Consolidation and strength Properties of Macau Marine Clay

Transcription

1 Consolidation and strength Properties of Macau Marine Clay by T.M.H. Lok and X. Shi Faculty of Science and Technology, University of Macau

2 1. Introduction The development of Macau economy has led to more and more demand on land resource. Today reclamation is a popular method to solve this problem not only in Macau, but also in Hong Kong and Singapore. In order to estimate the degree of consolidation and the amount of settlement after reclamation at a specified time, it is necessary to know the consolidation properties of marine clay, which is commonly found under the reclaimed land in Macau. To reduce the time for a certain consolidation settlement, prefabricated vertical drain (PVD) is usually used, because it can shorten the length of drainage path; also the coefficient of consolidation in horizontal direction is larger than that in vertical direction in general. The consolidation properties, especially in horizontal direction of Macau marine clay are rarely explored up to now. The traditional one-dimensional consolidation test (incremental loading test, IL) has been commonly used to determine the consolidation properties of clayey soil based on the theory proposed by Terzaghi in the early 1920's. As its name suggests, in the incremental loading consolidation test the load is increased in increments and the resulting vertical deformations are measured as a function of time for each increment. Typically, one increment of load lasts for 24 hours. The coefficient of consolidation can be determined based on curve-fitting methods, such as logt and square-root t method. The equilibrium strain for each load can be determined from the measured deformations. Each datum consisting of one value of load (or stress) and the corresponding value of equilibrium strain is represented by a single point in the stressstrain curve. Continuous stress-strain curves can not be determined accurately from an incremental loading test because of the limited number of data points. Also, it generally requires one week or more to complete a test. During the last several decades, several alternative tests have been developed to overcome the practical and theoretical disadvantages of traditional incremental loading consolidation test. These include the controlled gradient (CG) consolidation

3 test (Lowe et al. 1969), the constant rate of loading test (CRL) (Aboshi et al. 1970), and the constant rate of strain (CRS) consolidation test (Smith and Wahls, 1969; Wissa et al. 1971). In the CRS test, the specimen is loaded at a constant rate of deformation (and hence at a constant rate of engineering strain). Drainage is permitted at the top of the specimen but not the bottom, resulting in the generation of excess pore water pressures throughout the specimen except at the top. The deformation, induced axial pressure, and pore water pressure at the bottom of the specimen are measured at certain time intervals throughout the testing process. Theoretical methods to estimate the vertical average effective stress and the coefficient of consolidation within the specimen based on the total vertical stress, the pore water pressure at the base of the specimen, the strain rate adopted, and the sample size have been developed (e.g., Smith and Wahls 1969; Wissa et al. 1971; Lee, 1981; Yune et al. 2004; Seah et al. 2002). Using the interpretation data, a nearly continuous stress-strain curve can be determined. Among these methods, CRS test based on the nonlinear theory of Wissa et al. (1971) has been widely used and incorporated into ASTM Standard D4186. The study by Lee (1981) has taken the strain rate effect into account and is adopted more and more widely. Yune et al. (2004) have derived the theory under CRS consolidation test with lateral outward drainage, while Seah et al. (2002) have developed theory for lateral inward drainage. These developments have widened the use of CRS tests and made it easy to get the horizontal compressibility behavior of clayey soil. Many researchers have conducted CRS tests (Seah et al. 2003; Moriwaki et al. 2003; Sheahan et al. 1997; Lee, K. et al. 1993), some together with traditional incremental loading test for comparison. Both kinds of tests gave agreeable results. Besides, CRS can be used as a method to determine the coefficient of permeability of clay (Moriwaki et al., 2003). The key problem is the selection of strain rate, since the compressibility of clay is strain rate dependent (Leroueil, 1996). There were no previous detailed studies about the compressibility of Macau marine clay, especially the compressibility due to horizontal drainage. CRS test may be a good method that provides reliable results for consolidation if proper strain rates are used.

4 On the other hand, there are a number of natural and man-made phenomena such as earthquakes, machine vibrations, pile driving, explosions, heavy traffic that produce fast monotonic and/or cyclic loading affecting civil engineering systems. To evaluate the effect of such loads, it is necessary to understand and quantitatively measure the dynamic and cyclic properties of soils involved. One of the most important parameters is the shear modulus at very small strain ( G 0 ). It is recognized that shear modulus ( G 0 ) at very small strain is a function of stress state, void ratio or OCR (over consolidation ratio) for clayey soil (Hardin, 1978; Rampello et al. 1997). The value of G 0 should vary during the consolidation process because of the build-up of effective stress. Many researchers have performed studies to explore the relationship between shear modulus at very small strain and consolidation state, and some empirical correlations were proposed (Fam et al. 1997; Shibuya, et al. 1997; Viggiani et al. 1995); however, there is no data about the very small-strain stiffness of Macau marine clay up to now. The value of G 0 can be obtained by several methods, such as resonant column tests, bender element tests, cross-hole and down-hole tests. Among all these methods, resonant column test is widely accepted, since it can measure the value of shear modulus and damping ratio. However, resonant column test is expensive and time consuming, while it was found that G 0 can be obtained from shear wave velocity measurements using piezoelectric transducers or so called bender elements more easily and directly (Dyvik et al. 1985). A bender element is a thin, two-layer plate that can be installed in most triaxial cells. Shirley and Hampton (1977) introduced bender elements to soil testing. Agreement has been obtained between G 0 measured with bender element and with resonant column (Dyvik and Madshus 1985; Brignoli et al. 1996).

