Cast in place concrete piles eliminate the need forpile driving machinery which can cause dangerous

v i b rations and precipitate landslides and which issometimes too costly for use on small jobs. Two simplenew techniques for installing cast-in-place pilespromise solutions to these problems.

CAST-IN-PLACE PILES WITH STEEL CORESSt e e l - c o red concrete piles cast in drilled shafts prove d

e f f e c t i ve in pre venting landslides on part of the new fre e-way construction in Se a t t l e, Washington, where usualmethods of driving piling we re considered too haz-a rdous because ground vibrations caused by the ham-m e ring might precipitate the slide. The method was de-veloped by the Washington State highway depart m e n t .

The problem became apparent when hairline cra c k sd e veloped in the area in which the contractor was pro-ceeding with exc a vation pile driving, and footing andcolumn construction for shoring to support an uphill re-taining wall. Fu rther investigation re vealed some dis-placement at the 108-foot elevation in an adjacent pierfooting exc a vation. Slope indicators we re installed andc o r related findings indicated that the area encompassed

by the slide extended to the south for approximately 400feet on a line just west of the proposed retaining wall.

The threatened hillside movement was part i c u l a r l ys e rious because it threatened closure of one of the city’smain art e rial streets and endangered a nearby apart-ment building. No movement had yet occurred in themain area of the proposed uphill retaining wall and theancient fra c t u re zone was 10 feet below the re t a i n i n gwall footing elevation. The immediate danger was thepossibility of the slide pro g ression to the east beyond thelimits of the wall, necessitating difficult remedial work indisturbed earth in a re s t ricted right-of-way width. Alsothe prospect of heightening a 30- to 40-foot wall or in-c reasing exc a vation depths in a critical area we re disqui-eting. Quick action was impera t i ve. The need to pre ve n tg round vibrations such as those induced by a pile-dri-ver led to the selection of a cast-in-place cylinder piletype of constru c t i o n .

This pro c e d u re re q u i red pre - d rilling of holes, placinga steel beam in each hole, and back-filling voids withc o n c re t e. The upper zone would thus be pinned to thel ower zone through the thin fra c t u red layer prior to anym ovement, thus maintaining the stability of the earth in

Cast-in-Place Concrete Piles

both zo n e s. Altogether 62 pilings we re designed va ry i n gin depth from 62 to 81 feet and re q u i ring 4,410 lineal feetof boring exc a vation, 2,900 cubic yards of ready mixedc o n c re t e, 9,700 pounds of re i n f o rcing steel and 1,453,000pounds of high-strength, low - a l l oy stru c t u ral steel Ib e a m s.

Steel casings 1 1/4 inches thick and 4 feet 9 inches indiameter we re used to case the holes to their full depthsto retain the sandy clays. Exc e s s i ve moisture in the bor-ings re q u i red steady pumping prior to placing concre t eand slough from a wet sand strata was not always avo i d-a b l e. Consequently it was found desirable to drill andcase to the upper limit of the sand zo n e, place the I beamand then dri ve it to position with an improvised man-d rel and hammer.

The contractor started out by using tremies down asfar as 70 feet, but lost them so frequently because ofhanging up on stud bolts that he finally placed the con-c rete directly into the cylinders. It was found that thec rew could place a 60-foot head of concrete beforepulling out the casing, so two casing lengths we re placedb e f o re pulling up. Because the earth pre s s u re came pri-m a rily from the ove r b u rden the holes in the top 25 feetwe re ove r- d rilled regularly to 4 feet 4 inches and widercasings put down, the standard casings inserted, the in-terstices backfilled with sand, and then the outer casingsre m oved before placing concre t e.

A cast-in-place face wall was hung on the front of thepiles and fastened to the steel beams in the cylinders. Todo this engineers placed a wood blockout on the fro n tflange of the steel beam down to the wall footing eleva-tion. The blockout was re m oved after exc a vation to sub-g ra d e, there by permitting face wall dowels to be we l d e dto the beam flange.

The Washington State highway depart m e n t’s AX mixwas used for the concrete in the piers. This mix is a re-finement of the A class giving a minimum ultimates t rength of 4,000 psi at 28 days. An admixture was used tog i ve early curing because the drilling of adjacent pileswould be necessary in two days—less than normal cur-ing time.

Ex p e rience on this project has shown the versatility ofthis type of design, and it appears certain that it will beused on other jobs where dangerous earth conditions ex-i s t .

CAST-IN-PLACE PILES WITH PRECAST CORESA British piling company has introduced a system

which uses precast sections for the core of the pile. Theresult is a better and simpler technique which has prove dsuccessful on seve ral difficult pro j e c t s.

A hole is bored in the ground to the re q u i red depth andlined, as necessary, with a steel casing made up in conve-nient lengths. The precast sections are then thre a d e dover a central steel tube to form the pile core. Su p p o rt isby a special release gear fitted to the lower end of thesteel tube; the tube is also made up in short lengths

Diagram of new piling system developed by aBritish piling company.

Workmen thread a precast section of the pile overcentral steel tube to form pile core.

s c re wed together as assembly pro c e e d s. Lowe ring intothe borehole continues as each section is added. Ma i nlongitudinal re i n f o rcement (4, 5 or 6 ro d s, 1/2 to 1 inchin diameter) is threaded through holes in the sections,and thus cannot be displaced. The steel lining of theb o rehole is withdrawn and a grout mix pumped in si-multaneously through the central hole in the sections.The mix is under pre s s u re to fill all voids both within thebody of the pile and between the core and the sides ofthe hole. Subsoil water is forced out as the level of themix ri s e s. The result is a high-strength core grouted to-gether and into the subsoil to form a solid pile encasedin a thick skin of concrete firmly keyed to the substra-t u m .

The sections are usually cast in lengths of 1, 1 1/2, and3 feet and diameters of 11 1/2, 14 1/2 and 23 1/2 inchesfor nominal pile sizes of 14, 18 and 26 inches (work i n gloads up to 40, 60 and 150 tons). Each section has aribbed annular surface to promote a good bond with thes u r rounding mix and is re i n f o rced with 1/4-inch spira lbinding at 6-inch pitch. The sections are cast of vibra t e dc o n c rete with a nominal 1 : 1 1/2 : 3 mix by vo l u m e. Ma x-imum resistance to attack by sulfates in the ground wa-ter or subsoils can be assured by the use of sulfate-re-sisting cements, but standard portland cement andother types of cement are often used.

All the equipment and components needed for thepiles are kept light and easy to handle. The vibration as-sociated with pile driving is, of course, eliminated, a biga d vantage in built-up are a s, and headroom and spacep roblems are largely avoided. Piles up to 90 feet long areregularly being formed in this way without difficulty.Each stage of the assembly can be checked aboveg round. Main use of the system at present is on smalljobs which do not justify the use of full-scale pilingequipment. Inclined piles are also being formed with thesystem. Assembly through water, for such stru c t u res asb ridge piers, is easily achieved by using concrete pipesections as a permanent cofferd a m .

View of some of the 62 cast-in-place piles taken afterthe exc a vation had been completed but before the facewall was cast. Wood knockout blocks visible on the ex-posed side of the piles we re re m oved to permit we l d i n ganchor bolts to the encased steel beams, the latter int u rn providing for the fastening of the wall panels.

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