identifying late quaternary coastal deposits in kyonggi bay,

7/30/2019 Identifying Late Quaternary Coastal Deposits in Kyonggi Bay,

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Geo-Mar Lett (2006) 26: 7789DOI 10.1007/s00367-006-0018-2

O RIG IN A L

Kyungsik Choi . Ju Hyong Kim

Identifying late Quaternary coastal deposits in Kyonggi Bay,

Korea, by their geotechnical properties

Received: 3 January 2006 / Accepted: 3 April 2006 / Published online: 26 April 2006# Springer-Verlag 2006

Abstract Based on cone penetration tests with porepressure measurements (CPTUs) and standard penetrationtests (SPTs), the geotechnical properties of five lithostrati-

graphic units were determined during the construction ofIncheon international airport on reclaimed macrotidal flatsin Kyonggi Bay, Korea. Two late Pleistocene non-marineunits (unit V and unit IV) display largest N values (cf.number of blows required to achieve a standard penetra-tion), reflecting coarse-grained and overconsolidated sedi-ments. Tidal channel and tidal flat facies (unit IIIb) consistof unweathered late Pleistocene tidal sand and mud. Thetidal channel facies is characterized by upward-decreasingcone resistance (qt) and sleeve friction (fs) with negative

pore pressures (ubt), reflecting a fining-upward texturaltrend. The tidal flat facies, by contrast, is represented byuniformly low qt and fs values with high friction ratios

(FRs), suggesting homogeneous muddy deposits. Twooverconsolidated units, a weathered late Pleistocene tidalmud (unit IIIa) and an early Holocene organic-rich non-marine mud (unit II), are characterized by high qt, fs, FRsand Nvalues, unit IIIa being much more consolidated thanunit II. Holocene tidal sands and muds (unit I) show thesmallestqtand fs values with positive ubt. These are slightlymore consolidated than the tidal flat facies of unit IIIb. Twounconformable boundaries (a sequence boundary and atransgressive surface) have also been identified on someCPTU and SPT profiles. The boundaries are indicated bygradual but sharp increases in qt, fs and N values with anabrupt drop ofubt, which indicates the contact between two

units showing contrasting rigidity. The regional patternproduced by the unconformable boundaries indicates thepresence of late Pleistocene valleys which pass through the

middle of study area. The location of the valleys seems tobe controlled by the antecedent basement morphology.

Introduction

With the growing demand for the construction of largeinfrastructures on soft ground such as tidal flats in thecoastal zone, geotechnical properties of Quaternarydeposits have been extensively utilized to assess thestability of the construction sites, as they provideinformation on subsidence rates, degrees of consolidationand liquefaction, etc. (Robertson 1986; Das 1994; Clark

1998; Ng et al. 2000; Liao et al. 2002; Lee et al. 2003). Inparticular, recent improvements in cone penetration testswith pore pressure measurements (CPTUs) have facilitatedvery detailed stratigraphic profiling (Robertson 1990;Robertson et al. 1992; Grunwald et al. 2001). Combinedwith traditional standard penetration tests (SPTs), thisgeotechnical method has proved useful in recognizingstratigraphically important boundaries such as formersubaerial unconformities (Chang 1991; Amorosi andMarchi 1999). Despite its high potential for stratigraphicand sedimentological studies, however, this approach hasto date been applied only in a few case studies (Moran et al.1989; Amorosi and Marchi 1999; Veyera et al. 2001;

Ricceri et al. 2002).During the last glacial period when sea level was up to

120 m below the present mean sea level (Chappell et al.1996), most continental shelves were subaerially exposed.As a result of this, the former marine deposits became

pedogenically modified. Exposed marine deposits aretypically characterized by low water contents and highundrained shear strengths (Segall et al. 1987; Ergin 1996;Barras and Paul 2000; Rodriquez et al. 2000; Tovey andYim 2002; Yim et al. 2002) and thus commonly definesubaerial unconformities (e.g. sequence boundaries). Suchunconformities are increasingly being recognized both on

K. Choi (*)New Ventures Department, Overseas E&P,Korea National Oil Corporation,1588-14 Gwanyang-dong, Dongan-gu,431-711 Anyang, South Koreae-mail: [email protected].: +82-31-3802234Fax: +82-31-3841275

J. H. KimGeotechnical Engineering Research Department,Korea Institute of Construction Technology,411-712 Goyang, South Korea
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the shelf and in the coastal zone along the west coast ofKorea (Lee and Yoon 1997; Choi 2001; Jin 2001; Park andChoi 2002; Lim and Park 2003; Choi and Dalrymple2004). Despite this growing number of reports, however,the spatial distributions of these boundaries are still poorlyconstrained, mainly due to a lack of cores and adequategeophysical survey tools, especially in the coastal zone.

