INTRODUCTION
General
The Song Hong, or Red River Delta (RRD), is the second largest and most densely populated delta in Vietnam, home to major urban centers like Hanoi, the capital city, and its satellite cities, including Hai Phong, which boasts the country's second largest international seaport Recent infrastructure developments, such as the construction of the Hanoi Metro line, highlight the ongoing growth in the region, with numerous additional projects planned for the near future.
Sustainable infrastructure systems, including roads, ports, and logistics facilities, rely heavily on effective foundation designs that are both economically viable and technically sound To achieve optimal foundation designs, it is crucial to understand soil behavior, particularly the characteristics of soft clays, and to implement appropriate foundation solutions.
A thorough understanding of the compressibility characteristics of soft clays, commonly found in many areas of the RRD, is essential to prevent potential geotechnical engineering issues, including inadequate bearing capacity, significant swelling and shrinking potential, and soil settlement This knowledge not only enhances the safety and economic viability of structural designs but also allows for a broader range of applications.
Problem statement
The geological conditions of the Red River Delta (RRD) primarily consist of fine-grained compressible soil layers, including clay, silty clay, silt, and organic peat soils A study by Yen et al (2021) highlights the presence of transgressive alluvial deposits—comprising silt, sand, clay, and gravel, often rich in organic matter—in boreholes across Hanoi, Hung Yen, Thai Binh, and Nam Dinh provinces From a geotechnical engineering perspective, these alluvial deposits are highly compressible and exhibit increased permeability in their bedding plane direction, which can accelerate horizontal drainage when subjected to load, potentially compromising foundation stability (Barron, 1948).
The progressive settlement of foundations on clay is primarily caused by changes in water content, which occur slowly due to the soil's inability to rapidly transmit water to adjacent permeable layers This results in a time lag between external force changes and water content adjustments (Terzaghi, 1943) Consequently, large-scale construction projects such as high-speed transportation systems, high-rise buildings, and underground works like metro tunneling require ground improvement efforts before construction begins.
Ground improvement techniques, such as the use of prefabricated vertical drains (PVD), require precise measurements of consolidation coefficients in both horizontal and vertical directions These coefficients, specifically the radial coefficient of consolidation (c r) and vertical coefficient of consolidation (c v), are crucial for accurately calculating excess pore water pressure and determining ultimate consolidation settlements.
Figure 1.1 Dissipation of excess pore water in both horizontal and vertical direction by installing PVD in underlying soft clay layer
Excess pore water pressure (u) at any time after loading can be calculated by using the following Equation 1.1.
(1.1) where c r and c v are radial and vertical coefficients of consolidation; r = radial coordinate; u = pore water pressure.
Settlement of the improved ground at any time can be calculated by using the following Equation 1.2.
(1.2) where S c = ultimate primary consolidation settlement; U t = degree of consolidation at a particular depth; u 0 = initial pore water pressure, t = time.
In Vietnam, the coefficient of consolidation (c r) is typically estimated using TCVN 9355:2012, which can lead to significant inaccuracies in both c r and c v values due to variations in soil structure and the influence of applied pressure levels and test types This issue is illustrated in Figure 1.2, which depicts settlement problems at a construction site in Hai Phong city, arising from the misestimation of c r values compounded by construction quality control issues To ensure accurate assessments in ground improvement projects utilizing prefabricated vertical drains (PVD), it is essential to obtain c r values through experimental consolidation tests with horizontal drainage.
Figure 1.2 Settlement problems found in a construction site from Hai Phong owing to the overestimation or underestimation effect of c r values c r (2 to 5)c v (1.3)
A systematic study of the compressibility characteristics of soft clays from RRD is essential for preventing geotechnical engineering issues and enhancing structural designs This understanding can provide significant economic and technical benefits across a broader range of applications.
Necessity of the study
The Song Hong or Red River Delta (RRD) has been occupied by major cities including the capital city-Hanoi, Hai Phong, Hai Duong, Thai Binh, etc.
