WO2023131936A2

WO2023131936A2 - Bioinspired skirted footing and its method of installation

Info

Publication number: WO2023131936A2
Application number: PCT/IB2023/051788
Authority: WO
Inventors: Ravi Sankar Jakka; Khalid Bashir
Original assignee: Indian Institute Of Technology Roorkee
Priority date: 2022-01-05
Filing date: 2023-02-27
Publication date: 2023-07-13
Also published as: EP4460615A2; WO2023131936A3

Abstract

The present invention relates to the field of bioinspired geotechnics to provide an alternative to conventional vertical and inclined skirted footings and their method of installation. The invention provides tree root inspired substructure with improved load carrying capacity and a method of installation. Tree root inspired substructure herein defines a square or rectangular or circular or strip footing with closely spaced vertical/inclined micropiles. This hybrid substructure takes advantage of depth effect, width effect, arching effects, compaction and relative ease in installation on level and sloping grounds as compared to conventional skirt/bucket foundation. The micropiles attached to the traditional footing are spaced such that the trapped soil in-between behaves as a plug and major load shearing/transfer takes place at the level of tip of micropiles. Some load distribution also takes place along the micropiles and underneath the footing. The uplift, moment, lateral and vertical load carrying capacity gets enhanced due to the increase in the depth of foundation without much efforts on excavation. The proposed foundation can be cast-in-situ or precast or hybrid. Micropiles of the footing could be installed by either driving or boring. Micropile material can be solid/hollow steel or reinforced/unreinforced concrete, while the footing can be made up of steel plate/frame or reinforced/prestressed concrete. Appropriate selection of a bioinspired skirted footing saves a lot of material and construction time as compared to conventional skirted footing, leading to cost savings.

Description

BIOINSPIRED SKIRTED FOOTING AND ITS METHOD OF INSTALLATION

FIELD OF INVENTION:

The present invention relates to the field of biogeotechnic inspired foundations. The present invention in particular relates to biogeotechnics based system for vertical and inclined skirted footings and their method of installation.

DESCRIPTION OF THE RELATED ART:

Civil engineers globally search for alternative methods to improve the load-carrying capacity of a footing resting on the soil. In sites with poor bearing capacity or heavy structural loads, the conventional shallow foundation does not seem to be adequate. Thus, we either need to reinforce/improve the soil or adopt deep foundations.

Reference may be made to the following:

US8668408B2 relates to the skirted foundation for penetrating soft material. In this patent, the installation method of the skirted foundation is described in detail. The main motive of this disclosure is to increase the load carrying capacity with the provision of skirts. Holes are provided to the skirt for easing the penetration of skirts in softer material.

This patent highlights the use of short skirts and duct piles in combination, thereby reducing the cost of construction and installation of the foundation of offshore wind power facilities. US10851513B1 relates to the combined offshore wind power foundation with duct piles and a bucket. This patent highlights the use of short skirts and duct piles in combination, thereby reducing the cost of construction and installation of the foundation of offshore wind power facilities.

CN110700239A relates to a method for using tree root pile reinforcing foundation, comprising the following steps: (A), on the subject foundation for drilling a root pile hole, at the same time protecting wall prevents drilling into the sleeve hole collapse; (B), the drilling well after tree root pile hole into a cleaning device cleaning operation for many times, and ensures the hole overflow rinsing out the cleaning device, (C), the root pile hole of in aggregate, and putting into the grouting pipe and injecting water, ensure the hole clean, (D), the tree root pile hole to the cement grout or concrete, and tree root pile construction is finished, the purpose of the invention is to provide a method for using the root pile reinforcing foundation, can be convenient, reinforcing the foundation, not limited by the field. CN110528556A relates to a new type of foundation and construction method of bionic based on the root, belonging to the foundation engineering earthquake field. To solve the problem of the traditional single pile in pile foundation in the presence of seismic sinking and pile and bearing platform connection failure, damage and so on with soil sideways. Related research at home and abroad is only limited to upper structure about bionic innovation of earthquake- proof structure, no bionic base to root, more than related theoretical formula, experimental and simulation data. Structure design by bionic technology can improve the building foundation, reinforcing the research and design-related aspect. The invention, through setting different directions at the specific position of the pile foundation, the bionic structure root length to discrete stress position distribution of the foundation pile, realize multi-force transmission path, relative to the pile foundation having better robustness, it can well solve the depth based on the strong damage of the epicentre

The innovation involves the provision of three sets of four orthogonal micro-piles at different depths to the vertical pile forming the multi-layer inclined pile root system structure. Though it certainly enhances the load carrying capacity of the vertical pile, the construction process is very tedious and demands advanced excavation machinery.

US8974150B2 relates to apparatuses and methods for installing structures (e.g., foundations, footings, anchors, abutments, etc.) at work sites, such as difficult-access work sites. In some instances, a rotating drill assembly is assembled over a target location to excavate a radial array of batter-angled shafts associated with the target location in preparation for installing a radial array of micropiles. An operator utilizes the rotating drill with a foundation pile schedule/decision matrix to design and install the radial array of batter-angled micropiles. This disclosure also describes techniques for designing, fabricating and installing structural caps to be coupled to the installed radial array batter angled micropiles. These structural caps are lightweight and, thus, more portable to difficult-access sites where they are coupled to the micropiles, forming a foundation for the structure to be installed at the difficult-access site This disclosure describes the installation of Inclined micropiles to enhance the stability to the tower foundation under different types of loading conditions. It mainly focuses on installing battered (inclined) micropiles at inaccessible locations. This disclosure also presents the designing, fabricating and installing structural caps to be coupled to the installed radial array of battered angled micropiles.

