Due to the rapid development of civil engineering, especially road engineering, it is increasingly inevitable to use the areas characterized by problematic soil and water conditions. Therefore, structures located in such areas require shallow and deep foundations or soil improvement. The methods of soft soil improvement have been classified in many different ways. Chu et al. [1] divided them into five groups: ground improvement without admixtures in non-cohesive soils or fill materials, ground improvement without admixtures in cohesive soils, ground improvement with admixtures or inclusions, ground improvement with grouting type admixtures and earth reinforcement. The selection of the technique of soil improvement mainly depends on the soil type, area, depth and treatment required, soil properties, availability of materials, availability of skills, local experience and local preferences, environmental concerns and economics [2,3,4].
One of the improvement methods classified by Chu et al. [1] as ground improvement with admixtures or inclusions technique is the use of geosynthetic encased columns (GEC). GEC columns are designed to reinforce organic soils and very soft cohesive soils in which other technologies are inapplicable. Columns are constructed of a seamless geosynthetic cylindrical reinforcement sleeve, usually sand-filled. Geosynthetic encased columns can be performed by displacement or excavation methods. The displacement method improves the subsoil even before the load is applied. The reason for this phenomenon is a reduced soil porosity resulting from installing a closed-cast pipe. The effect of the GEC column implementation technique on its performance was presented by Meyer et al. [5]. The choice of the GEC installation method depends on the soil conditions and area characteristics and affects the surrounding soil-column interaction [5].
The guidelines for GEC column designing are contained in Recommendations for Design and Analysis of Earth Structures using Geosynthetic Reinforcements – EBGEO [6]. The EBGEO include detailed information on soft strata surrounding the GEC column and characteristics of the column, such as its minimum diameter, minimum and maximum length, fill and geosynthetic casing. In addition, according to EBGEO, the load-bearing soil deposited below the GEC column base should be characterized by the constrained modulus
The soft strata and load-bearing soil should be tested in detail. The precise identification of the subsoil and its parameters determine the correct reinforcement design, including establishing the column length. The installation of the geosynthetic encased column base in soft soils can be related to excessive settlements and bearing capacity failure at the construction, initial consolidation or exploitation stages. No clear design guidelines for reinforcement using floating GEC columns are available. However, they are frequently performed for economic and construction reasons.
The parameters of improved subsoil should be determined using various methods, including field and laboratory tests. However, the detailed investigation mainly relates to very soft soils characterized by variable properties even within one deposit and their ability to change properties with time [7, 8]. In practice, soil tests are usually limited to macroscopic analysis and basic soundings. Restricted investigations are caused by economic aspects and reduction of cost investment.
One of the most commonly used field investigation methods is the cone penetration test with pore pressure measurement (CPTU). The method has been successfully used for decades to determine physical and mechanical soil properties. However, properly determining geotechnical parameters based on CPTU data requires high-level skills, experience and knowledge. The most significant difficulty is the interpretation of the measurements obtained [9]. Nevertheless, a wide range of relationships for interpreting CPTU data is available in the literature [10,11,12,13,14,15]. According to those correlations, the geotechnical parameters depend on cone resistance (
The study aimed to determine the geotechnical properties of the subsoil related to the geosynthetic encased column installation. The constrained modulus and effective friction angle were established based on the CPTU data. The results were compared with the EBGEO requirements. The analysis mainly referred to the properties of load-bearing soil deposited below the GEC column base.
The subsoil under one of the sections of the bypass in north-eastern Poland was analyzed (Figure 1). The area under consideration was about 200 m long. Organic soils, mainly peat and mud with a variable thickness of approximately 3–8 m, were found in the considered subsoil. Organic soils were characterized by high variability of physical and mechanical properties and by very soft consistency. Below the organic soils, the glacial sediments in the form of very soft and soft cohesive soils with the local addition of organic soils were deposited. The water content and, therefore, the liquidity index of cohesive soils decreased with depth to a value corresponding to a firm consistency. The groundwater level was at a depth of 0.3 m to 1.2 m below the soil surface. Piezocone tests were performed at 12 points from S1 to S12 to a depth of about 12 m. The test points were located every 15 m along the section under consideration. The geotechnical cross-section of the analyzed subsoil and CPTU test locations are shown in Figure 2.
