Effective use of polymer in foundation piling
This document provides a framework for using long-chain PHPA polymer in foundation piling. It draws from articles and studies from the US and experience across New Zealand and Australia.
There are multiple PHPA polymers with differing levels of effectiveness related to their viscosity and mixing ability.
A PAC polymer may be required to support a PHPA polymer when being run in porous formations to support the development of a filter cake and reduce the volume of fines in the column.
This document is to support engineers and contractors, to enable them to be confident that the load-carrying capacity of the pile is not adversely affected during construction, and to ensure that an approved slurry is not detrimental to the structural capacity or life of the pile. I would encourage any engineers or contractors, to work in partnership with a company that specialises in providing technical support and advice, on the effective use of polymer in foundation piling.
Advantages
Highly concentrated; reduced chemical additions
Requires less mixing/processing equipment than bentonite
Reduces footprint on-site compared to bentonite
A PHPA does not create a filter cake, so there is no risk of reduced skin friction
Can reduce disposal costs
The product is environmentally safe
Cleaner pile (both base and sides) before a concrete pour
Allows for a more uniform displacement to concrete
Reduces chipping and cleaning of poured concrete
What is a PHPA polymer?
A PHPA polymer is a long chain molecule consisting of many simple molecules (monomers) linked together. PHPA polymers are mainly used in drilling applications to prevent clay hydrations and swelling, but they also cause a dramatic increase in fluid viscosity. This increased viscosity is utilized to maintain open shaft stability.
When used in foundation piling, PHPA polymer reduces the fluid loss rate in the shaft through viscous drag. A reduced loss rate allows the level in the shaft to be maintained above the water table, creating a positive pressure differential to maintain borehole integrity.
The example PHPA polymer we refer to in this document is M-I SWACO POLY-PLUS RD/DRY. POLY-PLUS RD/DRY is in paper bags and is water-soluble. It can be broken down using Sodium Hypochlorite. The PAC polymer referred to is M-I SWACO POLYPAC R or UL.
M-I SWACO is the world’s leading drilling fluids supplier. They lead the way in developing products and have developed suitable products to support polymer and bentonite-based solutions to help deal with different ground conditions and when problems occur.
Some companies market their PHPA Polymer as being unique, providing some aspect that is different such as being a vinyl polymer. In the past, when information was more difficult to assess, misleading crazy claims had the potential to convince inexperienced operators. If it sounds too good to be true, it is.
Mixing
We are not ready to create a suitable fluid even with the best make-up water. Soda Ash is used to adjust the pH and treat Calcium and Magnesium. We need the pH between 8.5 and 9.5 for the best effect.
Then POLY-PLUS RD/DRY is added slowly through a venturi hopper. POLY-PLUS RD/DRY dosage and the desired range of viscosity of the slurry will be designed by Blick to suit the formation. The initial mixing enables each polymer grain to get separately water-wet to initiate hydration to its full chain length.
The polymer slurry should be agitated until it sufficiently develops to be self-suspended using mechanical agitation.
Other products may be added to achieve desired properties, such as POLYPAC R or UL, depending on the formation. This begins with the Geotech site investigation. Documentation should be present with soil logs and passed onto the contractor as part of the design stage.
Early contractor involvement at the test drilling stage is a significant advantage in determining relevant information.
Contaminants
Polymer slurries are sensitive to groundwater, soil, cement, and water contaminants. Calcium, magnesium, chlorine, and organic matter are some contaminants that will affect the polymer slurry. These contaminants often become part of the slurry from the soil or groundwater.
PHPA polymers are also sensitive to pH, with an optimum range being 8.5 - 9.5.
Maintenance
The slurry level should be maintained two to three meters above the water table to overbalance hydrostatic soil pore pressure and maintain soil stability. If the slurry drops below the specified level, the operation should be paused and the proper slurry level re-established before proceeding. In some situations, we may recommend that the slurry be maintained at less than two meters above the water table; to reduce the rates of fluid loss. It should be noted that PHPA polymers will shear degrade, resulting in reduced viscosity. Once hydrated mechanical shear should be limited to prevent this.
The point of reference for maintenance of the slurry level should be the water table. The casing does not change the requirement to keep the slurry level above the water table. Attempts to excavate or hold open excavation in saturated or unstable soils with inadequate slurry head pressure may result in soil collapse below the casing. The stabilising fluid is added to the shaft before encountering the groundwater table.
