CONSTRUCTION U N D E R SPECIAL SOIL CONDITIONS

PILE FOUNDATION CONSTRUCTION CHARACTERISTICS IN THE

ALASKA OIL PIPELINE

D. I. Fedorovich and Yu. O. Targulyan UDC 624.154.546:69(211-17)

Design of piles, the techniques for their construction in permafrost soils, and the organization of piling work itself

abroad substantially differ from the methods adopted for their use in the North of our nation.

Considerable experience with pile foundations in permafrost has been acquired in the U.S.A. as a result of construction

of the 1220-mm diameter Alaska oil pipeline, the northern part of which, some 600 km long, passes through a zone with

permafrost soils. In this area, the oil pipeline is overground, and permafrost soils are used as bases in accordance with the first

principle, that is, in the frozen state. In the U.S.A., for permafrost soils use is made basically of bored--drop piles, that is, piles

sunk into previously bored holes of large diameter.

The bored---drop piles used in Alaska were made from 457.2-mm diameter steel piles bought in Japan. The lower end of

the pile is covered with a welded plate, and the space between its surface and the hole is filled with sand. At the top the pile has

a steel cover. The part of the pile located in the permafrost soils has a ring-shaped transverse projection or grooving (in the

U.S.A. it is called a "corrugationS). The corrugations, 38 mm in both height and width, are located at spacings of 30.5 cm at

depths 3-4.5 m. It is not necessary to make them above, since the working part of the pile is located at the bottom. Morever, the

corrugations weaken the cross section of the pile, which may cause it to be insufficiently strong under the action of the bending

forces in its upper part.

In the southern part of the oil pipeline, which passes through an area with high-temperature permafrost soils, in each

pile use is made of two cooling self-regulating steam--liquid devices (the so-called Long's piles, of which there arc about 120,0~X)

units). In the northern part of the oil pipeline, which passes through regions with low-temperature soils, the pile supports are

not equipped with such devices.

The cooling devices substantially increase the pile reliability, since under their action lhc temperature of the soil base

around the pile is lower than under natural conditions for a period of almost one year. However, in the U.S.A. the bearing

capacity of single piles in oil pipeline supports on soil bases is determined from their strength characteristics in the design one-

year period without taking into account the action of these installations. In the determination of the bearing capacity of the base

of a vertically loaded friction pile account is not taken of the pressure on the frozen soil under the lower end of the pile or of

the pressure transmitted by the ring-shaped projections. All this contributes to safety for increasing the reliability of pile

foundations. It should be noted that if the pile bearing capacity were determined in conformity with our norms (SNIP 2.02.04-

88), the end and the corrugations would be taken into account and the pile length in the permafrost soil would be shorter by

about 20--25%.

To sink steel pipe piles about 46 cm in diameter to the design depth of 61-cm diameter holes in Alaska, large, high-

efficiency drilling machines are used. Drilling is dry and mechanical, even hole walls being obtained, and the soil base heating is

minimal.

The hole diameter is larger than the pile diameter by 15 cm, that is, it significantly exceeds the diameter (5 cm) which

is required by the USSR norms for bored--drop friction piles. In our country it is endeavored not to exceed this dimension, in

connection with additional drilling costs and mcrease in the soil grout volume and in the duration of the freezing-together

period of the piles. In the U.S.A. this problem Is approached in a substantially simpler manner, which is due to the existence of

a stock of high-efficiency drilling machines and to the endeavor to ensure high-quality filling of the spaces between the piles and

the hole walls with sand. Piling work is carried out at any time of the year, and labor is paid on a worked-time basis.

After driving the hole, a hollow steel pile with a hermetically closed lower end is installed in it. If the hole contains

water, and installation and filling of the space between the hole and the pile is carried out immediately after sinking the pile,

that is, before the water begins to freeze, then the pile is not removed.

VNIIOSP. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 6-8, September-October, 1991.