In the course of the construction of Incheon international

airport on reclaimed macrotidal flats in Kyonggi Bay, westcoast of Korea, thousands of geotechnical penetration

probes and boreholes were sunk to assess the stability ofthe construction site. In combination with huge trenchsections excavated for the foundations of the passengerterminal of the airport, this project provided unparalleledopportunities to study the geotechnical properties of thelate Quaternary coastal deposits in this area, with well-constrained sedimentological information (Choi 2001).The purpose of this paper, therefore, is to demonstrate thatthe late Quaternary stratigraphy in the study area, in

particular unconformable boundaries resulting from sub-aerial exposure of the former seabed, can be identified on

the basis of geotechnical properties generated from SPTand CPTU data.

Study area

The study area is located in the inner part of Kyonggi Bay,the largest macrotidal embayment along the west coast ofKorea (Fig. 1). Before reclamation, the area consisted of agently sloping tidal flat developed around a number ofoffshore islands (Fig. 1). The tidal flat was incised bynumerous tidal channels which were particularly prominent

along the east coast of Youngyou Island (Fig. 2).Overlying basement rocks of Precambrian gneiss and

Jurassic granite, the sedimentary succession comprises upto 40 m of heterolithic tidal deposits and intercalated

palaeosols (Fig. 3). These deposits have been divided intofive lithostratigraphic units based on sedimentary faciesand stratigraphic position (Choi 2001; Park and Choi2002). In ascending order, these are late Pleistocene non-marine gravelly sands and muds (unit V), late Pleistocenenon-marine organic-rich sands and muds (unit IV), latePleistocene tidal sands and muds (unit III), weathered earlyHolocene non-marine organic-rich muds (unit II), andHolocene tidal sands and muds (unit I; Fig. 3). Unit III was

further subdivided into two parts based on the degree ofweathering. The upper part, unit IIIa, consists of weatheredtidal sands and muds whereas the lower part, unit IIIb, iscomposed of unweathered tidal sands and muds.

Fig. 1 Locality map showingthe geographic setting of thestudy area. Bathymetry is inmeters below present mean sealevel

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Materials and methods

Cone penetration tests with pore pressuremeasurements (CPTUs)

CPTUs, so-called piezocone tests (Robertson 1986), werecarried out at 20 sites in the construction area where it iscovered by 3-m-thick sand fills (Fig. 2). CPTUs were madeat borehole sites where sediment logs are available. At eachlocation, the CPTU probe was pushed to a depth of 2228 m below the surface (25 m on average). By this method,

a cone at the end of a series of rods is pushed into theground at a constant rate of 2 cm/s (ISSMFE 1989). Thetest device consists of a 60 cone having a 10 cm2base areaand a 35.7 mm base diameter, with a 150 cm2 frictionsleeve located above the cone.

Measured between the cone tip and the friction sleeve,pore pressure is the sum of the calculated static equilibriumpore pressure and the change in pore pressure which iscreated at cone penetration. In each case, recordings ofdepth, cone resistance (qc), sleeve friction (fs), and pore

pressure (ubt) were made at 25 cm intervals. The cone tipresistance measurements were subsequently corrected for

the equal area effect qt (Lunne et al. 1985), which is givenby the equation

qt qc ubt 1 a

where a is the net area ratio (0.75 in the present case).A friction ratio (FR), defined as the ratio (in percentage)

between fs and qc, was calculated to estimate sedimenttexture (e.g. Amorosi and Marchi 1999). The CPTUmeasurements conform to the United States ASTM (1995)standard and the ISSMFE (1989) reference test procedures.

