During installation of foundations formed from bore- and cast-in-place piles (including pileswith lower broadening of the shaft) in soils prone to slump-type settlement, their bearing capacity islower with respect to the soil than to the material of the shaft, i.e., a significant portion of the concretein the piles does not function, resulting in increased construction costs. A new pneumatic form, whichensures the opening of a recess of a given diameter over the length of the pile shaft, has been devel-oped to bring the capacity of the piles with respect to the soil and material of its shaft into correspon-dence. This will lead to a savings of concrete, and lower the bearing capacity of the pile shaft, approx-imating it to that of the pile with respect to the soil.

The pneumatic form is an inflated balloon consisting of a rubber chamber and jackets fabricatefrom tire rubber manufactured for heavy truck transport (a chamber-free variant is possible), in which acompressor is used to supply air under a pressure of up to 0.2 MPa (Fig. 1).

A template, which provides for a rubber blank at the terminal sections, is cut from dense paper ora thin cardboard, and shaped triangular cut-outs, which form the ends of the chamber after gluing of theballoon, are made prior to fabrication of the chamber (jacket). It is recommended that the height of thecut-outs be equal to the diameter of the form, for example, for experimental piles with a diameter of 800-800 mm. The cut-outs are glued to thin strips formed from the same material, i.e., a rubber sheet 1.5-2.0mm thick. A tire valve for pumping of air from a compressor is glued to the upper end of the chamber.

To ensure the article's strength, the chamber blanks are glued with a rubber glue. Unwoven syn-thetic materials (USM), for example, synthetic geotextile materials (SGM), are recommended for thejackets of the hollow bore- and cast-in-place piles. For these purposes, "Bidim," which is manufacturedby the Rhone-Poulenc Company (France) and which is fabricated from polyesters with a weight of 210g/m2, thickness of 1.9 mm, a tensile strength of 160 N/cm, and a relative elongation of 65%, are mostacceptable. In addition to the "Bidim" USM, it is possible to use Terram, which is manufactured by theICI Company (Great Britain), and is fabricated from polypropylene with a weight of 280 g/m2, thicknessof 1 mm, tensile strength of 94 N/cm, and relative elongation of 50%, etc.

Soil Mechanics and Foundation Engineering, Vol. 49, No. 2, May, 2012 (Russian Original No. 2, March-Apr., 2012)

The possibility of using pneumatic forms to install bore- and cast-in-place piles-envelopsis examined. The technology and methods of its preparation, as well, as test results arecited.

PROCEDURE FOR INSTALLATION OF BORE- AND CAST-IN-PLACE PILES-ENVELOPS WITH USE OF PNEUMATIC FORMS

Yu. L. TimofeevRostov-on-Don, Russia.

UDC 624.157.21

Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, pp. 21-24, March-April, 2012.

0038-0741/12/4902-0068 ©2012 Springer Science+Business Media, Inc.68

TECHNOLOGY AND WORK PRODUCTION

Domestic USM are fabricated from a polymer melt with a weight of 500 g/m2, thickness of 3.5-4.0mm, tensile strength of 100 N/cm., and relative elongation of 150%, rendering it suitable for the cham-bered and chamber-free pneumatic forms.

The thermally hardened "Typar" material (United States) with a weight of 136 and 200 g/m2,thickness of 0.46 and 0.6 mm, strength of 103 and 166 N/cm, and relative elongation of 45 and 52% isconsidered the USM most qualified for a chambered pneumatic form. This material is lightweight, andstrong, and exhibits low adhesion to concrete.

To accelerate inflation of the pneumatic form, strips of a conducting carbon-graphite fabric,which are connected to an electric power supply and heat the reinforced-concrete shaft of the pile, areglued to the internal surface of the external form. With the appearance of nano-carbon materials, nan-otubes in the amount of 0.001-0.0001 wt. %, which increase the strength of the material by 20-50%, areadded to the outer shell of the pneumatic form during its fabrication. The nanotubes also make it possi-ble to heat the form and shaft of the pile.

Initial data for analysis of a chambered pneumatic form for the installation of hollow bore- and cast-in-place piles 600, 800, and 1,200 mm in diameter are adopted from analysis of a 12-m-long form jacket,and a jacket diameter (in the inflated state) of 240 mm for piles 600 mm, 400-800 mm, and 600-1,200 mmin diameter. These dimensions are assigned from analysis of a 60-mm protective layer of concrete, Class20AII longitudinal rods for the reinforcing cage, and the possibility of positioning a concrete-casting tube.

