US3830069A

US3830069A - Novel method for constructing subjacent foundation

Info

Publication number: US3830069A
Application number: US00384141A
Authority: US
Inventors: P Lin
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-07-30
Filing date: 1973-07-30
Publication date: 1974-08-20
Anticipated expiration: 1991-08-20

Abstract

Trenches are formed around the periphery of the area intended to be reinforced. Openings are then drilled from the surface, such openings being in rows, and selected rows being of different depths. By using freezer points the soil is then frozen solid. There is next formed a series of spaced channels extending through unfrozen soil from one trench to the opposite trench and after being excavated the channels are filled with hardenable material such as concrete. The sections which are adjacent to these hardenable sections may be then thawed and excavated and thereafter filled with hardenable material to form a complete mat the surface of which is adapted for sustaining soil loads and the weight of any structure thereon. The trenches are then filled with soil or hardenable material.

Description

' United States Patent Lin [451 Aug. 20, 1974 NOVEL METHOD FOR CONSTRUCTING SUBJACENT FOUNDATION Primary ExaminerRobert R. Mackey Assistant Examiner-Alex Grosz Attorney, Agent, or Firm.lohn A. Young [57] ABSTRACT Trenches are formed around the periphery of the area intended to be reinforced. Openings are then drilled from the surface, such openings being in rows, and selected rows being of different depths. By using freezer points the soil is then frozen solid. There is next formed a series of spaced channels extending through unfrozen soil from one trench to the opposite trench and after being excavated the channels are filled with hardenable material such as concrete. The sections which are adjacent to these hardenable sections may be then thawed and excavated and thereafter filled with hardenable material to form a complete mat the surface of which is adapted for sustaining soil loads and the weight of any structure thereon. The trenches are then filled with soil or hardenable material.

10 Claims, 11 Drawing Figures CONCRETE OR g BACK -FlLLED SOIL PATENTED M1620 I974 SIIEEI NF 2 BACKGROUND OF THE INVENTION Because of the overstressing of soil by a foundation (frequently accompanied by non-uniform distribution of soil pressures), or because soils are inadequate to sustain surface loading, objectionable settling can occur for buildings and the like. It is a common remedial practice to employ corrective action by the construction of grillages, needles, piers, caissons, piledriving, etc. Unfortunately these conventional shoring techniques are complicated, are frequently uneconomical, and for that reason are unsatisfactory solutions for settlement problems of existing structures. For even further reasons, it becomes extremely critical to avoid pile driving where there is a shortage of working space and where disturbances are intolerable because of the possibility of cracking existing structure and endangering neighboring foundations.

For these reasons, conventional underpinning methods are frequently contraindicated. A classic example is the leaning Tower in Pisa which will not readily admit of any of these foregoing techniques and which is unfortunately now settling at a dangerous rate from which can be projected the ruination of the building before too long a period. Because the conventionally used systems for underpinning are unacceptable there must be developed new and different systems for providing a subjacent foundation which will obviate the difficulties of the prior art techniques including shoring, etc.

Where used in the specification, the terms subjacent foundation" is meant to include a foundation which is disposed at a level below an existing foundation level but not necessarily directly below or in contiguous relation with the existing foundation.

OBJECTS OF THE INVENTION It is an object of the present invention to develop a new and improved technique adapted for subjacent support of existing structures and which will preclude settlement or at least retard it to an acceptable rate.

In a further object of the invention, to use subjacent foundations to eliminate or reduce excessive and unequal settlement of existing or future structures, and also for the construction of a future structure on soil with an underlying strata of low bearing capacity such as quicksand, and undesirable clay formation. By constructing first a subjacent foundation below the undesirable soil formation and using a method to be described, the undesirable soil formation above the subjacent foundation can be consolidated by grouting methods or by lime, etc. in order to increase its bearing capacity. Therefore the subjacent foundation serves four objects;

I I. It is used to transmit the structure load to soil strata which have high bearing capacity.

2. It is used to distribute the structure load over an extensive subjacent foundation area, resulting in low soil pressure.

