Water resistance of soils, Soil dissolving - Ground science

Water resistance of soils

Water resistance (or waterproofness) of soils is called their ability to maintain their mechanical strength and stability in the aquatic environment. In the waterproofness of soils, both their physicochemical and physical-mechanical features are manifested. The conditions of interaction of soil with water can be static (calm water) or dynamic (moving water flow). In the first case, the result of this interaction in dispersed soils is their soaking, in rocky soils - softening, in the second case - erosion of soils. In accordance with this, the water resistance of soils is characterized by by their soaking, softening and washability.

Soil Dissipation

Soak is the ability of soils to soak in calm water to lose their connectivity and turn into a loose mass with complete loss of strength. Soaking up of soils occurs as a result of gradual weakening of non-waterproof structural bonds between elementary particles or soil aggregates during their hydration. Dissipative soils, as well as weakly cemented sedimentary and artificial soils with soluble, non-waterproof or clay cement, possess the ability to soak.

The parameters of soil soaking are [50]:

1) time of soaking (/ p ) - the time interval during which a sample of soil placed in water loses connectivity and disintegrates on structural elements of different sizes;

2) miscible speed (v p ), estimated by the relative mass loss Δm/mо of the sample in time Δ /, where mо - initial sample mass;

3) the character of soaking , estimated visually in excavations or on samples, reflects a qualitative picture of the decay of the soil.

Determination of water absorbency . Soil wetting is determined on samples with unbroken and broken structure with the aid of the PGG tool (Figure 5.7).

Fig. 5.7. The device for determining soil soiling [143]

To determine the soiling of undisturbed soil by a soil sampler, cylinders of 30 mm in diameter and 30 mm in height (or cubes of 30 30 30 mm) are cut from the ground monolith. Simultaneously, a sample is taken from the soil under investigation to determine its initial humidity.

In determining the soiling of the disturbed structure, the air-dried ground is crushed and sieved through a No. 05 mesh. The sieved soil is poured with water and a dough of such consistency is prepared in which it does not adhere to the hands when rolling. From the prepared dough, the soil is excised in the same way as from the soil monolith.

The cube or cylinder is mounted on a grid with holes of 1 cm which is suspended in a container of water (distilled or water from the sampling point). The samples placed in water begin to soak. When describing the nature of the soaking of the sample, a description is given of the shape, particle sizes (large, small lumps, flakes, dust), the sequence of their decay. Entries in the journal are made at the following intervals: 1, 30 minutes, 1.6, 24 and 48 hours, using the terms from Table. 5.4 and 5.5.

Table 5.4

Description of the behavior of a soil sample placed in water

The term

Description of the soil after 24 hours of soaking in water

Degree

Stable

No changes

1

Enough

Stable

Several cracks or sample surface is slightly crumbled

2

There are many cracks, crumbles into small pieces, the surface of the sample is crumbling

2

Unstable

The sample breaks up or almost the entire surface of the sample crumbles

4

The sample passes into a slurry or decomposes to sand

5

Soil dissipation criteria

Table 5.5

Time of sample soaking

Nature of soaking

Total soaking in 1 min

Instant

More than 80 ... 90% of the volume in 30 minutes

Very fast

More than 50% of volume per hour

Fast

More than 50% volume in 6 hours

Slow

Less than 25% of the volume in 24 hours

Very slow

Less than 10 % volumes in 48 hours

Practically non-wetting soil

The percentage of soil decay at any time is calculated by the formula:

where P is the decay of the soil,%; Н - initial numerical mark; H p is the numeric elevation in the process of soaking.

The experiment is considered complete if the sample gets wet and falls through the mesh to the bottom of the vessel or for a long time it will keep its state unchanged. If the sample does not get wet after 48 hours, a description is made and the test is terminated. Some weak rocks do not disintegrate immediately after extraction from water, but only after drying.

The value of the index of soaking in soils depends on their chemical-mineral composition (mineralogy of particles, the presence of water-soluble salts, the composition of exchangeable cations), structural and texture features (the nature of structural bonds, dispersion, texture, etc.), moisture-density, composition and the concentration of the aqueous solution interacting with the soil.

The composition of soils determines their structural features, the nature of the structural bonds and, consequently, affects the soaking. At the same time, natural cements contained in soils, for example, water-soluble salts, carbonates, gypsum, humus, etc., have a great influence on the nature and speed of soaking. Dissolution of salts on the contacts of particles leads to the disintegration of aggregates and soil soaking.

Wetting also depends on the composition and nature of the addition of soils. Most rocky soils are practically non-waterproof and only soften when saturated with water. The speed and nature of soaking dispersed soils is greatly influenced by the granulometric composition, which largely determines the nature of their porosity and, consequently, their water permeability. Macroporous, easily water permeable and usually have weak structural cohesion, soils have a high rate of soaking. On the contrary, thinly porous, little water-permeable and dense soils with an increased value of structural cohesion are distinguished by high water resistance and slow soaking. The presence of macro- and microcracks in the soils promotes their soaking. Primers with broken addition are characterized by a much higher rate of soaking compared to undisturbed rocks, since the former differ from the latter by a lesser connection.

Dry soils or soils with a low moisture content, as a rule, soak much faster than undersaturated differences. According to VA. Priklonsky, for each type of clay is characterized by some "critical" humidity, which can be used to judge its water resistance. If the moisture content of the clay is below the critical, the soil will soak; The soil with a higher humidity (above the critical one) practically does not soak. The magnitude of critical soil moisture increases in proportion to the growth of their exchange capacity (in montmorilloniton clay it is about 50%, in kaolinitic clay it is about 25%). This is due to the fact that, on the one hand, the bound water filling the fine pores at low humidity prevents the rapid penetration of new portions of water. On the other hand, capillary water at low humidity of clays also contributes to their capillary connectivity, which gradually disappears with full water saturation.

A certain influence on soaking is also caused by jammed air in the pores of the ground. With rapid hydration of the soil, a significant part of the air is trapped in the pores with absorbed water. If the compressed air pressure in the pores exceeds the tensile strength of the contacts, the compressed air destroys the ground and the bubbles come out. This process is typical for soaking loess and loesslike soils. Thus, with gradual hydration and water saturation, soils show greater water resistance than with the rapid development of these processes, since in the latter case more trapped air is formed in the pores. [50]

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