Sediment cores and standard penetration tests (SPTs)from boreholes

Sedimentary and SPTs logs (N values; cf. below fordefinition) from 130 boreholes were used to investigatespatial patterns produced by unconformable boundariesand sediment thicknesses. The boreholes were sunk using ahydraulically powered drill (KACA 1996). In particular,sediment cores from 15 boreholes were utilized forstratigraphic analysis and calibration of the CPTU data.

Fig. 2 a, b Detailed bathy-metric map (a) of the study areawith positions of boreholes andCPTU sites (b). Open circlesdenote CPTU sites of which thedata are discussed in the text butnot shown in the figures.Bathymetry is in meters belowpresent mean sea level prior toreclamation which commenced

in 1992

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Cored samples 1030 cm in length and 5 cm in diameterwere analysed for colour, texture and sedimentarystructures. The N value, defined as the number of blows

required to achieve a penetration of 0.3 m using a 63.5 kgdriving mass or hammer falling free from a height of760 mm (ASTM 1992), was determined at 1.5-m intervals.

Undrained shear strength from field vane tests (FVTs)and CPTUs

Field vane tests (FVTs) were performed at two boreholes,with a Geonor vane to provide reference values ofundrained shear strength (Su) for the CPTU tests. Thevane used in this study had a diameter of 50 mm and aheight of 100 mm. The vane was rotated at a rate of 6/min

until soil failure, to measure undisturbed shear strength(ASTM 1996). Remoulded shear strength was alsodetermined to yield sensitivity ratios, by rotating the vanethree times after failure. Estimations ofSu from the CPTUswere carried out using the equation

Su qt 0 Nk

where Nk is the cone factor and v0 is the total in situvertical stress. In the present study, an average Nk value of14.7, generated from 56 CPTU measurements, and field

vane tests were used for the estimation (Kim et al. 2001;Kim 2002).

Grain size analysis and soil classification

After organic matter was removed with 10% H2O2, thegrain size of the sediment was determined using theconventional sieving technique for the sand fraction, and aSedigraph 5100D for the silt and clay fractions. Sedimenttypes were determined according to Folk (1974).

Results

Basement morphology and spatial distribution

of sediment thickness

Near the shore, basement rock occurs at depths less than15 m below mean sea level (MSL). It gradually deepenstowards the middle and south of the study area (Fig. 4a)where it reaches its greatest depth at >40 m below MSL.Between the islands, the basement displays a prominentvalley, its axis trending northwestsoutheast (Fig. 4a). Thevalley is wider and deeper in the south, and shallower andnarrower in the north. Between Sammok and Yongyouislands, the valley displays a saddle structure reaching adepth of less than 20 m below MSL (Fig. 4a).

Fig. 3 Cross section illustrating the late Quaternary stratigraphy of the YoungjongYongyou tidal flat. Note the high relief of the sequenceboundary (SB1) and the transgressive surface (TS1) between the Holocene and late Pleistocene sequences. See Fig. 2a for the location of thecross section

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The spatial pattern in the distribution of sedimentthicknesses reflects basement morphology, sediment thick-ness being greatest where the basement is deepest (Fig. 4b).The maximum sediment thickness (>40 m) is found in thesouthern part of the study area between the Youngjong andYongyou islands (Fig. 4b). Near the coast and between theSammok and Yongyou islands, sediment thickness is lessthan 20 m.

Geotechnical properties of the basementand sedimentary units

Basal soil

The basal soil, which occurs mostly 35 m below MSL inthe CPTU test area, consists of weathered Jurassic granitecomposed of poorly sorted gravelly sands containing some

(


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Unit V and unit IV (late Pleistocene non-marinedeposits)

Unit V consists mainly of gravelly sands with smallamounts (>u0) areanother characteristic feature, the pressure increasing

linearly with depth, thereby characterizing a remarkablyhomogeneous mud succession (Fig. 6). Shear strengths

measured from FVTs (CPTUs) range between 0.4

0.7 kg/cm2 (0.40.6 kg/cm2).

