Selection of the Type of Dynamic Action and Determination of the Soil Resistance

In selecting the type of dynamic action, it is necessary to take into account the following basic factors: mass, dimensions arid shape of the elements driven or extracted, soil conditions and the technological characteristics of the work, performed.

We shall examine the influence of each of the said factors on the choice of the type of dynamic action. From the basic point of view there are no limitations with regard to the mass of the element driven; however, in selecting the type of dynamic action it is necessary to bear in mind that longitudinal vibrations can be the most effective in the relationship of the force of gravity to the magnitude of the compelling force, equal to 0.35 – 0.5.

The creation of vibration machines with a compelling force of more than 5000 kN (there is such experience) involves substantial constructional difficulties, associated with the need for assuring their durability. When longitudinal-rotational vibrations are employed, the ratio Q/P_o, can be increased to 0.5 – 0.8. With regard to the use of rotational vibrations, there are no limitations in this case with regard to the force of gravity since the perturbation acts in a plane perpendicular to the action of the force of gravity.

The mass of impact-vibrational machines is limited by the conditions of their durability or service life. The maximum mass of the vibration exciter can be taken in the combination of the longitudinal impacts with the action of static force or in the combination of longitudinal impacts with rotational vibrations. In these cases, with preservation of a high driving ability, it is possible to reduce the impact velocity of the vibration exciter and the mass of the latter can amount to up to 0.6 – 0.8 of the magnitude of the compelling force.

When two-impact regimes are used, the requirement on the limitation of the vibration exciter mass is sharpened and it can amount to 0.1 – 0.3 of the magnitude of the compelling force.

Experience reveals that based on the conditions of assuring the required service life, the mass of the vibrating hammer should not exceed 5 t.

Application of the vibration method is most effective with a relatively small cross sectional area of the driven element, which in engineering calculations is estimated as the magnitude of the specific pressure p on the soil (see Chapter III). With a value of p < 0.2 MPa and observance of the other conditions, impact-vibrational regimes are more effective than vibrational ones.

With a round or circular cross section of the driven element the most effective sinking and extraction are achieved with longitudinal-rotational vibrations and longitudinal impacts in conjunction with rotational vibrations. When longitudinal-rotational and rotational vibrations are used, it is necessary to check the driven tubular elements for strength and rigidity under the action of torsional moments. During the sinking of long elements (more than 25-50 m) it is expedient to use longitudinal vibrations.

All types of vibrational action are most effective in water-saturated sandy soils and also is soft- and fluid-plastic clayey soils. All forms of impact-vibrational action are most effective in low-moisture sands and also in stiff-plastic and semihard clayey soils.

In gravel soils and in soils with stony inclusions longitudinal-rotational and rotational vibrations can be effective in the cases where the driven tubular elements are equipped with a serrated cutting tip.

The sinking and extraction of a metallic pile are primarily effected with longitudinal vibrations. Impact-vibrational sinking of a pile is used when there is a substantial soil resistance. Two-impact regimes (impacts upward and downward) are used for extracting a pile that has been in the ground for a long time. The sinking of a pile-shell is carried out with longitudinal vibrations and the extraction of the cohesive soil from its cavity, with longitudinal and rotational vibrations.

The fitting of casing pipes in the impact-cable drilling of wells is carried out with a combination of longitudinal impacts and static force. Utilizing longitudinal vibrations and static force (vibrational forcing in) or longitudinal impacts and static force, the piles are driven, for example, in the construction of supports under electrical transmission lines, in the nonditch laying of pipelines, etc.

The use of longitudinal-rotational and rotational vibrations is recommended in the installation of pile supports near- existing construction.

The magnitudes of the dynamic frontal R and lateral F resistances of the soil (kN) at the limiting sinking depth can be calculated with the formulas: F = F_st /(k_σk_H) and R = R_st/k_L , where k_σ and k_L are coefficients dependent on the soil type and its physical state, k_H is a coefficient dependent on the directionality of perturbation (it is determined with the data of Table 3), Fst and Rst are the lateral and frontal static resistances of the soil, respectively, according to the Construction Standards and Regulations 11-02-03.85.

Table 3 Dependence of the coefficient k_H on the type of soils and directionality of the perturbations

The value of the coefficient k_σ for water-saturated sandy soils is: — 4.5 (coarse), — 5 (of medium coarseness) and 6 (fine); for wet sandy soils, coarseness values less by 1, respectively.

In clayey soils with a consistency of 0.25 < I_L ≤ 0.5 k_σ is taken to be equal to 2.5 for loams and 2.0 for clays, with a consistency of 0.5 < I_L ≤ 0.75 k_σ is equal to 3.0 for loams and 2.2 for clays and with I_L > 0.75 k_σ is 4.0 for loams and 3.0 for clays.

The coefficient k_L for low-moisture sands and clays with a consistency of 0.25 < I_L ≤ 0.5 is equal to 1.0, for wet sands and clays with 0.5 < I_L ≤ 0.75 k_L is 1.2, for water-saturated sands, 2.0, and for clays with I_L > 0.75 k_L is equal to 1.5.