DEFORMATIONS IN THE BASES OF SLEEPER FOUNDATIONS

Yu. F. Tugaenko and S. I. Kushchak UDC 624.153.523

The first investigations of discontinuous foundations, which include sleeper foundations, were carried out by E. A. Sorochan [i] and they were continued by M. I. Fidarov [2]. The technical and economic advisability of such foundations was theoretically and experimentally substantiated by them. However, investigations of the deformations in the bases of the dis- continuous foundations were not carried out. In this connection, the writers performed in- vestigations of such deformations for two sleeper foundations with different block arrangements. The total area of the underside of all the elements of one of the blocks is 1.05 m 2, and for the block it is 1.08 m 2. Each block consists of five identical elements measuring 0.14 x 1.5 m each, which are rigidly joined to each other. In each block there are three elements in the center and two elements at the edges, at distances which prevent their mutual influences (test No. 2). In the other block, there is one element in the center and two at each edge, which are spaced at distances equal to their width (test No. 3). Schematics of the disposition of the elements in the block and their dimensions are shown in Fig. i.

Beneath the foundation underside there were the following layers: i) loesslike loam, 0.8 m thick; 2) loess, 2 m; and 3) loesslike loam, 2 m. Under natural water content conditions, the loesslike soils of the mass had type I collapsibility. The index properties of the soils according to data from explorations performedby the Odessa Branch of the UkrGIINTIZ Institute are presented in Table I.

The investigations were carried out in a 1.4-m-deep trench. The test foundations were placed on a pit 0.15 m deep below the trench bottom on a 2-3-cm-thick leveling sand bed. To eliminate the collapse properties of the loess soils, the experiment was continued under soak- ing of the soil base, which was started 15 days before loading the test foundations. The water was supplied to the pits on a 24-h basis. The degre e of saturation of the soil, determined at a depth of 1.5 m, ranged from 0.75 to 0.78.

The test foundations were loaded by means of 2-ton cast-iron weights. The loading in- crement was of 0.04 and 0.06 MPa. Each increment was maintained until stabilization took place, which was assumed to occur when a settlement of 0.i mm per day was reached.

During the experiments, the contact pressures on the underside of the sleeper elements were measured by means of electrical resistance strain gauges, mounted in groups of three gauges on each element along its longitudinal axis. The extreme gauges were installed at a distance of i00 mm from the ends, and the central one was installed at a distance of i00 mm from the transverse axis. The gauge membrane planecoincided with the foundation underside

plane.

The layer displacements of the soil were measured with the aid of magnetic marks instal- led in a hole in accordance with the procedure recommended by I. Sey~ek [3] and improved at the OISI.

The deep marks were placed along the vertical axis at spacings of 10-20 em in the middle of the foundation and along the center of the unloaded area inside the block (Fig. 2). Below the unloaded area observations were performed in order to establish whether tNere was inter- action between the foundation elements.

It is well known that the depth of the compressible mass and the settlement, other con- ditions being equal, become larger as the foundation width increases. In sleeper foundations having different widths and joined into a single block, the settlements are equal. Consequ- ently, in the narrow foundations the settlement is larger, and in the wide foundations it is smaller than for separate transmission of equal pressures. Thus, narrow foundations "unload" wide foundations. On their undersides, the pressure is higher than on the underisdes of wide foundations.

Odessa Civil Engineering Institute (OISI) Translated from Osnovaniya, Fundamenty i Mekh~- nika Gruntov, No. 2, pp. 9-11, March-April, 1986.

42 0038-0741/86/2302-0042512.50 © 1986 Plenum Publishing Corporation

TABLE i

Soil parametem

Specific gravity Demity , ~/cm= DZy_ de , / c m s NaturallY{eft conzent Void ratio Modulus of deformation of soil,MPa: At natural water content . . . . Saturated An~les of intern'al friction, deg Cofiesion, MPa

a b

La~'er No.

I 2 8 2.68 2.67 2.68 1,75 1,64 1,78 1,51 1,34 1,49 0~16 0,15 0,18 9,78 0,92 0,80

10,00 7,90 8,90 4,80 4,30 5 40

22 22 0,017 0,008 0,027

0 ~ 0.2 Pmn, MPa 0 ~I ~ Pmn, MPa

< ' - - . o III and IV i

9 F i g . 1 . S c h e m a t i c s o f a r r a n g e m e n t s of f o u n d a - t i o n e l e m e n t s and g r a p h s o f r e l a t i o n s , a) S e t t l e m e n t o f f o u n d a t i o n s u n d e r mean d e s i g n p r e s s u r e Pmn f o r t e s t s Nos . 1 , 2 , and 3; b) d e p t h s o f z o n e s o f d e f o r m a t i o n u n d e r a c t i o n o f Pmn i n s e c t i o n s I - V I ; c) c o n s t r u c t i o n o f f o u n d a - tions in testNo. 3; d) the same, test No. 2.

