The influence of delta formation mechanism on geotechnical property sequence of the late Pleistocene-Holocene sediments in the Mekong River Delta - PubMed (original) (raw)

The influence of delta formation mechanism on geotechnical property sequence of the late Pleistocene-Holocene sediments in the Mekong River Delta

Truong Minh Hoang et al. Heliyon. 2016.

Abstract

The aim of the study was to characterize a variety of microstructure development-levels and geotechnical property sequences of the late Pleistocene-Holocene deposits in the Mekong River delta (MRD), and the paper furthermore discusses the influences of delta formation mechanisms on them. The survey associated the geotechnical engineering and the sedimentary geology of the late Pleistocene-Holocene deposits at five sites and also undifferentiated Pleistocene sediments. A cross-section which was rebuilt in the delta progradation-direction and between the Mekong and Bassac rivers represents the stratigraphy. Each sedimentary unit was formed under a different delta formation mechanism and revealed a typical geotechnical property sequence. The mechanical behaviors of the sediment succession in the tide-dominated delta with significant fluvial-activity and material source tend to be more cohesionless soils and strengths than those in the tide- and wave-dominated delta and even the coast. The particular tendency of the mechanical behavior of the deposit succession can be reasonably estimated from the delta formation mechanism. The characteristics of the clay minerals from the Mekong River produced the argillaceous soil which does not have extremely high plasticity. The microstructure development-levels are low to very high indicating how to choose hydraulic conductivity value, k, for estimating overconsolidation ratio, OCR, by the piezocone penetration tests (CPTU). The OCR of sediments in the delta types strangely change with depth but none less than 1. The post-depositional processes significantly influenced the microstructure development, particularly the dehydrating and oxidizing processes.

Keywords: Geology.

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Figures

Fig. 1

Fig. 1

Map of the sedimentary environments of the MRD (Ta et al., 2005), an investigative plan layout including the CLM1, VLM1 (Truong et al., 2011), BT1, BT2, BT3 (Ta et al., 2002b), and CTM1 and TAS1 (Takemura et al., 2007) sites.

Fig. 2

Fig. 2

Selected photographs of sedimentary structures from the CLM1 core: a) (at depth −34.0 m below the a.p.s.l) brown seams of very fine sandy silt with 8 mm in thickness, and gray mud seams, which are rhythmically alternated, b) (-27.35 m) Peaty seams, organic material, and bioturbation, c) (-25.2 m) layers of lenticular, discontinuous sand to sandy silt and bioturbation, d) (-22.9 m) lenticular laminae and mica flakes, e) (-15.33 m) flaser, continuous, parallel laminae, mica flakes, humus matter, and burrow, f) (-14.53 m) flaser, wavy, discontinuous parallel laminae, g) (-7.3 m) discontinuous parallel laminae, lenticular bedding, shell, burrow, and organic material, h) (-7.25 m) shells, i) (-4.8 m) discontinuous parallel laminae, lenticular bedding, and organic material, j) (-1.9 m) organic materials and peaty seam, k) (-1.02 m) clayey silty pebbles, which are dry whitish gray and 5 cm in diameter, and organic materials, l) (+0.63 m) mixing of silt, clay, and very fine sand, reddish, yellowish, and brownish gray, iron oxides.

Fig. 3

Fig. 3

Summary of the lab geotechnical test results on the CLM1 core.

Fig. 4

Fig. 4

CLM1 site displays: Geological column, CPTU and SPT results.

Fig. 5

Fig. 5

VLM1 site displays: Geological column, CPTU and SPT results (Truong et al., 2011).

Fig. 6

Fig. 6

BT1 site displays: Geological column (Ta et al., 2002b), CPTU and SPT results.

Fig. 7

Fig. 7

BT2 site displays: Geological column (Ta et al., 2002b), CPTU and SPT results.

Fig. 8

Fig. 8

BT3 site displays: Geological column (Ta et al., 2002b), CPTU and SPT results.

Fig. 9

Fig. 9

Compression curves of the e-log σv' relations which were obtained form the IL and CRS tests on undisturbed and reconstituted cohesive soil specimens of the CLM1 core of: a) The below facies associations, b) The flood plain and natural-levee facies associations.

Fig. 10

Fig. 10

Changes of the void ratio, which was caused by recompression on the effective overburden stress from the oedometer tests on the Caolanh and Vinhlong cohesive specimens.

Fig. 11

Fig. 11

Plasticity chart of deposits of the CLM1 core (this study), VLM1 (Truong et al., 2011), and TAS1 and CTM1 (Takemura et al., 2007) cores.

Fig. 12

Fig. 12

Relationship between the void indices Ivoand the effective overburden stress σvo'on the Caolanh, with data of the Vinhlong (Truong et al., 2011), Cantho and Tanan cohesive soils (Takemura et al., 2007).

Fig. 13

Fig. 13

Values of OCR with depth from IL, CRS and CPTU tests for the cohesive soils at the CLM1, VLM1 (Truong et al., 2011), BT1, BT2, and BT3 sites.

Fig. 14

Fig. 14

Selected images on the TSS of the CLM1 core: a & b) Natural levee sediment, iron and calcium cements, large angular quartz fragments (Q) 0.35 mm in length, iron oxide. The VLM1 core: c & d) Delta front sediment, Q with even sizes and organic materials, general greenish gray and black joining in bond. e & f) Delta front sediment, contiguous part between sandy and clayey seams. g & h) Prodelta sediment, plain clay seams. i, j & k) Marsh sediment, parallel clay seams and hydromica sticks, opaque iron oxide and calcites. l & m) Marsh sediment, plentiful calcite and iron compounds and diatoms. n, o) Estuary channel sediment, iron cements, plentiful pyrite and calcite, small sizes and density of space pores.

Fig. 15

Fig. 15

Selected SEM images of the CLM1 core: a) Natural levee sediment (+0.28 m), chaotical clays, aggregated clays, small sizes and density of space pores. b) Flood plain sediment (-4.81 m), chaotical clay, aggregated clays. The VLM1 core: c) Delta front sediment (-11.1 m), flocculation clay and large space pore. d) Prodelta sediment (-21.01 m), parallel clay seams. e, f, & g) Prodelta sediment (-21.01 m), petri dish and tubular diatoms, pyrites in space pore of the petri dish diatom. h) Bay sediment (-22.05 m), plentiful large pyrites. i, j & k) Marsh sediment (-30.45 m), plain clays seams, plentiful pyrites. l, m & n) Marsh sediment (-37.3 m), parallel clays, very plentiful tubular diatoms on the exposed surface. o) Estuary channel sediment (-45.75 m), parallel clays, small sizes of space pore.

Fig. 16

Fig. 16

Observed cross section of test specimens for the IL and CRS tests.

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