204 0038-0741/91/2805-0218512.50 ©1992 Plenum Publishing Corporat ion

The installed pile is aligned and the spaces are filled with wet sand fed through a flume from the grout mixer. At the

same time, the pile is vibrated by a small clamp vibrator secured to its upper end which transmits to the pile circular vibrations

in a plan perpendicular to its longitudinal axis. The vibration action ensures advance of the sand filler to the bottom of the space

over the entire depth and contributes to compaction of the filler. The water which falls into the hole is removed from the top.

In this manner, the spaces in 95% of the piles sunk in the Alaska oil pipeline were filled with sand. Initially, use was

made of layer-by-layer filling of the spaces by applying vibration of the sand filler by means of an immersed vibrator (as done in

this country at Noril'sk). However, this method was abandoned, in much the same way as clone here, because of its complexity.

The lower end of the pile is hermetically closed in order that the grout poured into the spaces and subjected to vibration

does not enter into the pipe; otherwise, it would interfere with the placement of the cooling steam--liquid installations in the

pile. The pile hollow is filled with sand after mounting the cooling installations.

In contrast with our practice, when the bored--drop piles are sunk into the holes previously filled with grout the

American engineers install the pile on the bottom of the empty hole. The hole bottom configuration depends on the spudding

bit, which may be tapered. Moreover, fine pieces of frozen soil are left on the hole bottom. When the top end of the pile is

vibrated, its lower end is firmly placed on the hole bottom, and the small hollows, if they exist, are filled with sand. Thus,

independently of the design criteria, reliable transfer of the load from the pile end to the soil base is actually ensured.

Sometimes in our literature it is recommended to throw the bored--drop pile into the hole for firm support of its tip.

However, with modern crane equipment this is difficult, since smooth lowering of the load "at a reasonable speed" is normal for

crane-lifting equipment. After installing the pile in the hole and filling the surrounding space with sand, a special device -- a corrugator -- is sunk

into it, which develops a high pressure and makes transverse corrugations from the bottom upward in the metal pipe. Since the

corrugations compact the sand grout, they are constructed immediately after filling with sand the space around the pile. The

operations of installing the cooling devices in the pile and of filling the internal hollow of the pile with sand can be performed

significantly later. Instead of the corrugations, rings can be welded to the pile (at the factory or the construction site), as done in our

country. However, in the U. S. A. this method is not applied because of the labor-consumption and also because of the fact that

the number of corrugations for each pile must be determined more precisely together with its length during the hole drilling

process. There have been cases of pile rupture during the tests; for this reason, after constructing the corrugations, the pile

integrity is carefully controlled. If damage is detected in a pile, it is removed, the hole is reconstructed, and a new pile is placed

in it. If the damage is discovered after freezing-together of the pile, it is heated from within with steam for its extraction.

A bottom cap is always installed in the pile in order that the sand grout fed into the surrounding space fills only the

recesses and does not penetrate into the inside of the pile, forming a hollow in the space around it. During the start of construc-

tion of the oil pipeline, attempts were made to install hollow piles without bottom caps, but it was decided later to place them

and to control this rigorously. The work quality of the American constructors is very high, which is due not only to labor discipline but also to the

definite pride they have in the results of their work. Small changes in the methods applied by the American engineers to sink the bored---drop piles, as well as use of sand

filler in the pile spaces instead of clayey-sandy or cement-sandy filler made it possible for them to obtain the following advantag-

es: 1) reduction of the period of freezing-together of the piles, since the moisture content of the sand filler is lower by a factor

of 2--3 in comparison with the clayey-sandy filler; 2) increase in the pile bearing capacity by 20--25%, since the strength of

freezing-together of the pile surface with sand is higher than with clayey soil; 3) saving of cement by replacing the cement-sandy

grout, which is widely used in our country, by sand grout. In contrast with our construction practice, American engineers organize differently piling work in permafrost soils. The

share of costs for design-exploration work is higher by a factor of several units in their country and reaches 12% of the total

construction cost. For example, the use of any drilling machines is planned by means of computers based on prospecting holes

drilled along the pipeline axis, which are located at spacings of 80-150 m. However, such a careful preparation does not provide complete freedom from inaccuracies. During the start of the

drilling work, use is frequently made of a light drilling machine which is driven to heavy material (boulders, pebbles, rubble,

etc.), after which a heavy machine is used for finishing the hole drilling operation. Such a construction order is sometimes

applied also in our country, for example at Noril'sk and Vorkuta. The technical--economic effect in this case is determined by

high efficiency of all the drilling rigs, which reduce by 30--40% the labor, the time, and the cost.

