R E S U L T S O F I N V E S T I G A T I O N S O F T H E P R O C E S S O F

C O N S O L I D A T I O N O F D R A I N E D M U L T I L A Y E R E D P E A T Y

BASES ( JOINT S O V I E T - F I N N I S H E X P E R I M E N T )

P. A. Konovalov, V. G. Goncharov, Yu. N. Platonov, and F. F. Zekhniev

UDC 624.138:624.131.276

Results are given from many years of experiments to reveal efficient technology and compaction of a multiIay- ered peaty base with the help of various drains: factory-made (the "geodrain" type), and sand ones made

directly on the site, with cylindrical (round) and flat cross sections of equal area.

In accordance with the program of Soviet- Finnish scientific andtechnical cooperation in the field of construction on the theme "Construction of Foundations in Complex Geological Conditions," full-scale experiments were conducted on preconstruction compaction of soils at a site near St. Petersburg. The engineering-geological conditions characteristic of this region (peaty territory in the vicinity of Ol'gino) present the greatest difficulties for clarifying a group of questions connected

with the efficiency of drains, efficient spacing of them in plan, correspondence of predicted pressures and settlements, etc. Taking part in the work were staff members of the Scientific Research Institute of Foundations and Underground Structures (NIIOSP) (Yu. K. Ivanov), the Main Leningrad Construction Administration and the Planning, Design and Technological Institute of St. Petersburg, the Perusyhtym~i concern (YIT-Vesto, Kallio joint-stock company), the Finstroi joint-stock

company, the Technical Research Center of Finland, and the University (Helsinki). On the Finnish side, the leaders of the work were K. H. Korhonen and M. Tammirinne; on tile Soviet side, P. A. Konovalov. Participating in the experiment in

different stages on the Finnish side were H. Ratmayer, E. Slunga, N. Solovev, Mr Yhola, U. Antikoski, R. M~ikinen, and

M. Tiainen. Investigations of the compaction of water-saturated layers through their depth and in time with the use of vertical

drains have been carried out here in our country and abroad mostly on homogeneous masses of soft soils. In our tests, for the first time we investigated a multilayered peaty base, where the values of the moduti of deformation of component layers differed from each other by an order or more. Moreover, with respect to modern instrumentation the present experiment was

among best equipped and lasted five years. Provisions were made for comprehensive investigations of the six test sites' engineering-geological conditions. Figure

1 shows a diagram of the Ol'gino proving grotmd, with indication of the test sites and the points that were investigated. Soil properties were determined in field, as well as laboratory conditions. Soil tests were carried out for the purpose of obtaining

information for subsequent calculations of consolidation. Data from investigation of the characteristics of soiis composing the base to be compacted at one of the sites are

presented in Fig. 2. The results of sounding of the soil mass at site No. 6 and soil tests for rotary shear strength conducted by Finnish

specialists showed that the soil mass to be compacted is inhomogeneous through its depth. Moreover, it was established that

an arbitrarily incompressible layer (in relation to the overlying, highly compressible soils) of moraine lies at a depth of 8.6- 8.8 m, i.e., we have a multilayered compressible mass with final thickness H.

The Ol'gino proving ground was symmetrically divided into six test sites, which weresquaresectionswith sides I8 x

18 m in plant (see Fig. 1). Sites 2, 4, and 6, were assigned to Soviet experiments; and 1, 3, and 5, to Finnish ones. After it was broken up into sites, the whole territory around the sites was covered with "dornite" geotextile to prevent mixing of soils and organic masses, and to increase the passability of vehicles and machinery.

Scientific Research Institute of Foundations and Underground Structures. Planning, Design and Technological Institute, St. Petersburg. Central Asian Scientific Research Institute of Foundations and Underground Structures. Translated from Osnovaniya, Fundament?" i Mekhanika Gruntov, No. 5, pp. 18-23, September-October, 1993.

0038-0741/93/3005-0207512.50 ©1994 Plenum Publishing Corporation 207

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Fig. 1 Fig. 2

Fig. 1. Diagram of location of test sites (No. 1-6) and points where soils were investigated. 1) places where

monolithic specimens were taken (by the Finnish side); 2) cone penetration test; 3) sounding; 4) testing of

soils for rotary shear strength; 5 and 6) places where monolithic specimens were taken before and after

compaction, respectively.

Fig. 2. Results of investigations of the characteristics of soils composing peaty base to be compacted (control

site). Light circles) before compaction; dark circles) after compaction; 1, 3, 5) layers of peat; 2, 4, 6) layers

of loam.

