Method for constructing a duo concrete pile with pre-drilling, driving, refusal control and compression for special foundations of civil engineering structures

Abstract

The invention relates to a method for constructing a duo-concrete pile for deep foundations of civil engineering structures. This method comprises a pre-drilling step in weak soil layers using an auger, followed by the insertion of a precast concrete lower element into the pre-drilled hole. The element is then driven into anchoring soil layers, with lateral soil displacement to improve layer cohesion. The method also includes molding a cast-in-place concrete upper element over the lower element. A refusal check is performed to ensure that the pile is properly anchored in a soil layer sufficiently strong to support the structure's loads. Figure 1

Description

Title of the invention: Method for constructing a duo concrete pile with pre-drilling, driving, refusal control and compression for special foundations of civil engineering structures technical field

[0001] The present invention relates to the field of deep foundation techniques for civil engineering structures, in particular the construction of concrete piles used to transfer the loads of a structure to deep and stable soil layers when the surface layers are unsuitable for supporting the stresses of the structure. It relates more specifically to a method for constructing a dual-action concrete pile with pre-drilling, driving, refusal control, and compression for special foundations of civil engineering structures. State of the art

[0002] Deep foundations have played a central role in civil engineering for centuries, enabling the construction of large-scale structures on land with varying geotechnical characteristics. Concrete piles, used to transfer loads from a structure to deeper soil layers, are one of the most common solutions for ensuring the stability of structures on land where the surface layers are too weak to support the imposed loads.

[0003] Historically, the earliest pile foundation methods relied on wooden piles, driven in by hand or with primitive mechanical hammers. With industrialization and improvements in construction techniques, the use of concrete for piles has become widespread. Concrete piles can be manufactured on-site (cast-in-place concrete) or prefabricated in factories, then transported and installed on construction sites.

[0004] Today, there are two main methods for installing piles: drilling followed by concrete injection (bored piles) and driving prefabricated piles. These methods have advantages and disadvantages depending on the type of soil encountered.

[0005] Bored piles consist of drilling a hole using an auger, then filling it with fresh concrete. This technique avoids the vibrations and noise associated with pile driving, but it has drawbacks in poorly cohesive soils, where the walls of the hole can collapse, requiring the use of protective tubes. Furthermore, it can lead to excessive concrete consumption when the soil is too loose and difficult to contain.

[0006] Driven piles, on the other hand, are prefabricated and driven into the ground by repeated blows. This method is faster but can generate significant vibrations, which are problematic in urban environments or near other sensitive structures. Furthermore, in weak soils, driving can cause deformation or settlement of the surface layers, thus reducing the pile's load-bearing capacity.

[0007] A common variant of driven piles is the displacement pile, which improves the compaction of the soils traversed by displacing the materials laterally during driving. However, this solution can also accentuate defects in the non-load-bearing surface layers, which become pressed against the pile shaft, potentially compromising the overall stability.

[0008] Despite these advances, current methods still have limitations when it comes to penetrating poor soil layers to reach deep, resistant layers. In particular: • Bored piles often require additional concrete in the loose surface layers, resulting in additional costs. • Driven piles can experience negative frictional forces when non-load-bearing soil layers settle after installation, which affects the load-bearing capacity of the pile. • In both cases, quality control of the installed pile, particularly control of refusals, remains complex and often depends on field experience.

[0009] To address these problems, the invention proposes a hybrid solution, combining pre-drilling, pile driving, and refusal control, to ensure greater efficiency during pile installation, while guaranteeing that the load-bearing soil layers are reached without compromising the pile's integrity. Summary of the invention

[0010] The present invention aims to overcome all or part of the drawbacks of the prior art described above by proposing an innovative solution for constructing a duo-concrete pile intended for the foundation of civil engineering structures, particularly on ground with weak surface soil layers. This method combines pre-drilling, driving, refusal control, and molding of a cast-in-place concrete top element to ensure a stable and durable foundation.

[0011] An objective of the invention is to significantly improve the load-bearing capacity of piles while reducing the negative friction forces that can occur during the installation of piles on poorly consolidated ground.

[0012] To this end, the present invention relates to a method for constructing a concrete pile, known as a duo pile, for deep foundations of civil engineering structures, comprising a pre-drilling step of a hole in weak soil layers using an auger, a step of inserting a prefabricated concrete lower element into the pre-drilled hole, and a step of driving the lower element into anchoring soil layers. This method is notable in that it includes a refusal control step to ensure that the lower element reaches the appropriate bearing layer and a step of molding a cast-in-place concrete upper element above the lower element, and in that the driving causes lateral displacement of the soil, thus improving the cohesion and bearing capacity of the anchoring layers.