CN102051876B relates to a new tree- structure pile, wherein it comprises a hollow column (1), the upper end of the hollow column (1) is provided with a top cover (4), the lower end of the hollow column (1) is provided with a base (6), the side wall of the hollow column (1) is provided with an expansion joint(2), and the hollow column (1) is connected with the hollow column (1) is equipped with expanding agent. After this invention's construction process, the tree-shaped structure piles, telescopic joint extends outwardly like tree root in the construction process, and the outer telescopic joint extends, by pouring concrete and filling material, the pile has higher strength, integrally bonded with the surrounding soil, pile, the structure of its type and good design, wide application range, it can under the precondition of not changing the pile length and the cross-section area of the, obviously increases the contact area between the pile foundation and the surrounding soil layers, thus increasing the carrier force, frictional resistance and stability of the pile. Furthermore, pull-out and skid-proof performance of the pile are improved

This invention relates to the use of expansion agents, which extends radially and forms root type structure. This process increases the contact area between the pile foundation and the surrounding soil layers, thus increasing the carrier force, frictional resistance and stability of the pile.

CN201292534Y a root foundation of a power transmission tower, comprising a step located on the upside, wherein a spur pile is set below the step. The utility model using said the technological solution is composed of an upside step and four lower spur piles, and is similar to the root system of the tree in nature; the piles are in tight contact with the undisturbed soil, thereby the piles have good resistance to up-pulling and down-pressing load; the pile foundation formed by the piles is the same as the root, in particular, the pile foundation can resist great horizontal force and has good stability. The pile hole is excavated by using a Luoyang shovel normally used in archaeological digging, a worker excavates the ground, the personnel safety is guaranteed, the labour intensity is low, and the advantage of the undisturbed foundation soil such as good load capacity is fully utilized, the vegetation deterioration is reduced, the water loss and soil erosion are avoided, and the utility model is favourable for environmental protection. Compared with the plug-type foundation, every foundation has concrete saved by about 10%, and the integrative cost can be lowered by about 5%.

This method comprises the use of four inclined piles. Though similarity with the root system is claimed, full benefits of the root system could be derived from fewer members. As fewer piles are used, the capacity enhancement is only through individual piles.

Publication No. CN112431443 relates to a novel habitat building. The novel habitat building is characterized in that a box dam hospitalization concept in a traditional dwelling is imported into a high-rise building, and a new dwelling concept is formed: houses are arranged around the dwelling, the dwelling is close to the dwelling, the dwelling is homed at home, and the dwelling is living in a community; the building looks like a pine tree and is composed of a center pillar trunk, frame branches, an appearance tree crown and a foundation tree root; a center pillar is hollow and is used for mounting elevators and pipelines, exchanging air, ventilating and discharging waste sewage; the frame branches and the appearance tree crowns form floors and walls of the building through light steel keels, steel inhaul cables and frames, and the floors and the walls are separated into houses; a residence is distributed in the mode that the center pillar trunk serves as the center to form the house; a public area is formed between the residences and the center pillar, and complete public service facilities are arranged; and the geothermal effect and an air exchange system are combined through the principle of a smoke chimney with the hollow center pillar, so that air exchange and energy conservation are realized. Except that the central pillar of the main body structure is made of reinforced concrete, and other structural parts can be industrially produced and assembled and constructed on site so that the construction efficiency can be effectively improved, the cost can be saved, and the land can be saved.

Publication No. CN104674945 relates to a tree-shaped steel pipe column structure and a construction method. The tree-shaped steel pipe column structure comprises a buttress structure, a pre-embedded steel slab, pre-embedded steel columns, vertical steel columns and slant steel columns, wherein the buttress structure is arranged underground, foundation bolts emerging from the ground are anchored in the buttress structure, the pre-embedded steel slab is arranged at the upper parts of the foundation bolts in a sleeving manner, the pre-embedded steel columns are arranged on the pre-embedded steel slab, the bottoms of the pre-embedded steel columns are provided with foundation bolt holes, the foundation bolt holes sleeve the foundation bolts and are fixed, the vertical steel columns are arranged on the pre-embedded steel columns, the certain ends of the slant steel columns are arranged on the vertical steel columns, a latticed column supporting jig frame is arranged between each of the other ends of the slant steel columns and a bearing surface, and a slant support is arranged between the top of each latticed column supporting jig frame and the corresponding slant steel column. The steel pipe column structure is attractive and stable in structural support, and an inner space of the main building is utilized reasonably.

Publication No. CN101446091 relates to a root type uplift pile. The pile is positioned below a pit in a group arrangement, has the diameter of 80 to 200 mm, the pile length H of more than and equal to 5 m, and the spacing B of more than and equal to 5 d; and d is the diameter of the pile; the bottom part size h of the pile top of the root type uplift pile being anchored in the pit is more than and equal to 200 mm.

