Determination of Mechanical Properties of Soils Based on CPTU Data

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 E_oed higher than 5 MPa (E_oed > 5 MPa) and an effective friction angle φ’ higher than 30° (φ’ > 30°).

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 (q_c), sleeve friction (f_s) and pore water pressure (u₂) registered during the CPTU test or other parameters derived based on them.

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.

Figure 1.

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 K₀ and over-consolidation ratio OCR of coarse-grained (sandy) soils based on CPTU data is characterized by low reliability. However, the applicability and reliability of the correlations for deriving undrained shear strength s_u and OCR ratio of fine-grained (clayey) soils are high. Relationships used to estimate the constrained modulus E_oed and effective friction angle φ’ are characterized by moderate reliability [15].

Figure 3 shows the cone resistance q_c, sleeve friction f_s and pore water pressure u₂ measured in February and June 2014. The results obtained at test points S1–S12 have been averaged. Figure 4 shows the normalized cone resistance Q_t, undrained shear strength s_u, effective cohesion c’ and overconsolidation ratio OCR for a typical location (S7) for February and June 2014 tests.

Figure 3.

Figure 4.

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 q_c values obtained from the February and June 2014 tests are similar and, at some depths, nearly the same. However, a significant difference between the averaged results of the February and June 2014 tests can be observed in the sleeve friction to a depth of approximately 2 m below the ground surface. The difference is also seen in the distribution of the f_s values at depth. As observed in Figure 3, the averaged pore water pressure to a depth of 6 m measured in February and June is similar. In contrast, the u₂ values at a depth of approximately 11.5 m below the ground surface determined in June are three times higher than that measured in February. The difference could be caused by difficulties and problems in determining and interpreting pore water pressure measurements [28]. However, the differences may have been caused by different specifications of the used equipment and differences in the experience of the penetrometer operators, including imprecise determination of the test locations.

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.

4.1.

Constrained modulus

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 E_oed calculations were conducted based on Equation (2) for a general value of α_n coefficient α_n = 5 proposed by Mayne [12, 13]. Due to the various thicknesses of the soft soil layer along the section under consideration, the length of the GEC columns was also variable, ranging from 3.7 to 9.9 m. Therefore, the depth of the column base at each location is also shown in Figure 5.

Figure 5.

Based on Figure 5, it can be generally concluded that the constrained modulus increases with depth. A noticeable difference can be observed between the E_oed values calculated based on the February and June 2014 tests, especially at locations S4, S11 and S12. However, at points: S3 and S5–S9, the E_oed parameter is nearly the same for both tests at the whole depth. The difference between the constrained modulus calculated based on February and June tests at a depth of column base varies from 1% at test point S5 to 46% at point S12. However, the average difference considering all locations equals approximately 20%. It can be observed in Figure 5 that the constrained modulus calculated based on the February data at a depth of the GEC column base exceeds the EBGEO recommended value of 5 MPa at eight locations (S2, S4–S7, S10–S12). Considering the June research, only four test points (S3, S5, S10, S11) meet the EBGEO requirements. Therefore, it implies that the GEC columns are too short in most locations, which may affect the excessive settlements.

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.

4.2.

Friction angle

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.

Figure 6.

As seen in Figure 6, a large spread of R_f and B_q values is noticeable without any evident dependence on depth. Therefore, it assumed the constant value of the attraction parameter. The average friction ratio calculated based on the February and June 2014 data is similar and equals 2.35% and 1.85%, respectively. The average values of the pore pressure ratio are 0.023 for February tests and 0.113 for June tests. The R_f parameter is lower than that suggested for silts and clays by Tschuschke [27]. However, the pore pressure ratio corresponds to B_q values characteristic of silt. Therefore, the value of the attraction parameter was assumed to be 30 kPa. The obtained results of the effective friction angle in locations S1–S12 are presented in Figure 7.

Figure 7.

Generally, the same dependence of the effective friction angle on depth can be observed for all tests. Relatively high values of φ’ parameter at shallow depths can be related to the low excess pore water pressure Δu, affecting the low pore pressure ratio B_q.

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 φ’ values significantly differ from the required value of 30°. According to February 2014 tests only at S4 and S11 locations, the length of the columns is appropriate. Whereas for the June tests, these are points S4 and S5. The inappropriate length of GEC columns can result in bearing capacity failure.

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.