The tidal channel facies consists of a 26 m thick, fining-upward succession having a sharp base and a gradationaltransition to the overlying unit. Cross-bedded silty sands,herringbone cross-bedded silty sands, and flaser-beddedsilty sands constitute the lower part of the facies (Fig. 5b),whereas the upper part is dominated by wavy mud beds andrhythmically laminated mud (Fig. 5c). The base of thefacies is marked by the presence of a thin lag deposit (e.g.gravel, mud pebbles and shell hash; Fig. 5b). It gradesupwards into the weathered late Pleistocene muds of unitIIIa. A well-defined channel facies occurs between 10 and

16 m below MSL in the CPTU test area (Fig. 6). N valuesvary widely between 8 and 50 but sharply decreaseupwards (1016 m below MSL). They are largest at the

base of the tidal channel facies where texture is coarsest.The qt values fluctuate widely between 15200 kg/cm

2

whereas fs values range between 0.22 kg/cm2. FRs lie

between 0.3 and 2, being lowest at the base of the facies. Incontrast to the tidal flat facies, negative excess pore

pressures are typical for this facies (ubt


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values at the top and base. The qt values range between 10and 60 kg/cm2, decreasing rapidly with depth. This unit is

better defined by the fs values which vary from 0.3 to 3 kg/cm2. FRs range between 1.5 and 7. Pore pressure values aremostly negative (ubtu0). Shear strengths estimated fromFVTs (CPTUs) vary between 0.5 and 1.1 kg/cm2 (0.2

2.3 kg/cm2

).

Unit II (early Holocene non-marine organic-rich mud)

Unit II represents an early Holocene organic-rich mudwhich is typically less than 2.5 m thick. The upper part ofthe unit is pedogenically modified, displaying weaklydeveloped clay illuviation and authigenic sphaero-siderites(Fig. 5d). N values range mostly from 5 to 10, rarelyreaching 20 (YIII-3; 68 m below MSL). The qt valuesvary between 7 and 30 kg/cm2, exhibiting gradational

transitions at the top and the base. The fs values varybetween 0.05 and 0.5 kg/cm2, fluctuating moderately withdepth. As in the case of unit IIIa, this unit is also betterexpressed by its fs values. The FRs range between 1 and 4,and pore pressure values are mostly negative (ubt


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Fig. 6 a, b CPTU and SPT profiles at site YI-4 (a) and site YII-5 (b), with stratigraphic interpretation. The FVT data are plotted in the N-

value column of YI-4 (a). See Fig. 2b for the location of probes and boreholes

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pressures sharply take on negative values. The magnitudesof qt and fs are much smaller than those for the SBs. Suchsurfaces are also better defined by fs than by qt, as is thecase for the SBs (Figs. 6 and 7). Within the CPTU test area,such a boundary is present 1.53.5 m below MSL. In some

places, a TS coincides with an SB (YI-2; Fig. 7). The TSsare also identifiable on SPT profiles, represented by anabrupt increase ofNvalues (Fig. 6). In many places, the TScoincides with the boundary at which the N values firstdisplay a significant peak. Where unit II is absent, the TS is

Fig. 7 a, b Correlation diagram of CPTU profiles with cone resistance (a) and sleeve friction (b), depicting stratigraphic units andunconformable boundaries (see Fig. 2b for the location of probes and boreholes). OSOriginal surface, TStransgressive surface, SB sequenceboundary, WL weathering limit, CB channel base

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interpreted to be superimposed on the SB. As inferred fromthe CPTU profiles, the N value characterizing the TS issmaller than that for the SB.

Spatial distribution of an unconformable boundarymapped by SPTs

Based on the 130 SPT profiles and logging data, anunconformable boundary was mapped for the study area(Fig. 8). This boundary corresponds to a depth at which the

N values first display a prominent peak. As explainedabove, the boundary is likely to represent a transgressivesurface which locally merges with a sequence boundary.The boundary is generally situated deeper towards themiddle of the YoungjongYongyou tidal flats and alsotowards the offshore (Fig. 8). It occurs as deep as 14 m

below MSL at the north-western limit of the study area(Fig. 8). Considering that unit II occurs only locally and iscommonly less than 2 m thick, the morphology of the

boundary is considered to mimic that of a sequence

boundary. The spatial distribution of the boundary outlinestwo valley systems which have developed in the north andin the south of the study area. The northern valley is betterdeveloped than the southern one, the two valley systems

being separated by an area where the boundary occurs asshallow as 2 m below MSL, as observed in the middle ofthe YoungjongYongyou tidal flat. The locations of thevalleys correspond with those observed in the basement(Figs. 4 and 8), which implies that the relief of theunconformable boundary has been inherited from theantecedent basement morphology.