Publication No. JPS58184068 relates to improving exfoliation resistance of a build-up welding layer in build-up welding austenitic stainless steel to the inner face of a tower tank, etc. made of carbon steel by providing a foundation root of austenitic stainless steel containing balanced Si and B. Constitution: In build-up welding austenitic stainless steel to the inner face of a tower tank, etc. made of carbon steel or low alloy steel, following foundation root is build-up welded first prior to surface layer building up. That is, the foundation root is formed of austenitic stainless steel containing <=0.08% C, 0.5-3.0% Mn, 7-16% Ni, 16-25% Cr, <=0.75% Si, <=0.08% B by wt%, and containing Si and B to satisfy the expression.

Publication No. UA70515 relates to insulation material has base and insulation layers. At that, the base is made of one layer or several layers, mostly soaked with tars or polymers, insulation layer/layers, this is /are made mostly of polymer materials and additionally it includes an adhesive layer to provide adhesion to surface being insulated and anti-adhesive layer to exclude glueing of material in the roll.

Publication No. CN101994324 relates to a reinforced foundation and a method for improving the bending and shearing bearability of an existing building rigid foundation. The reinforced foundation comprises an existing rigid foundation and a bearing structure above the rigid foundation. The method comprises the steps of: embedding the bearing structure of the existing rigid foundation with steel bars, expanding the length and the width of the existing rigid foundation according to design calculation values, coating a layer of bonding agent on between a new and an old foundation interfaces, and pouring a new expanded foundation wider and longer than the existing rigid foundation to the periphery and the root part of the bearing structure of the existing rigid foundation, burying the embedded steel bars in the new expanded foundation through pouring, tamping concrete poured on the deletion parts on the upper part of the existing rigid foundation and the bottom of the new expanded foundation, and pouring into an integer with the new expanded foundation.

Publication No. CN212248181 relates to a building foundation reinforcing structure which comprises a foundation, a first cylindrical movable groove is formed in the middle of the foundation, a root column is in contact with the middle of the inner bottom wall of the first cylindrical movable groove, and a compression spring is fixedly connected between the side surface of the bottom end of the root column and the inner side wall of the first cylindrical movable groove.

Publication No. CN210658414 relates to a connecting joint of a concrete assembly type prefabricated foundation beam and a prefabricated lotus root beam. The utility model belongs to the technical field of prefabricated buildings, and relates to a prefabricated lotus-root- shaped beam, which comprises a prefabricated foundation beam and a prefabricated lotus- root-shaped beam, the prefabricated lotus-root-shaped beam is provided with a column body section, at least one side of the column body section is provided with a bearing part, the bottom surface of the bearing part is provided with a first embedded plate, and the top surface of the column body section corresponding to the bearing part is provided with a second embedded plate; a lap joint part matched with the bearing part is arranged at the end part of the prefabricated foundation beam, a third embedded plate is arranged on the top surface of the lap joint part, and a fourth embedded plate is arranged on the bottom surface of the end part of the prefabricated foundation beam; a first connecting plate is fixedly connected between the first pre-embedded plate and the fourth pre-embedded plate, and a second connecting plate is fixedly connected between the second pre-embedded plate and the third pre-embedded plate. Steel bars do not need to be bound on site, construction is easy, connection reliability is high, and connection efficiency is high.

Publication No. CN112081158 relates to a construction process for forming a reinforced composite pile foundation through advancing type grouting of a high-rise building. The construction process comprises the following construction steps that pile foundation holes are drilled in a raft foundation by using a drilling machine; the length of drill rods drilled into a soil layer below the bottom of the raft foundation is L, filling and grouting are carried out, gaps of soil bodies around the drill rods and through gap channels are filled with injected grout, or areas where the soil bodies around the drill rods are relatively not more compact are filled with the grout, and the grout is solidified within 10 s to 60 s; after solidification, tree- root-shaped grouting bodies or irregular grouting bodies are correspondingly formed; 1/2 L of the drill rods are retracted upwards, and pressure grouting is carried out; the grout is uniformly diffused to the periphery, and solidified to form short cylinders; the operation is repeated to a designed depth; and a short cylinder structure formed in repeated advancing and retreating process forms the reinforced composite pile foundation for completely supporting the raft foundation.

Publication No. CN109750694 relates to a tree root pile for underpinning and reinforcing an existing building foundation and a construction method of the tree root pile. In a traditional tree root pile underpinning structure, a section of expanding pile body is formed at the bottom of a foundation to be underpinned, and interface shearing and friction type connection between a traditional tree root pile and an existing building foundation are changed into pressure-bearing type connection between the expanding pile body and the bottom of the existing building foundation. Therefore, the force transmission between the tree root pile and the existing building foundation is more direct and reliable, and the settlement of the existing building after underpinning can be reduced.

Publication No. CN102433884 relates to a system and a design method applying tree-root piles, and pile side compacted grouting, which can be used for effectively reducing disturbance of foundation excavation on an adjacent surrounding building and preventing the surrounding building from sinking.

Publication No. CN212375839 relates to the building comprises a support, a round plate is fixedly connected to the support, a groove is formed in the round plate, a folding pipe is fixedly welded to the side wall of the support, a motor is fixedly welded to the folding pipe, a rotating shaft is fixedly welded to an output shaft of the motor, and the rotating shaft is fixedly welded to the side wall of the rotating shaft. Wherein the rotating shaft sequentially penetrates through the support and the circular plate, a connecting rod is fixedly welded to the rotating shaft, a rotating column is fixedly welded to the connecting rod, a rectangular frame is arranged on the rotating column in a sleeving mode, a square pipe is fixedly welded to the rectangular frame, and a tamping part used for conducting insertion tamping reinforcement on aggregate is connected into the square pipe in a sliding mode.

