Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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Non-invasive ground penetrating radar investigation of a failing concrete floor slab

Martin L. King, GPR Geophysical Services, PO Box 6056, New Plymouth, New Zealand
Dany P. Wu, GPR Geophysical Services, New Plymouth, New Zealand
Dr. David C. Nobes, Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, NZ

ABSTRACT

At a New Zealand wastewater treatment plant there are several large wastewater settlement tanks that are crucial to the operation of the plant. These concrete tanks are 50 metres in diameter and 8 metres deep with the base of each tank approximately seven metres below ground level.

It was discovered that the base of one of the tanks had become distorted, with the result that it could no longer carry out its function. This was a major problem for two reasons:

  1. The tank was important for maintaining sufficient throughput for the overall operation.
  2. There was a danger of contamination of the subsurface aquifers, which are the main potable water supply for the area, should a tank base failure occur.

In addition to the problems outlined above, due to the type of construction of these tanks, the floor could not be cut into or disturbed to any significant extent without risking catastrophic failure.

Ground penetrating radar (GPR) was used to accurately pinpoint those areas below the base of the tank where voids had formed; the subsurface voids were the root cause of the tank floor distortion. The information that was provided using ground penetrating radar enabled grouting repairs, through the floor slab, to be carried out using precisely positioned small diameter holes thus avoiding the risk of catastrophic failure of the tank base. GPR also allowed the grouting progress to be monitored until the repairs were completed.

Keywords:
ground penetrating radar, non-invasive mapping, concrete floor, voids

INTRODUCTION

This study concerns a New Zealand wastewater treatment facility which uses large settling tanks, each 50 metres in diameter, 8 metres deep, and with a holding capacity of approximately 14,000 cubic metres of untreated waste liquid. The concrete tank construction consists of a segmented precast concrete wall with a 300 mm thick concrete floor base (Figure 1). The tank floor base is constructed using an unreinforced 100 mm thick concrete site base slab overlain by a 200 mm thick, steel reinforced, structural concrete floor slab.

A problem with one of the tanks arose when the automatic sweep arm became misaligned and thus non-operational. A suspect section of the floor was inspected, which revealed a cavity beneath the floor of the tank. The concern was that a significant

Fig 1: Wastewater tank cross-section construction detail.

number of other voids might have formed under the tanks causing the floors to gradually become so distorted that rupture of a tank was eventually likely. Any failure of a tank base would create serious structural and ground water contamination hazards.

The context and mode of construction influenced the methodology. The tank bases slope toward the tank centres and have lattices of post-stressed steel rod reinforcing in the structural concrete bases. Due to the type of construction, it was critical that the concrete floor slab remain substantially intact during investigation and repair operations. Any sizeable disturbance of the floor slab would weaken the concrete and could cause the base to be pulled out of shape by the post-stressed steel, resulting in catastrophic tank base failure. The voided areas thus had to be detected, mapped and repaired without significantly disturbing the reinforced floor slab.

Method

Ground penetrating radar (GPR) was selected as the best technique to provide the information required in this instance. GPR is a non-invasive high-frequency electromagnetic geophysical technique for subsurface exploration (e.g., Davis and Annan, 1989). Generally resolution increases with increasing frequency whereas depth of penetration increases with decreasing frequency. A balance is therefore necessary to achieve sufficient penetration with suitable resolution. GPR has been successfully used for a variety of applications, including mapping of contaminants (King, 2000), buried foundations (Nobes and Lintott, 2000), burial sites (Nobes, 1999) and concrete foundation integrity (Annan et al., 2002; Giannopoulos et al., 2002).

In this case a GSSI SIR-2 GPR system, with a 900 MHz shielded antenna, was used to scan the tank base. Data collection timing was set at 8 nano-seconds giving a penetration of around 350 mm assuming an average signal velocity of 90 m/ms (a dielectric coefficient of about 12). Suitable high and low pass filters were used as well as time varying gain to enhance the data and resultant imaging.