3. It is used as a base for soil consolidation.

4. It is used to intercept the downward flow of grouting (cement, chemical, or clay).

After the weak soil formation is thus consolidated,

the foundation which receives directly the structure load is constructed on the consolidated soil.

A further object of the present invention is to provide a method for providing subjacent support of existing buildings wherein the subjacent support can be installed with minimal disturbance to the existing adjacent structures or so as to avoid damage to such structures.

A still further object of the present invention is to provide remedial underpinning or remedial subjacent support by providing interlocking sections of a hardenable material such as concrete to distribute the supported load through a larger area of such soil stratum and thereby reduce the soil pressure on the underlying stratum. The result is to eliminate or reduce further settlement of the surface structure.

An overall object of the present invention is to provide a new method for providing subjacent support by a system of partial excavations wherein such excavations occur in stages, with the soil being successively substituted by a hardenable material and in which the finished substitute consists of a mat of hard durable concrete which extends as a supportive subjacent foundation. g

Other objects and feature of the present invention will become apparent from a consideration of the following description which proceeds with reference to the accompanying drawings wherein an example of the invention is illustrated by way of illustration and not by way of limitation.

DRAWINGS FIG. 1 is a plan view of the surface intended to be reinforced against settling, the inner bound area indicating the foundation area for an existing structure;

FIGS. 2, 3, 4, 5, 6 and 7 illustrate progressive views of the method for providing subjacent support wherein the area is first formed with drillings in rows, selected rows being of shallow depth as shown in FIG. 2; FIG. 3 illustrating the insertion of freezer points within the drilled openings in FIG. 2; FIG. 4 illustrating the excavation of the unfrozen sections after freezing has occurred from step 3; FIG. 5 illustrating the substitution of concrete blocks in the excavated sections developed from FIG. 4; FIG. 6 illustrating the excavation of the sections between the horizontally extending rows of concrete blocks; FIG. 7 illustrating the completion of pouring concrete in the excavated sections or spaces between the previously provided horizontal concrete blocks to form a continuous mat of concrete;

FIG. 8 is a cross-sectional view taken on line 8-8 of FIG. land FIG. 4;

FIG. 9 illustrates the same cross-sectional view as FIG. 8 but after the subjacent foundation has been provided;

FIG. 10 is a schematic view showing the flow of refrigerant through the freezer points which are inserted within the drilled openings in FIGS. 1-3, 10, 11; and,

FIG. ll illustrates a somewhat different subjacent support than that shown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG.'1 an area designated generally by reference numeral 12 includes a subsection 14 which is inadequate to support the weight of the building in the area 14; and, further assuming that remedial action must be taken for an existing inadequate building-foundation at 14, there is first made a series of drilled openings 16 and second series of drilled openings 18, it be noted from FIG. 2 that drilled openings 18 are to a shallower depth than 16 and this is the pattern generally established throughout the area 12; namely, selected rows are alternately drilled to deeper and shallower depths for a purpose which will be later described.

The difference in the depths of

openings

16 and 18 determines the height of the wall of the frozen soil for supporting the frozen soil to be described. The height ofthe wall is determined by the excavation equipment, and method, and by the concrete placing equipment and methods.

After the pattern of drilled

openings

16 and 18 is established throughout the area 12 and in the noted pattern, the openings, are of a diameter to accommodate for freezer points 20, 21 (FIG. which are then inserted into the borings l6, 18. Typically, the spacing between bored openings is from 3 feet to 7 feet and, as noted in FIG. I all the openings are staggered The openings are equidistant from adjacent openings. These spacings will, ofcourse, vary depending upon the condition of the soil such as its conductivity, as well as occurrence of ground water.

The freezer points consist generally of an elongated housing 22 (FIG. 10) having an inlet line 24 connected to distributor pipe 26 and outlet line 28, connected to collector pipe 30 so that refrigeration plant 32 can cause circulation of refrigerant through inlet line 24 and opening 34 into the housing 22, in the direction of the arrows 39 to opening 37 in line 28. As shown,-the refrigerant from refrigeration plant 32 is continuously circulated until the soil bounded by frozen line 38 (FIG. I0) is thoroughly hardened.