The load on the pneumatic form is summed from the effect of vibrators mounted on the concretefunnel, the pressure of the concrete mix and its mobility, and the rate of concreting.

Pile jackets 800 mm in diameter were fabricated from the domestic material. The concrete fun-nel was built with two attached Grade IV104 electro-mechanical vibrators with a vibration frequency of25 Hz, exciting force of 6.2 kN, and power of 0.33 kW.

The pressure of the freshly prepared concrete mix against the lateral surface of the pneumaticform is approximately 0.15 MPa in its lower section. Considering the portability of the concrete mix,and the possibility of dynamic effects of the vibrators and different concreting rates, it is recommend-ed that a pressure of 0.19-0.20 MPa be maintained within the form.

69

600(800)240(400) 6060

3

4

2

1

51,

500

12,0

00

a b

Fig. 1. Pneumatic form placed in bored hole 800 mm in diameter (a), and reinforcing cage of bore- and cast-in-place pile 800 mm in diameter (b): 1) walls of hole; 2) reinforcing cage of pile; 3) pneumatic form; 4) steel collars; 5) compacted crushed stone in face of hole.

Prototypes of a form with a diameter of 800 mm, which were fabricated by the Ufa plant forrubber-engineered articles, consisted of a rubber chamber (rubber thickness of 1.5 mm) and jacketsformed from high-quality rubberized material 2 mm thick.

The procedure employed for the installation of these bore- and cast-in-place piles was investi-gated in Taganrog at a site comprised of loess-like macroporous clayey loams 13-15 m thick, which areprone to slump-type settlement. This layer is underlain by loess-like clayey loams not prone to slump-type settlement. Three holes 600 mm in diameter and 12 m deep, which were expanded to a diameter of800 mm by a pantographic expander, were drilled by an SO2 auger boring rig prior to installation of thehollow prototype bore- and cast-in -place piles.

Tests of the jacket material indicated that when the form-stripping strength of the surface of thepile is 0.2-0.3 MPa, the adhesion of the jacket (with no anti-adhesion lubrication) with the concrete is0.06-0.1 MPa; this makes it rather easy to strip the deflated form from the surface of the concrete.

Prior to filling with compressed air, the pneumatic form is secured by collars (see Fig. 1), whichare welded to the reinforcing cage of the pile, and is lowered together with the reinforcing cage into thehole after being filled with air. Here, the piles are concreted by the method of a vertically displaced tube(VDT), or by placing the concrete under pressure with use of a concrete pump. A drive pipe is initiallylowered into the hole to avoid failure of the soil, and is extracted from the hole during concreting.

Three versions of experimental samples of material for the jacket were tested: without lubrica-tion of the outer surface, with lubrication by a thin layer with a standard water-emulsion composition,and lubrication with a standard hydrophobic waterproofing composition used on a metallic form toreduce its adhesion with concrete.

The material sample for the concrete was maintained at a temperature of 20-22oC. At an air temper-ature below 5oC, it is recommended that additives that accelerate the set of the concrete be used, and anti-freeze additives employed at a negative temperature (potash, sodium nitrate, etc.). When the internal surfacetemperature of the jacket is low, a carbon-graphite fabric should be glued on with a type D-5 glue. Theform becomes thermally active, shortening the set time of the concrete to its form-stripping strength by a fac-tor of two-three. The carbon-graphite overlay is connected to an electric heating substation. The temperatureto which the jacket is heated should be no higher than 30oC. When the temperature increases, it is necessaryto use higher-temperature materials and control the cure time of the specimen with a chronometer. Afterevery hour, a specimen is stripped using a dynamometer to determine the adhesive strength of the contactlayer between the fabric specimen and concrete. To secure the dynamometer, loops (forms of the same mate-rial) are glued onto the material specimens of the jacket at the four corners. Ra−Rb(t) curves (Fig. 2) wereplotted on the basis of the investigations, where Ra is the strength of the contact layer between the sample ofform material and concrete in the presence of an adhesion lubricant in MPa, Rb is the force required to sep-arate the material of the form from the concrete with no lubricant in MPa, and t is the holding time of thespecimen comprised of the concrete and form material with an adhesion coating in h.

To evaluate the strength of the concrete for each experiment, we molded three cubes (from con-crete of the same composition) with an edge dimension of 100 mm, which were maintained under thesame curing conditions as the concrete in the piles. When the forces of adhesion between the form andconcrete were measured by a dynamometer, the strength of the concrete, which should be 0.2-0.3 MPaaccording to active norms, was determined by an ONIKS-2.3 instrument.

