The Influence of Grout Installation Parameters on Shaft Bearing Capacity of Screw-Injection Piles

Abstract

In modern piling technology screw piles are used as a type of deep foundation for engineering structures, with the principal benefit of using said piles is that they offer an installation method that is virtually noise and vibration free. This makes these piles ideal for construction works in urban areas where surrounding structures can be affected by vibrations that would be produced from the installation of a driven pile. Since their initial production in the 1980s in Europe, multiple variations of screw piles are on the market today. This Msc thesis focuses on the Screw-Injection piles (SI-piles) commonly known as Fundex piles. SI-piles are a type of partial ground displacement piles where only a portion of the soil surrounding the pile is being pushed radially outwards during the rotational motion of screwing in the installation process. The other portion is transported back with grout flowing to the surface.

The current Dutch practice already have NEN guidelines on how to predict bearing capacity for SI-piles. These guidelines consist of CPT-based methods with an empirical correlation factor, the $\alpha$ pile class factor, which helps to relate the bearing capacity to the soil surrounding the pile. Nevertheless, one aspect that is not well understood is the effect that different properties of the injected grout have at the soil-pile interface and for bearing capacity. In this thesis two grout properties are being manipulated which are the Water/Binder and W/C ratios of the grout mixture, and the injection flow rate of the grout with the purpose to see whether and/or to what extent a difference exists in the shaft bearing capacity for SI-piles.

A full-scale experiment was conducted on 15 piles in order to evaluate the effect of these varying parameters. This research is composed of four targeted variations of W/B ratio and two injection flow rates, 5 groups of 3 piles each to be more precise. The piles were subjected to a static pile load test in tension, which means that the bearing capacity is composed of mainly the shaft resistance of the pile. The analysis breaks down in four main parts to analyse the indirect relationships between the properties that are accounted for in the empirical parameter $\alpha$. These four parts include the assessment of the load-displacement behaviour of the SI-piles, assessment of radial soil stress (CPT data), assessment of the records during the installation process (torque, RPM), the grout properties during installation and after 28 and 56 days of curing, and lastly, the pile shape (volume) after extraction of pile.

The assessment of the load-displacement behaviour showed that the predictions using the NEN guidelines for bearing capacity were extremely accurate for most pile groups (above 0.970 measured/predicted ratio). But for the pile groups with higher W/B ratio and with the highest flow rate (Groups C and D respectively) the measured shaft capacity would be much lower. A direct relationship between the W/C and W/B ratio is difficult to conclude since for pile B2 and C1 that had the same W/C ratio, the difference in the measured/predicted ratio was about 21\%. In the case of flow rate it is entirely seen that a higher flow rate leads to a significant decrease in measured shaft capacity. The NEN suggests a value of $\alpha_t$ for SI-piles of 0.009, yet the shaft capacity for groups with a higher W/C ratio and flow rate could be better predicted with an $\alpha_t$ $\approx$ 0.00793. Additionally, another important research objective is to try to optimise the $\alpha_t$ parameter by comparing the $q_c$ values for the pre-installation, the average post-installation and minimum value of the post-installation CPTs. This resulted in the $\alpha_t$ derived from the pre-installation CPT to have a much lower Coefficient of Variation, CoV, of approximately 0.08 whereas the average and minimum post-CPT $\alpha_t$ had a CoV of 0.12 and 0.11 respectively.

The assessment of the soil stresses is comprised of an analysis of the changes in cone resistance, $q_c$, throughout the field. The analysed data collected shows that for varying W/B ratios there is no solid relationship that relates the change in $q_c$ after the grout installation. However, a higher flow rate seems to have a significant impact on the cone resistance, leading to a general decrease of $q_c$ after installation, having a decrease as low as -16.53\% for pile D1, whereas for all other pile groups there was an increase in $q_c$ after installation, increases as high as 30\% (pile A3).

The assessment of the records during installation include the analysis of the torque during the installation process. It is seen that in both cases, high W/B ratio and high flow rate, there is a decrease in torque, but the flow rate of 115 [l/min] had a more significant impact than the increase in W/B ratio.\\
% Moreover, the 2D interpolation analysis aimed to see how post-installation CPT data should be considered. Pre-installation CPT data is sufficient to make a prediction on bearing capacity, but in this analysis both situations are being compared. This comparison resulted in that the difference between the two is minimal, the maximum difference found was in the order of $\pm$5 MPa.

The assessment of backflow grout resulted in higher W/C ratios having higher increases in density of the backflow fluid, and that high flow rate leads to a lower backflow density, this was supplemented with the sand transport data which suggests that higher W/B ratios lead to more sand transport out of the soil body. Furthermore, a inversely proportional relationship was found between W/B and W/C ratios and both the axial and bending stresses; the same inverse relationship is found with the flow rate. Additionally, the shear stress of the grout and of the soil were compared in order to determine if the failure is purely geotechnical or also structural.

The pile shape assessment resulted in a higher W/B ratio leading to a higher pile diameter, regardless of the flow rate during injection. There is also a very clear, almost perfectly linear, relationship between the mean diameter of the extracted pile and the measured shaft capacity. However not all piles were extracted and this includes piles installed with the highest W/C ratios (group C) and thus the aforementioned relationship has only been shown for a limited set of piles.