Thus the reinforcements used by various researchers includes metal strips, geogrids, prestressed geotextiles and geocells, stone columns, jet grouted columns, dynamic compaction, vibro flotation. Application of these techniques often turns out to be either very costly or restricted by the condition of the site. Thus there is a need for an alternative solution for improving the load carrying capacity of the soil.

Structural skirts, used in the skirted foundation, also known as bucketed foundations, are routinely used to support huge gravity offshore structures even in soft marine deposits because of short installation time, economic feasibility and satisfactory performance under cyclic loading. Similarly, the addition of structural skirts to the edges of shallow foundations offers significant improvement in the load carrying capacity. Thus compared with a traditional monopile solution to offshore structures and piles to building foundations, the skirted foundation design reduces the steel/concrete weight by half. However, installation of a skirted/bucket foundation in case of onshore structures is not so easy. The construction process is very tedious and demands advanced excavation machinery. If fact, it is not practical to install inclined skirted foundations.

In order to overcome the above listed prior art, the present invention aims to provide a biogeotechnics based alternative system along with its installation as a replacement to vertical and inclined skirted foundation. The present invention addresses the prior art issue by adopting a group of micropiles mimicking the geometry of the tree roots system to replace the skirts. The system adopts the principles of biogeo technique. Closely spaced micropiles mimic the root system of trees and serve as the vertical/inclined skirts to the foundation enhancing the overall load carrying capacity of a shallow foundation.

OBJECTS OF THE INVENTION:

The principal object of the present invention is to provide a practical solution to the method of installation/construction of inclined skirted footings based on the concept of bioinspired geotechnics, by replacing skirts with closely spaced micropiles mimicking the tree root system.

Yet another object of the present invention is to provide an innovative foundation based on the principle of biogeotechnics, which improves the load carrying capacity of a shallow foundation.

Still another object of the present invention is to provide cost-effective solution to improve the load carrying capacity and hence the footing performance.

Yet another object of the present invention is to provide tree root inspired footings that, apart from increasing vertical load carrying capacity, also increases lateral and moment carrying capacity of the footing significantly compared to vertical skirted footings.

Still, another object of the present invention is to provide biogeotechnics based footings that are easy to install and place. SUMMARY OF THE INVENTION:

The present invention relates to a bioinspired geotechnics based system and method for the shallow foundation. An inclined skirted shallow foundation is provided for onshore structures. The system adopts the principles of biogeo technique. Micropiles serve as inclined skirted foundations. It uses micropiles to mimic the root system of trees and enhance the load carrying capacity of a shallow foundation. Thus the invention provides an effective solution to the method of installation/construction of skirted footings, which are otherwise theoretical and non-practical. The micropiles with optimal spacing mimic the functioning of the roots of a tree and provide stability to the foundation under all different types of loading. Micropiles not only confines the soil but also act as structural members taking tensile, compression and frictional forces.

BRIEF DESCRIPTION OF THE INVENTION

It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments.

Figure 1 shows the configuration of bioinspired inclined skirted footing with varying spacing and inclination.

Figure 2 shows the model foundations: skirted footing (left), bioinspired skirted footing with vertical micropiles with 4D spacing (center) and bioinspired skirted footing with inclined micropiles with 2D spacing (right)

Figure 3 shows variation of load-carrying capacity with varying length of micropiles: (a) S=3D, (b) S=4D, (c) S=6D.

Figure 4 shows variation of load-carrying capacity with the angle of installation of micropile: (a) L/B=0.5, (b) L/B=1, (c) L/B=1.5, (d) L/B=2.

Figure 5 shows variation of load-carrying capacity with spacing between micropiles (S/D): (a) L/B=0.5, (b) L/B=1, (c) L/B=1.5, (d) L/B=2.

Figure 6 shows variation of load-carrying capacity with the length of micropiles (L/B): (a) θ=0°, (b) θ=10°, (c) θ=20°, (d) θ=30°

Figure 7 shows experimental model load-settlement curves for plate, skirted footing and bioinspired footing with fixed L/B=0.5 and S=3D.

Figure 8 shows the experimental model load-carrying capacity with micropile installation angle for bioinspired footing with fixed L/B=0.5. Figure 9 shows experimental model load-settlement curves for bioinspired footing, representing the influence of spacing between the micropiles with fixed L/B=0.5 and θ=15°.

Figure 10 shows experimental model load-settlement curves for bioinspired footing, representing the influence of the length of micropiles (L/B) with fixed S=4D and θ=15°.

Figure 11 shows the detailed method of construction.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention relates to a system and method for the shallow foundation. An inclined skirted shallow foundation is provided for onshore structures. The system adopts the principles of biogeo technique. Micropiles serve as inclined skirted foundations. It uses micropiles to mimic the root system of trees and enhance the load carrying capacity of a shallow foundation. Thus the invention provides an effective solution to the method of installation/construction of skirted footings, which are otherwise theoretical and non- practical. Micropiles not only confine the soil but also act as structural members taking tensile, compression and frictional forces.

