above mean
sea level, and a very slight slope from east to west and from
north to south as seen in Fig. 1.
After the last glacial maximum (LGM) period at approxi-
mately 18 000 – 20 000 years BP, the sea level rose rapidly
and most of the large deltas in Southeast and East Asia were
initially formed, including the MRD (Stanley and Warne
1994). The sea level in the Mekong Delta during the LGM
Received 25 July 2006. Accepted 25 July 2008. Published on the
NRC Research Press Web site at cgj.nrc.ca on 8 October 2008.
P.H. Giao.1GEPG Program, School of Engineering and
Technology, Asian Institute of Technology (AIT), Bangkok,
Pathumthani 12120, Thailand.
N.T. Dung. Department of Geotechnical Engineering, Dong A
University, 840 Hadan2-dong, Saha-gu, Busan 604-714,
Republic of South Korea.
P.V. Long. Vina Mekong Engineering J.S. Co., 44 Dang Van
Ngu, Phu Nhuan, Ho Chi Minh City, Vietnam.
1Corresponding author (e-mail: hgiao@ait.ac.th).
1514
Can. Geotech. J. 45: 1514–1524 (2008) doi:10.1139/T08-077 #2008 NRC Canada
was about 120 m below the present sea level (PSL) and
reached to a high-stand level of approximately 4.5 m above
the PSL at about 4000–6000 years BP. After the high-stand
sea level period, the sea level fell gradually, leading to Hol-
ocene evolution of the present-day delta.
According to the studies by Nguyen et al. (2000) and Ta
et al. (2002), the MRD area is mostly covered by Holocene
deposits which resulted mainly from two successions during
the last 5000–6000 years. Because of the flat topography, ti-
dal progradation was predominant in the MRD after the
high-stand period (Nguyen et al. 2000). The progradation
process has been continuing and widening the delta south-
eastwards and southwestwards, typically represented by the
Ca Mau Peninsula where soft clay is commonly found from
10 to 20 m thick. Nguyen et al. (2005) reported that the ages
of the clays in this continuously prograded peninsula are
from 1000 to 5000 years BP, decreasing seawards from the
northern peninsula to the present coastline.
To date, a comprehensive study of the distribution of
clays in the delta has not been undertaken due to the exten-
sive area involved. However, classified as a tidal-dominated
delta, Holocene deposits of the MRD are typically character-
istic of marine deposits. From a geotechnical point of view,
the uppermost layer of soft clay is a very important consid-
eration for construction projects. To have a systematic over-
view of this soft clay deposit, one needs to build a
computer-aided geotechnical database. The data from vari-
ous site investigation project reports have been collected
and reanalyzed. A geotechnical section following an approx-
imately north–south direction is plotted in Fig. 2, showing
that the soft clay deposit is found from the surface to a
depth varying from 10 m at Long An in the north to 50 m
at Can Tho and about 20 m at Ca Mau in the south.
Study location
Ca Mau Province is the southernmost province of Viet-
nam, located from latitude 8830’Nto9810’N and longitude
104880’E to 10585’E (Fig. 1). The province adjoins Kien
Giang and Bac Lieu provinces in the north, the South China
Sea in the east and south, and the Gulf of Thailand in the
west. Ca Mau province covers an area of 5 208.8 km2, ac-
counting for 1.57% of the total area of Vietnam, and 13.6%
of the MRD. As seen in Fig. 1, the province follows the
coast for 251.7 km (accounting for 7.72% of the coastline
of Vietnam). Surrounded by three large sedimentary basins,
Malay-Tho Chu, Cuu Long, and Nam Con Son, as seen in
Fig. 3, Ca Mau province has a high petroleum potential and
is considered one of the largest centres for oil and gas facili-
ties in Vietnam.
One example of the major oil and gas projects is the gas–
power–fertilizer (GPF) complex for the electricity network
in the Mekong Delta. The study site, referred to as the GPF
site, covers an approximate area of 1.2 km2and is located in
the U Minh District, Ca Mau Province. As most of the Ca
Mau area is covered by very thick and soft clay deposits,
the site must be properly investigated to help ensure a good
foundation design against failure and differential settlement.