1) Thus, the RRD includes many industrial zones (parks) and expressways where soft ground in large scale must be improved before the construction of facilities.
2) Consolidation parameters, especially c r and c v are important parameters in estimating ultimate and time-dependence settlement values of the ground.
3) There were no systematic experimental studies on c r and c v , and their correlations of the clays in the RRD.
Objectives
This study aims to assess the horizontal coefficient of consolidation in clays by conducting consolidation tests with both central (c r,CD) and peripheral drains (c r,PD) at various test sites in the delta The research will utilize established methods to evaluate these coefficients and will also rank the reliability of these methods based on the findings.
- To examine the c r,PD /c r,CD ratios from analytical solution on ideal soil and from experimental data on actual soil and the influence of some parameters to the ratios.
- To evaluate compression index (C c ), recompression index (C r ) and preconsolidation stress ( p ) of the clays from the test sites and develop possible correlations for the parameters.
Scope of the study
- Perform physical tests, consolidation tests with different drainage types (IL method), and numerical simulation of consolidation test.
- Analyze the experimental (lab and field) and numerical test results.
Outline and structure of the thesis
Collect information about geography and geological condition of RRD in order to understand the composition of soft clay layers
Perform Literature review on the geological condition of RRD
(emphasizing on the composition of soil types in sediment deposits, especially soft clay)
Collect information about past experienced geotechnical engineering problems based on the effect of underlying soft clays layers in RRD
Perform Literature review on the compressibility or consolidation parameters (e.g., c or CD), c , s ꞌ, OCR, C, r(PD v p
OCR, C c , C , etc.) and previous study r
Identify the problem statement and necessity of the research
Collect required data from available existing test sites and perform required lab tests
Analyze and evaluate the data obtained from test sites for individual compressibility parameters
Make conclusion and recommendation based on the obtained results
Analyze and evaluate the data obtained from test sites for individual compressibility parameters
Evaluate c by r(PD or CD) using existing eight methods.
Rank the methods base on R
, C , c two standardized C r (Silva’s method) methods based on the result from Oedometer test. c c c
, based on the result from CPTu test.
Figure 1.3 Flow chart shows the outline of the thesis (a) general framework of the research; (b) flow of the analysis steps
Figures 1.3a and 1.3b outline the comprehensive research framework and the analytical steps undertaken The initial phase involved gathering information and conducting a literature review on geological conditions and associated geotechnical engineering issues This foundational work highlighted the necessity for the research To further support the identification of compressibility characteristics, an extensive review of relevant theories pertaining to compressibility parameters was conducted Ultimately, essential data for analyzing consolidation parameters—such as c r, c v, C r, C c, σꞌ, OCR, and C p—were collected from the study sites and assessed to determine their characteristics.
Chapter 1: It provides a general introduction about the requirements of this research with its objectives and scopes of the reseach work.
Chapter 2: (i) Literature review about geographical and geological conditions of the RRD; (ii) Fundamental consolidation theory and consolidation parameters; and (iii)
Literature review about the previous study from 3 rd Intake student were mentioned in this chapter.
Chapter 3: In this chapter, procedure of the analying steps were mentioned.
Chapter 4: This chapter provides required information about field tests and laboratory tests that were performed and all the analyzed results.
Chapter 5: This chapter highlights the facts that were found during the analysis and specific conclusions and recommendations on them.
LITERATURE REVIEW
Geographical conditions of the RRD
The Red River Delta (RRD), the fourth largest delta in Southeast Asia, is situated on the west coast of the Tonkin Gulf and was formed through sedimentation over the Holocene period, approximately 9,000 years ago Covering an area of 12,620 km², the delta has an isosceles triangle shape, extending 150 km from its apex in Viet Tri City in the northwest to the southeastern coastline along the Tonkin Gulf, with a width of around [insert width].
