In the last two decades, the use of a prefabricated vertical drainage known as PVD has been recognized as a very effective and efficient soil improvement method in preloading sites composed of soft soil deposits [1, 2]. Smear zone is a disturbed zone in the ground developed during PVD installation using a mandrel. The insertion of the mandrel disturbs the soil around the PVD area to a certain extent and consequently reduces the permeability of the soil k in this area. The permeability reduction in turn inhibits significantly the horizontal consolidation of the soft soil, known as the smear effects which eventually degrade the effectiveness of the PVD [3, 4]. Due to their significant impact, many researchers have been interested in examining the smear zone characteristics [5-11].

There are two propositions to delineate the smear zone, i.e. the two-zone and the three-zone hypotheses as presented in Fig. (1). The two-zone hypothesis, described among others by Bergado et al., (1991) [6] and Chai and Miura (1999) [12], divides the ground around the PVD into the smear zone and the undisturbed zone. The three-zone hypothesis, proposed among others by Sharma and Xiao (2000) [3], divides the ground around the PVD into 3 zones, i.e. the smear zone, the transition zone, and the undisturbed zone.

Fig. (1). Schematic figures of a 2-zone theory (a), a 3-zone theory (b) a 2-zone cross section A-A (c), and a 3-zone cross section B–B (d)

The main parameters proposed for characterizing the smear zone are the extent ratio and the permeability ratio. The extent ratio s or s’ is the ratio of the radius of the smear zone r_s to the radius of the drain r_w or the radius of mandrel r_m, respectively. Hence s = r_s/r_w or s’ = r_s/r_m represents the range of disturbance due to the penetration of the mandrel on the treated ground. The permeability ratio κ is the ratio of the horizontal permeability k_h at the undisturbed location to that at the disturbed site k_s. Hence κ = k_h/k_s represents the degree of disturbance due to penetration of the mandrel on the soil permeability. These ratios become the 2 key values in PVD planning which will in turn affect the rate and duration of consolidation. The values can be estimated using analytical technique, field data back calculation (or numerical method), laboratory testing, or field testing.

A comprehensive standard method for determining them has not yet been established. Field data back calculation (or numerical method) and laboratory testing are among the more frequent applied methods. Researchers who characterized the smear zone with back-calculation of field data did simple curve fitting in their analysis [6-10]. Several other researches exercised field data back calculation coupled with finite element analysis [11-19], however, field investigation combined with laboratory test could also be conducted for the same purpose [20]. Alternatively investigators performed laboratory testing to obtain the characteristic values [3, 6, 9, 21-34]. Those whose results are compared in this study can be seen in column 2 in Tables 1 and 2, for field data and laboratory testings consecutively.

Due to the limitation of the soil lab model, the s’ and κ values resulting from laboratory testing results can not be directly used for planning without proper insights. This study aims to document and evaluate what components in the field that are extremely difficult to model in the laboratory and what disturbances may occur on the ground when the PVD works. This study includes historical data of a trial embankment for an illustrative purpose. It is hoped that the review made in this study will illuminate the subject on the smear zone especially in terms of field data and laboratory testing so that further research and advances in PVD technology may be assisted.

The field work starts usually from land clearing and stripping. These are followed by spreading the geotextile separator layer and the sand blanket layer on top of the separator layer, and then the sand blanket can be compacted. Subsequently, installation of geotechnical instruments, including pneumatometer piezometer, water standpipe, settlement plate, and inclinometer, is made. Lastly, PVD installation and preloading can be conducted.

The factors that may cause different disturbances to the soil during the PVD work include depth of installation, thickness of sand mat, patterns of installation, PVD interval, mandrel shape and dimension, height of embankment, and speed of installation. These factors vary between locations which are indicated in columns 3 up to 10 in Table 1, and their influences on the smear zone characteristics are discussed later in this paper.

As previously mentioned, the characteristics of the smear zone are normally estimated by the back calculation method using the data resulting from the geotechnical instrument. The estimation is conducted based on the obtained time dependent settlement data which are then fitted with Eq. 1 or processed with a numerical program. However, Bo et al., (2001) [39] did field testing to determine the values of s’ and κ using field apparatus. Table 1 summarizes 16 data set on soil improvement conditions from various locations using PVD. Note in the table that the extent ratio and the permeability ratio are given in the last two columns.

Tests in the lab are conducted without referential standards. Nevertheless, the shapes of laboratory soil tanks used are typically cylinders and boxes. Previous researchers had different h/d ratios from one and another as shown in column 3 in Table 2. Note that h and d are the height and the diameter of the cylindrical soil tank, consecutively. In contrast, Tran-Nguyen and Edil (2011) [26] developed the box-shape tank which then further utilized by Sengul, et al. (2016) [33]. Before placed in the soil tank, the sample is tested for the basic soil properties including moisture content w, liquid limit L_L, plastic limit P_L, void ratio e, spesific gravity G_s, unit weight soil γ, compression index c_c, and swelling index c_s.

The soil sample to be tested in the soil tank is divided into two types, i.e. disturbed and undisturbed. Because it is difficult and costly to make a sample that is not disturbed in a large scale, most laboratory tests use disturbed soil samples, employed among others by Sharma and Xiao (2000) [3], Chai et al., (2013) [28], and Sengul et al., (2016) [33]. On the other hand Rujikiatkamjorn et al., (2014) [29] used a relatively large of undisturbed sample with the aim of capturing more realistic smear zone characteristics within the intact soil structure, whereas Indraratna et al., (2015) [30] used a comparatively small, undisturbed sample directly from the field for oedometer testing. See Table 2 for more complete conditions and results of 17 previous laboratory studies on the smear zone.