Discussion

Pedogenically modified muds of high shear strengths havebeen reported from several locations along the west coastof Korea (Park et al. 1995, 1998; Choi 2001, 2005; Limand Park 2003; Choi and Dalrymple 2004). The con-solidated mud is interpreted to represent a subaeriallyexposed late Pleistocene tidal deposit (Choi 2001; Lim and

Park2003). The shear strength of the mud is typically 2

4times higher than that of the overlying unexposedHolocene tidal deposit (Park et al. 1998; Lim and Park2003). Based on various proxies including texture, colour,chemical and clay mineralogical composition, and strati-graphic position, unit IIIa is correlated with the consoli-dated mud (Choi 2001). Shear strength estimates based onFVT and CPTU measurements indicate that unit IIIa hasshear strengths which are at least three times larger thanthose of unit I (Fig. 6). In addition to unit IIIa, unit II is alsohighly overconsolidated. Negative or close-to-zero pore

pressures further suggest that unit IIIa and unit II consist ofoverconsolidated and fissured mud (Mayne et al. 1990;

Chen and Mayne 1996). The high shear strengths of unitsIIIa and II are due mainly to desiccation during subaerialexposure. The low water contents in units IIIa and II

provide more compelling evidence of desiccation (Choi2001). The pervasive occurrence of pedogenic signals inunits IIIa and II, such as illuvial clay coatings and increasesin immobile chemical elements, further suggests that theseunits were subaerially exposed for at least a few thousandyears (Choi 2005).

Cone parameters (qt, ubt and fs) and N values indicatethat unit II is less overconsolidated than unit IIIa. Given thesimilarity of textural composition between unit II and unitIIIa, the differences in cone parameters and Nvalues seem

to be related solely to differences in the degree ofoverconsolidation. It is obvious that unit IIIa hasexperienced more mechanical compaction than unit II

Fig. 8 Map showing the depthto the unconformable boundarywhich correlates either with thetransgressive surface or with thesequence boundary

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because the former is overlain by the latter. Considering theshallow burial depth (less than 10 m), however, it isunlikely that this alone can explain the difference inconsolidation. The stratigraphic analysis suggests that unitII has been subaerially exposed for thousands of years,which is much shorter than the estimated exposure time forunit IIIa (Choi 2005). Since mud generally consolidatesquickly after subaerial exposure (e.g. Amos et al. 1988;

Barras and Paul 2000), this difference in exposure timeseems to be insignificant.

In general, clay mineral composition and pore-waterchemistry is known to influence the shear strength of clays(Moore 1991). Thus, kaolinite-rich clay is more easilyconsolidated than montmorillonite-rich clay, and high

pore-water salt contents result in an increase in the residualshear strength (Moore 1991). Since the two units have asimilar clay mineralogy, clay mineral composition isunlikely to explain the difference in rigidity between thetwo (Choi 2005). On the other hand, unit IIIa is inferred tohave a higher pore-water salt content than unit II becausethe former is interpreted to be a tidal deposit, whereas the

latter is considered to be a freshwater deposit (Choi 2001).Although high pore-water salt contents are known to causeincreases in residual shear strengths of clays by up to 40%(Moore 1991), this is not sufficient to explain a nearlythreefold difference in the degree of overconsolidation

between unit IIIa and unit II. On the other hand, chemicalweathering processes such as oxidation, reduction, andhydrolysis might partly account for the difference inconsolidation (Yoshida et al. 1991). Since enrichment inferric iron oxide is more prominent in unit IIIa than in unitII, cementation by iron oxide should be more pronouncedin unit IIIa. However, further studies are needed todetermine the relative influence of these possible control

factors.The results of this study clearly show that CPTUs and

SPTs are effective in identifying unconformable bound-aries such as transgressive surfaces and sequence bound-aries in the study area. This can be explained by the factthat the sedimentary units above and below the boundarieshave a contrasting rigidity, despite their almost uniformgrain size. The CPTU and SPT measurements thus stronglyreflect the degree of overconsolidation. However, wheresuch consolidated muds are absent or sandy sedimentsoccur, recognition of the boundaries would be difficultsolely on the basis of geotechnical properties. Although the