[0013] This process has the advantage of ensuring optimal pile placement by reaching the most resistant soil layers, while guaranteeing enhanced stability thanks to the lateral displacement of the soil, which compresses the anchoring layers and improves their load-bearing capacity.

[0014] According to one aspect of the invention, the method is characterized in that the lower prefabricated concrete element is reinforced with a reinforcement cage comprising continuous tie rods along the entire length of said lower element, said tie rods extending to the head of the pile, and frames placed close together at the level of the head of the lower element, so as to resist driving forces.

[0015] Thus, the lower prefabricated concrete element can be subjected to significant driving forces without risk of breakage or deformation, which ensures reliable installation in compact or hard soil layers. The reinforced structure maximizes the durability and strength of the pile by ensuring a uniform distribution of mechanical stresses.

[0016] According to one aspect of the invention, the method is characterized in that the upper element is molded inside a temporary casing, said casing being removed after filling with concrete, allowing the connecting helmet to ream the wall of the hole to form a slightly enlarged section around the upper element.

[0017] This increases the precision of the fit between the prefabricated lower element and the cast-in-place upper element. The connection between these two elements is reinforced by the connecting cap, which guarantees a perfect joint while ensuring precise molding of the upper element. This precision improves the pile's load-bearing capacity by minimizing the risk of structural weakness at the joint.

[0018] According to one aspect of the invention, the method includes a step of avoiding the negative friction forces generated by the settling of the surface soil layers, these forces being reduced by pre-drilling the non-load-bearing layers before driving.

[0019] The advantage of this step is that pre-drilling limits the negative frictional forces that could compromise the stability of the pile after its installation, particularly in soft or poorly consolidated soils. Indeed, this step minimizes the risk of overloading the pile and ensures that the loads are correctly transferred to the deep anchoring layers, thus increasing the lifespan of the foundation.

[0020] According to one aspect of the invention, the method is characterized in that the lower prefabricated concrete element is driven in by hammering until the refusal value defined by a control of the blows of the dry hammer is reached, thus ensuring that the lower element is anchored in a layer of soil sufficiently resistant to support the loads of the structure.

[0021] This step offers the advantage of rigorous control over the pile installation. Controlling refusals ensures that the lower element is properly anchored in the bearing layer, thus providing a stable foundation suitable for the structure's load. This eliminates uncertainties related to pile penetration and guarantees that the soil's geotechnical characteristics have been correctly taken into account.

[0022] The fundamental concepts of the invention having been set forth above in their most elementary form, other details and features will become clearer upon reading the following description and with reference to the accompanying drawings, which give, by way of non-limiting example, an embodiment of a method for constructing a duo-concrete pile with pre-drilling, driving, refusal control, and compression, in accordance with the principles of the invention. Presentation of the drawings

[0023] The figures are given for illustrative purposes only to facilitate a better understanding of the invention without limiting its scope. The various elements may be represented schematically and are not necessarily to scale. Throughout the figures, identical or equivalent elements are identified by the same numerical reference.

[0024] It is thus illustrated in:

[0025] [Fig.1]: a longitudinal cross-sectional view of a duo concrete pile according to the invention, showing the lower prefabricated concrete element and the upper cast-in-place concrete element;

[0026] [Fig.2]: a cross-sectional view BB of the upper element, showing its circular section;

[0027] [Fig.3]: a cross-sectional view AA of the lower element, showing the reinforcements integrated into the pile;

[0028] [Fig.4]: in (a) a schematic view of pre-drilling using a helical auger in the weak soil layers, in (b) a view illustrating the insertion of the prefabricated element in the pre-drilled hole, with the temporary casing, and in (c) a view illustrating the negative friction forces to be avoided during the settlement of unconsolidated layers;

[0029] [Fig.5a]: a detailed view of the connection helmet between the lower element and the upper element;

[0030] [Fig.5b]: a view illustrating the molding of the upper element during the removal of the temporary casing;

[0031] [Fig.6]: a view illustrating the driving of the lower element and the lateral displacement of the compressed soil around the pile;

[0032] [Fig.7]: a diagram of the execution phases of a duo concrete pile, from pre-drilling to molding of the upper element;

[0033] [Fig.8]: a longitudinal cross-sectional view of the completed duo pile, after removal of the temporary casing and molding of the upper element in fresh concrete. Detailed description of implementation methods

[0034] The present invention relates to a method for constructing a duo-concrete pile, intended for deep foundations for civil engineering structures, comprising a prefabricated concrete lower element and a cast-in-place concrete upper element. This duo-pile is designed to overcome the limitations of existing foundation techniques by combining pre-drilling, driving, refusal control, and compression of the soil layers traversed.

