Interpretation of Reflectograms of Seismic Acoustic Flaw Detection of Piles

. The paper presents the results of continuity study of secant piles in excavated pits at microtunnelage by means of seismoacoustic flaw detection. The piles under study were placed in the work pits and the reception pits excavated for transfer of utilities of the urban networking infrastructure. The importance of the quality control is stipulated by safety assurance of the performance during utility re-routing. The piles provide for the important function of pit wall reinforcement against soil slides due to groundwater pressure. Reflectograms were provided of the auger-cast grout piles under study with subsequent analysis and confirmation of the design length and continuity of the pile trunk. The results of the test demonstrate a correlation of the soil uniformity with the signal regularity. On the interface of two soil layers of different density, the sound wave is reflected and partially passes further generating characteristic false defect peaks. The provided results are of practical value during reflectogram processing, allowing for experimental recognition of interferences and exclusion of flaws.


Introduction
Annually, about 100 km motorways are built in Moscow City, including not just the traffic-bearing surface, but also complex engineering structures with tunnels, bridges and multi-level interchanges.In 2022-2024, it is planned to build more than 272 km motorways, 87 bridges, tunnels, cross-overs and 48 pedestrian crossings.
The construction of traffic infrastructure facilities often encounters problems with re-routing of utility networks.The most up-to-date utility transfer method without damage to the existing urban infrastructure is microtunnelage.The essence of the microtunnelage technique is that the tunneling in the soil is performed by a tunneling machine.A powerful jack station installed in the shaft provides for pipeline routing.Modern non-destructive road crossing makes it possible to build tunnels with over 1000 m length without intermediate vertical shafts.Boosting jack stations installed inside the pipeline provide for the required propulsion force not exceeding the maximum limit values for each section.A special laser guidance system, installed in the tunneling equipment allows for precise tunnel routing over large distances and for curved tunnel routing.
The advantage of the microtunnelage is the possibility for utility routing in the most complicated circumstances, where conventional methods could not be implemented without special soil reinforcement measures.
Microtunnelage techniques envisage excavation of work and reception pits where the major works are performed.
For provision of construction performance safety in the work and in the reception pits, an essential pre-requisite is the verification of the continuity and of the length of the piles reinforcing the pit walls [1][2][3][4].
For the time being, the seismoacoustic flaw detection is one of the most up-to-date pile continuity test methods in the global practice.This method is based on the sound propagation theory (high and low frequencies) in solid body.This method is free of the well-known drawbacks of the existing methods which is the essential advantage of this evaluation method of the pile continuity.
However, in Russia, this method is rather rarely applied due to absence of codes on the basis of which contractors could utilize it and submit the test results to construction surveillance authorities.As a rule, the seismoacoustic flaw detection is used in combination with other methods [2][3][4][5][6][7].
In addition to the absence of codes ruling the testing procedure, the seismoacoustic flaw detection has a very poor corps of research studies.The studies of the past years were aimed at the description of the physical process of propagation of oscillations in a solid body [8][9][10][11][12][13].Typical reflectograms provided by manufacturers scarcely allow for high-quality and correct processing of the obtained results by the operator.
It is the purpose hereof to present and to interpret the results obtained during the evaluation of the continuity of piles in work and reception pits in the course of utility transfer.

Methods and Materials
The seismoacoustic flaw detection is used for continuity evaluation in the field independent on the type of the piles.The method is based on the hardware recording of the response of the pile under test to external impacts with specified pulse parameters.The signal is recorded by means of an accelerometer sensor connected to a special computer with a software package for the digitalization and the interpretation of the obtained data (Fig. 1).The results of the data processing are visualized as computer graphs (reflectograms) showing the pile length, the continuity, the damages and the arrangement thereof.
A hammer strike on the pile end imparts a longitudinal acoustic compressional wave propagating along the pile trunk with a velocity c.The acoustic specifications of concrete structures greatly differ from those of dispersed soil.Piles made of concrete feature relatively low energy losses of attenuation and re-emission in the rock mass.The acoustic waves are reflected back on the interface boundary (concrete -flaw, concrete -soil, etc.).The time interval between the hammer strike and the reflection from the interface boundary is measured by the sensor and is equal to the time t required for the wave propagation in the pile trunk twice along the entire length l (downwards and upwards).

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The pile length l is determined based on the time interval t of the wave propagation.The wave propagation velocity is thereby assumed unknown: As per Newton's second law, the propagation velocity of the elastic wave is dependent on the medium specifications: The defects are characterized by changes of the cross-sectional area E of the pile trunk and of concrete specifications ρ (Fig. 2).It is common knowledge that, in materials of higher strength, the propagation velocity of elastic waves is higher [1][2][3][4][5][6].