Installation is completed readily using the cast in situ or prefabricated micropiles. In case of use of prefabricated micropiles, system would be ready to take the structural loads immediately after the installation. After the insertion of micropiles, the footing will be either constructed or placed such that sufficient reinforcement of micropiles will be lapped within the concrete of the footing. In the case of the cast in situ, the casing can be left in the ground permanently to facilitate structural connections, for seismic design considerations, or for other design considerations. The configuration of bioinspired inclined skirted footing with varying spacing and inclination is shown in Fig. 1. A bioinspired skirted footing comprises substructure base (1) square or rectangular or circular or strip footing with closely spaced vertical/inclined micropiles and multiple micro piles (2a, 2b....) of different lengths attached to the base at different inclinations 0° to 90° and at a different spacing from each other which is in ration of 1D to 6D, where “D” represents the diameter of micropile, which act as inclined structural skirts fixed to the edges of the foundation. Thus use of micropiles in place of structural skirts, would be material-efficient, energy-efficient and economical as compared to traditional skirted footing, other ground improvement options as well as deep foundations.

The substructure herein defines a square or rectangular or circular or strip footing with closely spaced vertical/inclined micropiles. The vertical or inclined closely spaced micropiles are of varying lengths. Micropile material can be solid/hollow steel or reinforced/unreinforced concrete, while the footing can be made up of steel plate/frame or reinforced/prestressed concrete.

Inclined skirted footings not only increase vertical load carrying capacity but also increase lateral and moment carrying capacities of the footing significantly as compared to vertical skits. However, their installation is very tedious and expensive. The present invention provides an effective solution to the method of installation/construction of inclined skirted footings, which are otherwise theoretical and non-practical. This is a novel method of constructing a new foundation using the number of micropiles, which performs very well in different loading environments.

The present invention is based on the use of optimally spaced multiple micro piles, which act as inclined structural skirts fixed to the edges of the foundation. Thus it provides an alternative to skirted/bucketed foundation and can be constructed without much difficulties. The invention describes the novel method of constructing a new foundation using a number of equidistance micropiles, as an alternative to inclined skirted foundation. A radial array of batter-angled micropiles is drilled or driven and resembles the natural tree root structure. The invention uses micropiles to mimic the functioning of the roots of a tree and provide stability to the foundation under all different types of loading. The proposed foundation uses micropiles to mimic the root system of trees and holds the soil, which enhances the loadcarrying capacity of a shallow foundation and offers solutions to many challenging problems. These micropiles constrain the soil between them and increase the effective depth of the overall foundation, hence the footing performance. Use of inclined micropiles as compared to vertical micropiles confines much more soil. Thereby, they increase not only the effective depth but also the effective width of a shallow foundation.

This is an effective solution to the method of installation/construction of skirted footings, which are otherwise theoretical and non-practical. It presents an innovative shallow foundation based on the principles of biogeotechnics. Biogeotechnics is comprised of both bio-mediated and bio-inspired technologies, and offers solutions to many challenging problems. Micropiles serve as the inclined skirted foundation. The proposed foundation uses micropiles to enhance the load carrying capacity of a shallow foundation. The addition of inclined micro piles to the edges of shallow foundations offers significant improvement in the overall performance of existing foundations. Further, micropiles not only confines the soil but also act as structural members taking tensile, compression and frictional forces. These inclined micro piles which replicate the tree root system constrain the soil between them, increase the effective width of a shallow foundation and improve the footing performance. Thus, the proposed invention comes under the domain of bio-inspired getotechniques. Apart from increasing vertical load carrying capacity, the proposed bioinspired foundation also significantly increases the lateral and moment carrying capacity of the footing compared to the strip foundation or existing skirted foundation. The performance of this invented foundation depends on micropile dimensions, spacing between micropiles and the optimum inclination of micropiles.

METHOD OF CONSTRUCTION:

Micropiles, as well as footing, can be made of reinforced concrete following two methods of construction: Precast or Cast-in-Situ and two methods of installation: Driven and Bored. After the insertion/construction of micropiles, the footing will be either constructed or placed, such that sufficient reinforcement of micropiles will be lapped within the concrete of the footing. Drilled pre-cast/cast-in-situ micropiles are particularly useful in limited-access situations adjacent to vibration-sensitive structures, where driving of micropiles is not feasible. Further, the drilled micro piles are very beneficial when micro-piles are to be installed in relatively dense and/or obstruction-laden fill and/or hilly terrains. On the other hand, driven precast concrete micropiles are preferred in loose soils and they also help in rapid installation of the bio-inspired footing. However, they are to be designed to withstand high driving stresses. In fact, forming micropiles through driving, helps in socketing micropiles firmly within the ground, which further enhances the load carrying capacity and stability of the overall foundation. The proposed bio-inspired skirted footing would be material-efficient, energy-efficient and economical compared to other ground improvement options and deep foundations as well as traditional skirted footings. The detailed methods of construction for Cast-in-Situ (Bored/Driven) and Precast (Bored/Driven) are given below: Bored Cast-in-Situ: A simple method of construction of a micropile involves a total of eight steps as shown in Fig. 11. Step 1 involves predrilling a hole (with a casing if the hole is collapsible) as per design specification. Predrilling a hole can be done with normal auger if the sites are inaccessible for heavy machinery. The drilling can be done vertical or inclined according to design specifications and requirements using augers or drill bit as per the feasibility as shown in step 2. Step 3 and 4 involves lowering a solid steel-round textured reinforcement bar with centralizers into the bore and filling the cavity with cement grout, typically through tremie methods either under gravity or high pressure. Then the casing is gradually withdrawn, creating a bond zone between the grout and the surrounding soil. In case of loose soils, casing can be left in place and then micropile can be constructed as shown in Steps 5 and 6. The same process of construction is repeated for other micropiles following the spacing and inclination as per design specification and is shown in step 7. Finally, the reinforcement of the micropile cap/footing is placed and concrete is casted as per design specification as shown in step 8.

