CONSOLIDATION CHARACTERISTICS OF A SILTY CLAY: CONCERNING THE EFFECT OF SOIL DISTURBANCE

In  this  paper,  undisturbed  specimen  of  a  silty  clay  constituting  of  Phu  Bai formation  (ambQ21‐2 pb) was  collected  from boreholes  in Hue  city and  surrounding areas. The soil, under both undisturbed and disturbed conditions, was then subjected to standard one‐dimensional  consolidation  tests  with  7  loading  increments.  It  is  shown  from  the experimental results that the time to the end of primary consolidation (EOP), determined by Log Time method (tLT) and 3‐t method (t3T), decreases with the load increment and under the same vertical stress, the primary consolidation of disturbed silty clay finish at a shorter time than those of the undisturbed one. The coefficient of secondary consolidation (Cα) increases with  the vertical  stress and  reaches  the maximum values before decreasing. The obtained values of Cα = 0.005  ‐ 0.020  suggest a  relatively  low  secondary compressibility of  the  silty clay constituting of Phu Bai formation.


Introduction
As an important aspect in the field of soil mechanics, consolidation characteristics of clays under external load, both static and cyclic patterns, have been studied extensively for a long history. Consequently, the effects of different loading conditions such as loading duration, frequency, intensity and pattern on the consolidation characteristics of soils have been clarified [1]. In which, secondary consolidation due to re-arrangement of soil particle after the dissipation of pore water pressure shows more difficulty to observe and therefore, problems related to the magnitude and the rate of such a settlement component are still insufficiently understand [2], meanwhile the secondary compression is considered as a main factor leading to the difference in the compressibility of clayey soils induced by static and cyclic loading [1].
Recently, by using oedometer test under both standard consolidation and in shear box of the cyclic simple shear test apparatus, the differences in the consolidation characteristics have been clarified for undisturbed and disturbed clays at different Atterberger's limits with and without cyclic loading history [3,4].
Organosedimentary materials are considered as disabsentable components within Quaternary stratigraphy of delta-coastal plains through Vietnam. In Thua Thien Hue province, dark coloured and weak organosedimentary soils are named as Phu Bai formation (ambQ2 1-2 pb) with their specific characteristics including high organic content, low bearing capacity and shear resistance, high compressibility. Moreover, besides spreading nearly continuously, Phu Bai formation also has its own very changeable thickness and distributing depth. In Hue city and surrounding areas, the soils are stratified close to ground surface and therefore causing adverse effects on buildings' foundation stability, historical monuments' lifetime as well as reduced technical and the economic efficiencies of construction in the areas [5][6][7][8]. Obtaining undisturbed specimen of such soft soils is always challenge and even under the undisturbed condition, the initial soil structure and skeleton are commonly affected during the sampling.
Therefore, an observation of the effect of the soil disturbance on the geotechnical properties in general and on the consolidation characteristics specifically is important for foundation design on soft ground.
Because of the most common component constituting of Phu Bai formation [5], the silty clay (hereinafter called as Phu Bai silty clay) is therefore used in this study. The soil was subjected to one-dimensional standard consolidation tests with 7 loading increments ranging

Experimental aspects
In order to prepare testing specimens, the Phu Bai silty clay was taken from 6 boreholes at two construction projects in Hue city and used for this study. Index properties of tested soil are shown in Table 1.  Firstly, soil samples from boreholes were collected, transported and restored following Vietnamese Standard [9]. The sample was then trimmed and used for oedometer test (Fig. 1). In this study, the one-dimensional consolidation test was carried out under Japanese Standard (JIS A 1210) [10] and under this standard, 7 load increments ('v) were applied (from 'v = 0.125 kgf/cm 2 to 'v = 16.0 kgf/cm 2 ) and the load increment ratio was set as unity ('v/'v = 1) with the loading interval of 24 hours [10]. Situations of the consolidation test at different stages are shown in Fig. 2. In order to prepare disturbed specimen, disturbed soil was made directly from the undisturbed one by mixing with de-aired water to form slurry having water content of about 1.5wL. The slurry was kept for one day under the constant water content and was then poured into the double-specimen ring which was designed for preparing disturbed specimen (Fig. 3a).
The slurry was then subjected to the load increments of 'v = 0.03, 0.06, 0.125 and 0.25 kgf/cm 2 with the duration of 24 hours for each increment. Thereafter, soil specimen was released from the load (Fig. 3b) and then used for standard consolidation tests (Fig. 3c). Procedures and the performance of standard consolidation test on disturbed specimen are similar to those used for undisturbed one.

