The investigations of the physical properties of tested soils carried out in a laboratory allowed to describe the geological structure of the Stegny’s subsoil (Fig. 3). Down to 4.3 m from the surface, there appear dark-yellow homogeneous medium and fine sands that exhibit alternate deposition in the vertical direction. Within these sands, the ground water level is at 3.2 m in depth. Beneath the sands, a complex of Tertiary clays may be found. Pliocene clays consist mostly of clays and silty clays, however there it must be noted that the clays overdominate the silty clays. It is the evidence of a mechanical variability of the clay complex yet on a small section of the profile, which may be seen in Fig. 3 [13].

The bed of clays reveals a clear stratiform structure characterised by different colouring of particular layers at the same time. The horizontal stratification of the clays indicates a natural arrangement of formations and lack of glacitectonical dislocations in the form of xenoliths. The investigated soils, in a firm state, transit into a stiff state along with the thickness. Four different layers of clays may be distinguished there. The shallowest are deposits of the dark-grey clay laid down on the flaming clay of a rusty-red colour. The typical element of the geological structure is an inserted pad of silty motley clay of yellow colour and thickness of 1.2 m. Deeper are motley clays with numerous bright yellowish stains, from which two layers were selected with respect to the differences in geotechnical parameters. The boundary separating these layers runs at the depth of 10.0-10.5 m.

LABORATORY TESTING

In most countries, including Poland, the base for a classification of soils are suitably selected and realised identification researches. For this purpose, the mechanical composition of soils is determined by sedimentation methods. The water content, Atterberg’s limits and related dependencies are very useful in the identification and classification of soils. Additionally, as an auxiliary test, the determination of the soil and dry soil density is realised. Such tests were carried out on disturbed and undisturbed samples at the Geoengineering Department of Warsaw Agricultural University (SGGW) in 1999–2000. Soil samples were taken from two boreholes with seven different depths from 5 to 11.5 m, distanced by 1 m. The results of the investigation are shown in Tab. 1.

The consistency limits: liquid w_L and plastic w_p ones were examined simultaneously on the same material taken from 15 depths of the geological profile. The results of the investigations are arithmetic averages of 2 tests for the limit w_p, and – in the case of the limit w_L, – being determined by Casagrande’s method with 6-9 tests repetitions.

The water content w_n in cohesive soils was estimated by use of the classical oven-balance method. The samples have been analysed within a couple of hours since the sampling-moment in field. The fast examination eliminated the possible changes of the water content inside samples, resulted by the process of decompression and the influence of atmospheric factors. Totally, there were 150 measurements that gave 34 results in the form of mean values from 5 tests on the average. Taking into account the water content and Attenberg’s limits, the liquidity index I_L and consistency index I_C were calculated. The results are shown in Fig. 4.

The in situ tests concern to relatively the same depths, where the sampling for the laboratory tests had been carried out. To recognize a subsoil structure, standard Marchetti’s dilatometer tests (DMT) were carried out in 7 profiles. The results of the laboratory measurement of the liquidity index had been accepted as a model, referring to the in situ tests.

FIELD TESTING

The flat dilatometer test (DMT) was developed by Prof. Silvano Marchetti in the middle of 1970’s [7]. The construction of the dilatometer, which is shown in Fig. 5, was applied in field testing. The crucial part of the Marchetti’s dilatometer is a steel blade, where an elastic membrane is mounted. This membrane is deformed by the pressure of the gas supplied there. The device is also equipped with a measuring-control unit, which enables to read the pressure-values.

A standard testing with the DMT consists of two phases, i.e. pushing the flat blade into a subsoil and carrying out a measurement. The dilatometer test are usually carried out every 0.2 m and the circular steel membrane is expanded horizontally while the pressure required for specific horizontal movement is recorded. After a measuring tip had been inserted into the subsoil, the values of a gas pressure by the three positions of a membrane may be measured. These values present a readout A, B and C.

The results of test are presented against depth and generally include three parameters: the material index (I_D), horizontal stress index (K_D) and dilatometer modulus (E_D). These parameters are being used by the description of the sort of a soil and its geotechnical parameters [15].