As is apparent from Fig. 2, the adhesive strength (adhesion) of the specimens of the form mate-rial with no additional lubricant is low and amounts to 0.06-0.1 MPa by the time that the concrete spec-imen had attained its form-stripping strength; this is entirely permissible. Continuing investigations haveindicated that the piles were stripped of the form rather easily.

At a different air temperature, it is recommended that similar tests of prototype forms be repeat-ed to determine the forces of adhesion between the surface of the form and concrete.

The assembled form was tested with allowance for a safety factor of 1.5, i.e., at an air pressurecorresponding to 0.3 MPa. Here, observations were conducted for possible deformations of the form, of

E

70

which there were none. Moreover, its diameter was measured every 0.5 m. No geometric dimensionswere observed to deviate from their design values.

The pneumatic form was tested during installation of three prototype bore- and cast-in-placepiles 800 mm in diameter in dry loess-like macroporous clayey loams prone to slump-type settlement inTaganrog. According to the test results, the form possesses adequate strength, and its deformability fallswithin acceptable limits. The piles were installed in a Type-I soil with respect to proneness to slump-type settlement.

The pneumatic form with a diameter of 400 mm and length of 12 m was delivered from themanufacturing plant in the form of rolls. In the reinforcing cages (diameter of 640 mm, length of 12.5m, and 20AII reinforcing rods), the unfolded form was pulled through the previously welded steel col-lars, and air was pumped via a compressor to a pressure corresponding to 0.2 MPa (see Fig. 1).

The form unit and reinforcing cage so obtained were then lifted by crane and lowered into a pre-pared hole (Fig. 3).

71

t, hour Ra, MPa

4.0

3.0

2.0

1.0

0

0.12

0.10

0.08

0.06

00.1 0.2 0.3 Rb, MPa

1

2

3

Fig. 2. Diagram showing dependence of forces of adhesion between surface of pneumatic form with concrete and type of coating with adhesive composition on form: 1) surface with no lubrication; 2) lubrication with water-emulsion composition; 3) polymer-base hydrophobic waterproofing coating.

Fig. 3. Unit containing reinforcing cage of pile, and pneumatic form being lowered into hole.

The unit established in the hole (reinforcing cage-form) was straightened, and braced by metal-lic cramps to provide a protective 60-mm layer of concrete.

The piles were concreted with use of a vertically displaced concrete-casting tube with readilydetachable sections. The concrete was fed into the receiving funnel (bin) of the tube via a BP 1.6 typeof bucket transported by a self-propelled crane with a lift capacity of 15 tons. The time required to con-crete the prototype hollow piles with a concrete volume of 4.1 m3 is approximate 1 h, considering allpreparatory operations. In the process of concreting the piles, the three above-investigate control cubeswere molded to determine the strength of the concrete under laboratory conditions.

Structural and technico-economic indicators of the prototype hollow bore- and cast-in-place pilesare presented in Table 1 (where Dp is the diameter of the pile, A, A0, and AS are the cross-sectionalareas, respectively, of the pile, the concrete core of the pile replaced by the pneumatic form, and the 1620AII longitudinal rods of the reinforcing case, Nsh, Ncp, and Npc are the bearing capacities, respectively,of the shaft of the pile, the concrete core replaced by the pneumatic form, and the reinforced-concreteshaft, and Vc is the amount of concrete saved due to installation of the pneumatic form). The designcompressive strength of the reinforcement RSC is 280 MPa.

After the concrete in the pile had set to the stripping strength (0.2-0.3 MPa), the air was releasedfrom the form, and the latter extracted from the cavity of the pile. The quality of the surface of the cav-ity created by the pneumatic form was evaluated by its inspection using a mirror suspended from a cordat a 45o angle with illumination provided by an electric lamp (Fig. 4).

Visual inspection of the cavity within the shaft of the pile indicated that the concrete surface hasno deformations, and corresponds to standard requirements. Thus, the form is suitable for the installa-tion of hollow bore- and cast-in-place piles, whereupon the average saving of concrete in piles withdiameters of 600, 800, and 1,200 mm is approximately 25%.

72

Dp,mm A, cm2 A0,

cm2As,cm2 Nc, tons Ncp,

tonsNpc,tons

Vc,m3

Vc,%

600 2,826 615 50.24 561.3 66.9 470.0 0.54 15.9

800 5,024 1256 50.24 886.7 185.9 701.8 1.51 25.0

1,200 1,672,992 2,826 50.24 1,816.1 418.2 1,388.0 3.39 25.0

TABLE 1

Fig. 4. Finished pile-shell at experimental proving ground in Taganrog.