10.4.1 Laboratory Results

Wetting of unsaturated soil reduces the suction in the soil, the pore pressure ap­proaches the pore air pressure and the effective stress is reduced. Because of this, increasing moisture is associated with decreases in shear strength, stiffness and re­sistance to plastic deformation in all soils and aggregates and we can observe a decrease in bearing capacity and lower moduli of elasticity and increases in de – formability under the same applied loading.

10.4.1.1 CBR Tests

This influence of moisture on the bearing capacity of soils can be easily observed in the simple CBR test. This test measures the resistance of the compacted soil to the penetration of a piston, to evaluate its bearing capacity. CBR values for three typical soils related to water content are presented in Fig. 10.8. The sensitivity to moisture variations is particularly important for the silty sand (a).

Decrease of bearing capacity and of unconfined compression strength of clay with increasing water content can also be observed in Fig. 10.9.

The CBR test gives a good indication of the moisture sensitivity of subgrade soils, but its results are only qualitative. The influence of moisture on the resilient modulus and resistance to permanent deformations of soils and unbound granular materials can be studied using repeated load triaxial tests.

There are many ways of evaluating pavement structural capacity or adequacy and it is very common to perform deflection measurements with non-destructive testing equipment (COST Action 325, 1997). Once again, there is usually no knowledge of the pore pressure or pore suctions in the soil or pavement layer being assessed, so a total stress interpretative framework is necessary even though an effective stress framework would be more desirable.

There are several reasons for deflection measurements to be carried out: for quality assurance, to evaluate the bearing capacity of the unbound material of the granular base, sub-base and subgrade layers, to identify weak parts of the road, to investigate reinforcement requirements, to establish priorities for road strengthening and for research purposes.

Deflection measurements can be performed in various ways, using:

i) static deflection measurement equipment;

ii) automated beam deflection measurement equipment;

iii) dynamic deflection measurement equipment;

iv) deflection instruments with a harmonic load; and

v) deflection measurement equipment with an impulse load.

i) Static devices include: static and dynamic plate loading (bearing) tests and Benkelman beam.

Plate bearing test. This involves measurement of the deflection caused by a known static load applied through a hydraulic jack on the pavement layer surface by a circular plate. Circular plates are of specified diameters -300,450 and 600 mm with loads up to 60 kN. It is a slow and laborious test and needs a heavy vehicle acting as stationary reaction frame against which the hydraulic jack reacts. The plate test device may be mounted inside a van or at the rear of a lorry.

Dynamic plate bearing test. This is performed by lifting and dropping a known load from a known height onto the circular plate and measuring deflections. In this way, the dynamic effect of vehicles to the pavement is simulated.

The two preceding plate bearing tests are generally only used for mea­surement of bearing capacity of unbound layers.

Benkelman beam. This device consists of two main parts: a stand and a beam (Fig. 10.4). One end of the beam rests on the road surface but the other one is connected to a dial test indicator. The beam is suspended on

Fig. 10.4 Benkelman beam: (1) dual tyres of a loaded vehicle, (2) tip of the beam, (3) ball bearing, (4) adjustable support legs, (5) dial test indicator, (6) stand (Tehnicne specifikacije za ceste, TSC 06.630, 2002). Reproduced by permission of Direkcija Republike Slovenije za Ceste

the stand at two thirds of its length from the end being in contact with the road surface. In this way, when this end moves in one direction, the other end moves in the opposite direction half of that movement and it will be measured by dial gauge. The tip of the beam is placed between the dual tyres of a loaded vehicle. The vertical movement of the surface is then recorded as the truck moves slowly away from the loading area as this results in the rebound of the deflection that was caused by the application of the loaded vehicle. The rebound deflection/deflection ratio depends on the condition of the underlying road layers. For example, high water content caused by thawing can result in a relatively low ratio. If the deflection bowl is large, as in the case of weak subgrades, and the supporting legs are placed in the deflection area, this may produce inaccurate measurements.

ii) Automated beam deflection measurement equipment is represented by the Lacroix Deflectograph. It was developed in order to carry out measurements more quickly than by the time-consuming Benkelman beam. The measurement beam is automatically displaced along the measurement direction, while a mea­surement vehicle proceeds at slow speed. Equipment includes a T-shaped frame towed between the axles of lorry, enabling deflection to be measured in both wheel paths simultaneously (Fig. 10.5).

