Reportes estáticos

Sections 2.2 and 2.3.1 outlined the basic types of tube samplers currently available. Each type varies not only in their mode of operation, but also in the tube dimensions (diameter and thickness) and cutting edge geometry.

a b c

Figure 2.7 - Cutting Shoe Geometry: a) Attachable, b) Built-in and c) with a Liner

Dw Ds Dc Ds Dc Dw

20 This can be created by two methods: using an attachable cutting edge (expensive and makes the tube thicker - Figure 2.7a) or machining the end of the tube to become sharper or curved (Figure 2.7b).

As the sampler cuts into the soil, the tube is subjected to normal and frictional forces, both internal and external (Figure 2.8a) which will govern the extent of mechanical deformation the soil will be subjected to. Hvorslev (1949) states that the inside wall friction is the single most important source of disturbance during sampling. Upon removal, the vertical forces are reversed and therefore cause destructuring in the opposite direction (Figure 2.8b). It is of note that while friction is problematic during sampler driving, during retrieval it is beneficial, since without it sample recovery would not be feasible.

With

N= Normal Force, F= Friction, Q=Driving Force, Ub=Water or Air Pressure, Pt=Tensile Strength, W=Weight of Sample, Qp=Edge Resistance. Subscripts: e=external, i=internal Figure 2.8 - Forces acting on the Soil in the Sampler: a) during Driving and b) during Retrieval from the Ground (from Hvorslev, 1949)

Friction increases with penetration into the ground and can eventually cause severe mechanical disturbance (Hvorslev, 1949). In its least severe form, samples suffer from significant edge drag-down (Figure 2.6), while where the internal friction between the soil and the tube increases sufficiently to impede the sampling process, a soil plug is formed within the tube, and the forces on the surrounding soil are comparable to those created by a piston sampler (Figure 2.5b). This can result in severe distortions to the soil within and below the sampler. The important sampler dimensions are illustrated in Figure 2.7; these influence

21 the interaction between the soil and the tube as the sampler is pushed into the ground. Three parameters linked to the sampler geometry in BS EN ISO 22475-1:2006, the Area Ratio (AR), Inside Clearance Ratio (ICR) and Outside Clearance Ratio (OuCR) have been defined as:

% 100 Equation 2

% 100 Equation 3

% 100 Equation 4

With: Dw= Outer radius of the cutting shoe, Dc= Inner radius of the cutting shoe, Ds= Inner radius of the sampling tube, Do= Outer radius of the sampling tube. In many sources, Dw (the outer diameter) is referred to as B, and this notation is used in this thesis.

This means that a sampling tube with a high area ratio will have a comparatively thick cutting element which will produce relatively large deformations during its travel due to the large amount of soil being forced out of the way of the tube itself. A tube with a high inside clearance will have a large internal diameter in comparison to the cutting shoe. Since the amount of soil entering the tube is largely dependent on the diameter of the cutting shoe, the soil within the sample will be able to expand upon entry. This reduces the amount of friction between the soil and the tube for some of the travel, but is associated to disturbances caused by stress relief. An additional geometry parameter has a known effect on disturbance: the OCA (Outside Cutting edge Angle). This governs the apparent “sharpness” of the sampler, and hence the extent of deformation caused to the soil on the outside surface of the sampler.

Since some tube geometry parameters govern the amount of friction between the soil and tube walls, and the expansion which the soil is allowed upon entering the sampler, they are directly linked to the extent of mechanical sample deformation during the driving and retrieval of the sampler from the ground and later during extrusion of the soil from the sampler. It must be noted that where these mechanical deformations cause particle rearrangement, the deformations are predominantly plastic, and it is therefore not possible to recover the original shape and particle arrangements by reversing the direction of the sampling tube (Barnes, 2000).

