A Equitable Construction Paying Field - Does it Exist Anymore - Does Anyone Care: A New Screwpiling Case Study

Until industry players make the conscious decision to educate themselves on what constitutes a level paying field with respect to tenders, there is little hope if any, of the end user (the client) ever seeing optimum value for money and a reduction in building defects any time soon. Those of us that still hold the appropriate PI & PL insurance policies to support the quality of their workmanship & engineering capabilities need to shout it from the rooftops.

Little is understood about the principals of "Gauge Theory" (refer to previous LinkedIn Article, Understanding & Acknowledging Gauge Theory ).

In this article, I will demonstrate why fundamentally it is so important and which in turn will ensure a future construction contract achieves "Best Possible Outcomes" for stakeholders.

Actual Demonstration Project

A commercial development in WA needed to be constructed on a well known "P" class site where half the site consisted of imported uncontrolled (non-engineered) fill to a depth of 5.0-6.0m overlying saturated medium dense sands. The other half consisted of a shallow sandy layer over saturated peaty sands to a depth of 5.0-6.0m. The site was tested & identified as PASS (Potential Acid Sulphate Soils). The local' has a previous history of sulphuric acid buildup due to incorrect peat removal processes (mid-2000's) but groundwater & soils were later treated through the use of lime and pH levels were stabilised. None the less, designers of steel piles needed to be aware of the PASS classification & design in sufficient sacrificial steel loss for the specified 75 year design life of the buildings.

Screwpiles were selected as the preferred piling option by the project engineers. Historical geotechnical records showed considerable variability in the management of previous infill civil works on the immediate area so we elected (at no cost to the developer/builder) to conduct our own comprehensive onsite geotechnical testing to accurately determine the geotechnical capacities of a screw piled solution. This enabled our piling design engineers, Foundation Engineering Pty Ltd, to optimise the piling design yet still satisfy the Australian Piling Code AS2159-2009. Due to the variability of the ground conditions part of that optimisation required a 1% of pile quantity Static Load Testing Regime prior to piling commencement to confirm pile capacities in accordance with the geotechnical reduction factor used in the design calculations. We also knew the area very well as over the past 20 years we have conducted at least 10 previous screwpiling projects in the immediate area. None of those projects have shown any signs of settlement. We had the benefit of referring to all that historical data as well.

The Engineering

The project engineers specified 18 different pile compression load case requirements, ranging from 150-950kN. Based on all the data a minimum screw piling design depth for all but the heaviest loaded piles was 6.0m, with the larger piles founding @7.0m. An absolute minimum pile embedment depth of 6.0m for any pile founding level was established. A corrosion classification of "Moderate" (high end = 0.03mm/yr x 75 = 2.25mm x 2 on helical plates, x1 for pile shaft external surface as shafts were to be concrete filled) was calculated as necessary to satisfy the corrosion loss. Steel shaft wall thicknesses needed to be increased, as did helical plate thickness. Specified pile shafts varied from 114 x 6 through 219 x 12.7 G350 ERW CHS. Helix G300 plate thickness ranged from 20-32mm. Helix diameters ranged from 450-750mm. A total of approximately 450 piles were required.

In brief summary the critical factors for pile design were:

1. Identify the depth of uncontrolled/none-engineered fill & or peat (Result: variable 5.0 to 6.0m)
2. Identify water table (Result: 1.5 to 2.5m)
3. Identify corrosion classification (Result: Moderate 0.03mm loss p/yr)
4. Identify MINIMUM pile founding depths (Result: 6.0 to 7.0m)
5. Identify bearing capacity of soil at founding depth
6. Specify MINIMUM helix bearing diameters & plate thicknesses (Results: single 450/20 through to double 750/32mm)
7. Calculate punching shear & interface required concrete beam MINIMUM cover above pile head (Result: 18 variables)
8. Concrete filling of pile shafts (yes/no?: Yes)
9. ISL Static Load Tests to confirm pile serviceability (yes/no?: Yes @1% of volume, meaning 5 tests)
10. MAXIMUM pile location eccentricity (+ or - 50mm transverse of footing centreline maximum)
11. Accurate & detailed piling installation logs

Quality Assurance

An audited & currently accredited quality assurance management system was to include, but not be limited to the following:

• Structural engineers screwpile design & installation & testing certification
• Continuity of screwpile component & document traceability
• Continuity of screwpile manufacturing & recorded documentation
• NDT (None Destructive Testing) & certification of helix to pile shaft interface fillet welds
• 1% minimum ISL Pile Load Testing to satisfy Piling Code AS2159-2009
• Detailed & accurate piling installation results & location logs
• Formal review of installation results by piling design structural engineers for Installation Certification
• Project outcome review with client upon completion

The Tender

A total of 3 internal engineering analysis reviews and 3 pricing revisions were conducted prior to tender presentation. Considering we are at the head of the game (25 years of expertise) we were more than confident that our pre-contract work place us in the best possible position to win the work. Our price was sharpened to the point where has anything unusual be encountered on site any margin would have disappeared at a concerning rate...