The pressures on the undersides of sleeper elements having different widths and working in the system of a unified block were obtained in the experiments. In test No. 2, the mean measured pressure Pm on the underside of the extreme (narrow) elements amounted to 131%, while for the central (wide) element it amounted to 85% of the mean design pressure Pd obtained by dividing the load by the total foundation underside area. In test No. 3, as a result of the interaction between the extreme elements of the block, the zone of deformation in their soil bases was larger than in the central single element, As a result, the pressure on the under- side of the central element amounted to 123% while in the four extreme elements it amounted to 87% of the mean design pressure. Figure 2 shows diagrams of the layer displacements in tests Nos. 2 and 3. Since the increase in the pressures on the underside of the narrow founda- tions outstrips the mean design values, the deformations in their soil bases increases more slowly. Thus, in test No. 2 the pressure on the underside of the extreme foundations is higher by a factor of 1.54 than in the central foundation. Under these conditions, the deformation zone depth in the central foundation is larger by a factor of 2.7 while the strain is smaller by a factor of 2.7 than in the soil base of the extreme narrow elements.

In test No. 3, the extreme foundations were located at clear spacings equal to their width. Under the firstloading increments, an independent deformation zone was developed be- neath the underside of each element. The layer displacements beneath the unloaded area bet- ween the elements were not recorded. Starting from a mean design pressure of 0.i MPa, accord- ing to the displacement data for the deep marks, increase in the deformations beneath the un- loaded are due to the foundation interaction was established. The nature of the increase in

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TABLE 2

Pararneters

Test No.

foundatiom deformations for the same • , I m~as~red~ess____~

I under- ~idth l'~ . J , I side area b. and | I~ ~ ~ ~ I.~ , , ~ Ha SlH. I F , m ~ ~ lement a i a m e t e ~ j ~ ~ , ~, . . . . c m -

No. d, cm 1 ~ , ~ ~ ' ~ /

1,0

1,08

1,05

118 14 44 I4

2× I4~

0,289 8,80 1~2 0,072 0,182 0~39 t,85 9381 0,~0 0,28L 0,239 6,80 0,069 0,19~ 0 ) P.39 1,80 81 0)04~ 0,275 0,239 3,80 65" 0,058

ftalbtd)

1,08 2,21 2,23 2,21 2,32

deformations for the same sett lement

0,~00 0,245 0 ~248 0,~80 0,280

0,208 0,344 0,=44

cm

4

sitka IHalb(a)

1~ 0,037 0,~88 0,1~ 2,640 0,0¢8 :~,090

Assumed maximum value of H a under interactionconditions. tAssumed total width of two elements without taking into account the distance between them.

f 7/ IV V VI

20 5,mmTO 0 ~/~M ~mm 2# 0

"7,' , V v . ,, : , - .

4" I ' )),-- -..c. . , ) m S ) js, mm Z g E

L" #

o ~o 5,mln

F , i 0 ~0 ~mm

• c m

Fig. 2. Diagrams of layer displacements along axes of foundation elements, a) In test No. 3 at sections I, IV, V, and VI, and below unloaded area of section II for the following mean design pressures, MPa: i) 0.156; 2) 0.194; 3) 0.251; 4) 0.289. b) in test No. 2 at sections I, II, and I!I for the following mean design pressures, MPa: i) 0.151; 2) 0.188; 3) 0.244; 4) 0.281.

the deformations along the axis of the extreme foundation and in the space between them is characterized by diagrams constructed from the deep mark displacement data (see Fig. 2). In this test, the deformation zone depth at the extremeelements for the last loading increment waslarger by a factor of 1.8 and the strain was smaller by the same factor than in the soil base of the central narrow foundation. These relations vary with the growth in the pressure which causes the deformations to increase because of the foundation interaction (Figs. 2a and ib).

Figures la and b show graphs of increase in the settlement and the deformation zone depth due to the mean design pressure. Table 2 presents some parameters of the deformations in the soil bases of the foundations, obtained from the investigations.

Thus, for an equal measured pressure in tests Nos. 2 and 3, the strain in the soil base of the narrow foundations is smaller by a factor of 1.38 than in the wide central foundations, as well as than in the base of the foundations under interaction conditions. In circular

44

foundations~ the strain is larger by a factor of 1.5-2.4 than in strip foundations. For equal settlements, the strain in the soil bases of the narrow elements exceeds by a factor of 1.7-2.25 the strain for foundations workingunder interaction conditions.

The investigations made it possible to develop and use in the construction practice a foundation consisting of prefabricated sleeper--beam elements under the supports of an indus- trial building. The presenceof beding moment loads determined the foundation design in accord- ance with the schematic of test No. 3. For this arrangement of the elements, the reduced pres- sures caused by the vertical load at the extreme elements increase under the bending moment action~ This foundation underwent successful field tests at the construction site under the action of vertical and bending moment loads.

Use of prefabricated foundations of the sleeper--beam type in a metal cutting shop in Odessa permitted reducing the consumption~of materials by 18% and the cost by 67,000 rubles~

i,

2.

3.

LITERATURE CITED

E. A. Sorochan, Prefabricated Foundations of Industrial and Residential Buildings [in Russian], Gosstroiizdat, Moscow (1962). M. I. Fidarov, Bases andDiscontinuous Foundations [in Russian], Izd-vo "Ir," Ordzhoni- kidze (1973). I. Sey~ek, "Field test of soil deformation beneath foundation," in: Transactions of Fifth European--Danube Conference on Soil Mechanics and Foundation Engineering [in English], CSSR, Bratislava(1977), pp. 275-287.

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