22 When the sample is removed, it is typically sheared off by rotation from the ground below and retrieved from the borehole. This creates torsional and tensile stresses within the sample and can cause the sample to fail. The retrieval can create a vacuum below the sample which can also create some amount of destructuring, especially in soft and sensitive soils. Extrusion of the sample in the laboratory can be conducted in the same direction as the soil entered the tube, to avoid a complete reversal of stresses on the sample. When a thin tube or liner has been used, this will be cut into small lengths so that the sample can be retrieved from the tube without excessive frictional disturbance. However the cutting of the metal tube causes a significant amount of vibrations which may cause additional damage. All of the above can result in severe mechanical deformation of the sample, which is apparent as a destructuring of the soil and a disturbance at the edges of the sample. The extent of mechanical deformation largely depends on the aforementioned sampler geometry parameters (thickness, area ratio, inside clearance ratio, and edge of cutting element OCA), and as a general rule:

‐ High values of tube thickness are associated with high disturbance because of the

amount of soil displaced during tube penetration.

‐ High values of Area Ratio are associated with high disturbance due to the amount of excess soil allowed into the sampler. The AR is essentially the ratio between the volume of displaced soil and that of the sample. High values of AR are also linked to higher penetration resistance and therefore disturbance to the sample.

‐ High values of edge of cutting element, OCA (i.e, blunter tubes) are associated with high sample disturbance.

‐ High values of Inside Clearance Ratio allow stress relief during tube driving and increase disturbance since the sample is allowed to expand upon entry into the tube.

‐ A low ICR implies increased friction between the sampling tube and the soil. While friction is undesirable during tube penetration since it results in some extent of edge disturbance, a certain amount of friction is necessary to recover the sample. Friction causes some amount of disturbance, which has been observed to be significantly greater during driving than during sample recovery (Clayton et al, 1995). Similarly,

23 the soil beneath the borehole may be heavily remoulded due to the drag-down effect of the tube sampler.

‐ Extrusion strains have been found to be small in comparison with penetration

induced strains (if strains are due to stress relief, Baligh et al, 1987, and if they are

due to the mechanical action of the extruder, Chung et al, 2004).

It was mentioned previously that liners could be fitted to the inside of the sampling tubes to facilitate transport and handling of the soil once removed from the tube. The effect of installing a liner is illustrated in Figure 2.7c and Table 2.6. The apparent tube diameter is reduced, which has the effect of reducing the inside clearance and increasing the area ratio. Table 2.6 - AR, ICR and OCA of thick-wall, thick-wall with liner and thin-wall Samplers (from Gosling and Baldwin, 2010)

Feature Thick Wall open tube (U100)

Thick Wall open tube with plastic liner

Thin-Wall (values required by EC7) Outer Cutting Edge Angle 10° 7° < 5° Area Ratio 29.4% 47.1% < 15% Inside Clearance Ratio 1.34% 1.27% < 0.5%

Current sample quality requirements are such that most types of commonly used open tube samplers such as the U100 (Table 2.5) are rarely able to produce undisturbed specimens. There is an on-going debate as to whether this method of sampling can remain usable (Baldwin and Gosling, 2009, 2010, Gosling and Baldwin, 2010). Thin wall samplers produce less disturbance, but are only used in soft to stiff soils since harder materials and the presence of stones tend to cause damage to the tube. This highlights the need for the development of a resistant tube sampler able to collect stiffer soils.

Table 2.5 shows the complexity of sampling disturbance, which is dependent not only on the chosen sampling method, but on the ground conditions themselves. This is particularly relevant when considering the development of equipment to satisfy strictly defined quality requirements. Hvorslev (1949) recommended the following for sampler design and selection:

‐ The ICR should be tailored to the soil being sampled but should be no larger than 0.75-1.5% for long samples and 0-0.5% for short samples. This lets the sample

24 expand laterally and is small enough for the sample to develop adhesion to the sampler for easier retrieval

‐ The AR should be no more than 10% for open-drive samplers (more for piston

samplers)

‐ The OuCR should be zero

‐ The OCA should be no more than 10°, but 20-30° should be allowed very close to

the edge to avoid damage the cutting shoe.

A numerical and analytical study by Siddique (1990) reiterated the need to limit the AR to 10%, and expanded on the above by proposing an ICR limit of 0.5% for general use, and a maximum OCA of 5°. Other studies, such as those backing the International Society for Soil Mechanics and Foundation Engineering Subcommittee on Problems and Practices of Soil Sampling (1965), suggested using a combination of AR and OCA (Table 2.7).

Table 2.7 - Combinations of Area ratio and OCA (taken from Clayton et al, 1995)

Area Ratio (%) OCA (°)

5 15 10 12 20 9 40 5 80 4