REDUCTION OF THE VIBRATIONS CAUSED BY TRANSPORT TO THE GENERAL POST-OFFICE OF MOSCOW (part 2)

Dr. Sc. (Eng.) V.A. Ilyichev,

Dr. Sc. (Eng.) L.R. Stavnitser,

Ph. D. V.Y. Shishkin

Research Institute of Bases and Underground Structures (NIIOSP)

In accordance with this methodology, a computer program has been developed and calculations have been made for the reinforcement of the soil as a reinforced base (1) for the foundations of Moscow General Post-Office in order to design foundation soil reinforcement with cast-in-place piles. The development of crushed-stone piles installation technology involved determination of the optimal number (n) of piles to be installed under the foundation as well as determination of the driving depth (S) for crushed-stone compaction in a well. It was discovered that S value should not be less than 1.5 m while n value – should not be less than 10. In this case the respective value of estimated resistance of reinforced base Ra sufficiently exceeds the average pressure under the bottom of the foundation equal to 250 kPa. Within the framework of the base reinforcement project, it was accepted that n=12 which increased the estimated deformation module up to 21 MPa, so that additional foundation settlement would not exceed 2 cm. This is less than the permissible value for this building.

Since the Post-Office building is exposed to the vibrations caused by transport, the objective was set to examine its foundation instrumentally. This involved gauging transportrelated vibrations before and after base reinforcement as well as measuring the vibrations caused by the technological impact of the pneumodrilt during wells construction and sand-and-crushedstone mix compaction.

A set of apparatus, including SM-231 type three-channel vibrometer (vibrograph) of RFT brand, Germany, with an integrating amplifier and a power unit, KV-12 type piezoelectric accelerometers (operating frequency band – 1…250 Hz); H-115 oscillograph with ultraviolet registration of the processes examined. Complex calibration has been peformed for each channel at VEDS-400 vibration-testing machine within the frequency band that was twice wider than the transport-related vibrations spectrum. Vibration transducers were installed in three mutually perpendicular directions at foundation edges in specially dug exploration-pits.

The oscillations registered before the base reinforcement showed that their amplitude level did not exceed 1.5 mkm that is not dangerous for the load-carrying capacity of the building structures. The amplitudes of the oscillations caused by subway trains and road transport remained comparable, being predominantly horizontal, parallel to Myasnitskay street. The biggest amplitudes of the subway-related oscillations were observed in the high-frequency part of the spectrum (45-75 Hz) while the biggest amplitudes of the road-transport-related oscillations were specific for low-frequency vibrations (1.5…3.0 Hz).

Technological vibrations resulting from base reinforcement work were registered in several regimes of pneumodrilt operation: in the beginning of well construction and at various depths as well as in reverse regime during pneumodrilt extraction from a constructed well and also during repeat driving of the pneumodrilt back into the well for compaction of sand-andcrushed- stone mix in the process of cast-in-place pile construction. Oscillations frequency band turned to be narrow (25…30 Hz) and almost independent from pneumodrilt driving depth. As a rule, the biggest amplitudes were registered in the initial phase of driving. Their maximal values did not exceed 3.8 mkm for vertical components and 3.5 mkm for horizontal ones (which is perceptible within a radius of 3…4 m from the well), being great less than the permissible limits which is not dangerous for the load-carrying capacity of the foundations and the building structures.

Upon the completion of base reinforcement with cast-in-place piles, instrumental analysis of foundation vibration parameters was performed on the basis of the measurements provided by vibration transducers installed at the same point as they were at the first stage involving vibration exploration before the start of the reinforcement work. These measurements comparison showed a significant decrease in the level of transport-related oscillations. In particular, before the base reinforcement the maximal amplitude of the vertical components of oscillations reached 1.5 mkm while after the reinforcement it reached 0.25 mkm only, while the respective horizontal components of oscillations decreased by 6…10 times.

It is worth noting that the reinforcement work most considerably decreased high-frequency vibrations caused by subway trains.

Before the base reinforcement maximal amplitudes of oscillations could be clearly observed within the range from 45 Hz to 75 Hz while after the reinforcement horizontal components with 0.15 mkm amplitudes could be observed within the range from 25 to 31 Hz only.

After the base reinforcement the maximal amplitudes of the vibrations caused by road transport were registered on 8…14 Hz frequencies that were slightly higher than before the reinforcement (1.5…3.0 Hz) but they did not exceed 0.25 mkm for vertical and 0.2 mkm for horizontal components which is several times less than the level of the respective amplitudes of the oscillations registered before the cast-in-place piles construction.

REFERENCES

1. Gorbunov-Posadov M.I., Ilyichev V.A., Krutov V.I. [1985].

Bases and Underground Structures. Reference Book for a esigner. Edited by E.A Sorochan, U.G. Trofimenkov. Moscow, Stroyizdat.