New laboratory devices for testing soil samples (II)

(continued from the 219 – November 2024 issue)

PATENTED LABORATORY EQUIPMENT FOR THE DETERMINATION OF THE SOIL SHEAR STRENGTH

1. Device for the direct shear test along an imposed vertical plan of soil samples

In most of the geotechnical laboratories the determination of the soil shear strength is performed using triaxial equipment and direct shear box along an imposed horizontal plan (EN 1997-2:2007), (STANCIU et al., 2016). All direct shear test equipment available at the day (EN 1997-2:2007), (STANCIU et al., 2016), have in common one or more shear boxes for the shear of the specimen along an imposed horizontal plan, loaded under the normal force that develops the normal stress (₁) onto the failure plan. The loading of the cylindrical and prismatic specimens onto the imposed horizontal shearing plan is made by adding weights to a beam loading frame or by pneumatic/hydraulic devices.

The model for the evaluation of either the bearing capacity or slope stability, described by any author, considers that the failure of the ground develops along cylindrical surfaces under the direction of either a circle or a logarithmic spiral (STANCIU et al., 2016), Figure 9. The present devices for direct shear along an imposed horizontal plan bear the disadvantage that during the shearing process of the specimen, the anticipated stress state is similar to the one corresponding to the (M) point or the horizontal tangent to the failure surface (Figure 9), and the obtained results (φ; c) in a single point are valid for all the (i) points of the failure surface, including the emergent and final zones, as plans more closely to the vertical direction than to the horizontal direction.

A new device was designed, made as a prototype and patented as a direct shear box with a vertical imposed shear plan corresponding to cubic soil specimens (60 x 60 – 60 mm) for determining the shear strength parameters (φ; c) of the soil within the emergent zone of the failure surface.

Figure 9: The soil typical loading conditions along the failure surface with the corresponding tests to determine the soil shear strength

The general disposition of the new shear test apparatus consisting of the assembly of five main parts is presented in Figure 10; the five parts are the following:

• A – the group of levers (1), loads (2), counterweights (3) and vertical screw (4) for the application of the load/axial force (N) on the soil sample;
• B – the group for the application and measurement of the shear force (T) fitted with a marked screw reducer (5), the drive wheel (6), dynamometric rings (7), dial test indicator gauge/displacement sensor for the deformation measurement (8), with special coupling bushing (9) and half-cutters left (10);
• C – the shear box of the sample along the vertical plan, by an innovative approach, with half-cutters and shearing/cutting knives and the corresponding devices;
• D – the plate base (11), with slide tracks of the shear knives/split half-boxes, support posts, knives guides, dial test indicator gauge and their support;
• E – the steel plate – supporting the previous parts, including the shear box, assembled on the supporting steel frame of the shear test apparatus and potentially of the shear engine, electrical engine and of the gearbox to actuate the reducer and for the setting of the shear rate.

Figure 10: The general setting of the new shear device of the cubic specimens along an imposed vertical plan

The loads acting on the soil cubic specimen and the specimen sheared are presented in Figure 11.

Figure 11: The sheared specimen under the imposed actions (N and T)

Part C, representing the new shear box and its constituent parts are presented in Figure 12.

Figure 12: The new shear box of the specimens along the vertical plan and its constituent parts

The cubic soil specimen that is going to be sheared along the 1-1 plan is inserted into the shear box C constituting by the adjoining of two shear half-boxes (2 – right, and 3 – left). Subsequently, two vertical shear knives (7 – left, and 8 – right) are inserted, with thresholds for driving them by the half-boxes. At the base of the box, in the gap formed after joining the two half-boxes, two rectangular stone porous plates are placed, and two perforated copper plates (or solid grid plates with adhering teeth) on top of them; the same positioning will be repeated for the top part by inverting their positions, the copper perforated plates and then the stone porous plates on top. On top of these, two semi-pistons (12) are placed, equipped with centrally positioned bearing, with a recess each, in which two bolts (15) are introduced for the distribution of the vertical forces (N/2) of the semi-pistons. These forces will be taken over by the device (16) which includes a bearing (17) in its own axis, with a central hollow (18) and two round semi-glides separated by a central compressed spring (18), to constantly maintain the vertical loads (N/2) throughout the vertical shear of the soil specimen. The box C equipped in this way sits on a base plate and fits, pushing it, with the four groves, from the base of the shear boxes (2 and 3) on a sliding track, provided with four bearings for each half box, simultaneously with the positioning of the slides (23), from the upper part of the shear knives (7 and 8), on some of their sliding tracks, followed by ensuring the contact of two guiding devices (25), by acting on the cogs (27), through their bearings (26), until the two support posts of the vertical load group (A) touch each other, and by inserting a special bushing into the box housing (2) ‒ to the right and fixing it with two blades, thus ensuring the coupling with some dynamometric rings of the shear system (D). The shear half-boxes (2 ‒ right, and 3 ‒ left) are connected through the two bolts (33), screwed on the wall behind them, through the arms (34) and bolts (35) of the device (36), for operating the boxes/shear knives by inserting it, through the recess (37) of the bearing (38) in the central pivot (39), fixed in the base plate (E) hole.

