Amir

Measuring the Inclination of Deep Foundations Erez, Joram

MEASURING THE INCLINATION OF DEEP FOUNDATIONS

Amir, Erez1, Amir, Joram2

1 President, Piletest.com Ltd. [email protected] 2 Chairman, Piletest.com Ltd. [email protected]

Abstract The allowable deviation of piles from verticality is mentioned in practically all piling

specifications, with typical values ranging from 1.33 to 2 percent. Similarly, specifications also limit the tolerance of raked piles from their specified inclination. This restriction is easily understood for piled retaining walls which are required to remain water tight or serve as walls for underground parking garages. Exceeding the above limits in piles foundations can introduce large bending moments and shear forces (Amir, 2012) and may even lead to structural failure in piles designed strictly for axial loads.

Still, the above specifications are seldom enforced due to the lack of convenient testing apparati. In this paper, we describe the BIT (Borehole Inclination Tester), present its details and operation, and show a variety of field results.

1. INTRODUCTION 1.1 Classification of test methods for inclination

Several methods exist for testing inclinations of deep foundations. Those can be classified according to timing of test or sensor positioning: Timing of test During drilling, providing information during the drilling process Post drilling, checking the verticality of freshly drilled borehole, dry or slurry-filled After casting - testing the finished foundation Sensor positioning Pendulum-based devices - a pendulum lowered into the fresh borehole senses the

distance to the walls using either: o Feeler arms: A set of mechanical arms which follow the borehole walls in

several positions (typically four) around the pendulum o Remote sensing: A set of distance meters measure the distance to the borehole

walls using laser (in dry holes) or ultrasound (in wet boreholes) Centralizer-based devices keeping a mechanical device aligned with the borehole axis

and measuring its inclination. Pendulum based systems, in addition to inclination, usually provide additional information about the borehole shape and volume. Those systems, however, cannot be used to test raked piles or slender boreholes where the permitted deviation is larger than the borehole diameter. Centralizer based systems only provide information about the borehole inclination. The main challenge with those systems is the size and weight of the centralizer needed to test full-size boreholes. Additional challenge is the rotation of the centralizer.

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2. OVERVIEW OF THE BIT SYSTEM The BIT system falls into the category of centralizer based devices. It is dual purpose: During drilling, it can be mounted on the drilling bucket that acts as centralizer. In the finished pile, it can be lowered inside the access tubes which are routinely installed for crosshole ultrasonic testing (ASTM D6760 - 14). In large boreholes, the BIT system overcomes the challenge of centralizer size and weight by using the drilling bucket as centralizer and the drilling rig to move it up and down. BIT overcomes the centralizer rotation by continuously measuring azimuth and compensating any rotation using trigonometric calculations. In finished piles, BIT uses a specially designed centralizer (Figure 1 item 3) that fits into access tubes as small as 40mm ID.

The system consists of five main parts (Figure 1). 1. The upper unit which contains the electronic circuitry and is mounted inside a

100 m. cable-reel. 2. The sensor, containing a bi-axial (X-Y) MEMS inclinometer, a MEMS gyro and

a thermometer, all packaged in a compact waterproof housing. For drilling monitoring the lower sensor is rigidly mounted on the drilling bucket crossbar

3. For finished pile testing, the sensor is mounted on a stabilizer that acts as both a centralizer and a rotation-suppressor.

4. A wireless depth meter 5. A hand-held computer for recording and presenting data.

Figure 1. BIT system components.

3. PRINCIPLE OF OPERATION

3.1 Principle of operation - Drilling configuration

After the hole is drilled to the desired depth, the sensor is fixed on the bucket cross bar and inserted into the hole while pointing to the reference North. The depth, inclination and azimuth are measured at ground level and then at predetermined stops until the drilling bucket has reached the bottom of the hole. The procedure is then repeated on the way up, with readings taken at the same stops as before. During the whole procedure, the rig operator should keep Kelly bar rotation to a minimum (although some degree of

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unavoidable rotation is tolerable). When the bucket returns to the surface, it is aligned back with the reference North, and a final reading taken. Typical test duration is in the order of 10 to 15 minutes. 3.2 Principle of operation - Access tube configuration

The sensor with the centralizer is inserted into the access tube pointing to the reference North. The depth, inclination and azimuth are measured at pile head level and then at predetermined stops until the unit reaches the bottom of the tube. The procedure is then repeated on the way up, with readings taken at the same stops as before. When the unit returns to the surface, it is aligned back with the reference North, and a final reading taken. 3.3 Error assessment

Since deviation readings are taken at intervals going down and up again, the final accumulated deviation on the top of the pile/borehole (which should theoretically be zero) serves as an indication for any inaccuracies during the test. In case those inaccuracies are larger than a desired value, the source of the error could be investigated and the test can be repeated. 4. TYPICAL TEST RESULTS 4.1 Testing finished pile using CSL access tubes

A 45m deep, 1.2m diameter pile supporting a power station structure was installed with 2 plain metal 50mm CSL access tubes. BIT testing on both tubes showed nearly identical

deviations in both tubes (differences could be attributed to inaccuracies in the access tube placement within the cage). The maximal measured deviation from vertical was 0.19m at the depth of 27.5m (0.69%). The test closure error was 0.025m (


4.2 Testing a finished retaining wall using CSL access tubes

50 piles with a 0.6m diameter and 15m deep, serving as a cut-and-cover tunnel walls were installed with 3 CSL tubes each. BIT testing was done on all 150 tubes to the depth of 7m in order to plot the deviation of the wall into the tunnel space. Average testing time per tube was 5.5minutes Average closure error was 0.02m Deviations of 0.2m into the tunnel space were found in some piles, providing the owner and contractor with valuable information long before the tunnel was excavated. 4.3 Testing fresh boreholes

A 40m deep, 1.8m diameter single pile supporting a railway bridge was tested at several stages during the drilling during process and at the maximum depth of 40m. Drilling was done using Bentonite slurry using several buckets to overcome the soil and hard rock layers. The max deviation found was 0.52m (1.27%) at 40.5m towards 245 (WSW), Test closure error was 0.1m Note that test closure errors in those conditions is larger than in CSL access tubes probably due to the unavoidable tolerances between the bucket and borehole walls. The results are presented in top view (Figure 3) and in side view (Figure 4). The shaded triangular areas represent the no-go zone according to the project specifications.

0.30m0.10m

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Figure 3. Top view.

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0.10m 0.25m 0.40m

Offset [m]>> WSWENE


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5. CONCLUSIONS The system presented is a versatile and portable instrument for quickly measuring the deviation of bored piles from the vertical with accuracy within 0.1%. It may therefore assist the geotechnical engineer in enforcing the specification for pile verticality. The system may also serve piling contractors who have to meet strict verticality tolerances in contiguous and secant piled walls. In addition, it can help the contractors to evaluate the suitability of specific rigs to difficult site conditions, such as the presence of boulders or rock. REFERENCES

1 Amir, J. M., Amir, E. I (2012): Testing of Bored Pile Inclination - The 9th International Conference on Testing and Design Methods for Deep Foundations, Kanazawa, Japan, 211-216

2 ASTM (2014): Standard Test Method for Integrity Testing of Concrete Deep Foundations by Ultrasonic Crosshole Testing D6760-14, http://www.astm.org/Standards/D6760.htm

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