The basis of this interpretation is an empirical correlation between the dilatometer indices and the soil parameters being sought. The empirical correlations have been established to obtain proper estimation of the soil type basing on the material index (I_D). The similar correlations have also been worked out to describe relatively simple relationships between the horizontal stress index (K_D) and the coefficient of lateral earth pressure (K₀) as well as between the dilatometer modulus (E_D) and the compressibility modulus (M).

Since the obtained correlations are influenced by many factors, the derived relations should be verified and adopted to local conditions. To determine the model values of a given soil parameters, which serve to scale the dilatometer, another methods of research are necessary – both laboratory and field tests. The scaling may be carried out also in the calibration chambers. In the research of the liquidity index I_L of clayey deposits, the laboratory investigations of the water content w_n and consistency limits (w_p and w_L) turned out to be helpful [12].

The research of the state of Pliocene clays in Stegny allowed for the determination of empirical relationships, which describe the liquidity index. To find the form of the proposed relationships, the method of multiple step regression as well as the mean values of the dilatometer modulus E_D from 7 profiles of DMT have been used [15].

The results indicate the increasing trend of the dilatometric module E_D along with the thickness. As well, the scattered values of E_D increase. It disables the description of the state of soils solely by using the E_D modulus (Fig. 6). The horizontal stress index (K_D) is the other analysed parameter. It assumes the constant value K_D = 5.6 in the whole profile. Because of the lack of relationship between (I_L) and (K_D), the horizontal stress index has been omitted in the analysis of results. However, the material index (I_D) turned out to be an appropriate parameter. It shows a decreasing trend in the upper and lower zone of subsoil. Both parameters, E_D and I_D, are inversely to the thickness and the I_L index. This observation is confirmed by a one-factor regression. In the upper zone the modulus (E_D) dominates (R² = 0.77), while in the lower one – the I_D index (R² = 0.81).

ANALYSIS OF TEST RESULTS

From the analysis of mechanical composition of Pliocene clays, one concludes that a considerable decrease in the clay fraction is marked in the deposits while observing along the thickness. The silt fraction, however, increases what shows that two sedimentation cycles have taken place (Fig. 3). The first one is located at 4.3–10.0 m, the second – between 10.0 and 12.5 m. The character of the observed cycles indicates that the sedimentation took place in a water environment (bigger particles deposit first, then the finer ones). The exception is silty interbedding with 30-34% content of the clay fraction [12], [13].

The changes of mechanical composition in these two cycles correspond to variations of the water content w_n. The large similarity of changes characterizes also the liquidity index of clays. The changes of the state of these soils mainly result from the character of the water content.

There are two visible diminishing trends of the liquidity index. Stegny clays, being in a firm state, transit into a stiff from along with the thickness (Fig. 4). Consistency limits and the plasticity index I_p do not reflect the character of the variations of I_L index, because they do not describe the soil structure. They are determined on remoulded soil samples.

The comparison of the in situ tests and laboratory method gives the possibility of verifying the existing empirical formulas connecting the liquidity index I_L and the dilatometer modulus E_D. The performed analysis has shown very significant differences. Thus, it is necessary to adapt the relationship described above to the local conditions of Stegny site. The effect of this analysis is the elaboration of the new methodology of measurement of the liquidity index from DMT test.

The associate influence of the (E_D) modulus and (I_D) index on the liquidity index (I_L) is finally pronounced on the basis of the multiple linear regression. Both parameters (E_D and I_D), assume the normal distribution. The multiple linear correlation coefficients calculated for the both cases (in the upper and lower zone) are similar: r > 0.93. Both the first and the second parameter show the dispersion of the values. To describe a soil medium a ratio E_D/I_D has been introduced, which has a high coefficient of determination R² = 0.9 (Fig. 6). The proposed empirical formulas are completed in Tab. 2.

For elaborated formulas the high agreement of the results from the field and laboratory tests was observed. The coefficient of determination in every case is higher than R² = 0.8. The comparison of the results obtained in the dilatometer and laboratory tests is shown in Fig. 7.

CONCLUSION

For geotechnical engineers, who apply the theoretical rules in practice, the liquidity index is probably the most important parameter which rules the behaviour of clayey deposits. The consistency state of cohesive soils may be determined on the basis of the interpretation of dilatometer tests’ results supplied by the laboratory analysis of the liquidity index.