The geosynthetic encased columns with a diameter of 0.8 m were designed to improve the soft soils deposited in the analyzed area. The columns were spaced in a triangle grid with a dimension of 1.97 m, corresponding to an area ratio of 15%. The displacement method was used for the GEC columns installation. According to the design, the column base should be at least 0.5 m in load-bearing soil characterized by the constrained modulus higher than 5 MPa.
Piezocone tests were carried out at the design stage in February 2014 and were repeated in June 2014 during the construction process by another company. A second series of CPTU tests were performed ten days after the installation of approximately 20 GEC columns near the S11 and S12 locations. The re-investigation was related to problems after the partial GEC columns installation. The problems were related to the excessive settlement of improved subsoil.
Many correlations have been developed to determine physical and mechanical soil properties based on CPTU test results. These correlations apply to a wide range of soil types and vary in their applicability and reliability. Robertson and Cabal [15] presented a five-steps scale to assess the relevance of CPTU for deriving soil parameters. The researchers found that the determination of the in-situ stress ratio
The constrained modulus is one of the parameters characterizing soil compressibility. Its value varies with the effective stress, soil type and soil structure [17]. In the CPTU interpretations, the constrained modulus of various soil types may be described by the following expression [11, 18, 19]:
Sanglerat [19] reported that
The constrained modulus may also be expressed as a function of the net cone resistance
Generally,
For fine-grained soils,
According to Senneset et al. [21], semiempirical relationships using CPTU data provide a relatively good prediction of the constrained modulus of clayey soils.
The determination of effective strength parameters of cohesive soils from CPTU data is characterized by low accuracy [24]. However, the method for establishing the effective friction angle based on CPTU parameters considered the most accurate is the method presented by Senneset et al. [25] and supplemented by Sandven et al. [26]. The equation suggested by the researchers is as follows:
Tschuschke's approach [27] can simplify the selection of a parameter when only CPTU data are available. The researcher presented the approximate friction ratios
According to Mayne [12], the effective friction angle of fine-grained soils can be obtained from the equation below.
Equation (5) is applicable only for the following ranges of parameters: 20° ≤
Figure 3 shows the cone resistance
It can be observed in Figure 3 that the average cone resistance increases with depth, which is directly related to the occurrence of organic soils and the decreasing liquidity index of the cohesive soils deposited below the organic soil layer. The
It can be seen in Figure 4 that similar values of undrained shear strength and effective cohesion were obtained based on the February and June tests. The normalized cone resistance and overconsolidation ratio differ depending on the test date, reaching lower values for the June 2014 tests.
The values of the constrained modulus obtained at test points S1–S12 for the February and June 2014 tests are presented in Figure 5 as a function of depth. The
Based on Figure 5, it can be generally concluded that the constrained modulus increases with depth. A noticeable difference can be observed between the
The EBGEO recommendations do not include specific requirements for the depth of soil testing below the base of the GEC column. However, as Figure 5 shows, the depth of CPTU tests is too shallow in some locations, reaching the bottom of the column or slightly deeper. It mainly applies to points S6, S8 and S11.
Equation (4) was used to determine the effective friction angle of the analyzed soils. The attraction parameter a was established based on the friction and pore pressure ratios shown in Figure 6.
As seen in Figure 6, a large spread of
Generally, the same dependence of the effective friction angle on depth can be observed for all tests. Relatively high values of
In most locations, the February and June results are similar. The average difference between the effective friction angle of soil at a depth of the column base for both investigations considering all locations equals approximately 12%.
As shown in Figure 7, even for the maximum value of the attraction parameter assumed for silts, the effective friction angle of soil deposited below the GEC column base does not meet the EBGEO requirements at most locations and
Based on Figures 5 and 7, it can be concluded that the minimum value of the constrained modulus of soil deposited below the GEC column base equal to 5 MPa is too low compared with the effective friction angle of 30°.
The following general conclusions can be derived from the performed analysis:
A noticeable difference can be observed between the February and June 2014 test results. Both investigations established that the constrained modulus and effective friction angle of the soil deposited below the GEC column base did not meet EBGEO requirements at most locations. EBGEO requirements should be completed with information on the minimum depth of soil testing under the geosynthetic encased column base. The minimum value of the constrained modulus of soil deposited below the GEC column base required by EBGEO is too low compared with the requested value of the effective friction angle. Determination of the constrained modulus and effective friction ratio based on CPTU data is possible. However, the results should be compared with the results of laboratory tests.