Fig. 1. The Mekong River Delta (MRD) and location of Ca Mau Province.
Giao et al. 1515
#2008 NRC Canada
Fig. 2. Geotechnical cross section of the MRD (along the broken line indicated in Fig. 1). Latitude N and longitude E are given in parenth-
eses for each borehold (BH).
Fig. 3. Ca Mau Province and the surrounding sedimentary basins (after PetroVietnam 2008).
1516 Can. Geotech. J. Vol. 45, 2008
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Geotechnical investigation
Drilling and sampling
Soil investigations at the study area were carried out in
2001 and 2003. In the 2001 feasibility investigation stage,
25 geotechnical boreholes were drilled. Undisturbed samples
for laboratory tests were taken from all the boreholes at 2 m
intervals. The drilling depth in this stage varied from 34 to
80 m. In 2003, an additional 14 geotechnical boreholes, 14
field vane shear (FVS) tests, 20 cone penetration test
(CPTU) profiles, and six vertical electric soundings (VES)
were carried out inside the fertilizer plant area. Wash boring
was used in drilling. An Osterberg piston sampler was used
for taking undisturbed samples from the upper soft clay
layer; the sampler outer and inner diameters and length
were 89, 85, and 800 mm, respectively. A Shelby sampler
was used to take undisturbed samples from the lower stiff
clay layer; the sampler outer and inner diameters were 76
and 72 mm, respectively. The sampler tubes were sealed
with paraffin wax and then carefully transported to the geo-
technical laboratory in Ho Chi Minh City.
Subsoil profile
The soil profile and basic geotechnical properties up to a
depth of 40 m are summarized in Table 1. The average pa-
rameters are also plotted in Fig. 4. There are three main soil
units as follows: (1) The upper very soft to soft clay layer
(from the surface to about 17 m) is identified as grayish
black and organic soft clay with a low unit weight. It is rec-
ognized that the upper soft clay layer is very homogeneous,
as indicated by the CPTU profiles shown in Fig. 4. (2) The
lower stiff to very stiff clay layer (from 17 to 34 m) is iden-
tified as grayish blue, yellow, brownish red clay with a high
unit weight and lower water content. (3) The stiff sandy clay
(from 34 m downward) is identified as grayish blue, brown
stiff sandy clay. Its physical parameters given in Table 1 are
quite similar to those of the stiff clay layer, the main differ-
ence being the presence of numerous sand intercalations in
this layer.
Some of the basic geotechnical characteristics of the Ca
Mau clays are presented in the following sections.
Compressibility characteristics
The compression and recompression ratios (CR and RR,
respectively) are defined as the ratio of the compression in-
dex (Cc) and recompression index (Cs) to (1+eo), respec-
tively, where eois the initial void ratio. Figure 4 shows the
CR and RR, two important parameters in the calculation of
the compression of a soft clay layer. The CR is about 0.3 for
the upper soft clay and 0.12 for the lower stiff clay, and the
RR is about 0.09 for the upper soft clay and 0.06 for the
lower stiff clay, respectively. The ratio between CR and RR
is slightly greater than 3 for the soft clay and about 2 for the
stiff clay. These values appear somewhat low, however, and
require further investigation.
Activity
As seen in Fig. 5, the activity values (Ac) vary from 0.3 to
0.9 and are grouped in two clusters. The first cluster corre-
sponds to the upper soft clay layer with a clay fraction of
50%–75% and a plasticity index (PI) ranging from 30% to
40%; both kaolinite and illite are presumed to be the pre-
dominant clay minerals in this layer. The second cluster cor-
responds to the lower stiff clay with a wide range in the clay
fraction of from 20% to 70% and a plasticity index from
10% to 25%; kaolinite may be the predominant clay mineral
in this deposit.