Before filling of a layer of sand 80-100 cm thick, surface markers were placed on the surface of the dornite to

observe deformations of the whole proving ground. Then, the whole territory was covered with a layer of medium-grained

sand with content of clay particles less than 0.75%. On sites No. 1, 3, and 5, the Finnish specialists sunk factory-made strip drains of the "geodrain" type with their own

mechanical-action drain-sinking unit based on a "LOKOMO-24" hydraulic excavator crane. The drain was a plastic corrugat-

ed core 3.5 x 100 mm in size, wrapped with "Turag" polypropylene geotextile.

At site No. 1, 238 drains were sunk to a depth of 9-10 m on a rectangular grid with a distance of 1 m between

centers; and at site No. 3, I50 drains to the same depth at intervals of 1.5 m. On the basis of their own test, the Finnish

specialists believe that outflow of water from lower and upper layers can be accelerated by additional installation of short

drains in the upper part of the soil mass to be compacted. In connection with this, at site No. 5 drains were made 9.5 and

4.0-4.5 m long. The long drains (64 of them) were spaced at intervals of 2 m; and the short ones (154), at intervals of 1 m.

In plan, the drains at sites No. 3 and 5 were located on a triangular grid. The time of sinking one drain to a depth of 9.5 m

was 40-45 sec. Site No. 2 was assigned to round drains installed with the help of a casing pipe 35 cm in diameter. At site No. 4, flat

sand drains with a rectangular cross section 15 x 60 cm in size were installed with the help of a special mounted construc-

tion [1]. At sites No. 2 and 4, 23 each round and flat drains were installed to a depth of 9.8-10 m. They were located on a triangular grid in plan, at intervals of 2.5 m, and their length corresponded to the thickness of the layer to be compacted.

Consequently, according to the scheme of their operation they were classified as "ideal." At site No. 6, drains were not

installed, i.e., it was the reference site. The drains were made of medium-grained sand according to the technology developed

208

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290

600

800

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-9

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Fig. 3 Fig. 4

200 400 600 800, s, rnm

5

Fig. 3. Development of settlements of test sites in time at an experimental proving ground with drains. 1)

with strip drains at intervals of 1 m; 2) with round drains; 3) with strip drains at intervals of 1.5 m; 4) with

flat, sand drains; 5) with strip drains of different lengths; 6) control (without drains).

Fig. 4. Nature of deformation of peaty base through its depth in the process of a rise in the intensity of its

loading in time. Solid line) site No. 6, broken line) No. 4; 1) with load of 13.6 kN/m 2 on the twenty-fourth

day; 2) with 34 kN/m; on the thirty-fourth day; 3) the same on the one-hundred-thirty-seventh day; 4) with

51 kN/m 2 on the three-hundred-fifty-fourth day; 5) the same on the eight-hundred thirty-first day.

at the NIIOSP, together with trust No. 28 of the Main Leningrad Construction Administration, and were sunk with the help

of an SK-4361A construction crane. All preparatory work at the site was carried out in the summer-fal l period.

After the drains were installed at the sites, depth and surface markers were set up for measuring settlements in the

process of compaction of the base soils. As a rule, the depth markers were placed on the roof of layers of the soil mass to be

drained and were intended for measurement of layer-by-layer displacements of the soil in the base being compacted. Readings

of the markers' elevations were taken before the application of a load (embankment filling) and after application of the load,

daily during the period of intensive compaction, and then after 3, 7, 14, and 30 days.

Displacements of the markers were recorded with a TN-3 level by short rays from one stand in relation to a datum

point installed beyond the bounds of the proving ground.

Surplus pore pressure in the base being compacted was measured with PDS-3 string pore-pressure gages made by

"Soyuzglavavtomatika" and PTsP-1 recording equipment. The pore-pressure gages were preliminarily calibrated in a special

casing with the help of compressed air.

The sensors' reading were taken automatically, and then converted to units of pressure with the help of calibration

graphs. The pore-pressure sensors were installed at characteristic points of the soil mass to be compressed after the drains

were installed, and their readings were taken immediately after their installation, then after embankment filling, daily for a

month and subsequently once a month. Besides that, at each of our sites Finnish specialists installed a KP-5B control pore-

pressure gage made in Japan.