As indicated in FIGS. 2 and 3 the spacing of the bored openings and freezer points are such that the horizontal circles in soil effected by adjacent

bored openings

16 and 18 will intersect with each other. Consequently, all but selected sections 41 (FIG. 3) of the subjacent soil are frozen into a hard rigid mat and walls except, the unfrozen portion below the freezer points in selected openings 18, there being the soil portions in the vicinity of reference numeral 41 (FIG. 3).

The refrigerant which is circulated can be brine, freon, liquid nitrogen, etc. depending upon design requirements and the freezer point sizes can vary but are typically from 3 inches to 8 inches in diameter, the small sizes being for soils with more conductivity, larger diameter freezer points are conversely used for low soil conductivity or where the velocity of ground water is high.

The frozen layer of ground designated by the term mat of frozen soil having reference numeral 43 is generally bounded by the ground surface and bottom of bored holes l6, 18 or irregular level 60 (FIGS. 3, 4, 5, 6 and 7). During the time when the unfrozen soil is excavated in the spaces 50 (FIG. 4) soil and structure loads are supported by the mat 43 which in turn is supported by the wall of frozen soil 49 (FIG. 4). It should be noted, that even though frozen soil has high compressive strength, it is low in tension and shear strength. Therefore, allowable shear and flexual stresses of the frozen soil are the factors for determining the thickness of the mat of frozen soil 43. Of course, it is also deter mined by the underlying soil condition. The freezing can produce mats and walls of frozen soil regardless of weather, season and type of soil, as long as the ground water flow velocity is less than 6 ft/day.

The strength of the walls of the frozen soil 49 do not require sheeting, bracing, or strutting. This contributes to the convenience and savings of the invention.

After the freezing is effected at least two

trenches

42, 44, 46, and 48 are formed, these being excavations which are used to intercept underground water flow, to drain water from the soil. Insulation coverings are used during the freezing periods. The trenches are used to provide access for equipment and workmen to form transverse excavations within the sections 41 (FIG. 3) and section 80 (FIG. 5) which make up tunnels 50 (FIG. 4) and tunnels 86 (FIG. 6) of rectangular cross section. It should be noted that in the event that a building foundation is already in the subarea l4, drillings and freezer points are used to effect freezing thereunder by drilling through existing footings which rock drills or the like (not shown).

In any event, after the freezing is completed there is a continuous mat 43 of frozen soil from the approximate level (FIG. 3-7), to ground level. It should be noted that freezer points 20 or 21 extend above the surface of the soil to permit monitoring and to facilitate the freezing operation. Of course, during freezing operation all the freezer point above ground surface are insulated to minimize heat leakages and the insulation is preferably in the form of a foamplastic material.

After the freezing operation is completed and the continuous frozen soil mat and walls bounded by frozen line 45 is formed extending throughout the area 12, spaces 41 are excavated with equipment located in the trenches 42-46 to form hollow spaces 50 (FIG. 4) which are then filled with concrete to form blocks 52 (FIG. 5) which have longitudinal extending shear keys 54 extending longitudinally from one trench 42 to the other trench 46. The blocks 52 are reinforced with steel rods, etc. and after the concrete blocks harden, 2 inch openings (not shown) are then drilled through the frozen mat 43 (FIGS. 4-7) to receive pressured grouting which completely fills any unfilled spaces and cracks in the spaces 50 in order that the concrete blocks 52 are in complete and intimate contact with the mat 43. Thereafter, hot water or steam is circulated into the freezer points 20 in drillings l6 and the freezer points therein are then raised to the level of 60.

The spacing (FIG. 5) is then excavated to provide openings 82 (FIG. 6) and concrete is then poured in such spacings to form additional blocks 84 (FIG. 7) which interlock with blocks 52 thereby forming a continuous subjacent foundation (FIG. 7) which is substantially larger than the original footing 92.