Results and Discussion
For correct interpretation of the seismoacoustic flaw detection, a geological survey is pre-requisite.The geological data, including soil specifications, are provided by the contractor.As a result of the analysis of the spatial variability of individual parameters of the physical and mechanical soil properties, determined both in the lab and in the field, in the survey site located in Moscow City (interchange renovation), 4 geotechnical units (GTUs) are identified.
During the survey performance, drilling down to 23 m depth in the survey site discovered groundwater of super-Jurassic aquifer system.Groundwater was discovered by all wells in the depths from 5.2 m to 13.5 m.The aquifer stratum has a local head of up to 1.4 m.Steady levels are registered in the depth interval from 5.2 m to 12.2 m.The water-bearing materials are Middle Quarternary fluviolimnoglacial sands and Lower-Cretaceous sands, the top local confining bed is formed by Middle Quarternary fluviolimnoglacial loamy sands, the bottom confining bed is formed by upper-Jurassic loams, not discovered at drilling down to 23 m.The groundwater recharge takes place due to the infiltration of the atmospheric precipitation.
The coefficient of permeability of sandy soils has been measured in the laboratory with the following results: • for GTU-3 fine sands of middle density: 6.5 m/day; • for GTU-4 fine sands of high density: 3.5 m/day By chemical composition, the groundwater has sulfate-hydrocarbonate, magnesium-calcium composition, rather sweet, with moderate hardness (carbonate) with a mineralization of 0.3-0.4g/l, pH=6.9-7.3.In accordance with SP 28.13330-2017, the groundwater is not aggressive to concrete of all grades.The groundwater is medium-aggressive to the reinforcement of reinforced concrete structures, as per GOST 31384-2017.During seasonal increased atmospheric precipitation infiltration periods (spring snowmelt, etc.) emerging of temporary groundwater layers of upper ground table type in the upper part of the vertical section, as well as a level increase of the upper aquifer stratum.
Based on the geotechnical data, non-destructive road crossing must be performed by means of AVN 800 tunneling machinery complexes for utility transfer.
All pit works are carried out under forced intake and exhaust ventilation (PB 03-428-02 Section 12).
The outlet/inlet of the tunnel machinery complex envisage soil reinforcement by cement grout jetting.Such works are performed to provide for the occupational safety of the workers, the safety of the existing underground utilities and facilities, the surrounding development in the affected area, for protection of the pits against ground slides in the inlet/outlet of the tunneling machinery and loosening of the overlying GTU layers.The sheet pile wall of the pits is not withdrawn against the associated soil loosening when it is pulled out.
The utility routing by means of microtunneling will be performed in the following sequence: 1. Excavation of work and reception pits; 2. In work pits, start and buttress monolithic reinforced concrete walls are erected.Soil and cement elements (SCEs) are installed (Fig. 2).In addition, sealing rings shall be installed on AVN tunneling plant inlet and outlet; 3. Access stairs shall be envisaged for the workers to get down to the pit bottoms.Ladders are only admissible for access to/from pits with 5 m depth; 4. Tunneling plant installation in the work pit.The plant is put into the work pit by means of a mobile crane; 5. Tubes are supplied into the pit by means of a mobile crane; 6.The work tubes are drawn into the encasement by means of a jack station.After finishing of the works, the plant is withdrawn from the pit by means of a mobile crane; 7. Backfilling of the pit with sand; 8. Reclaiming works..0 is the specific gravity of water, kN/m3; H = 2.0+1.5=3.5 is the hydrostatic pressure on the sole of the horizontal ground water cutoff screen in natural conditions, m; gjet = 20.0 is the specific gravity of the GWC, kN/m3.The thickness of the horizontal ground water cutoff of cement-soil mix shall be at least 1.5 m.We assume the thickness of the horizontal cutoff equal to 2.0 m.
The calculation of the material strength of the soil-cement mix structures without reinforcement is performed in accordance with SP 63.13330.2018(par.6.4.8), under recognition of formula 5.7 of SP 291.1325800.2017like for concrete structures without reinforcement.Thereby, the design compression strength determined for soil-cement mix is assumed.The design tensile resistance is assumed zero (par.6.1.5 of SP 291.1325800.2017).
The design calculation of concrete elements under bending loads shall be carried out based on the following condition (formula 7.8 of SP 63.13330.2018): (5) ≤  М is the bending moment from external load, kN•m; Мult is the ultimate bending moment which can be accommodated by the element's cross-section (GWC), kN•m.
External load accommodated by GWC: In order to take a decision on the continuation of further works to excavate a pit with 14.2 m depth, seismoacoustic tests were carried out to confirm the design lengths and continuity of the piles (Fig. 2-5).Diagrams close to the reference ones cannot be obtained during real tests due to non-uniformities of the soil and the concrete.In this specific case, additional factors contributing to interferences are also neighbor secant piles and the soil reinforcement by cement grout jetting [13,14].To confirm that the piles have no flaws, they are visually inspected along the entire depth during the pit excavation (Fig. 5).During the visual inspection, no flaws were detected in any of the piles under test.

Conclusion
The re-routing of utilities is a rather complicated job requiring design documents developed based on the results of all types of survey affecting the safety of the construction (renovation) and the operation of the subject traffic infrastructure facility [14][15][16][17][18][19][20].
It is often impossible, in the design stage, to foresee in different construction stages all particulars required for consistent job progress and accounting of the network utility re-routing.Thereby, the owners of the network utilities demand in most cases a separate permit for the renovation of the utilities re-routed in the framework of the preparation of the construction site for linear facilities.
Pre-requisite for a valid interpretation of the results of the seismoacoustic flaw detection is a study of the geological section, and a preliminary instrumental measurement of ultrasound propagation velocity in each individual pile.
The obtained results confirm the influence of the soil uniformity on the signal purity.The less the structural uniformity of the soil, the more false characteristic peaks are obtained in the reflectogram [13,14].Also, cement grout jetting for soil reinforcement is an additional signal distortion factor.

Fig. 1 . 5 E3S
Fig. 1.Jet cementation of soils at the base of pits using "Jet" technologyTo provide for construction performance safety, ground water cutoffs (GWCs) shall be designed.The horizontal GWCs are designed in accordance with SP 291.1325800.2017(par.6.4.8).The thickness of the horizontal ground water cutoff screen based on the floating up conditions, is determined by the formula:

Fig. 2 .Fig. 3 .
Fig. 2. General view of the location of piles when strengthening the walls of the pit