Preconsolidation pressure and overconsolidation ratio
(OCR)
Some typical consolidation curves are shown in Fig. 6ato
illustrate the void ratio and stress relationships for the soft
clays at the study site. The oedometer test follows the stand-
ard procedure with loading pressure increments of 0.125,
0.25, 0.5, 1, 2, 4, 8, 16, and 32 kg/cm2. For each loading in-
crement, displacements of the specimens were recorded at
elapsed times of 0.1, 0.25, 0.5, 0.1, 2, 4, 8, 15, 30, 60, 120,
180, 360, 720, and 1440 min, respectively. After the final
load of 32 kg/cm2was maintained for 24 h, the load was de-
creased to 16, 8, 4, 2, 1, and 0.5 kg/cm2. The consolidation
curves are used to determin as accurately as possible the
preconsolidation pressure to calculate the OCR; Dung and
Giao (2005) have reviewed a number of methods and ap-
plied them to selected Mekong Delta soft clays.
In this study, the OCR was calculated based on both the
oedometer and CPTU test results. Figure 6bshows the OCR
calculated based on the oedometer test results using six dif-
ferent methods, all of which gave very similar values. For
the uppermost layer of weathered clay, the OCR ranges
from 2 to 5. For the soft clay found from a few metres be-
low the surface to a depth of 17 m, the OCR is approxi-
mately 1, indicating a normally consolidated clay. The OCR
then increases to about 3 at a depth of about 20 m for the
lower stiff clay and decreases to 1.2 at the lower bound of
this stiff clay layer, probably because of sampling effects.
Figure 6cshows the OCR profile estimated from CPTU
data based on the method of Chen and Mayne (1996). A
similar trend with depth is seen in Fig. 6b, but the OCR val-
ues are greater than those obtained from the oedometer tests.
CPTU cone tip resistance (qc) and field vane undrained
shear strength (Su,FV)
Figure 4 shows selected qcprofiles from the CPTU tests.
The value of qcranges from 0.1 to 0.7 MPa for the upper
soft clay layer and from 0.7 to 3.5 MPa for the lower stiff
clay layer.
The field undrained shear strength values of Ca Mau soft
clay were determined by vane shear tests at 14 locations us-
ing two types of vane, namely the Acker and Geonor. The
vane blade width (B) and height (H) were 13 cm
and 6.5 cm, respectively, for the soft clay layer to a depth
of 17 m and 11 cm and 5 cm, respectively, for penetrations
deeper than 17 m. In practice, however, the vane was unable
to penetrate more than 18 m, which is approximately the
limit of the upper very soft to soft clay unit. Figure 7 shows
the undrained shear strength (Su,FV) values that were calcu-
lated for the study site using the correction method proposed
by Aas et al. (1986). As the upper soft clay layer is very ho-
mogeneous, the Su,FV values vary from 5 to 25 kPa follow-
ing a linear relationship as given in Fig. 7 (i.e., Su,FV =
1.195D+ 4.392, where Dis the depth in metre). For the
Giao et al. 1517
#2008 NRC Canada
lower stiff clay, the undrained shear strength values can be
up to 90 kPa at a depth of 18 m.
Sensitivity
The sensitivity of Ca Mau clays was determined based on
14 field vane shear tests and plotted versus depth in Fig. 8.
The sensitivity varies between 2 and 6, with an average of
about 4. Thus, the Ca Mau soft clay can be classified as a
medium to very sensitive clay.
Soil disturbance analysis
Various researchers have considered a number of causes
of sample disturbance arising during the drilling to labora-
tory testing stages. This includes sampling procedure and
sampler type, structuration, aging and chemical bonding of
clays, plasticity index, macro-heterogeneities found in the
soil mass, and stress release (Chung et al. 2004). Despite
the fact that many factors can cause soil disturbance of clay
samples, few quantitative criteria are available to assess
their importance. Most existing procedures make use of
Fig. 4. Geotechnical profile of the subsoil at the GPF site.
Fig. 5. Activity of clays (Ac) at the GPF site.
Table 1. Basic geotechnical properties of the soil profile at the study site.