The new subjacent support being more widely extended encompasses a support area having a greater quantum of subjacent soil and therefore reduces the unit soil pressure. The new subjacent support 90 is located so that its center colinear with the center of the soil pressure at the bottom of the existing footing 92 and therefore the new foundation serves the function of distributing soil pressure uniformly to the underlying stratum of the subjacent foundation. Therefore excessive settlement at the level 98 is effectively precluded.

In the excavation steps for space 82 (FIG. 6)'described. frozen soil can be loosened by thawing or loosened while fully frozen by excavation tools, or by partial thawing followed by excavation. Thawing can be readily accomplished as previously described at the selected locations by steam point, water points, or electrical points. Laser beams, in conjunction with excavation tools can also be used.

The method and procedure described is most readily applicable with saturated sandy soils that are relatively easy to freeze and excavate and where there is no excessive supersaturation after thawing.

Upon freezing, water is converted to ice. On freezing, because water increases 9 percent by volume, there is a tendency for soil heaving. Artificial freezing in the present invention contemplates uniform heaving because of the trenches constructed around the working area. The heaving effect produced, will not propagate to the neighboring area.

If the freezing should be discontinued, the temperature should be raised gradually. For sandy underlying stratum, the melted water in 'the soil 'is quickly squeezed out by the structure and soil loads from the pores causing the structure settle gradually back to the level before freezing, and no difficulty will encounter. In case ofclayey soils, freezing and subsequent thawing may destroy the natural structure of soil and reduce its strength properties. Therefore, it is recommended that the underlying soil below the existing foundation and above the subjacent foundation should be subject to consolidation test under alternating freezing and thawing condition in preliminary laboratory analysis. During the testing, the sample should be under a compressive stress the same as an actual soil pressure. This test is used to ascertain that excessive settlement will not develop after thawing.

Furthermore, the gradation of the underlying soil below the existing foundation and the subjacent foundation should also be investigated, for gradation of soil is related to ice lense formation in frozen soils. lce lense formation depends not only on ground water table elevation, but also on the grain sizes of the soil. According to Terzaghi (Soil Mechanics in Engineering Practice, Terzaghi and Peck, John Wiley & Sons, Inc. Page l50), in soils with less than 1 percent of grain smaller than 0.02 mm, ice lenses are not formed under any condition which may be encountered in the field. It is not desirable to have ice lense formation, for subsequent thaws transform the soil containing the ice lenses into a zone of supersaturated material.

Quick-freezing can reduce heave and increase strength of the frozen soil. Therefore, the lower the coolant temperature, the better. Clearly, rapid freezing is especially useful in the fine-grained soil which tend to heave. Coolant of 40 F. orlower is desirable.

After the subjacent foundation is formed the soil above the foundation can be consolidated in any one or combination of methods including stabilization by lime, grouting (cement, clay or chemical), or heat treatment. In the event that lime stabilization is employed, and where clayey soils are involved, the procedure is generally along the following lines l first circulate hot water or steam into the freezer points 20, 21 after having withdrawn the freezer points 20, 21. Within the holes 16 and 18 I then add lime. Thereafter, I then drill additional openings at ground level which are spaced approximately 5-6 feet apart and of a diameter 69 inches. The openings are drilled to about 6 inches from the top of the subjacent foundation and hydrated lime or other material is then emptied dry into the openings and tamped in place to within about 2 inches of the surface. The hole is then plugged with asphalt, concrete or the like and the resulting grouting consolidates into a pillar-like structure which drys up the saturated melting soil, improves the bearing value and arrests settlement. The soil surface is consolidated by scarifying to a depth of about 6 inches and then hydrated lime is applied evenly to the surface of the soil. An amount of water is added with rotary equipment or the like and is intimately mixed with the soil.

Soil stabilization can also be produced by grouting. Grouting is recommended for the thawed soil and is especially applicable to the subjacent foundation situation. When grounding is applied to the soil, the downward flow of grouting (cement, chemical or clay) is stopped by the subjacent foundation, resulting in an effective soil consolidation and less materials are used.