The measurement starts when the rear wheels of lorry are at a certain distance behind the tip of measuring beam and ends when the wheels reach a certain distance in front of it. Then the entire T-frame is moved forward a specified distance in the direction of movement. The axial load can vary from 80 kN to 130 kN.

iii) Curviameter records the surface deflection under a dynamic load. The vertical displacement is determined in the right-hand wheel path by means of velocity sensors when the loaded twin-wheel rear axle of the measuring lorry passes over. Like the Lacroix deflectograph, loads can vary from 80 kN to 130 kN. The High Speed Deflectograph also belongs to this category. At a driving speed of 80-90 km/h, laser Doppler sensors measure the movement of the pavement surface under a 10-tonne axle.

iv) Deflection measurement by harmonic load can be performed by equipment that can exert a sinusoidal vibration in a road pavement. A well-known example of this type of equipment is the Dynaflect, in which the power is transferred to the road by means of crank at a specified frequency via two steel wheels. The deflections are measured by five geophones spaced at various offsets from the centre of load.

v) Bearing capacity assessment. The most common device to measure bearing capacity of roads is the Falling Weight Deflectometer (FWD). In a FWD test an impulse loading is applied to the pavement which is similar in magnitude and duration to that of a single heavy moving wheel load. The vertical response of the pavement system is measured on the surface with deflection sensors at different distances from the loading. Usually only the peak values are registered, see Fig. 10.6.

Frequently six or seven sensors are used but the spacing between the sensors can vary. Typical values are given in Table 10.1.

The deflections can be used to back-calculate the layer moduli of the structure. This requires information on the number of layers in the pavement structure and

their thicknesses. This is an inverse process where a single, correct solution does not exist. A number of software solutions exist to perform such back-analysis.

The deflections can also be used to estimate some simple parameters which are related to the condition of the pavement, some of the more common ones are:

SCI = D0 – D2 (10.1)

BDI = D2 – D4 (10.2)

BCI = D4 – D5 (10.3)

1 N-1

AREA = (Di – + Di )(n – n_0] (10.4)

D0

where SCI = the Surface Curvature Index, BDI = the Base Damage Index, BCI = the Base Curvature Index and AREA = the area of the deflection basin (all assuming radii as per the light pavement in Table 10.1).

A falling weight deflectometer is mounted on a trailer or in a van, so it needs a stable surface. On soft ground or in trenches the smaller portable light weight FWDs (Fig. 10.7) can be used. These equipments measure the central deflection, resulting in a surface modulus.

A general overview of conventional and some advanced numerical models used in practice has been given in Chapter 9, Section 9.4. Therefore, tests needed for the parameters of these models will now be presented.

• Resilient behaviour models

о Routine pavement design model: in practice much routine pavement design is carried out as catalogue – based design. The pavement is considered as a multi-layered elastic system with constant stiffness parameters in each layer.

о Advanced pavement models: RLT tests are required with variable confining pressure (CEN standard EN 13286-7 (2004)), they correspond to strain sta­bilization. Parameters are determined with curve fitting by the least squares method applied to the equations of the model and to RLT tests results in the stress – elastic strain planes (p, ev) and (q, eq). Both phenomenological models which describe behaviour from an observational standpoint (such as the k—6 and “Universal” Models) as well as theoretically-derived models (such as the Boyce and modified Boyce model) require this kind of testing. The more complex models require VCP test procedures (see Section 10.2.1, above).

• Permanent deformations models

о Analytical models: they require 3 monotonic triaxial tests for the rupture parameters and 3 VCP tests (q /p = 1, 2 and 3 for example) for the plas­ticity parameters. Curve fitting can be used with the least squares method applied to VCP tests results and to the analytical equation of the model

(evertical f(N)) in the plane (N, e^cal).

о Plasticity theory based models

• Bonaquist and Desai models

– Elastic behaviour: the parameters require RLT tests (q /p = 3) at various confining pressures (AASHTO T307-99 (2000)).

– Monotonic plasticity parameters: 3 monotonic triaxial tests till rup­ture for the rupture characteristics and hardening parameters.

– Approximate accelerated analysis: 1 RLT test with one stage at (q /P = 3) is required.

• Chazallon and Hornych model

– Elastic behaviour: RLT tests are required with variable confining pressure (CEN standard EN 13286-7 (2004)), they correspond to strain stabilization.

– Monotonic plasticity parameters: 1 oedometric test, and 3 triaxial tests till rupture are required.

– Cyclic plasticity parameters: 1 RLT test with one stage is required

(q /p = 2).

о Elasto-visco-plastic equivalent models

• Suiker elasto-visco-plastic model

– Elastic behaviour: these parameters are required for the initial state of the material. They require at least 2 monotonic triaxial tests at two different confining pressures when 100 cycles have been per­formed (q / p = 3).

– Monotonic plasticity parameters: they are required for the initial state of the material: 3 monotonic triaxial tests are required.

– Cyclic plasticity parameters: RLT tests (q/p = 3) are required.

о Mayoraz visco-plastic model

– Plasticity parameters require 3 triaxial tests till rupture and a RLT test

(q/p = 3).

о Shakedown models

• Perfectly plastic models

– Elasticity parameters: RLT tests (CEN standard EN 13286-7 (2004)) are required with variable confining pressure till stabilization.

– Plasticity parameters: 3 triaxial monotonic tests till rupture are re­quired.