[0035] Figure 1 shows a 100 duo pile composed of a prefabricated concrete lower element 1 and a cast-in-place concrete upper element 2. The lower element 1 is designed to withstand driving forces, notably through reinforced reinforcement 7 consisting of continuous tie rods extending along its entire length. This reinforcement also includes closely spaced stirrups in the top of the lower element, which reinforce the pile head to withstand repeated impacts from driving. The upper element 2 is molded directly onto the lower element after the latter has been driven into the ground, so as to form a monolithic block capable of transmitting the structure's loads to the deeper soil layers.

[0036] The connecting cap 6, located at the junction between the two elements, ensures the precise alignment of elements 1 and 2. This cap 6 covers the upper end of the lower element and guarantees a solid mechanical connection between the two parts. As illustrated in section BB in [Fig. 2], the upper element 2 has a circular cross-section and fits perfectly into the temporary casing 5. Section AA, shown in [Fig. 3], displays the reinforcement elements 7 integrated inside the lower element 1, as well as the general configuration of the pile in the ground.

[0037] The method of implementing the duo pile begins with pre-drilling, shown in [Fig.4]. This pre-drilling is carried out using a helical auger 16 which drills a pilot hole 8 in the weak soil layers 9. The pilot hole makes it easier to insert the lower prefabricated concrete element 1 into the soil, by reducing friction and minimizing the driving forces required for penetration.

[0038] As illustrated in step (a) of [Fig. 4], once the pre-drilling is completed, the lower element 1 is inserted into the hole 8, fitted with the temporary casing 5. The cap 6, at the base of the casing, covers the prefabricated element 1, ensuring alignment between the casing and the lower element. The casing 5 also acts as a guide for the ram 13, used to drive the lower element into the anchoring layers 10 of the soil. This method concentrates the driving energy on the lower element 1, avoiding the parasitic friction 11 generated by the non-load-bearing layers 9, as shown in (a) of [Fig. 4].

[0039] Phase (b) illustrates one of the key advantages of pre-drilling: it avoids the negative frictional forces 12 caused by the settlement of the superficial layers of loose soil 9. These downward parasitic forces can lead to an overload of the finished pile P shown in (c) and reduce the effective bearing capacity of the foundation system. Pre-drilling therefore helps to preserve the integrity of the pile and optimize its performance.

[0040] Figures 5a and 5b show in detail the molding process of the upper element 2. After the lower element 1 is driven into the anchoring layer 10, the temporary casing 5 is filled with fresh concrete. During the raising of the casing 5, the casing 6, whose outside diameter is larger than that of the prefabricated element 1, molds the upper element 2 with a cross-section slightly larger than that of the lower element 1. The continuous tie rods 3, integrated into the lower element 1 and protected by metal sleeves 17, are then immersed in the fresh concrete during the raising of the casing 5, ensuring structural continuity between the two elements. The connection of the lower element 1 and the upper element 2 is ensured by hooks 4, which grip the reinforcement cage of the element 1, thus forming a single block that can be driven simultaneously into the ground.

[0041] The system's effectiveness relies on the progressive driving of the lower element 1 into the anchoring layers 10, as illustrated in [Fig. 6]. The driving action, caused by the fall of the dry ram 13, generates a lateral displacement of the soil around the lower element 1, thus creating a compressed soil zone 14. This soil compression improves the cohesion and bearing capacity of the foundation, thereby ensuring greater long-term stability.

[0042] The different phases of the process for constructing the duo pile are described in [Fig. 7]. Phase 1 consists of positioning the auger on the location intended for the Pile. Phase 2 involves pre-drilling until the anchorage layers are reached. Phase 3 involves inserting the precast element into the borehole. Phase 4 then introduces the extension casing, which acts as a guide for the pile driver, allowing the precast element to be driven into the bearing layers, and also shoring the excavation walls. Phase 5 consists of filling the casing with fresh concrete once the anchorage depth is reached, and then removing the casing. Finally, Phase 6 prepares the pile head by leveling the concrete to the final level and placing the reinforcement bars in place for the subsequent construction operations.