Sampling and testing
When obtaining slurry samples, these should be taken from near the bottom of the excavation, from the upper portion, and from the slurry tanks at regular intervals to facilitate control of slurry properties. The main properties to be maintained are pH, water hardness and viscosity, density and sand content.
Fluid loss
If there are high rates of fluid loss (seepage of slurry into the soil), the polymer dosage and viscosity of the slurry may be increased. Alternatively or additionally, other products, including PAC Polymers and LCM, may be necessary.
Concrete displacement and fluid recycling
Upon reaching the final depth, the slurry column shall be allowed to stand static and undisturbed to allow sand to settle toward the bottom of the hole. Slurry samples shall be taken during this static period from the excavation's midpoint and within 60 centimetres of the base to determine sand content, viscosity, pH, and specific gravity. When sand content and specific gravity of near-bottom and midpoint samples are within specified maximums or when they stabilise and show no change over a 30-minute interval during which the excavation is static, the rebar and concrete placement may proceed.
While the concrete is being tremied, it will displace the polymer slurry upward. The displaced fluid is diverted to a holding tank. A piston pump will prime more quickly because of positive displacement and does not break down the polymer chains as a centrifugal pump.
Avoid pumping the last one to one and a half metres of slurry above the concrete interface into the holding tank, as this slurry will be contaminated from contact with the concrete.
Time is allowed for cuttings to drop out of the polymer slurry in the holding tank. The fluid is tested for viscosity, pH and hardness. By determining the viscosity of the slurry in the holding tank, you can calculate how much fresh product must be added. Commonly, it requires 25%-30% of the initial volume of POLY-PLUS RD/DRY to restore the slurry to its original viscosity for reuse on the next hole.
Stabilise the bored pile under fluids
Two types of pressure are exerted on the borehole during drilling; formation pressure resulting from the water table and hydrostatic pressure from the vertical mud column. Formation pressure can collapse the borehole if it is not overcome by hydrostatic pressure pushing back against the formation. Hydrostatic pressure is a function of the weight or density and depth of the drilling slurry pushing against the formation. To maintain an overbalance of hydrostatic pressure between the shaft and formation, the drilling slurry must “push back” against the formation with minimal rates of flow into the formation to prevent a drop in mud level. In unconsolidated, highly permeable formations, bentonite and filtration control polymers may be necessary. In this case, the hydrostatic overbalance in pressure occurs when the weight of the supporting fluid is in contact with the low permeability deposits (filter cake) placed on the sides of the borehole by the drilling slurry. The filter cake and the hydrostatic pressure control the formation pressure, reduce slurry loss and prevent caving, resulting in hole stabilisation.
A critical consideration is the impact of surge and swab pressures exerted due to the highly viscous polymer fluid compared to a bentonite-based system. Tripping speeds must be controlled when running in and out of the hole to prevent pressure from dropping below or increasing significantly relative to the formation pressure. Failure to manage this will lead to borehole collapse.
Maintaining the slurry level 1.5 to 2 meters above the surrounding groundwater level is necessary. Without this positive pressure exerted by the slurry column against the sidewall, formation pressures will cause the excavation to collapse.
Conclusion
A long-chain PHPA polymer is an effective method of stabilising a borehole in foundation piling.
Having the right mixing equipment is vital.
A great product is just the start. An experienced drilling fluids partner is vital to help at design, through execution and during project review.
More challenging ground conditions require a more complex mix of products to reduce risks, losses and collapses. When designing the job, Blick works closely with contractors, consultants, and councils to ensure the right solutions for a successful contract.
The cost between different providers is often tiny; the difference in technical advice and support available at design, through project execution, and during the review should be the primary factor in determining which provider to work with.
Blick has been a supplier to the drilling industry in Australasia for over 50 years. Blick and M-I SWACO work as a partnership for water-based drilling fluids across Australia, New Zealand, and the Pacific. They excel in support and advice.
Contact us today if you’re looking at optimising your performance and reducing costs through engineered fluid solutions. Our Australia team is available on +61 3 9068 5688 (Melbourne) or +61 8 6271 3575 (Perth) and New Zealand on +64 7 849 2366.
Appendix: Technical article testing properties of the polymer in bored pile
Many articles and books have been written on using Bentonite and Polymer in bored pile situations. Below is information from the ADSC (International Association of Foundation Drilling) from an article from the December/January publication of 1998. This article was co-written by Gary Matula, who recently retired from M-I SWACO, having developed many of M-I SWACO's modern range of products as their Lead Chemist, including POLY-PLUS RD/DRY.