5 For the determination of the consolidation properties in the vertical and horizontal direction of Macau marine clay, the CRS test was adopted because of its advantage over other methods together with conventional consolidation tests. Besides, the variation of shear modulus ( G 0 ) at very small strain during the primary and secondary consolidation was investigated by measuring the shear wave velocity using bender elements in the modified oedometer. The objectives of this research, and the means to accomplish these objectives, are summarized as follows: 1. Determine the vertical consolidation properties of Macau marine clay in oedometer together with some CRS tests in GDS Rowe cell for comparison 2. Determine the horizontal consolidation properties of Macau marine clay in oedometer together with some CRS tests in ELE Rowe cell for comparison 3. Determine the variation of shear wave velocity ( V s ) and very small strain stiffness ( G 0 ) during the primary and secondary consolidation process in modified oedometer equipped with bender element. Empirical equations are developed to correlate the values of V s and G 0 at the end of primary consolidation to the stress state and OCR (over-consolidation ratio). 4. The reconstituted and undisturbed Macau marine clay are used above for comparison.

6 2. Literature review Foundation design of a structure requires a reliable estimate of the magnitude and the rate of settlement. Building and bridges must be designed to withstand estimated differential and total settlements. Highway embankments must be designed to minimize settlements that produce uneven road surfaces and pavement distress. The consolidation behaviors of related clay are needed to make the design above more safe and economical. Besides, the behavior at very small strain is also of great importance in geotechnical engineering. For example, accurate definition of stress strain relationships for shear strains in the range of % is a prerequisite for accurate predictions of ground movements around deep excavations and tunnels (Burland, 1989), whereas knowledge of the stiffness of soils at strains in the range of % may be required in earthquake geotechnical engineering (Ishihara, 1996). The value of shear modulus at very small strain may not be a constant, which varies during the consolidation process and at different stress condition. Besides, the value of G 0 may be related to stress history and initial condition of clay, so if the consolidation and smallstrain modulus can be explored in detail with respect to Macau marine clay, it can provide useful information for understanding the engineering behavior of Macau marine clay. 2.1 Consolidation properties of soft marine clay Studies on the general characteristics of marine clays found in the coastal regions of various countries have been carried out to date by many researchers. Ag and Silva (1998) reported the consolidation and permeability behavior of high porosity Baltic seabed sediments, which is characterized by high void ratio, high organic content and large compression index. The geotechnical properties of Hong Kong marine clay have been reported by Yeung et al. (2002). It was shown that the predominant mineral of Hong Kong marine clay is illite and the clays are normally consolidated. The laboratory measured compression index C c of the uppermost 20 m of marine clays is approximately 0.6 and that of the alluvial clay is approximately 0.2. The coefficient of consolidation has exhibited considerable natural variability. Rowe cell consolidation tests with horizontal inward tests were carried out and it was shown that the coefficient of consolidation in 6

7 horizontal direction due to vertical loading is at least twice of that in vertical direction due to vertical loading. Chu et al. (2002) investigated the consolidation and permeability properties of Singapore marine clay by laboratory and in situ tests. The coefficient of consolidation and permeability of the soil in both the vertical and horizontal directions, c v, c h, k v and k h, respectively, were determined by oedometer, constant rate of strain (CRS), Rowe cell, piezocone (CPTU), flat dilatometer, self-boring pressuremeter (SBPT), and BAT permeameter tests. The values of c v are in the range of 0.5~2.3 m 2 /year based on oedometer tests. Two strain rates (1.25%/h and 4.49%/h) were used in CRS tests and the values of c v were calculated following the procedure of Lee et al. (1993). The values of c v from 1.25%/h CRS test agreed well with those from oedometer tests. While the strain rate of 4.49%/h was too high to get comparable c v at the undrained and drained surface, which supports the study by Lee et al. (1993). The value of c h, which was determined in Rowe cell tests and about 2~4 times larger than the value of c v, falls within a range of 2~4 m 2 /year and generally increases with depth based on CPTU test. The results of c h determined from Rowe cell and CPTU agreed very well and were generally smaller than those obtained from other in situ or laboratory test, such as SBPT, DMT dissipation test and oedometer test. The author also suggested that laboratory tests using Rowe cell and in situ tests using CPTU be the most suitable methods for the determination of c h. Above all, marine clay is characterized by high void ratio, high compressibility and low permeability. CRS consolidation test can provide reliable results for marine clay if proper strain rates are used. 7