Radar scans (profiles) were acquired along radial lines commencing at the tank wall and moving towards the tank centre (Figure 2). The resultant radargrams were then analysed and areas were identified that were interpreted to be underlain by voids.

Fig 2: Plan view of tank showing radar scan lines.

Results

The resultant data clearly identified areas where voids had formed (Figure 3). The voids will most likely be filled with air, or possibly water. In either case, the contrast in physical properties between the concrete base and the underlying voids will be significantly stronger than between concrete and sediments. The voids thus appear as "bright spots" with strong radar echoes, as seen on the right-hand side of Figure 3. It further enabled these demarcated areas to be classified as shallow and/or significant void zones. The site concrete/ structural concrete interface could also be seen and areas where the site concrete layer, under the structural layer, had collapsed into the void were obvious.

Fig 3: Radargram showing cross-section through tank concrete base. Scan taken prior to repair.

A scaled AutoCad drawing was used to delineate the detected voids (Figure 4), with the sizes designated large and small, enabling accurate targeting of sub-base voids for repairs without risk of serious damage to the tank floor. Precisely positioned holes, 50 mm in diameter, were drilled through the base, and specially prepared grout was used to fill the voids.

Fig 4: Plan view of typical void location plotted to scale using radargram results.

Results were plotted onto client's original AutoCad construction drawing.

The extent to which each grouting operation was successful could also be monitored by acquiring additional radargrams during and after the grouting process. A final radar survey was used to confirm successful elimination of the cavities. The strong contrast between the concrete base and the underlying voids disappears (Figure 5), because the physical properties of the grout and concrete are similar.

Fig 5: Radargram showing cross-section through tank concrete base. Scan taken after repair. Note the absence of strong reflectors in the grouted zone.

SUMMARY

Ground penetrating radar was successfully used to delineate the locations of voids beneath a wastewater treatment holding tank. GPR was also useful for monitoring the progress and success of the subsequent grouting operation. The use of GPR avoided the expensive, invasive and potentially destructive task of cutting through the concrete base, which could have exposed the surrounding subsurface sediments to the risk of tank collapse and consequent contamination of the subsurface.

REFERENCES

  1. A.P. Annan, S.W. Cosway and T. DeSouza, 2002. Application of GPR to map concrete to delineate embedded structural elements and defects. In S. Koppenjan and H. Lee (eds.), GPR 2002: Proceedings of the 9th International Conference on Ground Penetrating Radar, Santa Barbara, California. Paper number 4758-129.
  2. J.L. Davis and A.P. Annan, 1989. Ground penetrating radar for high resolution mapping of soil and rock stratigraphy, Geophysical Prospecting, 37: 531-551.
  3. A. Giannopoulos, P. McIntyre, S. Ridges and M.C. Forde, 2002. GPR detection of voids in post-tensioned concrete bridge beams. In S. Koppenjan and H. Lee (eds.), GPR 2002: Proceedings of the 9th International Conference on Ground Penetrating Radar, Santa Barbara, California. Paper number 4758-131.
  4. M.L. King, 2000. Locating a subsurface oil leak using ground penetrating radar. In D.A. Noon, G.F. Stickley and D. Longstaff (eds), GPR 2000: Proceedings of the 8th International Conference on Ground Penetrating Radar, SPIE Vol. 4084: 346-350.
  5. D.C. Nobes, 1999. Geophysical surveys of burial sites: a case study of the Oaro urupa, Geophysics, 64(2): 357-367.
  6. D.C. Nobes and B. Lintott, 2000. Rutherford's "Old Tin Shed": Mapping the foundations of a Victorian-age lecture hall. In D.A. Noon, G.F. Stickley and D. Longstaff (eds), GPR 2000: Proceedings of the 8th International Conference on Ground Penetrating Radar, SPIE Vol. 4084: 887-892.
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