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During pile sinking in Alaska, at each drilling machine a geologic engineer determines the basic physicomechanical

characteristics of the soil. In the drilling machines, which blow out the soil disintegrated at the face, they install picking-up

devices for the extracted soil. Based on the obtained data, they determine the sinking depth of each pile, and, consequently, the

hole drilling depth.

Data for establishing more precisely the pile length are available to the engineer in charge of the drilling machine.

Moreover, there is permanent radio communication with the design firm, and, when necessary, the engineer may receive

evaluations of the excavated soil from the viewpoint of its freezing-together with the piles, the pile bearing capacity, etc. If ice

layers are encountered during driving, the pile portion located within the ice mass thickness is excluded from the analysis.

Piles made from steel pipes are widely used in the North of Alaska for construction of foundations of different buildings

and structures. Frequently, for free operation of the construction equipment, the piles are cut at the ground surface level after

being sunk into permafrost soils, and later, immediately after erection of the building or structure, their upper ends are welded

again.

In pile foundation construction, a concrete lining is frequently placed to reduce the heat flow from the building to the

soil through the steel pile.

When buildings are constructed on extremely rigid steel piles, the problem of temperature deformation of the piles

arises. Instead of constructing close-spaced expansion joints in the building, the ends of the piles located most distant from the

center of the building are covered with teflon linings. In this way, slidelike support parts are obtained in the extreme piles.

However, the internal pile rows are rigidly connected to the structure and transmit the horizontal wind loads to the building.

When for the Alaska constructors it becomes necessary to erect on a pile foundation a structure fabricated previously

and transported to the construction site in ready form, the head parts of the piles are equipped with control devices by means of

which the areas of support on the head parts of the piles are carefully equalized in accordance with the project. Subsequently,

the control devices are frequently embedded in concrete.

Such piles can be used for assembling, on them, structures requiring especially accurate installation or subsequent

control of the spatial position of their components during the operation process.

In many cases, for construction on permafrost soils in accordance with the first principle in the U.S.A. use is made of

driven friction piles made as a rule from steel double-T wide-flange shapes. Use of driven steel pilcs is limited by high-tempera-

ture plastic- and hard-frozen clayey and sandy soils with a small quantity of gravel material. When such a pile is driven, some-

times the web or the flanges are damaged on hitting a small boulder, and the pile is moved aside or is even fully damaged. This

is not always detected during the driving process. Thanks to use, as driven piles, of steel shapes with open sections, they are

more widely applied than in our country, where driven piles have solid cross sections. Piles with open sections do not loosen

frozen soils for all practical purposes, since the area of the introduced cross section is small. Use of driven piles in permafrost

soils in Alaska is regarded as a forced solution when there are no sufficient drilling and other equipment, quick freezing-together

of the piles is not certain, and the structure must be urgently constructed in a remole xone. For these reasons, during the last

few ycars engineering companies are using bored--driven piles with increasing frequency, the sphere of application of which is

wider than for driven piles. This relates to high- and low-temperature permafrost clayey and sandy soils (to -2°C) with gravel

and pebble inclusions.

The cross section of bored--driven piles is similar to that of driven piles, that is, wide-flange double-T. The only

difference lies in the fact that two angles are tightly secured to the web of the double-T shape. Hence, after driving the pile the

hollows between the angles and the web remain empty. An opening is used for control of the integrity of the pile and more

precise establishment of its driving depth, determination of the temperature of the permafrost soil of the base along the length

of the pile, its freezing-together period, the restoration of the design temperatures, etc. Another opening is filled with an

unfrozen liquid, for example diesel fuel, and a thin steam--liquid cooling installation is placed in it to reduce the temperature of

the soil around the pile and to increase the pile bearing capacity. The diameter of the hole for the bored---driven pile is specified

so that the angles are fully inserted into it.