The residual polymer remaining in the borehole may reduce shearing resistance between concrete and either soil and/or reinforcing steel. Another concern is that polymers may form “oatmeal” (curdled mixtures of polymers and silts) that are not displaced by the concrete and may produce structural defects in the completed shafts.
Two 915mm diameter fully instrumented test shafts were installed using a PHPA polymer slurry to test these concerns. The first was at the University of Houston site (NGES-UH), and the second was at the Texas A&M University site (NGES-TAMU). Both shafts were drilled by introducing the slurry immediately at the beginning of drilling.
The effectiveness of polymer slurry during the construction of drilled shafts was presented based on four full-scale technique shafts at two National Geotechnical Experimentation Sites (NGES). It was concluded that the success of the polymer slurry in maintaining borehole stability is attributed to three main factors:
proper mixing,
careful monitoring of the slurry properties, and
introducing the slurry into the hole before reaching the piezometric surface.
The results showed that using PHPA polymer slurry, whether in cohesive or cohesionless soil formations, maintained the stability and side-wall profile of the open shafts (915mm diameter and 21m deep) for at least 18 hours when proper procedures were followed.
Ultimately, the success of a drilled shaft construction process is measured by the resistance or "capacity” developed by the concreted shaft.
The 21m deep shafts were equipped with internal Osterburg cells (O-Cells) at approximately mid-height. The O-Cell and its plates were designed to allow for the passage of the pump line for the concrete. Four PVC pipes for ultrasonic logging and two clear plastic pipes for fibre-optic logging. Embedment strain transducers (sister bars) were mounted at different levels on the reinforcing cage to measure the load transfer to the soil. The instrumented steel cage was lowered into the open shaft after completing the bottom cleanout, which was performed using a bucket with a vented bottom.
The shafts were concreted from the bottom up using a five-inch pump line while monitoring the volume of the poured concrete. Typically and as expected, the polymer slurry resulted in flocculation of the clay particles only at the University of Houston site, which is silt-rich.
The quality and homogeneity of the placed concrete were monitored using cross-hole sonic and fibre optic logging methods.
Results from the NGES-UH test shaft (cohesive soil)
In drilled shaft design, the basic approach to characterise the load transfer in side resistance for shafts in cohesive soils is to back-calculate the ⍺ - factor. The value ⍺ is equal to a fraction of the soil's undrained shear strength that is ultimately mobilised in side resistance, where high ⍺ factors mean that the drilling process produces relatively less soil degradation than low ⍺ factors. The back-calculated ⍺ factors for the slurry shaft at the University of Houston site, averaged over the depth of the shaft, ranged in dry or driven settings from 0.44 to 0.54, while the PHPA and Bentonite solutions ranged from 0.74 to 0.81. The ⍺-factor for PHPA polymer averaged 0.74, which is 35 per cent higher than the value recommended by the FHWA (O= 0.55). In other words, the actual peak load measured for the slurry shaft was 288 tons, while the predicted ultimate load (FHWA method) is 214 tons.
The polymer slurry shaft's average ⍺ -factor compares well with the two bentonite shafts. It is higher than the design value recommended by FHWA and other comparable shafts constructed under dry conditions.
Results from the NGES-TAMU test shaft (cohesionless soil)
For the design of shafts in sand, the FHWA recommends using a β-factor that correlates the unit side shear resistance to the effective vertical stress in the soil at a given depth. The average value of β calculated from the FHWA method is 0.90. The shaft design resistance is predicted as 380 tons. The polymer shaft had a measured resistance of 400 tons. The back-calculated β-factors for two other shafts drilled and tested at the same site using bentonite slurry were also compared. A Calcium Bentonite with less effective filter cake development provided an average β-factor of 0.48. A high-quality Wyoming Sodium Bentonite produced an average β-factor of 1.0. The PHPA slurry provided an average β-factor of 1.09.
The average β-factor for the polymer shaft exceeds the FHWA recommended value and compares favourably with values from other comparable shafts constructed using a Wyoming Sodium Bentonite slurry.
Findings
The use of PHPA polymer slurry does not decrease the capacity of the constructed shafts in the stiff clays of the NGES-UH site or the sand of the NGES-TAMU site. The ⍺-factor and β-factor were comparable to similar shafts constructed using Wyoming Sodium Bentonite slurry. In addition, the side resistance design factors that were back-calculated from the load test results of the polymer shaft exceed those recommended by the FHWA in both the cohesive and cohesionless soils.
The use of the polymer slurry did not impair the structural capacity of the shaft.