8 2.2 Very small-strain stiffness properties of clay Measurement of soil shear modulus at very small-strain The field and laboratory work on small strain behavior of soils resulted in the realization that the stiffness of soils under static loading could be classified according to strain levels. Atkinson and Sallfors (1991) divided strain levels into three categories: Very Small Strains (VSS), Small Strains (SS), and Large Strains (LS), as delineated in Table 2.1. It has been shown (Burland 1989) that the strain level around engineering structures lies in the small strain range, and thus the small-strain stiffness of geotechnical material is a key parameter in defining the material response to static loading. G 0 is also verty important in dynamic analyses such as those used to predict soil behavior during earthquake, explosions, or machine and traffic vibrations. Table 2.1 Strain level categories and limits (after Atkinson and Sallfors, 1991) Strain Category Strain Limits (%) Very Small Strain (VSS) <0.001 Small Strain (SS) to 0.1 Large Strain (LS) >0.1 These strain categories and limits delineate three zones of separate soil behavior. The stress-strain behavior is thought to be essentially linear elastic and the value of G 0 is nearly a constant in the VSS range. Upon reaching a strain threshold value near %, the stress-strain behavior becomes markedly nonlinear and the secant and tangent modulus begin to degrade in a hyperbolic fashion with increasing strain. This zone is designated as small strain and terminates at a strain of about 0.1 %. The onset of large strain occurs at about 0.1 %, marking the zone where the secant and tangent modulus are very small and the soil is approaching failure. Soil modulus in the LS range may be only 10 to 20% of that in the VSS region. There are several lab and in-situ testing methods to determine the shear modulus at very small strain. Some of them are summarized below: Resonant column test: this test can be used to evaluate the stiffness of soils at shearing strains ranging from % to 1%. However, since analyses of 8

9 resonant column tests are based on the assumption that the behavior of the soil is linear and elastic; analyses of the test data are strictly valid only in the region of very small strain (Isenhower, 1979). Bender element test: it involves transmitting and receiving shear waves using small electro-mechanical transducers known as bender elements (Shirley & Hampton, 1977). There are several advantages of the bender element technique, namely bender element can be installed in many devices such that the need for parallel resonant column tests may be eliminated, measurement and calculation of G 0 is much faster and easier than in resonant column device, and shear modulus at small and large strains can be compared directly on the same specimen. However, in bender element tests the strains are not constant throughout the sample because of both material and geometric damping. Field test: the shear wave velocity can be measured from the ground surface, using refraction surveys or Rayleigh wave techniques, or at depth, using crosshole or down-hole techniques (Stokoe et al. 1972; Anderson et al. 1975). Bender elements test is a relatively easy and direct method to determine shear modulus at very small strain. The transmitting element is excited by a change in voltage (causing it to bend) thus propagating a shear wave through the sample. This wave causes the receiving element to bend, producing a change in voltage that is recorded in a digital oscilloscope. Determination of the propagation velocity is then a straight operation that involves dividing the travel distance between the elements by the travel time of the shear wave. The shear modulus G 0 at very small strain in an elastic body is related to the shear wave velocity by G ρv = (2-1) 2 0 s Where: ρ =soil density; V s =shear wave velocity. 9

10 2.2.2 Existing empirical equations to estimate the very small-strain modulus As described previously, three state variables that have great impact on VSS modulus are mean effective stress, p ', the void ratio, e or the over-consolidation ratio, OCR (sometimes described as R 0 ). Many researchers have provided different empirical expressions for G 0 based on their lab test data. For sands, Wroth & Houlsby (1985) proposed a general expression relating shear modulus to mean effective stress in the following form; G p' = A( ) n (2-2) p p r r Where: G =shear modulus; A and n =constants, depending primarily on the nature of the soil and on the current strain; p r =reference pressure; p ' =mean effective stress. Experimental results from both dynamic and static tests on sands indicate that the value of n varies significantly with strain from values near 0.5 at very small strain to 1.0 at large strain. Hardin & Black (1968) assumed that the same relationship would hold for normally consolidated clays. Viggiani and Atkinson (1995) have shown that A =2088 and n =0.653 for normally consolidated speswhite kaolin with coefficient of correlation 2 R =0.996 and standard deviation σ =0.009 if taking the reference pressure p r =1 kpa under isotropic condition. Besides, A and n are not constants for a specified clay and dependent on the shear strain. After taking the OCR into consideration, Atkinson & Little (1988) found that the value of G/ p ' at a particular strain increased linearly with the logarithm of the overconsolidation ratio. These results can be expressed in the following form, 10

11 or G G = ( ) (1 log ' nc + c R0 ) (2-3) p p' G/ G = 1+ clogr (2-4) nc 0 Where G nc =stiffness of a normally consolidated sample at the same strain and the same mean effective stress; c =material constant; R 0 =over-consolidation ratio. Houlsby & Wroth (1991) expressed the variation of stiffness with over consolidation ratio using a following power function G m = ( G ) R nc 0 (2-5) P' P' or G log( ) mlog R Where: m =constant; 0 G = (2-6) nc Viggiani and Atkinson (1995) have given m =0.196 with coefficient of correlation 2 R =0.83 and standard deviation σ =0.021 based on their test results. A more general expression for G 0 of soils subjected to isotropic consolidation was proposed by Hardin (1978). It can be written as G0 = Sf() v OCR p p (2-7) k 1 n ' n a Where S =a dimensionless coefficient which depends on the nature of the soil; 11

12 f () v =a function of the specific volume; p a =atmospheric pressure; OCR=over consolidation ratio defined as the ratio of the maximum past stress to the current stress. Results of tests on soils in resonant column tests (Hardin & Drnevich, 1972) show that n is less than 1.0 and k increases from 0 to 0.5 as the plasticity index increases from 0 to 100. Jamiolkowski et al. (1994) proposed a relationship to consider the small strain modulus under anisotropic condition. = (2-8) G S f e p σ σ 1 nv nh nv nh 0 vh () r ' v ' h Where S vh =a material constant reflecting the current soil structure; f () e =a function of void ratio; σ ' v =effective stress in the shear wave propagation direction; σ ' h =effective stress in the particle motion direction; nv and nh =constants. Equation above can be used to interpret values of G 0 measured in conventional downhole seismic surveys on level ground or in laboratory tests using both vertically and horizontally cut specimens. The effects of the two principal stresses on G 0 are more or less equal to each other, implying nv = nh in equation above. Therefore, it becomes nv 1 2nv 2nv G0 = SvhK0 f() e pr σ ' v (2-9) Where: K 0 =coefficient of earth pressure at rest 12

