Patent Application
20170342674
The disclosure relates to improvements of a ground, notably but not exclusively a clay soil.
Various types of foundations are already known for erecting loading structures such as buildings or embankments for a highway. It is especially known to dispose piles in the ground above which the building is erected. Traditionally, a network of beams made of traditional reinforced concrete is then disposed at the top of the piles to support the floor of the loading structure, or the floor of such structure to which the piles are connected is designed as a slab able to sustain the local stresses generated by the supporting piles. If the piles are disposed in a mechanically weak ground, the load induced by the building is essentially transmitted by the piles to a harder deeper layer of the ground. Thus, substantially no charge of the building is transmitted to the weak ground and the piles must be designed to support 100% of the load of the building, leading to larger pile diameters and higher ratio of steel reinforcement in the piles. In addition, this generates high stresses and bending moments in the beams or slab above, requiring them to have a greater thickness.
To avoid these issues, it is also known to use rigid or semi-rigid inclusions, called “Controlled Modulus Columns” (CMCs). These inclusions have been used for decades to improve soils and control settlement of structures and embankments. CMCs are semi-rigid, cylindrical concrete columns typically installed in the soil with a hollow-stem lateral-displacement auger.
A plurality of such inclusions is generally disposed in the ground. A load transmitting layer is interposed between the ground and the load structure disposed thereon. As a result, only between 50% and 80% of the load of the supported structure is transmitted to the inclusions through the load transmitting layer, leading to smaller diameters for the inclusions and reduced stresses in the beams or slab supporting the loading structure.
U.S. Pat. No. 6,672,015 discloses a device for reinforcing a ground on which is disposed a loading structure, comprising a series of structural inclusions installed in the ground with a lateral-displacement auger and configured to mechanically reinforce said ground. The inclusions present a constant-diameter cylindrical shape.
The inventors have realized that traditional inclusions have the disadvantage of requiring a substantial amount of grout. Moreover, because a significant volume of soil is displaced, inclusions can give rise to ground heave. In scenarios where significant heave occurs, quality concerns arise, such as communication between inclusions during installation and vertical separation of the load transmitting layer and inclusions. In extreme cases, it may also lead to excessive tensile stresses in the inclusions. The specific measures traditionally taken to address these concerns, such as phasing of the installation of the inclusions or reinforcing them with steel, affect productivity and costs.
An object of embodiments of the disclosure is to provide a device for reinforcing a ground on which is disposed a loading structure, which is more environmentally friendly than traditional devices and which reduces the quality risks associated with excessive heave.
According to embodiments of the e, a device is provided for reinforcing a ground on which is disposed a loading structure. The device includes
a plurality of inclusions disposed vertically within the ground and configured to mechanically reinforce said ground, each inclusion of the plurality of inclusions having an axis and comprising a cylindrical core surrounded by at least one helical thread extending along the axial length of the cylindrical core, the cylindrical core defining an internal diameter of the inclusion and the helical thread defining an external diameter of the inclusion, wherein the internal diameter is between 250 mm and 450 mm and the external diameter is between 350 mm and 600 mm,
a load transmitting layer interposed between the ground and the loading structure disposed thereon, configured to transmit and distribute the load from the loading structure to both the ground and the plurality of inclusions,
wherein a ratio between a distance between axes of two adjacent inclusions and the internal diameter of said adjacent inclusions is between 4 and 14, and
wherein each of the inclusions is made from a material having a specified 28-day compressive strength between 5 MPa and 35 MPa.
In the following description, expressions “axial” and “radial” are considered in respect to the axis of the inclusion.
The 28-day compressive strength is defined in the Building Code Requirements for Structural Concrete, ACI-318-2008, published by the American Concrete Institute®.
By “specified”, it is understood that the 28-day compressive strength is the value indicated and used in the technical specifications and design documents of the inclusions.
Preferentially, the specified 28-day compressive strength is between 12 MPa and 27 MPa.
The long-term Young's modulus used for the design of the inclusions is calculated with the following formula:
E(psi)=⅓*57000*sqrt(f′c),
where f′c is the compressive strength at 28 days (psi).
According to a further object of the disclosure, the helical thread has a radial height between 30 mm and 100 mm.
According to a further object of the disclosure, the helical thread has an axial thickness between 30 mm and 80 mm. This axial thickness is measured at the junction between the thread and the core. Preferentially, the axial thickness is between 40 mm and 60 mm.
According to a further object of the disclosure, the helical thread has a pitch, and wherein a ratio between said pitch and an axial thickness of the helical thread is between 3 and 7. Preferentially, the ratio is between 4 and 6.
According to a further object of the disclosure, the helical thread has a cross section which is substantially rectangular, said cross section being taken in a longitudinal plane of the inclusion.
According to a further object of the disclosure, said material used for making the inclusions is a mortar or a grout.