The delta plain, located 146 km from Quang Yen town to the mouth of the Day River, is characterized by its flat terrain and an extensive network of rivers and streams Surrounded by mountains made up of Precambrian crystalline rocks and Paleozoic to Mesozoic sedimentary formations, this region showcases a unique geological landscape.
Figure 2.1 (a) Topological zoning of the RRD; (b) Three main sectors of RRD zoning according to the origins of its formation (Phach et al., 2020)
The Red River Delta (RRD) is divided into three main topographical units: the upper delta, middle delta, and lower delta The upper delta, situated 10-15 meters above sea level, features slightly eroded paleo-terraces on its northern side The middle delta, at an elevation of 5-6 meters, exhibits a gentle, wave-shaped surface with excellent drainage, comprising meandering rivers, levee belts, fluvial terraces, modern flood plains with grayish-brown clayey silt and sand deposits, and late Holocene mangrove clay (Thai Binh formation) The lower delta, located near the shoreline at 1-2 meters above sea level, consists of coastal flood plains, saltwater marshes, and sand bars The RRD encompasses 11 provinces and cities, including the capital city of Hanoi, Vinh Phuc, Bac Ninh, Ha Nam, Hung Yen, Hai Duong, Hai Phong, Thai Binh, Nam Dinh, Ninh Binh, Quang Ninh, and Ha Tay.
Geological conditions of the RRD
The geological conditions of the RRD encompass several factors, including tectonic activities and sedimentation characteristics; however, this study specifically examines the soil types present in the RRD The primary objective is to assess the behavior of soft clays in relation to heavy construction projects from a geotechnical engineering perspective.
Research on Holocene deltas indicates that their morphology and sedimentary facies vary due to changes in coastal settings and sediment discharge rates, leading to three dominant types: fluvial dominated, wave dominated, and tide dominated deltas The fluvial dominated delta features meandering rivers and levee belts, characterized by a higher fluvial flux and sediments comprising gravelly sand and mottled clay In contrast, the tide dominated delta includes abandoned tidal flats and marshes, with sediments containing shell and wood fragments The wave dominated delta is marked by sandy spits and beach ridges, influenced by strong wave energy from summer monsoons, resulting in tide-influenced sand and mud deposits rich in sand and wood fragments.
Figure 2.2 Cross section showing the five depositional Quaternary sediments from
Hanoi city area in north-east to south-west direction
The RRD basin is composed of Neogene and Quaternary sediments exceeding 3 km in thickness, with a subsidence rate ranging from 0.04 to 0.12 mm per year The Quaternary sediments, which unconformably overlay the Neogene deposits, originated around 1.9 million years ago and were shaped by five cycles of global sea level fluctuations linked to glacial and interglacial events These sedimentation phases can be categorized into five distinct phases.
1) Le Chi formation (early Pleistocene) composed of pebble, granule, sand, clayey silt, plant remains.
2) Ha Noi formation (from mid to late Pleistocene) composed of boulder, pebble, granule, dark-yellow sand, clayey silt.
3) Vinh Phuc formation (from late to Pleistocene) composed of sand, clayey silt, silt, clay, grey kaolin
4) Hai Hung formation (from late Pleistocene to early Holocene) composed of silt,clay, sand, plant debris, peat
5) Thai Binh formation (from late Holocene to present) composed of silt, clay, plant debris (Phach et al., 2020 and Tanabe et al., 2006).
The RRD has served as an alluvial delta plain for the past five thousand years, characterized by four paleoshorelines due to the middle-late Holocene regressive stage marked by sea level retreat Sediment analysis from various cores in Hanoi, Hung Yen, Thai Binh, and Nam Dinh provinces reveals that transgressive alluvial deposits directly overlay the eroded surfaces of the regressive alluvial rhythm (atTST/arLST) (Yen et al., 2021).