The concentrated vertical force (N) is applied through the screw that is placed in contact with the bearing (17) within the device (16), taking it from the vertical load group A (Figure 10) consisting of weights applied on the big lever that is balanced with the counterweight in contact with the small lever balanced through the counterweight. The sample shearing takes place by the shear system action B constituting of a reducer with worm screw mechanically or manually operated, with a wheel, crank, and dynamometric rings, following a standard procedure, simultaneously with the vertical settlement measurement using the dial test indicator gauge/displacement sensor. After the vertical shearing of the sample and resulting shear force (T) are obtained, the shear strength is calculated (τ_f =T/A), where A is the sheared cross-section, for the normal load N (stress σ₁ = N/A) and the stress σ₂ = K0N/A normal to the shear plan.

2. Equipment for the soil shear in plan deformation state

The compression tests of the soil specimens when in plan deformation state (σ₁≠0; ε₁≠0; σ₂≠0; ε₂=0; σ₃≠0; ε₂≠0), Figure 13, to establish the soil shear strength, are recommended for man-made and natural slope stability analysis, and for the dams, dykes and continuous foundations design.

Figure 13. The soil specimen in plan strain state

The patented testing equipment (STANCIU A., CIOARA St., 2014, Invention Patent RO 130870) consists mainly of the biaxial cell, Figure 14. a, the water and air tank Figure 14. b, and the vertical loading installation of the soil specimen.

Figure 14: Plan strain apparatus – schematic drawing: a) water tank; b) vertical loading system

The biaxial cell (Figure 15) consists of a metallic base plate (1) located at the bottom. On top of this two side plates made of 20 mm thick Plexiglas (2) and the pressure cells (3) are placed. The Plexiglas plate position is ensured by their insertion in special grooves laid down in the base plate and through profile angles (16). The pressure cells are jacked on the edges of the Plexiglas plates using screw presses (13). At the top of the biaxial cell, a steel plate is disposed (8) and fitted with three orifices with passage pieces (9) which are intended to allow the vertical sliding of the load piston (10). The unit of the cell thus formed is secured vertically through four tie rods (15) which by clamping press the base plate and the top plate on the edges of the Plexiglas plates. In the longitudinal direction, the cell position is ensured by means of pressure through the metal frame (14), and the screw presses (13) (STANCIU A., CIOARA St., 2014, Invention Patent RO 130870).

Figure 15: Plan strain apparatus – biaxial cell details

The vertical loading system (Figure 15) consists of a press operated by an electromotor which, through a threaded axis, transmits a constant speed of 7.78 mm/minute to the turn-table. The longitudinal loading system consists of a compressor and a water tank. Throughout the compressed air circuit, between the compressor and the water tank, a regulator is provided which has the role of allowing the desired pressure (σ₃≠0) to be regulated and, at the same time, it ensures constant pressure during the tests. This aspect can be constantly monitored with the help of a manometer installed on the top of the tank.

The vertical loading on the soil specimens in the biaxial cell is realized by moving the press plate at a constant speed, thus actuating the piston (10) disposed at the top of the cell. The measurement of the compression force acting on the sample will be carried out using the dynamometric ring (12) which is located between the piston rod and the press yoke, the maximum load of the dynamometric ring being 50 kN. Also, micro-comparators are placed on the rods of the vertical loading piston to record the imposed displacements, and implicitly the sample’s deformation in the vertical direction.

In the longitudinal direction, the sample will be subjected to a constant pressure provided by the pressurized water passing from the installation tank into the pressure cells (3). The latter are provided with special latex membranes (7) that ensure sealing at the soil sample interface (Figure 16). To monitor the pressure, at the top of the pressure cells, there are placed manometers (17) and screw valves for venting the system. At the rear of the pressure cells are located two orifices with passage and sealing parts (4) through which the rods of the perforated movable pistons slide (6). The movement of these pistons is achieved by turning the lever (5).

Figure 16: Plan strain apparatus – pressure cells

The prototype made at the Faculty of Civil Engineering and Building Services of Iasi provides the testing of soil specimens with dimensions of 200x100x400 mm. This equipment provides only the setting for specimen testing under in plan strain and unconsolidated-undrained (UU) conditions as it is not yet equipped with drainage and pore water measurement.

