New laboratory devices for testing soil samples (I)

The design of any construction type, their making in safety conditions for both static and dynamic actions, is not possible without the foundation ground investigation and testing. At this stage, an important role is played by the probes and samples extraction to be tested in geotechnical laboratories. The classical tests applied to the extracted soil samples result in obtaining the physical soil parameters but also the compressibility and shear strength respectively using the following tests: the oedometer test (TERZAGHI, 1925), the direct shear test along the imposed horizontal plan (CASAGRANDE, 1932), the triaxial test (BISHOP, 1962), the biaxial test (WANATOWSKI and CHU, 2006), with resulting classical parameters in both static and dynamic regimes (E_oed; m_v; C_c; j; j’; c; c’). Complementary to these classical tests, in the Geotechnical Laboratory from the Faculty of Civil Engineering and Building Services of Iasi, Romania, five new laboratory devices have been designed, made as prototypes and patented, for the samples and respectively the specimens testing to establish the mechanical parameters previously mentioned.

INTRODUCTION

The determination of the mechanical parameters (E_oed; m_v; C_c; j; j’; c; c’) that characterize the soil behaviour as foundation ground is presently made in all authorized geotechnical laboratories by standardized and harmonized classical tests at European level following the EN 1997-2:2007 with specificities at every country level. The tests (EN 1997-2:2007) have the purpose of establishing the soil strength by unconsolidated-undrained compression tests (STANCIU et al., 2016), triaxial compression tests, and direct shear tests in the box, respectively the parameters that define the soil compressibility.

To perform the types of specific tests to determine the above-mentioned soil parameters that define the soil strength and compressibility, specific devices have been designed and made as prototypes for each test type (oedometer, triaxial, shear box); the purpose is to provide the soil parameters determination in conditions as close as possible to those existing in situ, on cylindrical or prismatic specimens with the smallest disturbance from their initial structure. Five new devices for soil specimen testing in laboratory conditions have been designed, made and patented at the Geotechnical Laboratory of the Faculty of Civil Engineering of Iasi, Romania.

Thus, for the soil compressibility study as foundation ground, two patented devices were made: Device for determining mechanical characteristics of soils by axially symmetrical compression tests, process for testing in sampling nozzle, process for testing the samples and process one/two for providing specimen shapes (STANCIU A., LUNGU I., ILAS A., 2014, Invention Patent RO 133293); Oedometer with double action and device for the frontal loading, in steps, of specimens (STANCIU A., HERTA I.C., 2018, Invention Patent RO 133362). Three patented devices were made for the determination of the soil shear strength: Device for direct shear, along an imposed vertical plan, of soil specimens (STANCIU A., HERTA I., C., PREDOAIE C., 2020, Invention Patent RO 134239); Device for determining mechanical characteristics – Equipment for soil shear in plan deformation state (STANCIU A., CIOARA St., 2014, Invention Patent RO 130870); Installation with biaxial cell for determining the shear strength of soil specimens (STANCIU A., 2021, Invention Patent Application RO 00204/2021).

PATENTED LABORATORY DEVICES FOR THE STUDY OF SOIL COMPRESSIBILITY

1. Device for determining mechanical characteristics of soils by axially symmetrical compression tests: the Consolidometer

The basic principles of the Consolidometer consist in reducing the influence of the stress path specific to the classical laboratory tests, and performing the tests on specimens with lateral displacement partially obstructed by the surrounding soil, after bringing the specimen to the stress state and deformations similar to the in situ ones.

Regarding the stress path in the soil massif, a soil sample (Figure 1) is located at a certain depth (Figure 1. a) and has a specific stress and strain state. This specific state is changed by the sampling process, transport, and storage in the laboratory, through specimen assay and oedometer testing process.

Regarding the limitation of the oedometer testing, the methodology provides the induction of a specific stress and strain state, by applying successive loading steps, to a specimen with completely confined lateral movement. This aspect brings into discussion the evaluation of the value of the correction coefficient (M₀) for the transition from the oedometer deformation modulus (E_oed) to the linear deformation modulus (E_s). This topic will not be covered in the current article.