Driven Cast-in-Situ: A closed-ended hollow steel casing is driven into the ground. Then the solid steel-round textured bar with centralizers are placed into the casing and filled with concrete to create driven cast in-situ concrete micropiles. The casing can either be pulled out and reused as the concrete is being poured or left in place to form a part of the micropile. The same construction process is repeated for other micropiles following the spacing and inclination as per design specifications. Finally, the reinforcement for the micropile cap/footing is placed and concrete is casted as per design specification.

Bored Precast: Precast micropiles are casted using reinforced concrete at casting yard. The installation process of bored precast micropiles also requires drilling of bore using auger/drill bit similar to bored Cast-in-Situ micropiles. Step 1 involves predrilling a hole as per design specifications. Predrilling a hole can be done with normal auger if the sites are inaccessible for heavy machinery. The drilling can be done vertical or inclined according to design specifications and requirements using augers or drill bit as per the feasibility. Step 2 involves lowering of precast micropile into the pre bored holes. Step 3 involves filling the spaces between the micropile and bored holes with cement grout. The same construction process is repeated for other micropiles, following the spacing and inclination as per design specifications. Finally the reinforcement for cap is placed and the concrete is casted as per design specification as shown in step 8. Alternatively, structural plate or precast concrete slab can also be used as a micropile cap.

Driven Precast: The installation process of a micropile involves driving/hammering the micropiles to the required depth according to design specifications and requirements using a weight/hammer attached to a tripod arrangement. After insertion of micropiles, footing/cap will be either constructed or placed, such that sufficient reinforcement of micropiles will be lapped within the concrete of the footing. Precast micropiles should be designed to withstand handling and driving stresses. RESULTS AND DISCUSSION:

Numerical Study:

As mentioned above, bio-inspired skirted footing basically consists of a footing with micropiles placed at optimal spacing. Numerical simulations are carried out to show how closely spaced micropiles on the periphery of the footing are equivalent to a skirted footing. In this study, the capacity of skirted footings resting on cohesionless soils under vertical loading is investigated by means of a finite-element limit analysis (FELA) performed with OptumG3 (OptumCE, 2018). In this evaluation, the square footing width B is set to be 1.95 metre. Five different ratios of skirt depth (L) to footing width (L/B) were employed in the research: 0, 0.5, 1.5, and 2.0. There were five different angles of friction that were taken into account to analyse their influence. A range of inclinations of the micropiles (0°, 10°, 20° and 30°) were tested. Also spacing between the micropiles were varied from S=3D, 4D and 6D. The elastic plate element from the OptumG3 element library was used to mimic the footing, skirts and micropiles. We carried out a convergence analysis considering the different geometries and soil properties to determine the optimal domain size, which wouldn't impact the failure loads and their mechanisms. The movements in all directions are constrained at the base of the model, while only the horizontal movements are constrained on all the side boundaries. In order to examine the load carrying capacity of the footings, elasto-plastic model with Mohr-Coulomb (MC) failure criteria is adopted for modelling the material properties. Young's modulus, and poisson's ratio, unit weight, friction angle and the dilation angle are the considered soil material properties. Considered FEM models results from numerical simulations are presented in Figs. 3 to 6. It is worth noting from Fig. 6(a) that the load carrying capacity of bioinspired footing (S=3D) is slightly lower than the skirted footing. In other words the load carrying capacity of bioinspired footing (S=3D and L/B=0.5) is just 11% less than the skirted footing. While as for L/B>1, 1.5 and 2 the load carrying capacity of bioinspired footing (S=3D) is just 7-8% less than the skirted footing. Hence, it is interesting to note that, the load carrying capacity of skirted footing is replicated by the bioinspired footing by effectively spacing the micropiles.

Further for bioinspired footing (S= 3D and L/B=0.5) in Fig. 4 (a), it is seen that load capacity varies with θ, showing an initial increase and peak at θ= 20° followed by a milder increase or decrease with further increase in θ. It is clear that inclination of micropiles from 0° to 20° resulted in an increase of 29% in load carrying capacity whileas for further increase in inclination from 20° to 30°, nearly 5% increase in load carrying capacity was noticed for S=3D and L/B=0.5. For S=4D and L/B=0.5 case, inclination of micropiles from 0° to 20° resulted in an increase of 15% in load carrying capacity whileas for further increase in inclination from 20° to 30° nearly a decrease of 4.5% in load carrying capacity was noticed. Similarly for L/B=1.5 and S=3D, (Fig. 4 (c)) the inclination of micropiles from 0° to 20° resulted in an increase of 108% in load carrying capacity whileas with further increase in inclination from 20° to 30° nearly a decrease of 6.5% in load carrying capacity was noticed. For S=4D and L/B=1.5 case, inclination of micropiles from 0° to 20° resulted in an increase of just 29% in load carrying capacity whileas for further increase in inclination from 20° to 30° nearly a decrease of 14% in load carrying capacity was noticed. Main reason for less improvement with the inclination of micropiles in this case is primarily because of the inability of micropiles to confine the soils with an increase in the spacing.