13 3. Equipment and testing procedures 3.1 Introduction In this research program, the traditional consolidation test with oedometer and CRS test with Rowe cell were performed. In addition, oedometer was modified to equip with bender elements to measure the shear wave velocity in the consolidation process. Both reconstituted and undisturbed Macau marine clay were adopted for comparison. This chapter describes the method to prepare remolded samples in consolidometer and testing programs in detail. 3.2 Reconstituted Macau marine clay (RMMC) Consolidometer Fig. 3.1 Schematic configuration of consolidometer 13

14 A consolidometer (Fig. 3.1) was used to prepare the reconstituted samples. The consolidometer is a rigid-walled, stainless steel cylinder with inside diameter of 20cm and a height of 60cm. During the consolidation, the sample is loaded from the bottom by air pressure through a rigid cap, which is sealed against the chamber wall by two lubricated O-rings. Pore water is free to drain through two 0.5cm thick porous stones which are attached to the top cap and the base-plate, respectively. Therefore, water can drain from both sides of sample during consolidation. To prevent the migration of soil particles into the porous stone during consolidation, filter papers (0.014cm thick) were placed between the clay and the porous stones at the top and bottom of chamber Preparation method for RMMC The material for the preparation of RMMC was taken from an excavation site in Macau. Before making the slurry, the initial water content of the Macau marine clay (MMC) was measured, and then the amount of distilled water needed was calculated to make the water content about twice the liquid limit after mixing the MMC into slurry. In addition, 15g salt per liter distilled water was added in order to simulate the deposition environment in the sea based on the measurement of salt content of the sea water. MCC was then mixed into thick slurry and passed through 600μ m standard sieve to remove all large soil particles and shells. The friction between the rigid cap and the chamber is reduced by lubrication and then pushing the bottom cap back and forth for several times. If the cap can move smoothly at a small stable pressure, the lubrication is satisfactory. The friction is equal to the value of pressure which is about 30 kpa under which the cap begins to move smoothly. Before the porous stones were fixed in the top and the bottom of the chamber, they were boiled to reduce air bubbles inside. Then the slurry was poured into the consolidometer for consolidation. A small mixing machine was operated to reduce air bubbles contained in the slurry. A pressure of 1Bar (100 kpa) was applied to the slurry from the bottom cap, and water drained from both sides of chamber. The consolidation was monitored at different time intervals by measuring the movement of rod at the bottom. At the beginning the measuring time interval should be small, but later it could be several hours or more. The consolidation was stopped when the primary consolidation had finished based on the sample height vs. log time curve. One typical example is shown in Fig It generally needs several 14

15 days to more than ten days to finish the primary consolidation depending on the initial sample height and initial water content of the slurry Sample height (cm) Time (min) Fig. 3.2 Sample height of RMMC vs. log time After finishing the consolidation, the sample was carefully pushed out by hydraulic extruder and was cut into different sizes depending on the tests to be performed, but the peripheral part of the specimen was discarded because of disturbance. Each piece of soil was wrapped with cling film by several layers. The pieces were stored in a humid room at % relative humidity with stable temperature, keeping the water content constant until they were used for testing Index properties Clay index test provides the basic properties of clay. Besides, many correlations for some other soil properties such as compression index, the coefficient of earth pressure at rest were established based on index tests. Some values of index properties of MMC exist, and it is beneficial to compare the current ones to the previous. Table 3.1 summarizes the values of index properties together with values obtained by Carter (2001). The particle distribution of the source of RMMC was also determined and is shown in Fig

16 Table 3.1 Index properties for MMC Liquid limit (%) Plasticity index (%) Initial water Specific content (%) gravity RMMC 65~69 30~34 60~ Carter(2001) 60~75 30~35 60~75 N.A Percentage finer (%) Diameter (mm) Fig. 3.3 Particle distribution of the source of RMMC 3.3 Undisturbed Macau marine clay (UMMC) Sampling method Undisturbed samples were taken for consolidation test since the corresponding results provide the actual soil behaviors in the field and are more useful for construction design. Two different sizes of undisturbed samples were obtained with 76mm stainless steel Shelby piston tube samplers and U100 steel tube sampler with the length of 1m and 0.5m, respectively. The soil profiles at Taipa and Cotai are slightly different. The undisturbed clay samples at Taipa were taken at the depth of 3m to 6m, while at the depth of 6m to 13m at Cotai. After the sample was taken out from the bore-hole, both ends of the sample tube were sealed with wax in the field. Then the samples were kept in a room at % relative humidity with stable temperature. 16