The testing normally starts from the sample preparation stage. The sample preparation is carried out by mixing the soil with water until saturated, putting it into a consolidometer cell, extracting trapped air, and providing a pre-consolidated compression. This stage is followed by the PVD or sand drained (SD) installation. The insertion of PVD (or SD) utilized a round or rectangular mandrel with a size smaller than the actual size. Also, the insertion speed in the laboratory is usually smaller than that in the field. Finally the consolidation stress representing the preloading is given. Based on the results of consolidation happening in the lab, the determination of the smear zone characteristics can be done. Columns 10-11 in Table 2 shows the results of the 17 previous laboratory on the characteristics of the smear zone composed mainly of soft soils.

With reference to Tables 1 and 2, the extent ratio s’ in the smear zone varies between 2.0-5.0 in the field using back calculation and 2.0-6.3 in the lab using laboratory apparatus. On the other side the permeability ratio κ varies between 2.0-13.8 in the field using back calculation and 1.03-3.13 in the lab using laboratory apparatus.

It appears that the extent ratio s’ resulting from both methods, field data back calculation and laboratory testing, yields comparable range. On the other hand the permeability ratio κ resulting from the field data back calculation reveals a noticeably greater range if compared with that resulting from laboratory testing. The greater range of κ values in the field indicates that the level of disturbance in the ground varies greatly between locations. Also, the discrepancy in κ ranges implies that the laboratory testing can hardly model the high level disturbance of field conditions.

The position of the case study is at the location of the multi purpose container yard in Kuala Tanjung Port which is is located on the east coast of North Sumatra, Indonesia, fronting the Strait of Malacca. Fig. (2) shows the 13.5 ha of the embankment area of the case study site taken during preloading in June 2017.

Fig. (2). Photograph of the embankment area of the case study.

The soil investigation was carried out using a machine drill, SPT, undisturbed and disturb samplings, DCP, and laboratory testing. From the results of the soil investigation it is known that the basic soil layer is composed of soft clay and very soft clay as thick as 10 m, with SPT ≤ 2. This clay layer sits above a sandy clay layer having a medium consistency of 7 m thick with 4≤SPT≤5. This sandy clay layer is located above a dense sand layer of 3 m thick with 10≤SPT≤30, below which a very dense sand layer of 10 m thick rests with SPT≥60.

A geotextile separator is used over the 13.5 ha of the embankment area to separate the base ground with the embankment material. The data related to the PVD work in the case study site can be seen in the last row in Table 1. The geotechnical instruments used for the analysis encompassed 7 piezometers, 7 water standpipes, 44 settlement plates, and 8 inclinometers. The total height of the embankment, which has an average of 5.2 m, was achieved gradually in 14 stages as indicated in Fig. (3).

Fig. (3). Construction stages at the trial embankment location.

The consolidation induced settlement is calculated using the 1-D consolidation with the load distribution following the 2V:1H ratio and c_h=1,35 c_v.The degree of consolidation of the embankment is estimated by taking the smear zone effect into account and ignoring the well resistance.

The characteristics of the smear zone are determined through the back-calculation method using time dependent settlement data obtained from the case study site (Fig. 4). For the case of constant permeability in the smear zone, the settlement data are fitted with Eq. (1) using the parameter m of Eq. (4). For the case of parabolic permeability in the smear zone, they are fitted with Eq. (1) using the parameter m given by Eq. (5). Lastly, for linier permeability in the smear zone, they are fitted with Eq. (1) using the paramater m given by Eq. (6).

Fig. (4). Time-settlement relationships of field data and back calculation results.

Plotted against the field settlement data in Fig. (4), the results of the consolidation for the three permeabilities are obtained using the value of s’ = 3 and κ = 7. If the results in the figure are checked in detail, it can be seen that the discrepancies appear to be significant on days 85 up 135. However, the largest discrepancy is found to be with the parabolic permeability, followed by the linear and the constant permeabilities in a decreasing order. It is noteworthy that the extent ratio s’ = d_s/d_m= 3 is the value suggested by Chai and Miura (1999) [12] for locations with no test data. Meanwhile the value for permeability ratio κ=(k_h/k_s)=7 can be considered as a high representative value that reflects the level of disturbance in the soil during PVD installation.

In this study we have documented and evaluated the values of smear zone parameters, namely the extent ratio s’ and the permeability ratio κ, resulting from field investigation and laboratory testing. The assessment on the causes of the variation of the values of the smear zone parameters is made particularly with relation to the different conditions in the field and in the laboratory. A case study is given here to illustrate the procedure to estimate the values of both parameters by field data back calculation. The concluding findings can be summarized as follows.

Smear zone parameter values are quite different in accordance with their locations. With reference to the results of the previous studies it is found that the extent ratio s’ values vary between 2.0 - 5.0 and between 2.0 - 6.3 based on the back calculation and laborary testing, consecutively. On the other hand, the permeability ratio κ values based on the field data back calculation range from 2.0 to 13.8 which are quite large if compared with the κ range from 1.03 to 3.13 based on laboratory testing.

Three causes of the s’ and κ variations are suggested here. First, the laboratory testing is unable to model the complexity of field conditions. Second, there are no standards established to be used as references in the laborary testing. Third, the level of disturbance in the field is much greater than that in the laboratory testing and differs at each research location.

If fund is permitted, it is recommended that a trial embankment be applied using the same methods, materials and equipment that will be used on the actual embankment. Then smear zone parameter values will likely be obtained with a better accuracy for the real application of PVD.