The results of site and laboratory investigations show that the proposed formulas describing the I_L index with the use of dilatometer tests readings give a high coincidence between these results and the empirical values in heavily overconsolidated Pliocene clays. The comparison of calculated and measured values confirms the credibility of the applied methodology of the evaluation of the state of clayey soils. The flat dilatometer tests (DMT) are the easy way to evaluate sought soil parameters in the place of its natural occurrence. Taking into consideration the results of laboratory and in situ tests, it may be concluded that the existing formulae should be adapted to the local conditions and the history of soil deposits.

The Pliocene clays exist at the significant area in Poland, taking up ca. ¾ of the state area. Those soils are lying at various depths under the blanket of quaternary deposits and appear over the surface in places. The Pliocene clays are non-homogenous and show anisotropy of stress, structure and the geotechnical parameters [4]. Taking all that into consideration, the proposed formulas may be valid only in the research of clays in the firm state with the clay fraction content f_i = 30 ÷ 80%. As the next stage, the methodology should be broadened by the new elaborated examples of Pliocene clays in stiff and very stiff state for E_D/I_D > 30 as well as soft and very soft state for E_D/I_D < 10. A verification and eventual re-scaling of the Marchetti’s dilatometer is necessary in order to adopt it to the local conditions of subsoil (for different Polish regions and other kind of fine-grained soils).

REFERENCES

1. Barański M., 2004. The selected properties of clay according to field tests. Spec. Symp. Pliocene clays of Warsaw, ITB Warsaw, 15-29.

2. Borowczyk M., 2004. The behaviour of Pliocene clays in foundation trenches. Spec. Symp. Pliocene clays of Warsaw, ITB Warsaw, 31-47.

3. European Standard EN ISO 14688, 2002. Geotechnical investigation and testing – Identification and classification of soil, Brussels.

4. Frankowski Z., 2004. The occurrence of the clays from the Poznań formation in Warsaw. Spec. Symp. Pliocene clays of Warsaw, ITB Warsaw, 5-13.

5. Kaczyński R., Grabowska-Olszewska B., Borowczyk M., Ruczyńska-Szenajch H., Krauzlis K., Trzciński J., Barański M., Gawriuczenko J., Wójcik E., 2000. Litogenesis, microstructures and geological-ingineering properties of pliocene clays from Warsaw region. Project KBN nr 9T12B00616.

6. Kaczyński R., 2003. Overconsolidation and microctructures in Neogene clays from the Warsaw area. In: Geological Quarterly. Vol. 47, No. 1, Warsaw, 43-54.

7. Marchetti S., 1980. In-situ tests by flat dilatometer. In: Journal Geot. Eng. Division., ASCE, 106, GT3, 299-321.

8. Młynarek Z., Tschuschke W., Niedzielski A., 1997. The evaluation of state of consistency coherent soils exequted with method of cone penetration test. Proc. of 11^th Nat. Conf. Soil Mech. and Found, Gdańsk, vol. 2, 113-119.

9. Polish Standard PN 86-B-02480. The building soils – the definitions, symbols, the division and description of soils.

10. Polish Standard PN 86-B-02481. Geotechnics – Basic terms, symbols and units.

11. Redel C., Blechman D., Feferbaum S., 1997. Flat dilatometer testing in Israel. Proc. of the 14^th Inter. Conf. on Soil Mech. and Found. Eng. Hamburg, vol 2, 581-584.

12. Sobolewski M., 2002. Determination of flow water characteristic in cohesive soils on the basis in situ tests. PhD, Dep. Geoingineering SGGW, Warsaw. [in Polish].

13. Sobolewski M., Effect on non-homogeneity of Pliocene clays in the vicinity of Warsaw and their physical properties. EJPAU, 6(2), #1.

14. Szymański A., Sobolewski M., Massoud F., 1999. Estimation of mechanical parameters on the basis of in situ tests. Roczniki AR Poznań CCCX, Melioracja i Inżynieria Srodowiska 20, cz. II, 99-109.

15. Szymański A., Sobolewski M., 2003. The use of dilatometer test for determination of permeability coefficient in cohesive soils. VI Conf. Aktual problems science-research archtecture. Olsztyn-Kortowo, 381-387.

Accepted for print: 7.12.2006

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.