17 3.3.2 Index properties The Atterberg limits, the specific gravity and the particle size distribution were determined and compared with the reconstituted samples. The results for both sites were summarized in Table 3.2. The particle distribution of the undisturbed samples obtained from Cotai is plotted in Fig From table 3.2, it is found that the initial void ratio of UMMC at Taipa is much larger than that of UMMC at Cotai, since these two places have different profiles as described previously. From Fig. 3.3 and Fig. 3.4, it is observed that the fine content (diameter smaller than 0.075mm) of the sample taken from Cotai is about 75%, meanwhile, the fine content of the source of reconstituted sample is about 85%. It means that the sample taken from Cotai has more sand content inside. Table 3.2 Index properties for Macau marine clay Liquid limit (%) Plasticity index (%) Initial water content (%) Specific gravity UMMC (at Taipa) 52~55 29~31 60~ UMMC (at Cotai) ~ RMMC 65~69 30~34 60~ Percentage finer (%) Diameter (mm) Fig. 3.4 Particle distribution of the undisturbed samples at Cotai 17

18 3.4 Consolidation apparatus Traditional consolidation apparatus-oedometer Oedometer test is a conventional method to determine the compression index C c, recompression index C s, and preconsolidation pressure p ' c, which define the soil compressibility; the coefficient of consolidation c v, which characterizes the rate of primary consolidation; and the secondary compression coefficient C α, which defines the creep behavior. Fig. 3.5 Schematic of the oedometer used in this study The most commonly used loading system is the beam-and-weight mechanism. The loading stress is constant after applying a dead load at a certain step. There are two commonly used types of consolidation cells: fixed ring and floating ring. In this research program, the cell with floating ring was used, since the effect of friction 18

19 between the container wall and the soil specimen is smaller than that of fixed-ring container. Fig. 3.5 shows the schematic diagram of Casagrande oedometer apparatus used in this study Rowe cell consolidation system Consolidation tests with vertical drainage in Rowe cell were performed in the GDS Consolidation Testing System (GDSCTS) (shown in Fig. 3.8), while those with horizontal drainage were performed in the ELE Consolidation Testing System (ELECTS) (Shown in Fig. 3.9). Both systems were controlled by the software GDSLAB developed by GDS company. The fundamental components of the Rowe cell consolidation system are shown in Fig The system is composed of the GDS Rowe cell or ELE Rowe cell, the GDS pressure/volume controllers, data acquisition system and controlling computer. Two pressure controllers are linked to the computer and the test cell as follows One for axial stress and axial displacement control. One for setting back pressure and measuring volume change. In this research program, the pressure controllers with maximum pressure 2000kPa were used based on the pressure requirement. Schematic common arrangements and connections for GDS Rowe cell are shown in Fig The GDS Rowe cell was replaced by ELE Rowe cell when performing consolidation tests with horizontal outward drainage. 19

20 Fig. 3.6 GDS hydraulic consolidation system Fig. 3.7 Diagram of the Rowe cell consolidation hardware elements and its arrangement Two different kinds of Rowe cell were used in this program as described before. The schematic diagram of the GDS Rowe cell is shown in Fig Another kind is ELE Rowe cell, whose schematic diagram is shown in Fig The difference of these two cells is that the ELE Rowe cell is equipped with rim drain, which permits drainage when the top and bottom back pressure valves are closed at the same time. 20

21 Vertical Pressure Access Valve Vertical Pressure Chamber Back Pressure Access Valve Rubber Top Bag Porous Stones Soil Specimen Pore Pressure Access Vallve Pore Water Pressure Transducer Fig. 3.8 GDS Rowe and Barden cell (developed by GDS Company) Vertical Displacement Transducer (LVDT) Flexible Lead To Cell Pressure Controller Vertical Displacement Stem Air Bleed To Back Pressure Controller Ram Drain Cell Diaphragm Water Water Top Platen with Porous Stone Soil Specimen To Back Pressure Controller Base Porous Stone O-ring Seal Pore Pressure Transducer Fig. 3.9 Conventional Rowe cell (developed by ELE Company) Flexible porous disk or rigid porous disk can be used in GDS Rowe cell to form free strain (equal stress) or equal strain (free stress) boundary conditions when performing consolidation tests with vertical drainage (shown in Fig. 3.10). Similarly, flexible or rigid plate can be used in ELE Rowe cell to form free strain (equal stress) or 21

22 equal strain (free stress) boundary conditions when performing consolidation tests with horizontal drainage (shown in Fig. 3.11). The rigid ones were used in the study to form equal strain boundary condition as shown in Fig (b) and Fig (b), since CRS test method was adopted. Fig Free stain and equal strain for vertical drainage (a) Free strain with vertical drainage (b) Equal strain with vertical drainage Fig Free stain and equal strain for horizontal outward drainage (a) Free strain with horizontal outward drainage (b) Equal strain with horizontal outward drainage Both Rowe cells can be used to perform classic tests such as incremental loading (IL) which is similar to traditional oedometer tests or more advanced tests such as constant rate of strain (CRS) test or constant hydraulic gradient (CG) test under computer control The comparison between oedometer and Rowe cell The invention of Rowe cell is to overcome most of the disadvantages of the conventional oedometer apparatus when performing consolidation tests on lowpermeability soils, including non-uniform deposits. The design of the cell differs from a 22