Referring next to the embodiment of FIG. l1.with an existing foundation 92 made of high strength materials on a subarea 14 it is sometimes desirable that the mat of frozen soil extend from the bottom of an existing foundation to ground surface. In the case of the embodiment of FIG. 11 the bored openings 18 reach the level 60 which is the bottom of an existing foundation 92 the openings are all made except for subarea 14. In subarea 14 the openings are all made except for subarea 14. In subarea 14 the openings or bore holes reach the top of the existing foundation and no drilling is effected through the foundation for 18. The openings 16 extend from ground surface to the level 98 and the mat of concrete for the whole area 12 is produced the same as described in FIGS. 3-7. The existing foundation 92 is then. directly supported by the mat 90.

Although the present invention has been illustrated and described in connection with a single example embodiment it will be understood that this is illustrative of the invention and is by no means restrictive thereof. it is reasonably to be expected that those skilled in this art can make numerous revisions and adaptations of the invention and it is intended that such revisions and adaptations will be included within the scope of the following claims as equivalents of the invention.

What I claim is:

l. A process for providing subjacent support comprising the steps of:

forming peripheral excavations around the area intended to be supported, drilling and thereafter freezing the soil to provide a mat of frozen soil of any preferred depth providing stepped unfrozen subsurface sections, forming tunnels extending horizontally at subsurface level through said unfrozen subsurface sections and extending from one excavation to the next, thereafter filling the excavated portions with hardenable-supporting material, excavating the adjacent intervening sections disposed between the hardenable supporting material portions, and then filling the excavated intervening sections with additional hardenable material to form a substantially continuous subjacent platform throughout the area intended to be subjacently supported.

2. The process in accordance with claim 1 including the step of forming interlocking portions between adjacent hardenable weight-sustaining sections.

3. The process in accordance with claim 1 wherein the freezing is produced by freezer points which are inserted in place and adapted to receive a flow of refrigerant therein to effect soil freezing.

4. The process in accordance with claim 1 including the step of thawing and then excavating the spaces between the previously excavated and filled spaces containing hardenable material to effect a continuous subjacent support.

5. The process in accordance with claim 1 including the step of drilling through existing foundations to effect subjacent support by providing a mat of frozen soil at a depth directly below and surrounding the area intended to be reinforced.

6. The process in accordance with claim 1 including the step of forming a subjaeent support prior to the formation of a surface foundation.

7. The process in accordance with claim 1 including the step of drilling the soil after thawing thereof and the formation of said subjaeent support and filling such drillings with grouting to effect reinforcement of such soil..

8. The process in accordance with claim 1 including surface structure supported thereby.

Claims

1. A process for providing subjacent support comprising the steps of: forming peripheral excavations around the area intended to be supported, drilling and thereafter freezing the soil to provide a mat of frozen soil of any preferred depth providing stepped unfrozen subsurface sections, forming tunnels extending horizontally at subsurface level through said unfrozen subsurface sections and extending from one excavation to the next, thereafter filling the excavated portions with hardenable-supporting material, excavating the adjacent intervening sections disposed between the hardenable supporting material portions, and then filling the excavated intervening sections with additional hardenable material to form a substantially continuous subjacent platform throughout the area intended to be subjacently supported.

6. The process in accordance with claim 1 including the step of forming a subjacent support prior to the formation of a surface foundation.

7. The process in accordance with claim 1 including the step of drilling the soil after thawing thereof and the formation of said subjacent support and filling such drillings with grouting to effect reinforcement of such soil.

8. The process in accordance with claim 1 including the step of drilling openings directly below an existing foundation and surrounding such foundation and forming the subjacent platform at a depth which is in contiguous relation with the existing foundation.

9. The process in accordance with claim 1 wherein said peripheral excavations are backfilled with hardenable material.

10. The process in accordance with claim 1 wherein said subjacent foundation is disposed with its center substantially colinear with the center of pressure of the surface structure supported thereby.