Figure 1: Illustration of the effects of the stress path within a soil sample

In order to eliminate or reduce to the minimum the above-mentioned characteristics of the classical soil samples, the Consolidometer equipment for laboratory testing was designed and a prototype was made, which combines the plate load testing methodology with the oedometer testing and triaxial testing. Schematically, the component parts of the Consolidometer are presented in Figure 2 and respectively in Photo 1. Basically, the Consolidometer is made of a main loading system (P₁-P₄) for the reconsolidation of the soil extracted with the sampler (auger) (P₁₁) and, respectively, a secondary loading system (P₉-P₁₁) through which in the drilling hole from the large sample, through a mini-plate (Ø = 40 mm), the load is applied in loading steps as in the plate load test (EN 1997-2:2007) (Figure 3), obtaining the specific plate load test parameters (E_u, E_v, k_s, p_u, c_u, E), with partially confined lateral deformation. For the same purpose, the Consolidometer allows to be performed other two types of testing, namely Type II and Type III, with their similarities with the already known oedometer testing principles for the consolidation stage, plate load testing and triaxial testing for the actual proposed testing.

Figure 2: The Consolidometer and its main component parts as P1 – P4: main loading system; P5: dynamometric ring; P6: water reservoir; P7: tripod in contact with the top plate of P8; P8 & P11 (multiple sub-parts in direct contact with the sample): sampler, top and bottom plate, etc. P9 – P10: secondary loading system; P12 & P14: reinforcement parts; P13: stand for displacement transducers

Photo 1: The Consolidometer made as a prototype in the Geotechnical Laboratory of the Faculty of Civil Engineering of Iasi, accompanied by the presentation of its main component parts (P1 ¸ P14)

Fig. 3 (a), (b)

Fig. 3 (c), (d) | Figure 3: The Consolidometer Type I test, further named plate load test type with soil sample disturbance

2. The Oedometer with double action and frontal device for loading in steps of the soil specimen

In soil mechanics, the oedometer test is a classical test used for the determination of the soil parameters which describe the soil deformability/compressibility (C_c, C_r, E_def, m_v, a_v, E_oed). This test was designed to simulate the deformability and drainage of the foundation ground under the loads transmitted by constructions. The oedometer device used in all the geotechnical laboratories was designed based on the research made by TERZAGHI (1919-1925) and CASAGRANDE (Figure 4).

Figure 4: Typical oedometer set test: 1) one bench and three oedometers; 2) three consolidation cells (50 mm or 63.5 mm, 75 mm or 100 mm); 3) three gauges, either analogue or digital; 4) leaver loading arm; 5) support arm; 6) weight hanger; 7) weight set

The main component of any oedometer testing device, which remained the same from TERZAGHI/CASAGRANDE until the present day, is the consolidation cell presented in Figure 5, with its constituent parts.

Figure 5: The classical oedometer cell structure – transversal section: 1) soil sample; 2) oedometer cell; 3) fixing collar; 4) oedometer cell base; 5) and 6) porous stones; 7) loading device

In the classical oedometer testing device, the load is transmitted to the specimen using just one loading beam which provides different loading ratio. The specimen is confined by a metal consolidation ring which obstructs the soil from deforming in the horizontal direction (ε₁ ≠ 0, ε₂ = ε₃ = 0). Using the classical oedometer test, the parameters (C_c, C_i, C_a, t_a, m_v, a_v, E_oed) evaluated using the consolidation curves (stress – strain curve; stress – void ratio curve and consolidation curves) can be affected by errors which lead to different values when compared to the real in situ values and these errors will affect the final values of the computed/predicted settlement. In order to eliminate these outcomes of the classical oedometer testing device, a new oedometer testing device was designed, made as a prototype and patented (STANCIU A., HERTA I. C., 2018, Invention Patent RO 133362). The new oedometer applies two independent loads to the soil specimen using two concentrically set pistons with the help of two loading beam frames. The design and construction of a new type of oedometer, with double action and front loading device, in steps, for testing of a cylindrical soil specimen/ sample, as well as its components are shown in Figure 6.

Figure 6: New oedometer testing device main parts: 1) oedometer cell; 2) metal frame (bench); 3) the first loading lever; 4) weights (for loading); 5) counterweight; 6) sliding counterweight; 7) first loading frame; 8) rods of the first loading frame; 9) rod guides; 10) guide sheets; 11) loading tripod; 12) first sample loading frame; 13) cylindrical soil specimen (not shown); 14) consolidation transducer; 15) second loading frame; 16) counterweight; 17) weights (for applied load); 18) the second loading frame; 19) rods of the second frame; 20) oedometer cell support

The new consolidation cell structure and its components, designed and prototyped by adapting a classical cell, are presented in Figure 7. The other components from Figures 4 and 5 (2; 15; 16; 17; 18; 19; 20; 22; 23; 24) are specific to the classical oedometer (7; 8; 9; 10; …. etc.).