The equipment makes available the remoulding of the soil samples by compaction at the optimum moisture content (previously established by the Proctor test) directly into the biaxial cell. Three consecutive tests performed at various values of the (σ₃) stress (σ₃=100; 200; 300 kPa) the ultimate stress (s₁) results for ε₃=20%·H; the resulting Mohr’s circles provide the common tangent of Coulomb’s line with the corresponding shear strength parameters (φ; c) for the plan strain condition (ε₂=0).

3. Installation with biaxial cell for determining the shear strength of soil specimens

Many geotechnical problems, as previously shown, such as the stability of slopes, embankments, earth dams, retaining walls, and continuous foundations, require for analysis and design the determination of the shear resistance parameters (φ; c) in plan strain state (ε₁ ≠ ε₃ and ε₂ = 0).

There are several equipments with biaxial cells for testing soil samples of regular quadrilateral prisms, in plan strain state (WANAROWSKI and CHU, 2006). Some of them have been patented (VARDOULAKIS and DRESCHER, 1989; STANCIU A., CIOARA St., 2014, Invention Patent RO 130870).

In general, these types of equipment use a biaxial cell in the centre, in which the specimen encapsulated in an elastic rubber membrane has its lateral deformation obstructed by two rigid vertical glass walls, on its opposite sides.

The new installation mainly consists of a biaxial cell (B) and a connecting and coupling device (D), both of original design, which can be integrated into the structure of a classical triaxial apparatus (A, C, E, F), Figure 17.

Figure 17: The new equipment for in plan strain state testing of soils: A, C, E, F – the components of the classical triaxial apparatus; B – the new biaxial cell

The new biaxial cell (Figure 18) replaces the classical triaxial cell and allows the shear test for soil specimens of rectangular parallelepiped shape, Figure 18. a). The cell rests on a lower platen (1) and has two Teflon walls (2) that obstruct lateral deformations. Two pressure cells (3) are mounted at the ends of the lower platen for applying pressure in the longitudinal direction (σ₃ ≠ 0) by means of water and a rubber membrane, Figure 18. a).

Figure 18: The new biaxial cell: (a) parallelepiped specimen and the corresponding stress and strain state (b) the main components of the biaxial cell: 1) lower platen; 2) Teflon walls; 3) load cells; 4) quick pneumatic connector for the water line; 5) quick pneumatic connector for the air line; 6) upper platen; 7) mixed cylinder; 8) rectangular piston; 9) piston rod; 10) axial displacement transducer

A mixed (hydraulic and mechanical) cylinder (7) is mounted on the upper part of the specimen, resting on an upper platen (6), through which a vertical pressure (σ₁ = σ₃ + σ₁) is applied by means of a rectangular piston (8). Two quick pneumatic connectors are mounted on the top of the mixed cylinder: one for the water line (9) and one for the air expulsion (10). The axial displacement transducer (11) is mounted on the piston rod (9).

The connection of the new cell to the pressure system (A) of the classical triaxial apparatus is done under water-controlled pressure (σ₃) for the cell (3), by means of an original device (Figure 19). The pressurized water line is connected to the original device through the connector (6) and distributed through the quick push-in connectors (7) and (8) to the pressure cell and the mixed cylinder.

Figure 19: Device for connecting the plan strain cell to the pressure system (A) presented in Figure 17, for applying controlled water pressure (σ3): 1) duralumin support; 2) clamp for fixing; 4) manometer; 5) faucet; 6) connector to the pressure system (A); 7) quick connector for the pressure cells (6); 8) quick connector for the mixed cylinder

The assembled, equipped and connected plan strain cell is placed under the load frame (C) and connected to the data logger (E) and the computer (F) for reading the pressure-settlement results of the specimen.

The determination of the soil shear strength parameters (φ; c) is made based on testing three specimens from the same soil sample with different stress (σ₃). After each test is performed, the maximum normal stress at failure (σ₁) is determined and according to the standard triaxial test methodology, the critical Mohr’s circles are plotted. Using the strength envelope, the investigated shear strength parameters (φ; c) are obtained.

CONCLUSIONS

In this paper, five inventions that were designed, prototyped and patented, are presented for the first time together as a group, regarding the determination of the geotechnical parameters that describe the soil compressibility and the shear strength parameters.

These new devices/equipment that follow the classical ones – oedometer, direct shear box designed by TERZAGHI/CASAGRANDE, respectively BISHOP’s triaxial equipment, are complementary to those already existing in all the geotechnical laboratories around the world, with applications for specific situations or intended to reduce the disturbance effect during sampling and transportation as negative influences on the final testing results.

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.

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. 220 – decembrie 2024, pag. 132-1135