Simultaneously, flexible hoses were installed at the foot of all six sites for pulling through a horizontal inclinometer,

which was intended to measure settlements of the surface of the base being compacted. At all of the sites, asbestos pipes 160

mm in diameter were set vertically closer to the center to observe the water table (WT), with the lower end resting on the

roof of the base to be compacted. Subsequently, these pipes were used to take soil monoliths for the purpose of investigating

their properties after stabilizations of the embankments' settlements.

The compacting sand fills were built up in stages: in the first stage, to a height of 0.8-1.0 m; in the second, 3 m. In

the final analysis, they created pressure on the surface of the base to be compacted equal to 54 kPa. The process of filling

each site took no more than five days, which was arbitrarily taken as corresponding to "instantaneous" application of the loads.

209

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, q ~ "//li//i/~/////, ,'///i/////I "//////i//~ "////i//lJ "l////i "//~/////////} ~//////////~ "////j///~/I/,'~ ~ 100

~ I1 ~ ~ ,--.----" ~ ~ ~ 0 ZOO

~ : t ~ zoo

I ,...d. I

Fig. 5. Diagrams of settlements of contact surface of em-

bankment with base drained by round drains, with p = 54

kPa. A) line of contact surface after the action of a working

layer 1 m thick; 201-207 - numbers of surface markers.

Comparison of the physicomechanical characteristics of the sites' base soils taken in a natural state from different

strata, as well as after the formation of loading layers, made it possible to establish the following. The upper peat layer

tocated at the contact with the foot of the fill responded most actively- to the loading process. However, to evaluate qualitative

changes in the soils' characteristics it is better to consider buried soil layers. In layer No. 3 (see Fig. 2), which is represented

by we11 decomposed peat, the moisture content was reduced from 380 to 260%, and the void ratio, from 9.4 to 7.5, and even

as low as 5.2. Interesting changes were noted in the seepage ability of this peat layer. Thus, in particular, its coefficient of

consolidation in a vertical direction C v was practically" unchanged, but then the radial coefficient of consolidation C r dropped

by more than 1.5 times, from 17 to 10 mZ/year. Here, the hypothesis about a reduction in the draining role of peat inter-

layers in the process of consolidation of a base was sufficiently fully confirmed.

A number of changes were also noted in layers of loam, which possess higher density than peat. Thus, in the first

layer of loam the coefficient e decreased by 18-27%, which indicates significant compaction of it. The layer of loams lying

below proved to be extremely inhomogeneous in its properties and especially porous at its contact with the underlying layer

of peat. It is precisely in this area that noticeable changes in the soil's characteristics occurred in the process of consolidation.

There, the loam's moisture content decreased by half (from 50 to 20-25 %); the coefficient e was reduced by 25-50%; C v, by

almost two times; and C r, by as much as three times,

Unfortunately, the procedure for establishing quantitative changes in the soil's properties by taking specimens of it at

different times and subsequent comparison has shortcomings. In our opinion, it is more objective to evaluate the behavior of the base under a load according to data from observa-

tions of the displacement of a set of surface and depth markers, and also measurements of pore pressure made at the site over

the course of three years. If we consider the sites' settlements in the first four months (Fig. 3), then the overall pattern of their deformations

will look like this. Settlement of site No. 4, which was drained by flat sand drains, went most intensively. In the first 1.5

months, the rate of its settlements was 350 mm/month, then it dropped to 35 ram/month, and overall for the four months it

was 60% of the maximum settlement of the test sites in five years. If the settlement of site No. 4 in four months was 550

ram, then for the undrained site No. 6 during the same time it was equal to 400 mm. The settlements of sites with other types

of drains occupied an intermediate position in this interval. However, four months after loading of the drained sites different relationships began to show up in the rates of their

settlements, which subsequently led to the fotIowing results. After a year, it became obvious that the settlements of sites No.

2 and 4, which were drained by sand drains, had progressed very little in time and noticeably lagged behind the others,

including the control site No. 6. Draining of the base with strip drains provided an advantage over the undrained base for a

year and a half, when about 90 % of the total settlement had already occurred. -The Finnish side's suggestion, realized at site No. 6, that the base be drained with a combination of strip drains of

different lengths was fully justified. Already after four months, the settlements of the base drained in this way were prevalent

throughout the year. The settlement measured during this period was more than 95 % of its complete amount achieved in five

years. The efficiency of the suggested measure provides a basis for drawing specific practical conclusions.

210

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Fig. 6. Dissipation round

p = 5 ~ ~ ii

. . . . . ~N! I 9 t8 G--- Zg Z # ~r Ig ~ I~r

of surplus pore pressure in soils of site No. 2 with

drains. No. 1-3) pore-pressure gages.