Layer 1 (upper very
soft to soft clay)
Layer 2 (lower stiff to
very stiff clay)
Layer 3 (stiff sandy
clay)
Thickness (m) 16–17 15–20 —
Unit weight, g(t/m3) 1.40–1.60 1.72–1.98 1.70–1.85
Natural water content, Wn (%) 60–95 25–40 30–40
Liquid limit, LL (%) 60–80 40–50 40–50
Plastic limit, PL (%) 30–40 20–25 25–28
Void ratio, eo1.6–2.5 0.7–1.2 0.9–1.2
Compression index, Cc0.8–1 0.2–0.3 0.25–0.35
Swelling index, Cs0.27–0.3 0.12–0.15 0.12–0.17
Undrained shear strength, Su(kPa) 15–25 30–90 —
Hydraulic conductivity, k(cm/s) 5.510–7 3.510–7 —
1518 Can. Geotech. J. Vol. 45, 2008
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widely available oedometer test data and the assumption that
a more disturbed sample would have a lower void ratio for a
given effective stress level (Giao et al. 2004). Two fre-
quently used techniques to quantitatively evaluate soil dis-
turbance are based on calculation of the volumetric strain,
"vo (Andersen and Kolstad 1979), and the normalized void
ratio, De/eo(Lunne et al. 1997). These procedures were em-
ployed in this study, as shown in Tables 2 and 3. The results
of soil disturbance evaluation for the Ca Mau site are shown
in Fig. 9 and indicate that a number of samples were dis-
turbed, especially those taken in the interval 10–15 m and
around the 25 and 35 m depth levels.
Near-surface geophysical investigation
General
Two long-standing conventional direct current (DC) elec-
Fig. 7. Undrained shear strength by field vane shear (FVS) testing
of Ca Mau soft clay. D, depth; r2, coefficient of correlation; Su(Field),
uncorrected undrained shear strength from FVS; m, correction factor
proposed by Aas et al. (1986).
Fig. 8. Sensitivity of Ca Mau soft clay.
Fig. 6. (a) Consolidation curves for Ca Mau soft clays. (b) Over-
consolidation ratio calculated based on the oedometer test results.
(c) Overconsolidation ratio calculated based on the CPTU test re-
sults using the method of Chen and Mayne (1996). qT, cone tip re-
sistance; svo, overburden stress; 0
vo, effective overburden stress.
Giao et al. 1519
#2008 NRC Canada
tric survey techniques are vertical electric sounding (VES)
and electric profiling (EP). These techniques are of limited
use for mapping subsoil profiles consisting of alternating
clayey, silty, and sandy soils that have low electric resistiv-
ity. At the same time, the EI technique becomes a more and
more efficient tool to investigate the shallow soil layers
(Barker 1981; Overmeeren and Ritsema 1988; Griffiths et
al. 1990; Loke 1999). However, application of EI for site in-
vestigation of the soft clay deposits with reference to infra-
structure projects has been limited, a recent application
being the case of Pusan clays in the Nakdong River delta as
reported by Giao et al. (2003).
Electric imaging of the Ca Mau clay deposits
Electric imaging was applied in this study mainly to as-
certain its usefulness for site investigation of clays in the
Mekong Delta and particularly to detect the thickness of the
soft clay layer that used to have very low resistivity. The lo-
cation of the EI survey line is shown in Fig. 10. The field EI
procedure uses the Wenner array as shown in Fig. 11. The
initial unit spacing was 5 m, gradually increasing to a max-
imum spacing of 75 m over a 230 m line. The EI survey
employed an innovative semi-automatic procedure, using a
Resistivimeter SYSCAL R1 connected to a seismic cable
and a switching box made by the Asian Institute of Technol-
ogy. More details of this semi-automatic procedure can be
found in Giao and Adisornsupawat (2004).
The resistivity data collected from field tests were ana-
lyzed by the program RES2DINV, Malaysia (Loke 1999).
The results of an inverse analysis of the EI data obtained at
the Ca Mau site are shown in Fig. 12. The upper soft clay
was very clearly mapped, having an electric resistivity of
about 0.5–0.8 Um. Based on the results from EI analysis,
the subsoil profile of the site can be characterized as fol-
lows: (i) The resistivity of Ca Mau clays at the study site
mainly ranges from 0.5 to 3 Um, indicating a marine clay.