[0043] The table below presents kp coefficients used for calculating the pile's bearing capacity according to the pressuremeter method. These coefficients vary depending on the type of soil encountered and the pile installation method, with or without soil displacement. The NF EN 12699 standard, relating to displacement piles, is taken into account in this table, although the pre-drilling method of the present invention differs slightly from that of fully displacement piles. The kp values ​​are therefore adjusted according to the experimental results obtained on site.

[0044] [Table] Soil type. Elements used without soil compaction. Elements used with soil compaction. Clays, silts A 1.1 1.4 B 1.2 1.5 1.3 1.6 Sands, gravels A 1.0 4.2 B 1.1 3.7 1.2 3.2 Chalks A 1.1 1.6 B 1.4 1.8 2.6 Marls; Mammals 1.8 2.6

[0045] The kp coefficient values ​​provided in this table show the importance of the effect of soil displacement on the bearing capacity of piles depending on the different soil types encountered. For clays and silts, the values ​​increase moderately, with a stable improvement ranging from 1.1 to 1.6 depending on the intensity of the displacement. These soils, which have relatively consistent characteristics, benefit from a slight improvement in bearing capacity when the soil is displaced. In contrast, sands and gravels show a much more pronounced variation. Without displacement, the coefficients remain low (between 1.0 and 1.2), but with soil displacement, the values ​​rise drastically, reaching up to 4.2 in some cases. This shows that in less cohesive soils, displacement plays a crucial role in improving bearing capacity. As for chalk, the values ​​are More stable with or without displacement, the coefficients increase from 1.1 to 1.8 without displacement and from 1.6 to 2.6 with displacement. These soils, which are naturally more resistant, require fewer adjustments to optimize their bearing capacity, but still benefit from displacement. Finally, for marls and mamo-limestones, the kp coefficients start at relatively high values ​​(1.8 without displacement) and reach 2.6 with displacement, confirming that these soils are inherently capable of supporting significant loads, but that displacement continues to provide a bearing capacity gain, although less pronounced than in less resistant soils. Overall, soil displacement proves essential for improving pile bearing capacity, particularly in looser soils such as sands and gravels, while in harder soils such as chalk and marls, the effect is less critical but still beneficial.

[0046] Finally, [Fig. 8] shows the completed duo pile, after the removal of the casing 5 and the casting of the upper element 2 in fresh concrete. The pile thus formed has a slightly wider upper section compared to the prefabricated element 1, ensuring better load transmission to the anchorage layer 10. The tie rods 3, now embedded in the concrete, ensure structural continuity between the two elements, thus guaranteeing the performance and durability of the foundation.

Claims

Demands

1. A method for constructing a concrete pile (100) for deep foundations of civil engineering works, comprising: a pre-drilling step of a hole (8) in weak soil layers (9) using an auger (16), a step of introducing a lower prefabricated concrete element (1) into the pre-drilled hole (8), and a driving step of the lower element (1) to drive it into anchoring soil layers (10), said method being characterized in that it further comprises a refusal control step to ensure that the lower element (1) reaches the appropriate bearing layer, and a molding step of an upper element (2) of cast-in-place concrete above the lower element (1); and in that the driving causes a lateral displacement (14) of the soil, improving the cohesion and bearing capacity of the anchoring layers (10).

2. A method according to claim 1, wherein the lower prefabricated concrete element (1) is reinforced with a reinforcement cage (7) comprising continuous tie rods (3) along the entire length of said lower element, said tie rods extending to the head of the pile (100), and closely spaced frames at the level of the head of the lower element, so as to resist driving forces.

3. A method according to any one of the preceding claims, wherein the upper element (2) is molded inside a temporary casing (5), said casing being removed after filling with concrete, allowing a connecting helmet (6) to ream out the wall of the hole (8) to form an enlarged section around the upper element (2).

4. A method according to any one of the preceding claims, comprising a step of avoiding the negative friction forces (12) generated by the settling of superficial soil layers (9), these forces being reduced by pre-drilling the non-load-bearing layers (9) before driving.

5. A method according to any one of the preceding claims, wherein the lower prefabricated concrete element (1) is driven by hammering until the refusal value defined by a control of the blows of a dry hammer (13) is reached, thus ensuring that the lower element (1) is anchored in a soil layer sufficiently resistant to support the loads of the structure.

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