Into civil engineering - soil mechanics (3) the physical state indicators of soil
More generally speaking, the soil dominated by gravel and gravel is called coarse-grained soil, also called non-cohesive soil; the soil dominated by powder and clay is called fine-grained soil, which is generally fine-grained soil. The so-called physical state of the soil refers to the tightness and softness of the soil. namely
Coarse-grained soil - degree of tightness;
Fine-grained soil – the degree of softness and hardness or the consistency of so-called cohesive soil.
Let's analyze these two important engineering concepts.
1. The compactness of coarse-grained soil (non-cohesive soil).
The compactness of the soil usually refers to the content of solid particles per unit volume, and the more particles of the soil, the denser it is, and vice versa. The dry density and porosity ratio (or porosity) mentioned in the previous article are both indicators of density.
However, this method of expressing the density by "solid" content or pore content has obvious disadvantages for "soil", mainly because it does not take into account the influence of the important factor of particle size gradation. Particle size gradation, that is, the size of different particle sizes and their proportion in the soil. In order to better indicate the loose state of coarse-grained soil, the porosity ratio of the site soil is used to indicate the degree of compactness of the soil by comparing the pore ratio emin at the tightest time and the pore ratio emax at the tightest time that the soil can be achieved, and the index of measuring the compactness is called the relative density Dr, and the formula is:
When Dr=0, i.e=emax, indicates that the soil is in the most loose state;
When Dr=1, i.e=emin, indicates that the soil is in its most compact state.
The criteria for determining the compactness of coarse-grained soils by this index are:
But this method is too theoretical, not only difficult to measure, but also inaccurate. Therefore, the compactness of natural sand soil is usually tested in situ standard penetration at the site, which is indirectly determined according to the standard of the following table according to the number of hammers N, as shown in the following table:
The so-called standard penetration test method is to use a piercing hammer with a mass of 63.5 kg to drop freely along the drill pipe at a drop distance of 76 cm, and the tubular standard penetrator is hit by the corresponding number of 30 cm.
Fine-grained soils cannot be measured in the laboratory for emax and emin. There is also no maximum and minimum porosity ratio, so its density can only be judged based on its porosity ratio or dry density.
2. The consistency of fine-grained soil (clay soil).
Consistency refers to the degree of softness and hardness of the soil or the resistance to deformation or destruction caused by external forces of the mound (a bit like the concept of strength).
The demarcation water content of a clayey soil entering from one state to another is called the characteristic moisture content of the soil, or the consistency limit, and there are liquid limits, plastic limits, and shrinkage limits.
The plasticity index is equal to the difference between the liquid limit and the plastic limit, and the formula is as follows, which roughly represents the ratio of the weakly bound water mass that the soil can absorb to the mass of the soil grain.
An index that characterizes the relative relationship between the natural water content of the soil and the demarcated water content, that is, the liquidity index, is formulated as follows. It is often used as a criterion for remodeling the soft and hard state of soil.
The figure below shows the gradual reduction of the moisture content of a saturated clay sample. It is saturated until the plastic limit moisture content (point D). The soil sample is flowing at point A, and as the moisture content decreases, the volume of the soil sample shrinks, showing a straight line relationship (linear) of AE. where point C corresponds to the liquid limit and point D corresponds to the plastic limit. After point E (to the left), the water content and volume are nonlinear relationships, and the water content corresponding to point B is generally taken as the shrinkage limit. The M point is the intersection of the straight AD extension line and the ordinate coordinate, and its corresponding volume Vs is the total volume of the solid particles.