Figure 7: The new oedometer cell main parts: 12) the first loading top cap; 13) cylindrical soil specimen; 21) second loading top cap; 23) oedometer steel consolidation ring; 24) base cylindrical porous stone; 25) top annular porous stone; 26) top cylindrical porous stone; 27) consolidation cell base; 28) fixing collar; 29) fixing and guiding screws; 30) settle blocking piece; 31) oedometer cell walls; 32) tripod guiding holes; 33) holes for screw fixing and guiding

The upper/top porous stone, unlike the one from the classical oedometer, has two parts, one with an annular shape and the other one placed inside the annular one having a cylindrical shape (Figure 8).

Figure 8: Porous stones: a) annular stone; b) cylindrical stone

In conclusion, the soil sample/specimen testing using the new oedometer testing device follows two steps: the first step is the soil consolidation under the vertical pressure equal to the pre-consolidation pressure (σ_p‘), using the first loading beam frame (12); the second step is the classical consolidation/ compressibility testing which will load the specimen using the second loading frame (21) resulting in the output data to derive the characteristic curves.

REFERENCES

[1] BISHOP W.A., HENKEL J.D. (1962), The Measurement of Properties in the Triaxial Test, E. Arnold;

[2] CASAGRANDE A., ALBERT S.G. (1932), Research on the Shearing Resistance of Soils, Mass. Institute of Technology;

[3] EN 1997-2:2007 Geotechnical design – Part 2: Ground investigation and testing;

[4] STANCIU A., LUNGU I., ILAS A. (2014), Invention Patent RO 133293, Device for determining mechanical characteristics of soils by axially symmetrical compression tests, process for testing in sampling nozzle, process for testing the samples and process one/two for providing sample shapes;

[5] STANCIU A., HERTA I. C. (2018), Invention Patent RO 133362, Oedometer with double action and device for the frontal loading, in steps, of samples;

[6] STANCIU A., CIOARA St. (2014), Invention Patent RO 130870, Device for determining mechanical characteristics ‒ Equipment for soil shear in plan deformation state;

[7] STANCIU A., HERTA I., C., PREDOAIE C. (2020), Invention Patent RO 134239, Device for direct shear, along on imposed vertical plan, of soil samples;

[8] STANCIU A. (2021), Invention Patent Application RO 00204, Installation with biaxial cell for determining the shear strength of soil samples;

[9] STANCIU A., LUNGU I., ANICULAESI M., TEODORU I. B., BEJAN F. (2016) Foundations – Investigation and testing of the foundation ground, Vol. II, Technical Publishing House, Bucuresti, ISBN 978-973-31-2291-3;

[10] TERZAGHI K. (1925), Erdbaumechanic auf bodenphysikalischer Grundlage (Leipzig: Franz Deuticke) 689;

[11] VARDOULAKIS I., DRESCHER A. (1989), Bi-axial geomaterial test system, United States Patent No. 4885941, Washington, DC: U.S. Patent and Trademark Office;

[12] WANATOWSKI, D., CHU, J. (2006), Stres-Strain Behavior of a Granular Fill Measured by New Plane-Strain Apparatus. Geotechnical Testing Journal, Vol. 29, No. 2, Paper ID GTJ 12621, at: www.astm.org.

(to be continued)

Authors:

Anghel STANCIU, Professor Emeritus D.H.C.;

Irina LUNGU, Professor, Ph.D;

Mircea ANICULAESI, Lecturer, Ph.D ;

Oana Elena COLT, Lecturer, Ph.D ;

Florin BEJAN, Lecturer, Ph.D ‒ “Gheorghe Asachi” Technical University of Iasi, Romania

Andrei ILAS, Geotechnical Eng. Specialist ‒ Arcadis Excellence Center Romania S.A.

[Proceedings of the 17th Danube European Conference on Geotechnical Engineering (17DECGE), June 7-9, 2023, Bucharest, Romania – https://17decge.ro/]

…citeste articolul integral in Revista Constructiilor nr. 219 – noiembrie 2024, pag. 66-69