Figure 2.3 Alluvial delta plain with four paleoshorelines with ages of 3–2.5 Ka: 1.5–1
Ka, 0.7–0.5 Ka, and 0.3–0.1 Ka belonging to the highstand systems tract
A study by Tanabe et al (2006) analyzed the paleogeography and evolution of the Song Hong or Red River delta, utilizing 101 accelerator mass spectrometry (AMS) radiocarbon dates The research examined sediments from seven cores, each ranging from 30 to 70 meters in depth, which were categorized into four distinct sedimentation units.
1: Late Pleistocene shallow-marine sediments; (ii) Unit 1: Latest Pleistocene fluvial sediments; (iii) Unit 2: Holocene estuarine sediments; and (iv) Unit 3: Holocene deltaic sediments The locations of those seven tested cores are shown in Figure 2.4 The soil types that are found in those cores are mostly fine-grained soft soil types such as clay,clayey soil, silt, silty soil, sandy soil, etc.
Figure 2.4 Location of the seven cores (Tanabe et al., 2006 modified after Tanabe et al., 2003b)
Consolidation
Consolidation is the gradual process in which soil particles are compressed closer together due to sustained pressure, leading to the drainage of water from the voids between them As defined by Terzaghi in 1943, this process occurs when the water content in saturated soil decreases without being replaced by air Figure 2.5 demonstrates the behavior of consolidation in saturated clay through a spring and piston analogy.
Figure 2.5 Spring-cylinder model for consolidation in saturated clay or spring and piston analogy illustrating the principle of 1D consolidation (Das, 2010)
2.3.1 Three stages of deformation in accordance with time during consolidation process
There are three stages of time-deformations during consolidation under each load increment as shown in Figure 2.6.
Stage I, known as initial compression or short-term settlement, occurs immediately after load application and prior to drainage in laboratory tests This phase is characterized as immediate or distortion settlement in clay soils and immediate or elastic settlement in sand A portion of the elastic compression from this stage is recoverable once the applied loads are removed (Head and Epps, 2011) Additionally, it contributes to deviations in experimental curves from theoretical predictions during the early stages (Robinson and Allam, 1998), particularly in methods used to calculate the coefficient of consolidation in both vertical and horizontal directions.
Stage II is primary consolidation, in which pore water pressure are transferred into effective stress due to dissipaton of excess pore water from the voids in soil material. Primary consolidation selltement or long-term settlemt is the process of time- dependent compression and related to theoretical curves for most clays And Terzaghi’s consolidation theory is applicable to only this phase Not like in the case of immediate settlement, the soil could not reach back to its orginal state after removing applied pressure and it may leave a small amount of swelling in tested soil sample For the estimation of settlements, only primary consolidaion stage is used in many cases. Although it is the most significant phase among the others for inorganic clays, secondary compression phase is more significant for organic soils such as peats and highly organic clays if it is considered well over a long time period.
Figure 2.6 Time-deformation plot during consolidation under a given load increment
Stage III, known as secondary consolidation or creep, typically occurs in unclean sands and follows the primary consolidation phase, which involves the complete dissipation of excess pore water While there is some overlap between the two stages, secondary consolidation is characterized by irreversible deformation of the soil specimen after the removal of applied pressure due to the plastic readjustment of the soil fabric It is important to note that the estimation of settlements related to secondary compression is generally less reliable than those based on primary consolidation (Head and Epps, 2014).
2.3.2 Consolidation theory (vertical and horizontal drainage cases)
The first consolidation theory for saturated clay soils was proposed by Terzaghi in
1925 based on the following assumptions (Terzaghi, 1943; Head and Epps, 2011; Das, 2014; Budhu, 2011; Das, 2010; Briaud, 2013):
2 The clay is completely saturated
3 Water and solid constituents of the clay are perfectly incompressible
4 The Darcy’s law is strictly valid
5 The hydraulic conductivity (k v ) is a constant
6 The strains in the clay layer are one-dimensional and small
7 There is a linear relation between the v and the void ratio (e).
8 The soil is not viscous, thus not influenced by time and strain rate.
Based on the above assumption, coefficient of vertical consolidatation can be calculated with the following Equation 2.1.