23 conventional oedometer in that the test sample is loaded hydraulically by water pressure acting on a flexible diaphragm, instead of by a mechanical lever system (shown in Fig. 3.8 and Fig. 3.9). This arrangement enables samples of large diameter (up to 254mm diameter) to be tested, and allow for large settlement deformations. But the most important features are the ability to control drainage and to measure pore water pressure during the course of consolidation tests (shown in Fig. 3.8 and Fig. 3.9). Two drainage conditions (vertical or horizontal) are possible, and back pressure can be applied to the sample (shown in Fig. 3.8 and Fig. 3.9). The Rowe cell has many advantages over the traditional oedometer consolidation apparatus. The main features for these improvements are the hydraulic loading system, the control facilities, the ability to measure pore water pressure, and the capacity of testing samples of large diameter. Some of them are summarized below: Pressure of up to a large value (such as 2000kPa in this study depending on the capacity of pressure controller) can be applied easily even to large samples Drainage of the sample can be controlled, and several different drainage conditions can be imposed on the sample. Control of drainage enables loading to be applied to the sample at an undrained condition, allowing for full development of pore pressure. Consequently the initial immediate settlement can be measured separately from the consolidation settlement, which starts only after the drainage line is opened. Pore water pressure can be measured at any time The volume of water drainage from the sample can measured, as well as surface settlement. The sample can be saturated by applying increments of back pressure until a B value near unity is obtained, or by controlling the applied effective stress, before starting consolidation. The sample can be loaded either by applying a uniform pressure over the surface ( free strain ), or through a rigid plate which maintains the loaded surface plane ( equal strain ). 23

24 3.5 Consolidation testing programs Traditional consolidation tests Conventional consolidation test is the most popular method to determine the consolidation properties of clay although it takes one week or more to finish one test. In this program, the cell of 75mm in diameter was used to explore the consolidation behavior of reconstituted samples, while the cell of 50mm in diameter was used to explore the behavior of undisturbed sample, since the undisturbed sample was taken by 76mm diameter stainless steel Shelby tube and both cells have the height of 20mm.The remolded and undisturbed samples were cut in vertical and horizontal directions to investigate the consolidation behavior in corresponding directions due to vertical loading (shown in Fig. 3.12). σ v ' Vertical Direction σ h ' Horizontal Direction σ h ' σ v ' Fig Vertical and horizontal direction cut for sample The displacement was measured by Linear Variable Displacement Transducer (LVDT), whose accuracy is 0.001mm and measuring range is 10mm. The LVDT was connected to a digital signal conditioner, so the displacement could be read directly at any time. Before the test, calibration of LVDT was done by using a standard thickness block, and it was found that all the LVDTs were in good condition. A unit increment load ratio was adopted and each increment load lasted for 24 hours. Different loading sequences were used based on the estimation of preconsolidation pressure of the remolded and undisturbed samples. In some cases, 24

25 unloading and reloading were performed in order to explore the variation of shear wave velocity in these processes. During the consolidation process, the vertical displacements were recorded at time intervals of 0, 0.1, 0.25, 0.5, 1, 2, 4, 8, 15, 30, 60, 120, 240,480, 720, 1440 minutes after the application of dead load. Besides, four tests were lasted for about 3 months to explore the variation of shear wave velocity in secondary consolidation Rowe cell consolidation tests The Rowe cell consolidation test is becoming more popular because of its advantage as illustrated before. In general, during a consolidation test in Rowe cell, measurements of the following are observed and recorded at appropriate time intervals, which was controlled by the software GDSLAB. Diaphragm pressure Back pressure Pore water pressure Vertical settlement Volume of water draining out It is convenient to perform Rowe cell consolidation test with the aid of the software GDSLAB developed by GDS company. All the tests were performed according to Head (2006) Back pressure saturation method During specimen saturation, back pressure of 200 kpa was applied through both sides of the specimen in GDS Rowe cell (in ELE Rowe cell for horizontal drainage test, back pressure was applied through the rim drainage as shown in Fig. 3.9). Prior to the start of the vertical consolidation loading, the back pressure valves were closed, an increment of cell pressure was applied to cell diaphragm, and the pore pressure responses were recorded. When the B-value is larger than 0.96, the saturation is considered to be satisfied. 25

26 Constant rate of strain test with vertical drainage in GDS Rowe cell As described in chapter 2, the CRS method is attractive, so the tests in Rowe cell were performed with the CRS method. The strain rates of 2%/h was used for RMMC and 1%/h for UMMC based on the recommendation by Lee et al. (1993). Displacement increment was controlled by the software GDSLAB. Water was drained from the upper surface of sample through back pressure valve (shown in Fig. 3.8) and the pore water pressure was measured at the undrained surface by the pore water pressure transducer (shown in Fig. 3.8) Constant rate of strain test with horizontal outward drainage in ELE Rowe cell For the determination of c h, horizontal drainage to the periphery with equal strain loading (shown in Fig. 3.11(b)) is usually preferred using traditional incremental loading method with drainage at the top surface of sample and pore pressure measured at the bottom undrained surface of the sample. However, it is also time-consuming since it is similar to the incremental loading tests for determining c v as described above. In this study, the CRS test with horizontal outward drainage was adopted and the test setup was shown in Fig The porous plastic lining was boiled for at least 20mins to release all the air bubbles inside, after that it was put in deaired water before use. The valves B and D were closed to prevent vertical drainage and pore water pressure was measured at center base of the specimen during all the test process. Back pressure (200 kpa) was applied, and water drained from the rim drain F (as shown in Fig. 3.13). Based on the recommendation by Lee et al. (1993), the strain rate of 1% was adopted. Fig Test set-up for horizontal drainage in ELE Rowe cell 26