According to a further object of the disclosure, the load transmitting layer has a thickness between 0.3 m and 1.5 m, preferentially between 0.3 and 1 m. This thickness is measured between two adjacent inclusions.
According to a further object of the disclosure, at least one of said plurality of inclusions comprises first and second helical threads.
According to a further object of the disclosure, a ratio between:
an axial distance between said first and second helical threads; and
an axial thickness of the first helical thread,
is between 3 and 7. Preferentially, the ratio is between 4 and 6.
According to the figures, embodiments of the disclosure include a device 10 for reinforcing a ground on which is disposed a loading structure. In this non-limiting example, the loading structure is a building 12. Without departing from the scope of the disclosure, the loading structure could be an embankment, a road, or any other loading structure.
In the example of
The device 10 comprises a plurality of inclusions 18 disposed vertically within the ground 14 and configured to mechanically reinforce said ground 14. While only three inclusions are illustrated on
In
In
The cylindrical core defines an internal diameter D1 of the inclusion 18. In this example, the internal diameter D1 is 280 mm.
The helical thread defines an external diameter D2 of the inclusion 18, which in this example is 380 mm.
The helical thread 22 has a pitch p, defined as the axial distance between two successive threads, which is equal to 380 mm in this example.
Moreover, the helical thread has a radial height RH of 80 mm in this example, and an axial thickness AT of 75 mm.
This threaded geometry is achieved by using a displacement auger 100, an example of which being illustrated in
The displacement auger 100 is provided with one protruding cutting tooth 110 located near a bottom end 112 of said displacement auger and extending radially from the helical flight 104.
In this example, the radial height of the cutting tooth 110 is about 100 mm.
The displacement auger 100 is screwed into the soil to a predetermined depth, which gives rise to a lateral displacement of the soil. Such displacement increases the density of the surrounding soil, and as such increases its strength and bearing capacity. During insertion of the displacement auger 100, the cutting tooth cut a helical groove in the bore wall, which is immediately filled by the laterally displaced soil. After completion of the bore, the displacement auger is raised, the hinged cap 106 opens, and material is injected through the hollow cylindrical core 102 into the bore. At the same time, as the displacement auger 100 is raised, the same rotational direction is maintained. The cutting tooth 110 cuts a fresh helical groove into the bore wall around the stem. The groove is filled with the injected material at the same time as the central bore so as to make the threaded inclusion.
According to another example illustrated in
The benefit of two cutting teeth is the ability to create a double helix thread shape with the same pitch as with a single tooth. This provides a pull rate that is twice as fast, or the possibility to rotate the auger more slowly while maintaining the same pull rate in order to tackle firmer ground conditions that offer an increased resistance to the rotational movement.
Before or after the installation of inclusions 18, the ground 14 is covered by a load transmitting layer 30, also called “load transfer platform”. From
The purpose of the load transmitting layer 30 is to transmit and distribute the load from the loading structure to both the ground and the plurality of inclusions. In this example, the thickness of the load transmitting layer 30 is about 1 m, being measured between two adjacent inclusions.
Depending on the nature and characteristics of the ground to improve, the load transmitting layer transfers between 30% and 90% of the load of the supported structure to the inclusions, and typically between 40% and 70%.
After settlement, the upper ends of the inclusions 18 generally penetrate into the load transmitting layer 30, as illustrated in
In example of
Load tests have been carried out in Leland, N.C., USA in August 2015.
Table of
Inclusions DT-1 and TT-1 were each installed at 4 meters depth without tipping into a bearing layer of any kind. Thus, they were expected to plunge at relatively light loads, with relatively low load realized at the tip of the inclusions during plunging failure. The results of these two load tests validate that inclusions DT-1 and TT-1 having similar deflecting plots, with load tests DT-1 realizing about 156 kN at the tip at failure, and TT-1 realizing an about 120 kN at failure.
By dividing the remaining load capacity (i.e. difference between Ultimate Failure Load and Tip Load at Ultimate Failure) by the installed length, the skin-friction capacity per meter can be approximated. In this case, the skin friction capacity of DT-1 is about 64 kN/m, while the skin friction capacity of TT-1 is about 88 kN/m. As a result, threaded TT-1 skin-friction resistance appears to be greater than that of the comparable classical DT-1.
Making of inclusion DT-1 required about 0.51 m3, while the making of inclusion TT-1 required about 0.311 m3, which is 39% less in volume than for making DT-1.
Classical inclusion DT-2 and threaded inclusions TT-2, TT-4 were all installed 5.5 m deep, into a medium-dense bearing layer. The two threaded inclusions TT-2,TT-4, performed almost identically, as mentioned in the table of
Making of inclusion DT-2 required about 0.708 m3 of material, while the making of inclusions TT-2 and TT-4 required about 0.425 m3 of material, which is 40% less in volume than for making DT-2.
