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Site Investigation - Elementary or Supplementary to Pile Design?

posted Sep 8, 2021, 8:09 AM by jeffery jim   [ updated Sep 8, 2021, 8:33 AM ]

In previous short note on pile application and positioning, the main concern about piling and the effect on nearby properties, we have seen literature point to no specific values when it comes to safe distance for piling point from triggering damages to properties. All literature prescribe this to be tackled during site investigation and most of the time, this issue is often overseen or remain negligible to many designers.

2. Many structural engineers and contractors usually oblige with the cost envelope indicated or dictated for a project. In many cases, site investigation is a process which is deemed as supplementary to many as it only provides geologist's recommendation for pile type or furnish engineer with time and depth of possible ground in order to have proper ground treatment work for infrastructures.

3. This is wrong and horrendously despicable as this will lead to negligence and abusing the opportunity provided to have a comprehensive understanding of the site condition. Before a site investigation is conducted, design engineers should conduct site reconnaissance and desktop study in order to determine preliminary considerations before planning of a proper site investigation or exploration. Many engineers will use GIS software typically Google Earth for a mere peep on the site location and some to the extend of reviewing river and elevations. These are not able to substantiate proper planning of the exploration. The basemap for a preliminary stage should be based on digital elevation map which is widely available before making adjustment if the area is known to be involved with new developments nearby. GIS application like ArcGIS will help with some statistical analyses on terrain feature which is far more extensive that what Google Earth could offer.

4. The other relevant maps to be considered at areas which deemed critical would be the geological map and soil map. Soil Map may not be necessary since soil family is relatively similar if it sits on similar elevation or topography unless it sits on certain geological formation and era. Soil use report is another good source of understanding and often being used by many environmental consultants when generating their EIA report. Sadly, these kind of sources are usually obsolete and no longer in place due to socio-economic changes which taken place and certain changes in land husbandry within the region. Provision should be made with contamination that could take place making certain zone into brownfield and not as reported 40 years ago as greenfield.

5. BS 5930 Code of Practice for Site Investigation is a great manual, however this code speaks the fundamental of things and key concepts when talking about planning and execution of proper and comprehensive investigation. The interpretation of report from the work should be done by multidisciplinary engineer (preferably geotechnical engineer and a geologist) when it comes to irregularity of readings, arbitrariness of records; and temporal and spatial variability from soil/rock test and geological mapping. This goes back to geostatistics fundamental of random sampling with high volume of sampling for correlation or krigging, or perhaps purposive method. The research or investigation should be planned based on criteria and sampling method should be accurate. In this case, seasoned engineers and geologists will be useful when using purposive method based on localized experience and knowledge. This eliminate biases from total randomness to proximity with minimal or sparse outliers.

6. Most typical site investigation reports we come across in local projects involves soil/rock properties from field tests and sunken borehole logs (including limited groundwater level monitoring) apart from limited chemical tests. It is quite rare to have a report which includes geological mapping unless slope failure occurred during the construction phase which involves seismic hotspots and fault lines. It is even rare to see the use of various geological instruments to determine site condition such as the use of Electrical Resistivity Tomography to reconfirm readings from boreholes except for certain special projects (landfill and water dam in my previous projects).

7. There are so many test to be conducted and I will not be discussing about these testings as I have discussed about some of it in my past short notes or unless if there are specific follow-up inquiries about these test at site. I have rendered the actual practices of how site investigation have been treated as supplementary record during design instead of taking precedence to the whole design process especially the foundation which is the primary structural member in providing stability.

8. In order for a structure to be designed with sufficient loading capacity and efficient load propagation to bearing, the pile should designed based on the site condition. Boreholes should be located between 10 to 15m for critical water retaining structure and 30m for ordinary structure depending on imposed load category, partial safety factor and reliability differentiation. For infrastructure, boreholes may not be essential (it can be substantiate with Macintosh Probe, Trial Pit, Vane Test, Plate Test, etc.) except for locality with superstructures especially regions with possible geological deposit less than 30m depth and with seismic PGA value of 0.05G (add on microzoning test if lack of data or other geophysics testings).

9. Retrofitting of structure and introduction of new piles may need more reconsideration in planning which is similar to adjacent property damage protection studies. This involves a lot of parameters and conditions throughout different phases which may fluctuate momentarily, temporarily or periodically many parameters involving water and soil.

10. Once more, it is important for all engineers to revisit and reconsider processes in site investigation. This should take precedence above all and as the key reference in practicing value engineering. Incomplete data only points toward a direction where nothing can be validated and to substantiate the design. Worst, GIGOLO - Garbage in, Garbage Out, Laugh Out(loud).