(ii) As seen in Fig. 13, a distinctly higher resistivity up to 3–
4Um compared with that of the underlying soft clay layer
of around 0.5 Um indicates the existence of weathered clay
in the uppermost part of the soft clay unit, and this sublayer
of weathered clay is about 3 m thick. The higher resistivity
of the weathered clay was also found and noted by Giao
(2001, 2004, 2005) and Giao et al. (2003) for Korean and
Bangkok clays. (iii) The very soft to soft clay layer has a
very high water content and very low resistivity (0.5 Um),
and it is very well mapped to a depth of about 17 m. The
EI results thus agree closely with the results of the geotech-
nical investigation. (iv) Beneath the very soft to soft clay
unit, there is a layer of resistivity of around 1.5 Um, corre-
sponding to the lower stiff to very stiff clay (found between
18 and 35 m deep). (v) Underlying the stiff to very stiff clay
is another unit of resistivity from 2 to 3 Um, whose bottom
was not reached by the EI configuration employed in this
study. This belongs to the stiff sandy clay unit.
A comparison of the resistivity of the Ca Mau clays with
that of soft clays from other locations in the world is shown
in Fig. 13. It is clear that the resistivity of marine soft clays
varies in a rather narrow range from slightly less than 1 to
12 Um. There are also cases of Japanese clays with higher
resistivities, e.g., clays at Okayama have a resistivity up to
12 Um, the deep stiff clays at Osaka and underlying Kansai
Airport have a resistivity around 6 Um. One of the reasons
for many Japanese clays having a higher resistivity com-
pared with the other marine clays is due to the possible ef-
fect of diatoms as mentioned in Giao et al. (2003).
Integration of geotechnical and geophysical
data
An integrated geotechnical–geoelectric subsurface model
was constructed based on both geotechnical and EI data, as
shown in Fig. 14. The upper soft clay has a low resistivity
that is less than 1 Um, and the lower stiff clay has a resis-
tivity from more than 1 to 2 Um. Although there is only a
small difference in the resistivity of the upper soft clay and
the lower stiff clay, the boreholes provide a good calibration
for the geophysical data. It is interesting to note that the
uppermost part of the lower soft clay has a higher resistivity
than the rest of this unit and perhaps represents the very
weathered clay layer. The higher resistivity of the weathered
clay compared with that of the soft clay was noted by Giao
(2001, 2004, 2005) and Giao et al. (2003) for Korean and
Bangkok clays.
Comparison with well-logging results
An electric logging (16 inch short normal) test was con-
ducted at the study site with geotechnical boreholes during
Fig. 9. Evaluation of soil disturbance for clays at the GPF site.
Table 2. Soil disturbance scale based on values of the volumetric
strain, "vo (Andersen and Kolstad 1979).
Sample quality
Very good (A) Good (B) Fair (C) Poor (D) Very poor (E)
<1 1–2 2–4 4–8 >8
Table 3. Soil disturbance scale based on the normalized void
ratio change, De/eo(Lunne et al. 1997).
Sample quality
OCR
Very good to
excellent (A)
Good to
fair (B) Poor (C)
Very
poor (D)
1–2 <0.04 0.04–0.07 0.07–0.14 >0.14
2–4 <0.03 0.03–0.06 0.06–0.10 >0.10
1520 Can. Geotech. J. Vol. 45, 2008
#2008 NRC Canada
the soil investigation phase prior to the EI survey. Logging
curves including self-potential (mV), natural gamma (counts
per second), and resistivity (Um) are shown in Fig. 15.
The apparent resistivity values taken from the EI data are
also plotted in Fig. 15. It is observed that the well-logging
resistivity values do not change until a depth of 40 m. As
discussed previously, in the subsoil profile at the study site,
although it was easy to distinguish the upper and lower clay
layers using the physical and mechanical properties of the
soil profile, it was not possible to recognize the difference
between these two clay layers from the well-logging resis-
tivity curve. The main reason is probably due to the short
normal-logging configuration adopted being able to test
only in the invasion zone near the well bore, where the
physical properties are altered by the drilling process.