Then the degree of consolidation or percentage consolidation with the function of c v , h and t can be calculated as following Equation 2.2:
Based on Equation 2.2 and 2.3, vertical coefficient of consolidation (c v ) can be calculated as the following Equation 2.4:
(2.4) v t (length /time) where, c v = vertical coefficient of consolidation; T v = time factor for vertical consolidation; h or 2 = the length of the longest draiage path.
Typical coefficient of consolidation (c v) values for fine-grained soils range from 10 −3 m²/day to 10 −1 m²/day (Briaud, 2013) Field evidence shows that clay settlement rates often exceed those predicted by one-dimensional consolidation theory, as this theory overlooks horizontal pore water dissipation and does not account for actual geometric conditions Consequently, three-dimensional effects play a significant role, leading to potential inaccuracies in settlement rates calculated using conventional one-dimensional methods.
The theoretical framework for radial drainage to drainage wells, established by Barron in 1947, focuses on the consolidation theory, while McKinlay expanded this concept in 1961 to include drainage radially outward to a continuous peripheral drain Additionally, the horizontal drainage system employs two types of configurations: peripheral drains (PD) and central drains (CD), which are determined by the positioning of the sand drains.
In the context of radial consolidation with sand drains positioned at the periphery (PD case) and under free strain loading (FSL) conditions, the governing differential equation for excess pore water pressure, denoted as u(r, t), in the clay at a radius r is described by Equation 2.5.
In the context of radial consolidation with sand drains positioned at the periphery (PD case) and under equal strain loading (ESL) conditions, the governing differential equation that describes the excess pore water pressure u(r, t) in the clay at a radius r is represented by equation 2.6.
(2.6) where is average of u over the loaded area (A) of soil sample, and calculated as:
A from the PD-FSL Case (Silveira, 1951): n 1 (4B 2 T u u 0 4 e
B n from the PD-ESL Case (Scott, 1963): u u e ( 32 T r ) 0
Based on the above differential Equations 2.5 and 2.6, radial or horizontal coefficient of consolidatation can be calculated as the following Equation 2.10, 2.11, and 2.12.
(2.12) where, T v = time factor for radial consolidation case, U r = degree of consolidation, u 0 initial excess pore water pressure at t = 0, d e = diameter of the soil sample
In 1948, Barron introduced solutions for two boundary conditions: Free or Flexible Strain Loading (FSL), which occurs due to a uniform surface load distribution, and Equal Strain Loading (ESL), characterized by uniform settlement across all surface points.
The basic assumptions for the theory of drain wells with the consideration of well smear effect and well resistance effect for fine-grain soils are as follows (Barron, 1948):
(i) All vertical loads are initially carried by excess pore-water pressure (u).
(ii) All compressive strain within the soil mass occurs in a vertical direction.
(iii) Use the most economical pattern of drain wells.
(iv) The zone of influence of each well is a circle with the uniform load distribution on it.
(v) The thickness of the smeared zone is constant and homogeneous.
The governing differential equation for excess pore water pressure u(r, t) in clay, specifically for the radial consolidation problem involving a sand drain at the center (CD case) and free strain loading (FSL) case, is defined at a radius r.
In the radial consolidation problem involving a sand drain at the center (CD case) under equal strain loading (ESL), the governing differential equation for excess pore water pressure, denoted as u(r, t), in the clay at a radius r is established.
(2.14) where is average of u over the loaded area (A) of soil sample, and can be calculated as:
A from the CD-FSL Case (Barron, 1948): u u 0
(2.16) from the CD-ESL Case (Barron, 1948):
Based on the above differential Equations (2.13) and (2.14), coefficient of horizontal consolidatation can be calculated as the following Equation 2.18, 2.19, 2.20 and 2.21.