Thus, the ultimate load capacity of bioinspired footing as shown in Fig.4. was affected by both θ and S. The combined effect of micropile inclination and spacing was however significant, and it was observed from Fig. 3 and 4 that the values of the ultimate load of bioinspired footing were greater for smaller values of spacing (S) and with inclination angle (θ) between 10° - 20°.

From Fig. 5 and Fig. 6, it was observed that the values of the ultimate load of bioinspired footing were greater for larger values of L/B when micropiles are placed at optimal spacing (i.e S=3D). The maximum capacity in all the cases is attainted at S=3D. It is clear from Fig. 6a that an increase in the L/B of micropiles from 0.5 to 1 resulted in an increase of 101% in load carrying capacity and also an increase in the L/B of micropiles from 0.5 to 1.5 resulted in an increase of 230% and nearly 400% for L/B=2 in load carrying capacity.

From Fig. 3a, it can be noted that for S=3D and L/B>1 the load carrying capacity increases with the inclination from 0° to 20°, whileas it decreases with further increase in the inclination from 20° to 30°. For S=4D, the maximum load carrying capacity is achieved at θ=10°. For S=6D the maximum load carrying capacity is achieved at θ=0°.

Also from Fig. 3b the micropile groups constructed with a S=3D displayed a stiffer response than the groups constructed with S>3D. In addition, for S=3D, groups constructed with θ= 20° displayed an increase in capacity compared with groups constructed with θ=0°, 10° and 30°. This demonstrates the benefit of using S= 3D and θ=20° when a larger capacity or stiffer response is required. The above results confirm that both inclination and spacing between the micropiles plays an important role in improving the load carrying capacity. Therefore by using micropiles at optimum inclination and with optimum spacing, the load carrying capacity enhances significantly particularly at higher L/B. It's worth to note that the load carrying capacity of skirted footing is replicated by the bioinspired footing by using micropiles at an optimum inclination and with effective spacing between the micropiles.

Experimental Study:

An experimental study has also been carried out to verify the numerical findings of the study. The load carrying capacity and settlement of physical bioinspired foundation models resting on the sand were determined using a laboratory setup consisting of a test tank, sand raining hopper, and loading system. The 2.10 m x 1.20 m x 1.10 m tank had a see-through wall on one side, where the experiments were carried out. The boundary effects were nullified by keeping a safe distance between the boundary and the foundation. A servo-hydraulic linear actuator (double acting, double ended) with a 3T capacity that was installed there to supply the reaction forces required for vertical loading. A displacement transducer is installed within the servo hydraulic linear actuator to monitor settlements.

Table 1 provides the details of all the experimental tests conducted in this study and the tests are designated by the alphanumeric characters in their names. The letter “BF” indicates bioinspired footing with either vertical or inclined micropile cases. Whileas letter “SKF” indicates a skirted footing case and “SF” indicates surface footing. The number after letter BF/SKF/SF represents the inclination, designated as 0, 15 and 30. 0 represents vertical micropiles and 30 represents micropiles are inclined at 30°. The last number indicates the L/B (length of micropiles or skirts wrt width of footing. Width of footing (B) is 200mm for all the cases.), designated as 0, 0.5, 1 and 1.5. (0 represents there are no micropiles or skirts attached while 1.5 represents 300mm micropiles/skirts of length). Specially for bioinspired footing cases spacing between the micropiles is indicated by “xD” ( x= 2,3,4 or 6). “D” represents the diameter of micropiles, which is constant as 16mm. So 2D represents that the micropiles are spaced 32mm from each other. Hence, BF4D -10-1.5 represents Bioinspired footing spaced four times the diameter of micropiles (4xl6=64mm) from each other with 10° inclined micropiles with the length of micropiles as 1.5 times the width of footing (1.5x200= 300mm). Table 1: Details of experimental model tests carried out

Fig. 2 shows the detailed configuration of the plate, skirted footing and bioinspired model foundations, all of which were made of stainless steel. Three square-shaped plate with widths of 200 mm with 16 mm thickness were fabricated and used. Additionally, tests are also conducted using simply square plate alone to obtain the load capacity of the footing without any skirt and micropiles. For the micropile foundations, three installation angles (θ) of 0°, 15° and 30° and three pile spacing (S) of 2, 3, 4, and 6 times the micropile diameter (D) were considered. The diameter of the micropiles for all cases was 15 mm.

Loads were applied using a servo hydraulic linear actuator with the load increment 0.02 kN/s. The results from the experimental study are presented in Figs. 7 to 10. The load-settlement curves of the surface footing (SF) in Fig. 7 show that the ultimate state is reached after a certain settlement beyond which no further increase in the load capacity is observed. For bioinspired footing with inclined micropiles, the load-carrying capacity increased continuously without a clear indication of yielding or failure. These footings couldn’t be tested till the ultimate state due to the limitation of the reaction frame. Hence, in the present study, the load capacities of all the model foundations are compared, corresponding to 10% of raft width settlements.