In the fifth year of observations (after 1575 days), in the central part of the sites the following settlements were

noted: No. I - 832 ram, No. 2 - 663, No. 3 - 843, No. 4 - 773, No. 5 - 770, and No. 6 - 918 ram.

Data from measurements of layer-by-layer displacements of the base soil at various depths showed which layers of

the compressible mass were responsible for formation of its settlement.

As a result of observations of depth markers, it was established that the compressible mass of the layered peaty base

within the bounds of which soil displacements caused by the filtering overload are noted is 0.7-0.9 of the width of the

overload embankment on top. The increment of total settlement of the base under the load occurred mainly on account of

water loss and compaction of soils in the upper layers.

Draining of the base with flat sand drains was fairly effective and made it possible to more actively include deep-

lying water-saturated layers of soil in water loss. The measurements confirmed their higher efficiency on account of increased

perimeter, in comparison with round drains, with the same cross-sectional area, and a decrease in the "stopping-up" effect,

which was clearly manifested in round drains. The nature of deformation of the water-saturated base through its depth in the

process of a rise in the intensity of its loading and in time at the undrained site No. 6 and site No. 4, which was drained by

flat sand drains, is shown in Fig. 4. The effect of drainage for the bases with round sand drains was less than for flat sand drains. Half a year after

loading of the bases, a lag in settlements of sites No. 2 and 4 behind all the rest becomes noticeable. Initially, this circum-

stance can cause nothing but surprise. However, we must not forget that in solving drainage questions we simultaneously

carried out intensive reinforcement of the base with sand columns, which significantly altered the overall deformative abilit3 ~

of the once strongly compressible base. As a result, on account of a certain looseness of the sandy material packed in the

drains, in the initial period there was an overall settlement of sites No. 2 and 4, which then increased insignificandy in time

and, in the final analysis, was less than the settlement of site No. 6 in the fifth year. The reinforcement effect also showed up

at sites drained with strip drains, but to a lesser degree, probably due to their high flexibility.

Analysis of the behavior of depth markers in the upper peat layer showed that the type of drains and their interval

absolutely- did not affect its compaction, since the main receptacle of water contained in it is the sand overload itself.

Analyzing the rates of deformation of various layers of the base, we can note that they depend on the porosity and

seepage ability of each specific layer. Moreover, for layers of peat the rate of deformation is also affected by their degree of

decomposition. Thus, in particular, the efficiency of drainage of the base can be judged according to the rate of deformation

of its layers. A high rate of deformation is characteristic of the bases being drained, especially at the site with round drains.

It should also be noted that, regardless of the type of soil being compacted (peat or loam), the nature of change in rates is the

same for each site. A sharp change in the rate occurs in the first months; at the end of the second and beginning of the third

months, the rates are almost equalized at all of the sites, and they become constant. If we look at graphs of the changes in

pore pressures, then we can see a drop in them to 3-5 kPa. Consequently, the phenomenon of the soil skeleton's creep can be considered the reason for further settlement. Settlements on account of secondary consolidation of the soils do not exceed 5-

t0% of the total amount, which agrees with M. Yu. Abelev's conclusion [2], where, in contrast to our data, the settlement

due to secondary consolidation was 15-20% of the total.

Settlement of the contact surface of the overloading embankment can be judged according to the curves given in Fig.

5, which we obtained as a result of geodesic observations of the displacements of depth markers. In parallel, Finnish special-

211

U, kPa

20 ~'~ < . . . . . _~, ...... /

"10 4/ . . . . . ~ ;: . /" 0 "2

30 60 gO 120 lad 100 210 2~ t, days

Fig. 7. Dissipation of surplus pore pres- sure in layer of peat (No. 3) from p = 54 kPa. 2, 4, 6) numbers of sites.

ists monitored the position of the contact surface by pulling a horizontal inclinometer through a flexible pipe that they

installed on the contact surface before filling the site. In principle, the diagrams of contact displacements in our tests were

identical. As follows from the data obtained, uneven settlement is observed at practically all of the sites. The small sag in the

center of the sites is explained by the nature of the flexible plate's operation on the base being compressed, as well as by

drainage of the base, which cannot be said of site No. 6 (without drains), where smooth sagging at the contact surface was

not recorded, however a clearly expressed depression was observed in the center. Markers were placed under the slopes to

obtain information in the case of possible upward yielding of the soil, but such a phenomenon was not observed at any of the

sites. Settlements in the center of the sites exceeded the marginal settlements by 1.5-2.5 times (according to data from

inclinometers, this difference reached 3.2 times).