27 3.6 Bender element test Experimental system set-up As described in chapter 2, bender element test is an efficient technique to measure the shear wave velocity. Very small-strain shear modulus ( G 0 ) can be obtained based on G = ρv (3-1) 2 0 s The equipments required to perform the bender elements test are shown in Fig There are six important components in the setup: the oscilloscope, signal generator, power amplifier, filter, oedometer equipped with bender elements and the personal computer. Fig shows the whole system configuration. Fig The bender element testing system The bender element is a two-layer piezoelectric transducer that consists of two conductive outer electrodes, two piezoceramic sheets, and a conductive metal shim at the center (Fig (a)). The outer electrodes are typically made of deposited nickel 27

28 or silver. There are two types of bender elements: series and parallel. In the series type, the pole directions of the two piezoelectric layers are opposite to each other and the bender element is connected at the outer electrodes as shown in Fig (b). In the parallel type, the two piezoelectric layers have the same pole direction as shown in Fig (c). The ground cable is attached to both outer electrodes, and the core wire is connected to the intermediate metal shim. For the same applied voltage, the parallel-type connection provides twice the displacement of the series-type connection. Therefore, manufacturers often recommend the use of parallel-type bender elements as source and the series-type bender elements as receiver. However, the series type of bender elements were used as the source and receiver for convenience in this program. Therefore, the connecting method for the wire is the same as shown in Fig (b). Fig Bender elements: (a) schematic representation of bender element, (b) series type, and (c) parallel type All bender elements should be water-proofed with a thin polyurethane coat. Several coats may be applied if needed. Then an electric shield was needed to prevent cross-talk. Cross-talk is the electromagnetic coupling between source and receiver bender elements manifested as an output signal with an early component that is quasisimultaneous with the input signal. This crosstalk can be very important in 28

29 conductive soils such as wet clays on seafloor sediments. As an electric shield, a layer of silver conductive paint was spread over the surface of the coated bender element. The conductive paint must contact the shield in the coaxial cable, i.e., ground. The layouts of the bender elements at the top and bottom of the specimen are shown in Fig and Fig. 3.17, respectively. The encapsulated elements were placed within the pre-drilled holes in the top porous stone and the base porous stone. The fixed end of the bender was embedded into the hole by injecting epoxy into the hole using a syringe and needle. It is important to ensure that no air was trapped within the epoxy, as this could weaken the epoxy. Fig Layout of bender element at the top plate Fig Layout of bender element at the base pedestal The whole oedometer equipped with bender element is shown in Fig The shear wave velocity ( V s ) was measured during the process of consolidation test 29

30 to explore the variation of shear wave velocity due to the dissipation of pore water pressure and at the end of consolidation. The time of measuring the shear wave velocity is the same as the displacement measurement. Bender element (transmitter) Top cap Wire lead Soil specimen Oedometer ring Base plate Bender element (receiver) Fig Bender element test set-up in Oedometer Experimental procedure The effective travel length of the wave travel path L was taken as the distance between the tips of the bender elements (Viggiani & Atkinson, 1995). A single sinusoidal pulse with the amplitude of 10V was used as the source. Fig shows a typical trace of signals. The received and source signals have similar shape. The travel time of shear wave velocity between the two bender elements was interpreted as the interval between the source peak and the first peak of received signal distance (e.g. Δ t =5.56e-5s as shown in Fig. 3.19). This method can give similar arrival time to the results derived through a numerical analysis of the driving and received signals (Viggiani & Atkinson, 1995). 30

31 Receiver :V 1.E-02 8.E-03 6.E-03 4.E-03 2.E-03 0.E+00-2.E-03-4.E-03-6.E-03-4.E E E+00 2.E-05 4.E-05 6.E-05 8.E-05 1.E-04 Time (s) Transmitter:V Fig Bender element traces 220 Shear wave velocity(m/s) Wave number Fig Shear wave velocity vs. wave number at a typical state As described in chapter 2, the received signal is effected by wave number (defined as R = L / λ ) between the source and receiver bender elements. In this d program, it was found that the shear wave velocity converged when the wave number was bigger than 2 as shown in Fig Therefore, input signal of high frequency was used to ensure the wave number to be larger than 2 when performing shear wave velocity measurement. 31

32 4. Consolidation properties of Macau marine clay 4.1 Introduction As vertical drains are often used to accelerate the consolidation process in reclamation project, the permeability and consolidation properties of soft clay in both vertical and horizontal directions become very important. High quality undisturbed samples are not easy to obtain because of skill and equipment requirement, as well as time and cost. Therefore, in general, reconstituted samples compressed to the similar stress condition to the undisturbed samples in the field are adopted. In this research program, reconstituted and undisturbed samples were used at the same time and the anisotropic consolidation behaviors of Macau marine clay were explored in oedometer and Rowe cell. The quick consolidation method-crs test in Rowe cell was used because of its advantages described in chapter 2. All the consolidation test results of Macau marine clay are summarized in this chapter. 4.2 Consolidation behaviors of RMMC The reconstituted samples were prepared as described in chapter 3. There are small differences in the initial water contents of slurry before reconsolidation for different sample batches, since it is not easy and practical to make it exactly the same at every time. Anyway, the finial water contents of the reconstituted samples are similar and the corresponding values are between 60% and 62% Consolidation behaviors of RMMC in the vertical direction In order to get the consolidation properties of reconstituted Macau marine clay in vertical direction, six oedometer tests and two CRS tests in GDS Rowe cell were performed. Their physical properties of test specimens are summarized in Table 4.1. The samples were taken from different batches for different group numbers. No. G4OMLB03 means the third test of Group 4 Oedometer Multi-Loading with Bender element test and G3CRS01 means the first test of Group 3 in Rowe cell with Constant Rate of Strain method. Shear wave velocity was measured in four of the six oedometer tests and the corresponding test results are described in chapter 5. 32