• Kinematic hardening models

– Elasticity parameters: RLT tests (CEN standard EN 13286-7 (2004)) are required with variable confining pressure till stabilization.

– Plasticity parameters: 3 triaxial monotonic tests till rupture are re­quired for rupture parameters and 3 VCP tests (q /p = 1,2 and 3 for example) with 3 stages for each stress path.

As shown in Chapter 9, Section 9.5, a complete description of a material’s be­haviour necessitates an effective stress approach with the pore pressures (or pore

Number of load pulses Number of load pulses

Fig. 10.3 Typical results from permanent deformation testing where the accumulated axial strain is shown as a function of the number of load pulses in the permanent deformation tests (Erlingsson, 2000) suctions) being separately controlled, or monitored, from the applied pressures. Be­cause most road materials are coarse grained and partially saturated and/or above the ground water table, it usually proves impossible – and certainly it is impractical in most situations – to monitor the pore suctions during each transient pulse. For this reason, almost all testing programs determine parameter values for resilient and incrementally-developed plastic strain models in terms of total, not effective, stresses. Instead, test procedures typically seek to control the moisture or suction conditions so that data is collected at representative conditions.

As described above, unsaturated soils exert an attraction on water, either by cap­illarity in the pores, between grains of the soil, or by the physico-chemical effects. The energy necessary to extract water from the unsaturated soil, for example from a dense compacted swelling clay used for engineering barriers in nuclear waste, can be higher than 10 MPa. However, a granular soil will have a suction that is
several tens of kPa (Delage 2001). The following techniques are commonly used

for suction/moisture control:

i) Axis translation system

This system is based on ceramic low porosity stone, called High Air Entry Value (HAEV) porous stone. The principle of the system is that the pores of the ce­ramic are too small to de-saturate, even when a high air pressure is applied. In other words, the capillary air-water menisci located at the surface of the ceramic can resist an air pressure applied to it, thereby maintaining a continuous water column from stone to specimen. The system has been employed for determi­nation of the water retention curve (i. e. the soil water characteristic curve) (as in the Richards cell) as well as for suction control in oedometer and triaxial testing (Peron et al., 2007; Cuisinier & Laloui, 2004). The retention curve can be determined as follows. A saturated sample should be placed in the cell and the air pressure increased in a step-by-step progression. For a given air pressure, suction equilibrium should be achieved in a time period of a few days (typically 3-10). At the end of equilibrium phase, the sample should be quickly withdrawn and weighed to determine the water content. The sample is then placed into the cell and the air pressure increased to bring about a new equilibrium with a higher suction and lower water content and lower degree of saturation. The process is then repeated as desired.

ii) Osmotic technique

This was initially used by Kassif and Ben Shalom (1971) in an oedometer test to study expansive soils. Several authors extended this technique to a standard triaxial apparatus (Laloui et al., 2006; Komorik et al., 1980; Delage et al., 1987). The technique is based on drainage of the specimen caused by the process of osmosis. The soil sample is placed in a tube-shape cellular semi-permeable mem­brane, which is immersed in an aqueous solution of PolyEthyleneGlycol (PEG). Since the PEG molecules are too big to cross the semi-permeable membrane, the difference between concentration of the PEG solution and that of pore water results in an osmotic pressure. Thus, the suction value depends on the concen­tration of the solution: the higher the concentration of the polyethylene glycol (PEG) solution, the higherthe suctionof soil specimeninthe semi-membrane after the equilibrium state is reached (Dineen & Burland, 1995; Delage et al., 1998).

iii) Vapour phase method

Usually, this method is used to determine the water retention curve of soils with high suctions, though it can be used to bring a triaxial or other test specimen to a particular moisture-suction condition prior to mechanical testing. The method is based on the theoretical standpoint that water potential (i. e. suction) is related to a particular relative vapour pressure of the water in the soil-water system (Qian et al., 2006). The relative vapour pressure of water in equilibrium with the system is characterised by its relative humidity. Therefore, suction can be established by creating the relative humidity that is related to the concentra­tion of a solution identical with the composition of the soil water. The soil specimen should be placed in a desiccator containing an aqueous solution of a given chemical compound. Depending on the physico-chemical properties of the compound, a relative humidity is imposed within the desiccator. Water exchange occurs by vapour transfer between the solution and the sample via the vapour and a given suction is imposed when vapour equilibrium is reached. It should be noted that the technique generally requires a very long equilibrium time that can be a few months (1-2 and up to 6). Marcial et al. (2002) introduced an air circulation technique to reduce the equilibrium time from 6 months down to 2-4 weeks. Also, it should be noted that either the same products at various concentrations or various saturated saline solutions may be used.

In conventional practice, osmotic and vapour phase methods have quite often been combined to bring soil specimens to any desired point on the soil-water charac­teristic curve. The osmotic technique should be used to control low matrix suction (less than 2 MPa) and the vapour phase method for measurements of higher suctions (> 2MPa).