(2.21) where, T r = time factor for radial consolidation case; U r = degree of consolidation; u 0 initial excess pore pressure at t = 0; c r = coefficient of radial consolidation; d e = diameter
18 of zone of influence; d w = diameter of drain well; u 0 = excess pore-water pressure initial uniform; T r = time factor for radial consolidation case, U r = degree of consolidation.
Standardized methods to determine vertical coefficient of consolidation, c v
The following Table 2.1 shows existing methods to determine vertical coefficient of consolidation, c v (Lecture materials of Dr Nguyen Tien Dung, Lecturer, MIE Coordinator, VNU Vietnam Japan University)
Table 2.1 Standardized methods to determine vertical coefficient of consolidation, c v
T h 2 c v , where t 50 is corresponding to 50% of
50 primary consolidation (U = 50%); theoretical time factor T 50 =0.197
T h 2 c v , where t 90 is corresponding to 90% of
90 primary consolidation (U = 90%); theoretical time factor T 90 =0.848
Methods to determine radial or horizontal coefficient of consolidation for central
Table 2.2 presents various methods for determining the radial coefficient of consolidation (c r,CD) for central drains using the incremental loading method (IL) The eight methods outlined are also applicable for calculating the radial coefficient of consolidation for peripheral drains (c r,PD).
Table 2.2 Existing methods for the determination of c r from radial consolidation test with a CD using incremental loading
2 Matching log(d e 2 /t) vs U r method (Sridharan et al.
4 Non-graphical matching method (Robinson & Allam,
6 Steepest tangent fitting method (Vinod et al., 2010)
T c r r 90 t where t 90 is obtained from vs t 0.5 plot (approximately linear in a range of 20 U r 60%);
T r90 is obtained from theoretical curve of T r vs U r at
Plot of this theoretical equation is matched with experimental curve of d e 2 /t vs U r c r
8t inf where t inf is time at the inflection point from experimental vs log t curve; F(n) is function of n.
Plot of this theoretical equation is matched with experimental curve of vs t r100 , 0 , and c r
66 where t 66 is obtained at intersection point from two tangent lines on log( corr ) vs log(t) curve; T r66 is obtained from theoretical curve of T r vs U r at U r 66% at a given n.
T d c where t x is obtained at experimental vs log t curve at
x = 0 +(h/0.847)(x/100), 0 is corrected initial settlement; T rx is obtained from theoretical curve of T r vs U r at U r x (%) at a given n.
T d 2 c r r 50 e t 50 where t 50 is at 50 = 0.5(0 +100), T r50 is obtained from theoretical curve of T r vs U r at U r = 50% at a given n.
8 Full-match method (Chung et al.,
Note: d e = diameter of influence zone (= 2r e ), d w = diameter of drain (=2r w ), n = d e /d w
= r e /r w , F(n) = n 2 ln(n)/(n 2 -1) - (3n 2 -1)/4n 2 (Barron 1948), = measured settlement from radial consolidation test; 0 = corrected settlement at t = 0, 100 = settlement at
Methods to determine radial or horizontal coefficient of consolidation for peripheral
peripheral drain (PD) case, c r,PD
The following Table 2.3 shows existing methods to determine radial or horizontal coefficient of consolidation for peripheral drain (PD) case (c r,PD ) by using incremental loading method (IL).
Table 2.3 Existing methods for determination of c r from radial consolidation test with a PD using incremental loading
90 where t 90 is obtained from vs t 0.5 plot (approximately
2 Inflection point method (Ganesalingam et al., 2013) linear in a range of 20 U r
60%); = measured settlement from radial consolidation test. c 0.1605 r e 2 r t inf where t inf is time at the inflection point from experimental curve of vs log t.
Compression index (C c )
The compression index (C c) represents the slope of the linear segment of the e-log (void ratio vs logarithmic effective pressure) curve, which is crucial for settlement calculations While the e-log curve is typically regarded as linear in higher pressure ranges, making C c a constant, it can exhibit concave upward or downward shapes influenced by soil plasticity and initial water content.