Classical inclusion DT-3 and threaded inclusion TT-3 were each installed to a depth of 11 m, into a very dense bearing layer. The maximum capacity of the load test setup was reached in both cases without structural and geotechnical failure.
Making of inclusion DT-3 required about 1.39 m3 of material, while the making of inclusion TT-3 required about 0.85 m3, which is 39% less in volume than for making DT-3.
These results show that threaded inclusions with a 280-mm internal diameter, 380-mm pitch, 100-mm cutting tooth, and 380 mm external diameter performed similarly to traditional 400-mm diameter inclusions. Moreover, in most cases, skin-friction capacity of the threaded inclusions was slightly better than the classical inclusions. The performance of threaded inclusions is also expected to improve further with a reduced pitch dimension compared to the pitch dimension tested. In average, the material volume needed to make threaded inclusions was 39.2% in volume less than for classical inclusions.
Consequently, load tests performed in Leland show that threaded inclusions according to the disclosure have similar properties as traditional ones while requiring smaller quantities of material. Thus, the device according to embodiments of the disclosure has less impact on the environment and is less costly than traditional soil reinforcing devices.
Also, a network of threaded inclusions was built in Leland (Trial Threaded CMCs). 22 threaded inclusions have been installed along three rows. Adjacent inclusions are spaced at 2.4 m intervals.
In these examples, the material used to make the inclusions is a mortar having a specified 28-day compressive strength of about 27.7 MPa. This value is the one used to design the inclusions.
The long-term Young Modulus of the inclusions is (in psi) ⅓*57000*sqrt(f′c), where f′c is the compressive strength at 28 days (psi). A grout could be used instead of mortar.
The inventors have also run heave calculations, using three different methods, which show a significant heave reduction when using threaded inclusions:
According to the inventors, the “worst-case” scenario for heave is using a 450-mm inclusion on 1.5 meter spacing square grid with a depth of clay penetration of 15 m. The effective volume displaced to produce a 450-mm equivalent threaded inclusion is about equal to a 360-mm diameter traditional inclusion. Comparing heaves with the above three methods yields the following:
A. Heave Calculation with 450-mm Traditional Inclusion:
B. Heave Calculation with 360-mm Traditional Inclusion (Equivalent to a 330-mm Core Diameter Threaded Inclusion):
Consequently, replacing a 450-mm traditional inclusion by a 330-mm internal diameter threaded inclusion (having the same volume as the 360-mm traditional inclusion) results in a 35 to 47 percent reduction in heave.
Another simulation was carried out, based on a real project performed in Bellmawr, N.J., USA. The results are indicated in the table of
Two rough data points were used to estimate a “displacement ratio” (as used in Method #2 described previously) for this project conditions. According to the inventors, such method is one of the best ways to demonstrate the effects on heave of substituting traditional inclusions with threaded inclusions according to the present disclosure because the replacement ratio and the calculation method are checked against actual heave measurements.
The two data points are:
The displacement ratio estimated with these two data points is about 37%. This means that, for a given volume displaced in a unit cell equal to one spacing squared (2.3 square meters), 37% displaced volume can be expected to translate into vertical heave.
Then, predicted heave values were estimated using method #2 with a ratio of 37%, at square spacing ranging from 1.5 m to 2.4 m. Traditional inclusions were compared to threaded inclusions according to the disclosure of the same equivalent outer diameter. To achieve a 450 mm outer diameter inclusion, a 330 mm core auger would be needed for the optimal threading tooth configuration. Moreover, the equivalent volume of a 330 mm core threaded inclusion is about equal to a 360 mm traditional inclusion (which is what was used to estimate heave).
This method shows that heave is much lower when using threaded inclusions according to the disclosure.
As a result, another advantage of embodiments of the disclosure is to substantially reduce the platform heave, which decreases the quality risk associated with excessive heave.
As previously mentioned, the inclusions are preferentially from mortar. For instance, for a specified compressive strength (at 28 days) of 21 MPa, the following mortar composition can be used:
About 270 kg of cement;
Fly ash or slag;
Sand or stones (max 10 mm size);
Approximately 170 L water;
Additives such as super plasticizer, water reducer, etc. . . . .
Alternatively, the inclusions could be made from concrete.
Throughout the description, including the claims, the term “comprising a” should be understood as being synonymous with “comprising at least one” unless otherwise stated. In addition, any range set forth in the description, including the claims should be understood as including its end value(s) unless otherwise stated. Specific values for described elements should be understood to be within accepted manufacturing or industry tolerances known to one of skill in the art, and any use of the terms “substantially” and/or “approximately” and/or “generally” should be understood to mean falling within such accepted tolerances.
Where any standards of national, international, or other standards bodies are referenced (e.g., ISO, ACI, etc.), such references are intended to refer to the standard as defined by the standards body as of the priority date of the present specification. Any subsequent substantive changes to such standards are not intended to modify the scope and/or definitions of the present disclosure and/or claims.
It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims.