As is known, surplus pore pressure appearing under the action of an external load significantly affects the process of

consolidation of water-saturated soils. Measurement of the total pore pressure with pore-pressure gages, and of the WT in pipes specially installed for that

purpose made it possible to determine surplus pore pressure as the difference between total and hydrostatic pressure, and to

trace the changes in them. The experimental dissipation of surplus pore pressure at the investigated points is shown in Fig. 6. Pore-pressure

gages were installed at various depths in the middle of characteristic layers of the soil mass to be compacted. In plan, they

were located near the center of the drain. The maximum rise in pore pressure was observed immediately after embankment

filling. The process of dissipation of surplus pore pressures in the drained upper layers of site No. 2 (layers No. 2 and 3) is

characterized by three stages (periods): 1) an immediate increase in pore pressures after loading (1-2 days) and a rapid drop

in them (30-45 days); 2) a gradual reduction to a constant amount (2-3 months); 3) maintenance of constant pore pressure

(after 4-5 months from the beginning of loading).

In lower layers (No. 4, site No. 4), the second stage occurred more slowly and was not very clearly expressed in

general, i.e., the first and second stages were combined, as it were, and lasted 3-4 months. After that came the period of

constant pore pressure at the given depth. However, at site No. 6 these phenomena were not observed, and a very slow drop

in surplus pore pressure took place there. Constancy of pore pressure was not noted during all 12 months of observation,

although after 5-6 months from the beginning of loading the changes become negligible (0.5-1.0 kPa in 3-4 months).

It should be considered necessary to take into account the effect of this phenomenon on the process of occurrence of

settlements in time and on the strength characteristics of water-saturated clay soils. The cycle of measurements of pore

pressure through the depth of the soil mass to be compacted at site No. 4 for different moments in time, which was conduct-

ed immediately after filling of the overload embankment, showed that the total pore pressure rose sharply. Surplus pore

pressure at a depth of 6 m was 46 kPa; and at a depth of 2.6 m, t8 kPa. Such high surplus pore pressure in the middle of

clay layers to be compacted is due to slight seepage of pore liquid to the sand drain. The surplus pore pressure drops rapidly,

and on the fortieth day after loading it was 33 and 7 kPa, respectively for depths of 6 and 2.6 m. On the nirtety-eighth day it

was akeady equal to 15 and 4 kPa.

A graphic picture of acceleration of the process of seepage consolidation in the case of use of vertical sand drains can be obtained by looking at the dissipation of surplus pore pressure at any one depth of thxee sites simultaneously. Since the

layer of loam (No. 4) is thickest (4 m), it is interesting to make these observations in that layer. It was established that the

greatest value of pore pressure was noted at the site drained with flat drains. This is explained by the high rate of embank-

ment filling, when the layer of sand for the second stage of loading was ffiled in one working day. The minimum value of

212

initial pore pressure is observed at the base of site No. 6. As a result of drainage, there was a rapid drop in pressure in the

pore water. This process takes place almost identically at sites No. 2 and 4. A small difference is seen in residual values of

pore pressure. A clearer picture of the sharp change (decrease) in surplus pore pressure is presented in Fig. 7, which shows the

process of dissipation of surplus pore pressure in a layer of peat (No. 3) at three sites. In comparison with the layer of loam, there is a rapid drop in pressure in the pore water here, which can be explained by the effect of the use of drains, as well as removal of water beyond the bounds of the part of the base being compressed. However, if in the case of use of drains the residual pressure in the pore water is insignificant (4-5 kPa), then in the peat layer of the base of the site without drains it is I9 kPa. Slowing of the process of dissipation of pore pressure at all of the sites and constant residual pressure at sites No. 2

and 4 are due to a decrease on the soil's porosity and a corresponding reduction in the soils's seepage ability. On the whole, the changes in surplus pore pressure through the depth of the soil mass beiag compressed under the

action of an overloading embankment correspond to the nature of consolidation of soil tayers in time.

REFERENCES

i .

2.

P. A. Konovalov and F. F. Zekhniev, "New technology for installation of vertical sand drains," in: Bases of Struc-

rares [in Russian], Bmo, pp. 215-217. M. Yu. Abelev, Construction of Industrial and Civil Structures on Soft, Water-Saturated Soils [in Russian],

Stroiizdat, Moscow (t983).

213