Flow Mechanisms in Bored Piles

Added: 2026-05-21

[00:00:00]Hello and welcome to the numerical modeling highlight series by the joint EFFC DFI concrete task group and Swansea University. I'm Tom Mitchell, a research assistant from Swansea University. And in this video, I'll walk you through the flow mechanisms of fresh Tummy concrete as it's placed within board piles.

[00:00:24]Let's start with the unreinforced case.

[00:00:27]The examples shown in this video focus on a pile that's 16 m deep and has a diameter of 1.5 m. The flow processes are captured midway through the pore after the tremy has been split. What we're looking at here in this figure is a cross-section taken through the center of the pile where the coloring represents the velocity magnitude of the concrete and the black arrows represent the velocity vectors. All concrete simulated has a slump flow of 500 mm and all walls are modeled with no slip boundary conditions.

[00:01:02]As the fresh concrete exits the tre, it penetrates a short distance into the previously placed concrete before the flow is redirected, turning back on itself by 180° to flow upwards.

[00:01:16]During this flow redirection, the concrete spreads radially outwards towards the surface of the pile.

[00:01:23]After the flow redirection, purely vertical flow is seen across the full radius of the pile.

[00:01:33]Looking at the flow front where the flow front refers to the region of concrete approaching the support fluid, we observe the plug flow mechanism.

[00:01:44]where a uniform vertical flow pattern is maintained throughout the entire cross-section above the tremy exit.

[00:01:52]What this means in practice is that the concrete of the flow front is simply carried upwards as a plug and is maintained within this region leading to the flow mechanism to be known as plug flow.

[00:02:04]The concrete sitting at that flowfront will be from the very first batch poured and that remains true for the entire length of the paw so long as sufficient TI embedment is maintained during the TI breaking process.

[00:02:21]Now let's take a look at what happens when we add a reinforcement cage. The diagram on the left shows a visualization of a small section of the pile where the gray bars represent the reinforcement and the outer edge is the pile surface.

[00:02:37]The particular cage we've modeled has a cover zone of 75 mm, has clear spacing between the vertical reinforcement of 100 mm and has clear spacing between the horizontal reinforcement of 200 mm.

[00:02:52]The vertical reinforcement bars have a diameter of 40 mm and the horizontal reinforcement have a diameter of 20 mm.

[00:03:02]The figure on the right is again showing a cross-section through the center of the pile where the left side of the image is the slice taken between two vertical reinforcement bars while the right hand side is a slice taken directly through one of the vertical reinforcements.

[00:03:19]The initial flow behavior is the same as previously described in the unreinforced case. Concrete exits the tre, penetrates a short distance into the previously poured batch and then the direction of flow is reversed.

[00:03:34]But here the reinforcement cage limits the radial spread of concrete to the region inside the reinforcement cage.

[00:03:43]After flow redirection, purely vertical flow is observed within the reinforcement cage. Importantly, no fresh concrete enters the cover zone at this stage and there is no flow within the cover zone.

[00:04:01]Things change when we focus on the flow front. Concrete flows vertically within the reinforcement cage leading to the formation of a concrete differential at the flow front where the concrete level inside the reinforcement is higher than the concrete level inside the cover zone.

[00:04:18]This concrete differential then drives the flow of concrete horizontally into the cover zone at the flow front as shown by the growing horizontal component of the flow vectors within this region.

[00:04:30]Once the concrete enters the cover zone, it almost immediately comes to a stop, effectively becoming locked in place.

[00:04:38]Throughout the majority of the pile, there is no concrete flow within the cover zone, only at the flow front.

[00:04:46]What this means is that the concrete of the flow front is continuously consumed into the cover zone as the pore progresses and is replaced by concrete below.

[00:04:56]This mechanism is often referred to as bulging flow, but I believe it's more aptly described by the term consumption flow. The end result is that provided the pile is deep enough, the entire first batch ends up within the cover zone.

[00:05:15]So to bring everything together, there are two bulk flow mechanisms that exist within board piles. Plug flow or consumption flow.

[00:05:25]The observed bulk flow mechanism depends solely on whether a reinforcement cage is present within the pile.

[00:05:33]In an unreinforced pile, the flow front rides upwards as a plug from start to finish. This is plug flow. The practical consequence of this is that the first batch of concrete remains at the flow front throughout the entire pore.

[00:05:49]In a reinforced pile, however, the flow front is continuously consumed into the cover zone as the concrete level rises.

[00:05:57]This is consumption flow. This leads to the first batch of concrete being gradually consumed into the cover zone during the pour where it is no longer affected by flow.

[00:06:09]Exactly when and at which [clears throat] stage this batch will be consumed into the cover zone depends on many parameters including the cover zone, the reinforcement rate, the pile dimensions, and the fresh concrete properties just to list a few. Some of these parameters and their impacts will be shown in future videos.

[00:06:28]For more information on this research topic, including further videos and papers, scan the QR code on the following slide.