The role of the leader hole for the bored---driven pile consists in the following:

1) Establishment of the possibility of driving the pile (absence of large boulders, of considerable quantities of large-

fragment inclusions, etc.);

2) Easier pile driving, especially as regards the introduction, into the soil, of the part of the cross section which has a

widening caused by the welded angles;

3) Protection of the pile against movement to one side during driving.

206

It can be assumed that piles made from wide-flange double-T shapes strengthened by angles when use is made of 80--90

mm diameter leader holes are successfully driven also in our country in permafrost clayey soils with a temperature reaching

--2°C.

It should be noted that the term "bored---driven pile" has different meanings in our country and in the U.S.A. In our

country, it applies to piles with solid circular or square cross sections or to hollow pipe piles, but with closed ends, sunk into a

hole whose diameter is only somewhat smaller than the maximum cross=sectional dimension of the pile. The coefficient of

penetration-start is close to unity.

When the American constructors drive thin and smooth bored---driven steel piles with open cross section, the lateral

loosening of the soil is very small. During the process of vertical advance of the pile, the thin soil layer along its lateral surface

is thawed. In order that the thin ice film formed during thawing will not reduce the pile bearing capacity, the American engi-

neers lower the temperature of the soils around the piles by installing in each of them a cooling device, the cost of the use of

which is equivalent to lengthening the pile by 2--3 m.

Driven steel piles made from structural shapes with opcn cross sections could in many cases in our country substitute

the bored---drop steel pipe piles used for construction of oil--gas industry installations, electric energy transmission lines, bridge

crossings, etc. This would make it possible to substanlially reduce the drilling work volume, the construction periods, the pile

freezing-together time, and the pile sinking labor and cost, and at thc same time to increasc the pilc bearing capacity in compari-

son with bored--drop piles madc from steel pipcs.

In other countries, design--exploration work is not subjected to cost curtailmcnts, and savings are not made in equip-

ment acquisition, in construction expenditures, in construction site preparation, in pile dimensions and designs, and in careful

execution of the construction work and control of its quality. In the final analysis, economic advantages are obtained from

structural safety, operation costs, and uninterrupted operation of the structures. The problems of increasing the reliability of the

structures are basically solved not during their operation stage but during the design and construction process.

M E T H O D S O F C O N S ~ U C T I O N O N W E A K S O I L S W I T H O U T T H E I R

S T A B I L I Z A T I O N

S. V. D o v n a r o v i c h UDC 624.15:624.131.21

When a building under construction is h)cated on a base which includes weak soil layers whose deformation may be

inadmissible for the building, use is frequently madc of prcconstruction stabilization of the weak soils or of pile foundations,

which substantially increases the construction cost. In order to prevent the increased cost, at the VNIlOSP Institute several new

construction methods have been developed which ensure decrease in the soil base deformations not on account of stabilization

of the weak soils or of use of piles in them but of changes in the method of transmission of the loads from the building to the

base. The essence of these methods is described below.

1. Construction of Foundations on Compacted Subfoundation Pads

Underlain by Reinforcing Slabs

To substantiate this method, experiments were carried out on a trough measuring 8 × 8 × 8 m with the foundations

directly on loose sand and on pads. Figure 1 shows the relation between the settlement s and the load N for foundations having

width (diameter) b = 0.8 m on a 5-m-thick layer of weak soil (loose sand): in case 1 for a foundation located on the soil; and in

cases 2 and 3 for foundations on a 0.56-m thich sand pad underlain by a 0.15-m-thick sand--cement unreinforced slab. The width

1991.

VNIIOSP Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 8-11, September-October,

0038-0741 /91 /2805-0218512 .50 01992 Plenum Publ i sh ing Corpora t ion 207