33 Table 4.1 Physical properties of the reconstituted Macau marine clay in conventional and CRS consolidation tests No. e o r s (kn/m 3 ) G4OML G4OML G4OMLB G5OMLB G6OMLB G7OMLB G3CRS G3CRS Where: e o =initial void ratio; r s =initial saturated unit weight Conventional consolidation tests Conventional consolidation tests were used to explore the consolidation behavior of reconstituted Macau marine clay, since it is the most popular method although it takes more than one week to finish one test. Six oedometer tests with reconstituted samples taken from different batches were performed to determine the compressibility of Macau marine clay. The shear wave velocity was measured in all the tests except G4OML01 and G4OML02 in order to explore the variation of very small-strain shear modulus at different loading stages. Fig. 4.1 and Fig. 4.2 show the relationship between void ratio (e ) and vertical effective stress ( σ ' v ) for all the tests. It is found that the samples have fair repeatability although they were taken from different batches but prepared with similar procedure. 33

34 Void ratio G4OML01 G4OML Vertical effective stress (kpa) Fig. 4.1 Void ratio vs. log vertical effective stress for RMMC without shear wave velocity measurement 1.7 Void ratio G4OMLB03 G5OMLB01 G6OMLB01 G7OMLB Vertical effective stress (kpa) Fig. 4.2 Void ratio vs. log vertical effective stress for RMMC with shear wave velocity measurement The Logarithm-of-Time curve fitting method (Casagrande and Fadum, 1940) was found to be more reliable in interpreting the data and was used instead of the Square- Root-Time curve fitting method (Taylor, 1942). The relationship between coefficient of consolidation ( c v ) and vertical effective stress ( σ ' v ) is plotted in Fig. 4.3 and Fig It is found that the variations of c v obtained from the specimens with bender elements and without bender elements are almost the same, which means that the equipped bender elements have little effect on the consolidation process of specimens. The maximum value of c v is about 0.12 cm 2 /min which occurs in the OC stage. The values of c v in the OC stage are much larger than that in the NC stage and increase with 34

35 vertical loading stress after which is larger than the pre-consolidation pressure. It varies in the range of 0.02 to 0.05 cm 2 /min in the NC stage. Coefficient of consolidation(cm 2 /min) G4OML01 G4OML Vertical effective stress (kpa) Fig. 4.3 Coefficient of consolidation vs. log effective stress for RMMC without shear wave velocity measurement Coefficient of consolidation (cm 2 /min) G4OMLB03 G5OMLB01 G6OMLB01 G7OMLB Vertical effective stress(kpa) Fig. 4.4 Coefficient of consolidation vs. log effective stress for RMMC with shear wave velocity measurement The deformation-stress (void ratio-applied stress) and density-permeability (void ratio-hydraulic permeability) relationships are the two basic ones needed for consolidation analysis. Oedometer test is also a method to determine the coefficient of permeability ( k v ) indirectly based on Terzaghi s theory as shown below: k = (4-1) m cγ v v v w 35

36 Where: m v =coefficient of volume change for vertical loading; γ w =unit weight of water; c v =coefficient of consolidation. Fig. 4.5 and Fig. 4.6 show log v k plotted against log linear log-log relationship. The variation of log k v vs. log form of Eq.4-2 (Lambe and Whitman, 1979). σ which suggest an almost ' v σ can be expressed in the ' v k v Aσ = (4-2) ' B v Where: A and B =constants; k v =coefficient of permeability in vertical direction and σ ' v =vertical effective stress. Coefficient of pemeability (cm/min) E-04 G4OML01 G4OML02 1.E-05 1.E-06 1.E-07 Vertical effective stress (kpa) Fig. 4.5 Coefficient of permeability vs. vertical effective stress for RMMC without shear wave velocity measurement The parameters A and B can be determined based on the test results (shown in Fig. 4.5 and Fig. 4.6). Using the results of oedometer tests without shear wave velocity 36

37 measurement as shown in Fig. 4.5, Eq.4-3 was obtained with coefficient of correlation 2 R = k v = 9 10 ( σ ' ) (4-3) v Coefficient of permeability (cm/min) 1.E-04 1.E-05 1.E-06 Vertical effective stress (kpa) G4OMLB03 G5OMLB01 G6OMLB01 G7OMLB01 1.E-07 Fig. 4.6 Coefficient of permeability vs. vertical effective stress for RMMC with shear wave velocity measurement For samples with shear wave velocity measurement, there are some scatters at low stress condition as shown in Fig. 4.6, but the value of coefficient of permeability ( k v ) converged when the loading stress is large enough. After taking all the test results in Fig. 4.6 into account, the values of k v is correlated to vertical effective stress ( σ ' v ) as given by Eq.4-4 with coefficient of correlation 2 R = k v = 1 10 ( σ ') (4-4) v The relationship between void ratio (e ) and vertical effective stress ( stage for clay can be described below: σ ' v ) in NC e= e C log σ ' (4-5) i σ ' = 1 c v v 37