presence of two highly consolidated units is clearly

indicated by the cone parameters and N values, unitboundaries are often gradational on the CPTU and SPTprofiles. As a result, the boundaries are defined as zones,rather than sharp surfaces. Similar observations were madein late Quaternary deposits of the south-eastern floodplainof the river Po in Italy where the TS is represented by atransitional band (Amorosi and Marchi 1999). Thegradational boundaries are attributable to either self-weightconsolidation or an increase in water content duringtransgression. The former may explain the downwardincrease in rigidity within unit I. This would result insimilar shear strengths in the lower part of unit I and the

upper part of unit II, and thereby produce the gradationalcharacter of the TS on the CPTU profiles. Once a sedimentis subaerially exposed, its surface would be desiccated andunsaturated, resulting in a decrease in pore pressure and anincrease in shear strength. During the ensuing transgres-sion, however, the upper part of the consolidated unitswould be re-saturated. Increased water content would thuseventually again lower the shear strength in the upper part

of the unit (e.g. Tovey and Yim 2002). Another possibilityis weathering, which contributes to the softening ofoverconsolidated mud through an increase in soil moisture,

physical disruption and loss of bonding, and probably alsoprecipitation of expansive clay minerals (Moore 1991;Yoshida et al. 1991). This process may account for thegradational contact between units IIIa and II, since theupper part of unit IIIa may have experienced intensive

physical and chemical weathering. The presence ofabundant blocky peds in the upper part of unit IIIa alsosuggests repetitive swellingshrinking episodes (Choi2005).

It was furthermore shown that CPTUs are useful in the

characterization of sedimentary facies. Fining-upward tidalchannel facies and homogeneous tidal flat facies are

particularly well defined on the CPTU profiles. Theheterolithic nature of the tidal channel facies is illustrated

by the fluctuating cone resistance and sleeve frictionvalues. This is consistent with the fact that the channelfacies is composed of interlaminated sand and mud (Choi2001). The sharp textural contrast between the tidal flatmud and overlying tidal channel deposits in unit IIIb isclearly recognizable on the CPTU profiles (Figs. 5b and 6),implying that cone factors are highly sensitive to texturalchanges. The distinct contact between the landfill and unit I(designated as OS in Fig. 7) on CPTU profiles is another

indication of the susceptibility of cone factors to texturalcomposition.

The combination of cone factors, such as FRs, furtherprovides a powerful predictive tool for textural character-ization. For instance, unit IIIa has high fs values which arecomparable to those of the sandy deposit in unit IIIb.However, the very high FRs of unit IIIa (1.37), whichcontrast with the low FRs of the sandy deposits (0.32),suggest that unit IIIa would be composed mainly of mud.SPT profiles also proved useful in the detection ofunconformable boundaries, albeit with poorer resolution.

These findings demonstrate convincingly that, whencalibrated with continuous long sediment cores, geotech-

nical probes and boreholes can be used as efficient tools inQuaternary stratigraphic analysis.

Conclusions

From the results of this study, the following conclusions aredrawn.

When calibrated against core data, geotechnicalparameters obtained from CPTUs and SPTs are useful

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for the characterization of sedimentary facies andunconformable boundaries.

Fining-upward tidal channel facies and homogeneoustidal flat facies are particularly well-defined in CPTUsand SPTs.

Unconformable boundaries such as transgressivesurfaces and sequence boundaries are characterized

by gradual or abrupt increases in cone resistance,

sleeve friction, and N values with a drop in porepressure, implying a contact between adjacent units ofcontrasting rigidity.

Based on the spatial distribution of unconformableboundaries inferred from SPTs, late Pleistocene valleyswere identified in the north and south of the study areain Kyonggi Bay, Korea. Given the similarity of valley

positions, their morphology seems to have beeninfluenced by the antecedent basement morphology.

Acknowledgements This study has greatly benefited from thelogistic support of many engineering companies, including DongaEngineering Consultant, Daewoo Engineering, and Yooshin Co-

operation. The authors express their gratitude to these companies fortheir sustained support which made this study possible.

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identifying late quaternary coastal deposits in kyonggi bay,

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