Although electric well-logging might not be sufficiently sen-
sitive to differentiate the upper and lower clay layers, the EI
was able to map these clay layers very well and thus can be
considered in this case as a less time-consuming and more
economic solution compared with well logging.
Concluding remarks
(1) The geological conditions, including formation processes
of the Mekong Delta, were briefly highlighted. A typical
profile of the shallow subsoil profile was constructed for
a long section from north to south as shown in Fig. 2,
indicating the existence of a soft clay deposit in the Me-
kong Delta that varies from 10 to more than 50 m in
thickness.
(2) Geotechnical data at a coastal site in Ca Mau Province
were analyzed to determine clay activity, overconsolida-
tion ratio, undrained shear strength, sensitivity, and com-
pression characteristics. The analysis undertaken could
Fig. 10. Location of the EI survey line at the GPF site.
Fig. 11. Electric imaging procedure at the GPF site using the Wenner array. C1 and C2, current electrodes; P1 and P2, potential electrodes.
a, unit spacing.
Giao et al. 1521
#2008 NRC Canada
aid in effective characterization of the subsoil profile to
a depth of 40 m.
(3) For a depth range from the surface to about 40 m, the
subsoil at the gas–power–fertilizer (GPF) site of Ca
Mau consists of three distinct units: the upper soft clay,
the lower stiff clay, and the stiff sandy clay. The upper
soil layer is soft, medium to very sensitive. The typical
values of these soil layers are presented in Fig. 4.
(4) The upper very soft to soft clay layer (from ground sur-
face to a depth of 17 m) is of the most concern because
it can easily compress owing to the loading from struc-
tures founded on it. To avoid possible geotechnical da-
mage, including differential settlement, one needs to
carefully evaluate the soil parameters used in the founda-
tion design and settlement calculation. The soil distur-
bance that was observed in this study indicates that
samples taken from this layer can be easily disturbed, af-
fecting the quality of the testing results in the laboratory
and the subsequent soil characterization.
(5) An electric imaging (EI) survey, using the Wenner con-
figuration, was employed and was able to successfully
map the upper soft clay and lower stiff clay units. The
Fig. 13. Resistivity values of different clays from other locations in the world (modified and updated from Giao et al. 2003).
Fig. 14. Geotechnical–geophysical model of the GPF subsoil pro-
file.
Fig. 12. Results of EI survey at the GPF site.
1522 Can. Geotech. J. Vol. 45, 2008
#2008 NRC Canada
resistivity section of the Ca Mau clays matches quite
well with the geotechnical profile, as shown in an inte-
grated geological–geophysical model in Fig. 14. The
upper soft clay has a low resistivity of less than 1 Um,
and the lower stiff clay has a resistivity of from more
than 1 to 2 Um. Although there is only a small differ-
ence between the resistivities of the upper soft clay and
the lower stiff clay, the geotechnical boreholes provide a
good calibration for the geophysical data.
(6) Increased use of EI techniques is strongly recommended
in future site investigation and soil characterization of
Mekong clays, especially for those sites underlying oil
and gas facilities. Electric imaging could provide fast
and economic mapping over a large area, providing sup-
plementary useful geotechnical site investigation data.
For mapping of very soft deltaic clay deposits, EI can
be superior to the old technique of vertical electric
sounding (VES) in terms of resolution; it can also be
less time-consuming and more economic than shallow
electric logging.
Acknowledgements
Data collection and electric imaging fieldwork were
kindly assisted by the Geotechnical Engineering Laboratory
of the Asian Institute of Technology (AIT), Vietnam Petro-
leum Institute (VPI), VinaMekong Engineering Consultant
Company, Port Coast Consultant – Transport Engineering
Design, TEDI South Inc., and Ca Mau gas–power–fertilizer
project management board. Sincere thanks go to the re-
viewers and editors for their critical and valuable comments,
which have helped improving the manuscript.
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