Research and Calculation of the Load Capacity of the Bases of Metal Screw Piles According to the Torque Value Required for Driving Them

ABSTRACT

The results of investigation and improved method of calculation of the load capacity of the bases of metal screw piles under pressing and holding loads from the torque value required for driving them, developed on the basis of the performed investigation are presented. On the basis of literary sources [1-3, etc.] and the data of obtained in theoretical and experimental investigations performed in the Institute BelNIIS Republican Unitary Enterprise, it has been shown that between the torque M_k, kN·m, required for driving (hereinafter referred to as the torque M_k) of the metal screw pile (hereinafter referred to as the MS pile) into soil and its bearing capacity in case of both pressing-in (F_dt, kN) and pulling out (F_d, kN) exists a stable relationship F_d (F_dt) = k·M_k, where k is the transition coefficient, 1/m.

It has been determined that the Mk value and MS pile driving speed depend on the soil density, diameter of the pile shaft (d_p, m), blade (D_b, m), tip shape (either open or closed) and height of soil plug in the open tip, the influence of which on the MS pile driving speed and Mk value is bound with the relationship n = D_b/d_p. If n≥3, the M_k value is determined mainly by the soil resistance to driving the blade.

Provided that, the following factors influencing the Mk value required for driving the developed MS piles [4] and the typical ones [5] applied in the Republic of Belarus have been revealed. They include:
1. Driving method: manual, with the MS pile driving speed of 0.5-2 revolutions per minute (V, rpm); or mechanical with the use of mechanisms providing the rotational speed of more than 2 rpm.
2. Blade diameter and pitch. Should the ratio between the blade thickness and its pitch not exceed m ≤ 0.6, the direct proportion between the D_b and the soil resistance to driving the MS piles (V, M_k) is actually observed.
3. Shape and material of MS piles and relationship between these parameters. In particular, dimensions and construction of the blade (number of threads, their pitch and arrangement).
4. Soil type (its density, dampness, etc.). In case of mechanical driving of the MS piles, the blade immersion depth of up to 6 m affects, as a rule, insignificantly the M_k value and the blade sinking rate per revolution (pitch “a”).

According to the results of the performed investigations by means of in-place tests of the MS piles with the length of 1.5-8 m for pressing-in and pulling-out loads, the coefficient of transition from M_k to F_d (F_dt) has been determined for various types of soils with measuring the Mk values corresponding to them. For the purpose of monitoring and operational evaluation of the F_d (F_dt) values, the practical calculation method has been developed for the jammed in the soil MS pile of blade diameter D_b ≤ 800 mm and length 1 ≤ 8 m, which has ensured the improved accuracy of the existing calculation methods based on torque Mk, by at least 30%.

Keywords: the screw metal pile, tests, bearing capacity, torque of screwing up, method calculation, recommendations.

For citation: Kravtsov V. Research and Calculation of the Load Capacity of the Bases of Metal Screw Piles According to the Torque Value Required for Driving Them. In: Contemporary Issues of Concrete and Reinforced Concrete: Collected Research Papers. Minsk. Institute BelNIIS. Vol. 8. 2016. pp. 114-133. https://doi.org/10.23746/2016-8-7




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