Time to EOP of undisturbed and disturbed Phu Bai silty clay
Based on the settlement measurement with time, the vertical settlement in strain (v, %) is plotted against elapsed time (t, minute) in Fig. 4 for undisturbed and disturbed Phu Bai silty clay subjected to different load increments. In order to clarifying the consolidation characteristics, undisturbed and disturbed specimens should be subjected to the same load increment as seen on the undisturbed specimen in Fig. 4. On the other hand, the fisrt load increment of the consolidation test should be smaller than the pre-consolidation pressure of soil specimen and therefore, the first load of σ'v = 0.125 kgf/cm 2 should be applied to the disturbed one. The results indicate that the settlement increases with elapsed time regardless of the vertical stress and that, such a tendency is more unique on undisturbed specimen which shows slightly smaller total settlement after the final loading increment (16.0 kgf/cm 2 ). It is known that cementing materials originated during the sedimentation affect the properties of clayey soils and in nature, organic matter partly constitutes the representative cementing materials of the soil [1]. The compressibility of clays is therefore influenced by organic matter and the compression index (Cc) of organic clayey soils has been confirmed to be in proportional to the organic matter content [11]. Silty clay like the soil used in this study with high sand content usually shows relatively high permeability resulting in a relatively short time for pore water dissipation or to EOP. Meanwhile the settlements in Fig. 4 continue to increase within 24 hours suggesting that the time to EOP should be determined so that the secondary settlement could be observed for each load increment. Undisturbed specimen Disturbed specimen In order to estimate the time to EOP of the soil, the Log Time method [12] which is practically applied to estimate the settlement of clay layers and the 3-t method which is commonly applied for laboratory investigation [10,13]. The detail of these two methods and typical applications on different clays have been shown in our previous research [3,14].
Obtained results of the time to EOP determined by using Log Time method (tLT) are shown in Fig. 6 for undisturbed and disturbed Phu Bai silty clay. Despite of several scattering, the results indicate that tLT generally decreases with the vertical stress and that, under the same load increment, undisturbed specimen shows larger value of tLT than those on disturbed one. It is known that the rate of consolidation depend on the ability of water to flow through the soil matrix and with undisturbed condition, cohesion and the particle bond tend to prevent the movement of pore water reducing the consolidation rate compared with those of disturbed one. Based on a comparison of tLT with the time for completed dissipation of the pore water pressure which is a standard measurement of EOP, it is shown in the most cases that, tLT is considerably smaller and in laboratory where the primary consolidation should be confirmed for next experiment, the 3-t method is considered as an effective solution [13]. In Fig. 7, results of the time to EOP determined by 3-t method (symbolized as t3T) are shown for each load increment on undisturbed and disturbed specimens. Similar to the tendency in Fig. 6, t3T decreases with 'v regardless of the disturbance of soil structure and that, undisturbed specimens show larger values of t3T than those of the disturbed ones. When comparing the results between Fig. 6 and Fig. 7, t3T is evidently larger than tLT, then the ratio of t3T to tLT (t3TtLT) is plotted against 'v by symbol in Fig. 8. Despite of two scattering values (t3TtLT = 9.1 and 23.7), for the silty clay used in this study, t3T is at 3 to 7 times higher than tLT for undisturbed specimen and from 3.3 to 8.1 times for the disturbed specimen.

Coefficient of secondary consolidation of Phu Bai silty clay
By measuring the settlement through the secondary consolidation, the coefficient of secondary consolidation in terms of void ratio (Cα) can be determined as schematically shown in Fig. 9. In this figure, Cα is the slope of the curve of the void ration (e) versus logarithm of time (t) after the primary consolidation and defined by Eq. (1) as followed: where e1 and e2 are the void ratio corresponding to time t1 and t2 in the secondary consolidation stage (Fig. 9).
Relationships between C and 'v are shown in Fig. 10 for undisturbed and disturbed Phu Bai silty clay. It is seen that C firstly increases with 'v and reaches the maximum values of about C = 0.013 and C = 0.020, corresponding to the vertical stress of about 'v = 1 -2 kgf/cm 2 and 'v = 2 -3 kgf/cm 2 for disturbed and undisturbed specimens, respectively. Secondly, C decreases to smaller values. The values of C = 0.005 -0.020 suggest the low secondary compressibility of the silty clay constituting of Phu Bai formation. By carrying out a series of one-dimensional consolidation tests on an organic clay, Mesri [15] indicated that at small consolidation pressure, remolded specimens with a more dispersed fabric show smaller value of C compared with those of the undisturbed ones. However, C of remoulding specimens increases with 'v to a maximum value and then decreases and finally approaches to those of undisturbed specimens at high consolidation pressure. Meanwhile in Fig. 10, when 'v ≤ 1.0 kgf/cm 2 , the differences of C between undisturbed and disturbed specimens are unclear due to several scattering on the results. For higher consolidation stress, better results are obtained indicating the lower secondary compressibility of disturbed specimen than those of undisturbed one and such discrepancies seem to decrease with the increase in the vertical stress.

Conclusions
The main conclusions are as follows: 1. The silty clay constituting of Phu Bai formation (ambQ2 1-2 pb) under undisturbed condition shows slightly smaller settlements than those of the disturbed one, meanwhile the time to EOP of the former condition is longer than those of the latter.
2. The time to EOP, determined by both 3-t method (t3T) and Log Time method (tLT), decreases with the vertical stress regardless of the disturbance of soil structure. Ratio of t3T to tLT changes from t3TtLT = 3 to t3TtLT = 7 for undisturbed specimen and from t3TtLT = 3.3 to t3TtLT = 8.1 for the disturbed one.
3. The silty clay constituting of Phu Bai formation shows a relatively low secondary compressibility with the coefficient of secondary consolidation in terms of void ratio (Cα) is about C = 0.005 -0.020. In addition, the coefficient of secondary consolidation of the soil firstly increases with the vertical stress and reaches maximum values at 'v = 1 -2 kgf/cm 2 and 'v = 2 -3 kgf/cm 2 for disturbed and undisturbed specimens, respectively. Thereafter, C decreases as the vertical stress increases.