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Piles - Part 2. Application and Positioning

posted Sep 7, 2021, 6:29 AM by jeffery jim

Piles as we have learned previously is an engineering application which involves loads and stresses propagation from structure to substructure and subsequently to the ground. Today, an inquiry was made in regards to the safe distance of piling works to the nearest building.

It is a simple question but it is rather complex to be precise and can cause serious engineering, legal and economic implications if it is done the wrong way. As known to many, adjacent properties mainly structures can settle or crack and most of the time vibration is the main culprit. I would like to highlight that vibration is not the main concern as I will discuss in thematic paragraphs.

2. We often get a lot of feedback and concern when piling works is too close to adjacent properties or facilities. Standard specifications did not dictate any distance nor method in facilitate piling works with the site condition and this particular concern. In British standard (BS 8004), this is vaguely mentioned as part of survey and reconnaissance work on adjacent property. The European Standard (BS EN 1997) made reference to execution of geotechnical works on various types of piles (BS EN 12699, 12794 and 14199) prescribe similar considerations as per British Standard. American Concrete Institute (ACI 543) did not mention anything in regards to such requirement at all. In Malaysia context, the two standards which can be used bases for this and the compliance without defaulting Section 14 of Contract Form PWD203/203A. This particular section only highlights that damages to properties and other liabilities are to be handled by contractor and client shall be kept indemnified from all legal action(s) and demand(s) shall there be any damage to property or properties surrounding the site during the construction. Standard Specification for Building Works (SSBW) 2005 highlights the risks and possibilities from piling works and property damages. SSBW 2014 prescribes this issues to be mitigated or explained in method statement as part of the construction management or Project Quality Plan. Nevertheless, SSBW 2014 unnecessarily includes consultant and client into the picture as they are called to approved the method statement.

3. The question remains how far is too far and how close is too close? It is all very much depending on the ability of design engineers to interpret the site condition during the design stage and mobilize the right method to suit the proximity of surrounding structures and cost as well as ground condition. As recommended by earlier literature, all pile design should be robust and perform safely based on geological setting and geotechnical consideration. For developers or turnkey contractor, this can be mitigated by integrating the piling design as part of methodological construction method to resolve problems which may arise from damages succumbed by nearby properties.

4. To many, the simplistic approach for this problem is by introducing micropile or injection method for displacement piles. Injection pile is a great method which can reduce the vibration up to twenty times and can maintain pretty low cost as per conservative pile driving method with exception of space constrain and existing platform condition to operate. Micropile and bored pile on the other hand can be ridiculously expensive and time consuming. Then again, it is subject to financial ability of a client after conducting cost benefit analysis of a particular foundation design.

5.There are many other methods which can be considered however ground condition at the surrounding parallel to the properties should be first considered. One of the most common methods would be from the site investigation borehole log. Nevertheless, the limitation of a borehole log is its accuracy when we making interpolation of strata based on triangulation method as each borehole is only accurate and reliable for 10 meters. This is an issue when it comes to chaotic soil or geological condition.Trial pit methods is limited by the depth of the trench based on the excavator boom reach. The best method would be Electrical resistivity tomography (ERT) by placing a single line parallel to the structure or property intended for protection. This test should be done with additional info in regards to the property's structure and foundation design. With the probe from ERT in placed, the section can reach up to 50 depth and soil condition can be determined.

6. The reason why ERT should be used as part of supplementary test to boreholes is to check the resistivity of material underground. This will give clear indication of the position and depth of materials and saturation level based on correlation. The importance of establishing saturation condition and underground material is to generate possible direction of (vibration) Reyleigh wave, S wave and P wave and annotation it produced in circular or cylindrical condition. The other outcome which ERT provides would be the possible localized water saturation zone and aquifers. From soil taxonomy and pedology, infiltration rate can be projected which is essential when we are using various damping methods. This generates condition or phases which may cause settlement due to water saturation or even loss of mass as an outcome due to soil bulking factor.

7. Geological and geotechnical report from ERT will suggest the best way forward to mitigate issue in regards to property damage. There are standalone methods and some are combination of several methods. In-fill trench is one of the simplest method which works in a way which will halt vibration from Reyleigh wave and weaken P and S wave which travel in cylindrical or circular manner respectively. Nevertheless, the suitability is depending on cost for infill material. Water is the cheapest however may subject to pore pressure, infiltration and retention rate as well as other geotechnical issues on global stability - depending on resting or active phase/condition. If water is used, it is recommended that this only functional if it is more than 8 meter away from piling point and amplitude can be reduced by almost 0.8 by ratio if piling point is 12 to 16 meters away. Subsurface or ground water movement can be hazardous if the soil saturation changes and this lead to disequilibrium and settlement to nearby property especially if sits on shallow foundation. Differential settlement can cause damages to public and private infrastructures.

8. Another method is combination of piling method which employs the use of micropile and injection pile up to two or 3 grid lines at the edge of the project perimeter or boundary which will act as buffer before continuing other grids with hydraulic driven piles. It is similar to infill method with exception that this soldier pile-like application is used to suppress the annotation of vibration waves. It is surprising that smaller pile can triggers extraordinary vibration. Smaller pile is said to contribute to excessive vibration in comparison to its larger size counterpart as the result of resistance at the shaft and/or the shoe or toe. Hence, depth of soldier pile is subject to distance and suppression capacity based on soil condition.

9. One way which contractor or developer can consider as a confirmatory method of piling method selection is the use of geophones and other geo instruments. This test method is viable and recommended during the site investigation stage in order to find the right way forward in preserving and protecting nearby structure. Test has to be done at the earlier stage of the design or else, telltale sign will accommodate you with lawsuits and demands once the surrounding property or properties is/are damaged.

10. There is no straight answer and recommendation made by any standard specification as this question remains quite subjective. This depends highly on the soil condition as redundantly mentioned in all specifications and require geologist, geotechnical engineer and structural engineer considerations.