We knew one of our major competitors had pulled out due to site complexity & project size. We assumed, they just didn't do their homework properly & they scared themselves. Surprisingly the phone call came through that we were not successful at tender. We decided to keep a close eye on the works that were to follow as we were curious as to how anybody could have completed the works for a lesser price & still provide a quality Australian Standards outcome? In other words was what was going to be installed "apples for apples"? Had we made mistakes in our proposal? Was there something we could learn from this? And if so what? Months on we now know.

Gauge Theory Analysis

The following is a comparison summary which, when analysed with Gauge Theory principals, generates a clear picture as to what happened.

It became immediately clear that no pretender or pre-site mobilisation testing or analysis had been conducted by the successful contractor. The fabricated piles delivered to site were way too short & helical configurations were small. There appeared to be no understanding of the imported uncontrolled fill & they were unable to install the piles to what our engineers calculated to be the minimum required depths. They were quickly exceeding the torsional capacity of the pile shafts and decided to pre-drill the fill with large diameter augers in an effort to reduce the density of the immediate rubble limestone fill. This resulted in limited success. The original installed pile lengths were 4.0m. Soon after they decided to site weld/fabricate 2.0m long flanged pile extensions so as to found in the natural undisturbed medium dense sands @5.0-6.0m. It appeared the pile extension were a remedial fix as is indicated in the images below:

Furthermore it appears they were unable to achieve the minimum piling depths indicated previously as the images below clearly identify "Cut 4.8" on numerous large pile capacity locations. This indicated either: a) the torsional capacity of the installing equipment was insufficient to effect further pile embedment to the minimum depth, b) the torsional capacity of the pile shaft during installation had reached its maximum, c) the helicast type helix had bound-up or had rubble lodged in its flights resulting in false installation torque readings, d) any combination of the above. Again site testing proved minimum founding levels for these screwpile to be 6.0 to 7.0m depending on specific location. At 4.8m the screwpile is still founding in the uncontrolled fill "P" classification zone depth & future long term pile/footing settlement is a real possibility

Our further onsite observations concluded:

1. Insufficient allowance was made for screwpile shaft & helical sectional corrosion loss to accommodate a "Moderate" (high-end) corrosion classification. Pile sections were significantly lighter (thinner) and the thin spiral helical profile thicknesses had no practical means of accommodating a 0.03mm x 75yr of plate sectional loss, (2.25mm per side, i.e. 5mm total loss on helix's & external only on the pile shafts assuming the piles were concrete fill specified as ours were).
2. The largest helicast helix witnessed on site & welded to the large capacity screw pile shafts was 450mm dia. That compared with our single 600/25pl to double 750/32pl diameter plate helix's. An engineering analysis on the installed cast iron spiral helix's fails in bearing capacity even ignoring the incorrect founding depth indicated above.
3. No on-site ISL pile load testing was noted onsite. That being correct a further limiting geotechnical reduction factor should (must) be adopted thereby reducing the assumed (calculated) bearing capacity of the screw piles. This again brought into question, "Why Such Small Helical Size Configurations" were being installed? And again it all ignores the critical point of piles still founding in the upper "P" fill zones.
4. No "White Paint" was identified onsite indicating no quality checks were performed on welding integrity. A number of the welds we witnessed onsite were of significant concern. It appeared none of the on-site splice welding that occurred was subjected to NDT's.
5. Little regard was given to pile location tolerances. We noted numerous locations where location tolerances exceeded 50-100mm from centreline of footing load. We did not witness any recording of final pile positioning recording by any piling crew staff member. We could not determine whether there was a Piling Supervisor for the project? We did not see or witness the concrete filling of screwpile shafts prior to or during the pouring of the concrete footings. We were unable to determine whether sufficient/correct concrete cover minimums at the pile head & interface existed to accommodate the punching shear load requirements. This is often overlooked, or simply no understood, by many screw piling contractors.

We have immense concern regarding such conduct as noted above. There is little likelihood that the buildings will fall down any day soon. But there is a very real threat of the potential for long term & or differential building settlement, part of which is contained within design life serviceability as previously noted.

We are hopeful the actual project engineers read this article and are drawn to the conclusion that they should (if not already done so) check to see all the statutory, quality & building standards requirements have been met with respect to the performance of the screw piling installed.

At least now we know how we were beaten in Tender. Nothing more than a sad state of affairs & a continuing deterioration of industry standards & practises. Nothing changes. Other than it seems to be getting worse...