In 1979, research highlighted that highly compressible clays and soils with lower initial water content than at the liquid limit state can be analyzed using specific formulas, such as Equation 2.22 It was found that the compression index (C c) values derived from the e-log ( ˊ ) curve are generally lower than those obtained from liquid limit (LL) values, particularly as the plasticity index (PI) increases Additionally, both the compression index (C c) and recompression index (C r) tend to decrease due to disturbances caused by sampling and sample preparation processes.
Mathematically, compression index (C c ) is defined as follows:
(2.22) where e 2c and e 1c are corresponding void ratios at vertical effective stresses ' v2c and
' v1c in the virgin compression range (NC range) ( p < ' v1c < ' v2c ).
Figure 2.7 Idealized curve of e-log ( ˊ ) from oedometer test for determining compression index (after Mayne et al., 2001)
Recompression index (C r )
The recompression index (C r) is a crucial parameter for determining the ultimate consolidation settlement of a clay layer subjected to an increase in vertical effective pressure ( ′ ) This index can be visually represented through an idealized e-log(σ v ′) curve, as illustrated in Figure 2.7, derived from conventional consolidation testing.
Mathematically, the recompression index (C r ) is defined as follows:
(2.23) where e 2r and e 1r are corresponding void ratios at vertical effective stresses ’p v2r and
’p v1r in the recompression range (OC range) (’p v1r < ’p v2r < ’p p ).
Preconsolidation stress (σ'c) refers to the maximum vertical effective stress that a soil element has experienced historically, representing the greatest overburden under which the soil has consolidated This parameter, also known as preconsolidation pressure (σ'p or P'c), can be determined through both field tests, such as the CPTu dissipation test, and laboratory tests like the Oedometer test.
This research focuses on two methods for determining preconsolidation stress, specifically utilizing the one-dimensional consolidation test, also known as the odometer test.
Casagrande (1936)’s method is a classical method and widely use Procedure to determine preconsolidation stress or pressure from Casagrande (1936)’s method are as follows (Figure 2.8):
(i) by visual observation, establish the point a, at which the e-log pꞌ, OCR, C curve have a minimum radius of curvature.
(ii) draw a horizontal line ab.
(iii) draw the tangential line ac at a;
(iv) draw the line ad, which is the bisector of the angle bac;
(v) project the straight-line portion of the e-log pꞌ, OCR, C back to intersect line ad at f The abscissa of point f is the preconsolidation pressure
Figure 2.8 Casagrande (1936)’s method (Dung and Giao, 2005)
The drawback fact of this method is the maximum curvature point cannot be correctly identified because of the changing in void ratio with applied pressure before and after
Pꞌ, OCR, C c when the tested specimen is disturbed from drilling and sampling procedures.
Figure 2.9 (Silva, 1970)’s method (Dung and Giao, 2005) The main steps of (Silva, 1970)’s methods are as follows (Figure 2.9):
(i) draw a horizontal line at void ratio e = e 0 , where e 0 is the initial void ratio of the specimen.
(ii) extend the tangent line with consolidation curve in NC stage and obtain the intersection point with e 0 line at a;
(iii) from point a, draw a vertical line until it intercepts the test curve at point b;
(iv) from point b, draw a horizontal line until it intercepts the virgin consolidation line at point c The abscissa of point c is the preconsolidation pressure.
Overconsolidation ratio (OCR)
The overconsolidated ratio (OCR) is defined as ratio of preconsolidation stress ( p ) to the current vertical effective stress ( v0 ):
0 where, ꞌ, OCR, C p = preconsolidation stress; v0 = current effective vertical stress
Kulhawy and Mayne, 1990from Mayne, 2007, provided a correlation between the
CPTu test data and the overconsolidation ration (OCR) as follow (Figure 2.10):
v (2.25) where OCR = overconsolidation ratio; C OCR = a coefficient; ≅ 0.2 < COCR