The behaviour of skirted footing and bioinspired footing were studied with the help of loaddisplacement curves. In Fig 7, 9 and 10, the horizontal dashed lines indicate 10% of raft width settlements. It was seen from Fig. 7 that the load carrying capacity of skirted footing is 18.5 kN which is slightly higher than bioinspired footing (S=3D) as 17.2 kN. In other words, the load carrying capacity of skirted footing is just 7% more than the bioinspired footing with S=3D. It is interesting to note that the load carrying capacity of skirted footing is replicated by the bioinspired footing by effectively spacing the micropiles. Also inclination of micropiles will further increase the overall performance of the foundation. As seen from Fig. 7, in bioinspired footing with a micropile inclination of 15°, the load carrying capacity is increased significantly as compared to bioinspired footing with vertical micropiles. The load carrying capacity of BF3D-15-0.5 is 25% more than BF3D-0-0.5. However, further increase in micropile inclination from 15° to 30° doesn’t help much in increasing capacity. These findings are well supporting our numerical findings, where we have clearly seen an increase in bioinspired footing capacity increases significantly with an increase in micropile inclination upto about 20° and thereafter only a slight increase in capacity.

The ultimate load capacities for bioinspired footing with fixed L/B=0.5 and with varying S/D were obtained from Fig.7 and plotted in Fig. 8 as a function of installation angle (θ).

The ultimate load capacity of bioinspired footing as shown in Fig. 8 was affected by both θ and S. These observations are also similar to our numerical findings.

Further for bioinspired footing with S= 3D, in Fig. 8, it was seen that load capacity varies gently with θ, showing an initial increase at θ= 15° followed by a milder increase with further increase in θ and reaches a peak at θ=30°. It is clear that the inclination of micropiles from 0° to 15° resulted in an increase of 25% in load carrying capacity whileas for further increase in inclination from 15° to 30° nearly 8% increase in load carrying capacity was noticed. From Fig. 9 and Fig. 10, it was observed that the values of the ultimate load of bioinspired footing were greater for smaller values of S and higher values of L/B. It was interesting to note from Fig. 9 that the decrease in spacing from 3D to 2D leads to an increase of 23% in load carrying capacity for fixed L/B=0.5 and θ=15°. Also, it is clear from Fig. 10 that an increase in the L/B of micropiles from 0 to 0.5 resulted in an increase of 36% in load carrying capacity and an increase in the L/B of micropiles from 0 to 1 resulted in an increase of 84% in load carrying capacity.

The above results from the experimental study also confirm that both inclination and spacing between the micropiles play an important role in improving the load carrying capacity. Therefore by using micropiles at an optimum inclination and with optimum spacing the load carrying capacity enhances significantly, particularly at larger micropile lengths and all the benefits of an inclined skirted foundation can be replicated by the bioinspired foundation.

Numerous modifications and adaptations of the system of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the true spirit and scope of this invention.

Claims

We Claim:

1. A bioinspired skirted footing comprises substructure base (1) square or rectangular or circular or strip footing with closely spaced vertical/inclined micropiles characterized in that the solid/hollow steel or reinforced/unreinforced concrete, footing of steel plate/frame or reinforced/prestressed concrete and multiple micro piles (2a, 2b....) of different lengths attached to the base at different inclinations 0° to 90° and at different spacing from each other which is in ration of 1D to 6D, where “D” represents the diameter of micropile, which act as inclined structural skirts fixed to the edges of the foundation wherein closely spaced micropiles to a footing helps in plugging the soil and making it as a part of the footing, yields comparable load carrying capacity to the footing with structural skirts and thus replaces the structural skirts.

2. The bioinspired skirted footing and its method of installation, as claimed in claim 1, wherein the method of installation includes the following steps:

- predrilling a hole, vertical or inclined (with a casing if the hole is collapsible) as per design specification.

- then lowering a solid steel -round textured bar with centralizers into the bore.

- then filling the cavity with cement grout, typically through tremie methods either under gravity or high pressure.

- the casing is gradually withdrawn, creating a bond zone between the grout and surrounding soil or bedrock.

3. The bioinspired skirted footing and its method of installation, as claimed in claim 1, wherein the method includes following steps:

- After insertion/construction of micropiles, the footing will be either constructed or placed, such that sufficient reinforcement of micropiles will be lapped within the concrete of the footing.

- Drilled pre-cast/cast-in-situ micropiles are particularly useful in limited-access situations adjacent to vibration-sensitive structures, where driving of micropiles is not feasible.

- Further, the drilled mircopiles are very beneficial when micro-piles are to be installed in relatively dense and/or obstruction-laden fill and/or hilly terrains.

4. The bioinspired skirted footing and its method of installation, as claimed in claim 1, wherein precast installation process of a micro pile involves driving in case of loose and soft soils, or drilling a bore through soils, rocks, overburden, etc. and then placing and after insertion of micropiles, footing will be either constructed or placed, such that sufficient reinforcement of micropiles will be lapped within the concrete of footing. The bioinspired skirted footing and its method of installation, as claimed in claim 1, wherein in case of loose soils, the casing can be driven and then micropile can be constructed. The bioinspired skirted footing and its method of installation, as claimed in claim 1, wherein forming micropiles through driving, helps in socketing micropiles within the firm ground, which further enhances the load carrying capacity and stability of the over all foundation. The bioinspired skirted footing and its method of installation, as claimed in claim 1, wherein the skirt action by closely spaced micropiles increases vertical load, lateral load and moment carrying capacities of the footing. The bioinspired skirted footing and its method of installation, as claimed in claim 1, wherein the load carrying capacity also depends on the spacing of micropiles, length of micropiles, the optimum inclination of skirts/micropiles.