Compaction of peat masses by weakly filtering soil surcharges

COMPACTION OF PEAT MASSES BY WEAKLY FILTERING SOIL SURCHARGES

P. A. Konovalov, I. N. Kulebyakin, and S. Ya~ Kushnir

UDC 624.138.22:624.131.276

For preparation of construction on swamped and peat areas, wide use is made of weak soil compaction by means of a hydraulic-fill sand surcharge mass.

It is endeavored to carry out the hydraulic filling operation using suitably filtering sands of medium and coarse grain size, which ensures intense discharge of pore water from the compacted layer. However, in the oil-industry region of West Siberia and in many other regions, such sands are not available, and for compaction of peat soils it is necessary to use fine and silty sands with a coefficient of permeability equal to or less than in the top peats, which affects the compaction quality. For this reason, the need arose for determin- ing the consolidation characteristics of peat masses under weakly filtering soil surcharges, for which the Scientific-Research Institute of Bases and Underground Structures jointly with the Tyumen Civil-Engineering Institute singled out in the Nizhnevartovsk and Surgut region, during 1980-1982, nine test sites measuring 25 x 25 m, ~n~hose peat massesa layer of fine and silty sand (kp = 0.5-2.0 m/day), having different thicknesses, was placed by the hydraulic-fill method (Table i).

The site surface consisted of a layer of top weakly decomposed peats (R = 5-20%), whose coefficient of permeability varied with depth from 1.2 to 0.02 m/day. In all the sites, the peats were underlain by water-saturated sandy loams. The groundwater level emerged at the surface. The physicomechanical properties of the soils were determined both under laboratory conditions and by field methods (rotary cutting, sounding, and radiometry).

To measure the layer deformations, the sites were equipped with deep screw marks, which were installed at l-m vertical spacings in the compacted peat layer. The mark construction prevented the effect, on them, of the soil friction under displacement of the top layers of the hydraulic-fill base. The displacements were recorded by means of a level, with short lines of sight, from the same station with respect to a fixed mark.

During consolidation of the hydraulic-fill base, the pore pressure was measured by means of strain gauges of the D. S. Baranov's system and PDS-I string gauges made by the Soyuz- glavavtomatika Factory. The gauges were installed prior to the hydrualic filling at vertical spacing of 0.5 m in the compacted peat layer down to its center.

The changes in the physical characteristics of the soils (their density and water content) with depth in the hydraulic-fill base and with time were recorded by PPGR-I and VPGR-I radioisotope instruments. Radiometric holes were constructed prior to the hydraulic filling, which made it possible to establish the initial density and water content of the peat soil. In order to eliminate the effect of edge phenomena on the measured quantities, the deep marks, pore piezometers, and radiometric holes were located in the central portions of the sites (Fig. 1).

Thus, during the process of consolidation of the peat soil under the sand surcharge, the changes in the soil density and water content, the layer deformations, and the increase and dissipation of the pore pressure in the con~pressible layer were determined.

A characteristic graph of the layer compaction of the peat mass with time when loaded by a layer of hydraulic-fill weakly filtering soil is presented in Fig. 2.

Analysis of the site deformation observation results showed that the total settlement of the peat layer after 1-1.5 years amounted to 25-30% of its thickness and that it depended on the compacting pressure p. However, intense increase in this settlement was observed only

Scientific-Research Institute of Bases and Underground Structures. TyumlSl Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. 18-20, November- December, 1985.

0038-0741/85/2206-0233509.50 © 1986 Plenum Publishing Corporation 233

TABLE I

Site No. J ' Peat laye~l

! 2 31, 2,8 2,5

H y d r a u l i c - i o el 1 £ i l l layer, t "'*1 ~,5 1,4 2,o ~aiCrkne,~ t'- t . . . . . . . . . . . . . . . . . [

I 5 ] 6 7 8 9

2,4 £,3 2,3 2,2 2, !

0

-3,1

.6,3 ~

2

!

Fig. i. Schematic of a r r a n g e m e n t o f i n s t r u - merits in experimental hydraulic-fill site. a) Plan; b) arrangement of instruments with depth, i) Deep marks; 2) surface marks; 3) pore pressure gauges; 4) radiometric holes; 5) sand; 6) peat; 7) sandy loam.

up to a certain value of p. Further increase in p affected the settlement value only slight- ly. Moreover, all the tests showed that the peat mass compaction characteristics are uniform with depth. The top layer, about i m thick, was compacted most of all. Its relative com- pressibility was % = 0.25-0.35, whereas for the underlying peat layer it did not exceed I = 0.15-0.25 (Fig. 3). The cause of this nonuniform deformation is probably connected with the morphology of the peat mass, in which the top active layer has low density and high permeability [1-3].

The field tests showed that the nature of the compaction of the peat in the mass under the continuous surcharge differs somewhat from its deformation in the compression device, which, according to the general opinion, simulates a one-dimensional problem.

The experimentally established relation betwen % and p can be represented by a hyper- bolic equation

_ P ( I ) p + bp '

in which a and b are coefficients, the coefficient ~ having the dimensions of pressure.

The expression ~ + bp corresponds to the modulus of deformation E of the peat, whose value increases as the load increases. The coefficient a, which characterizes the soil structural strength, is similar to the initial modulus of deformation E s and varies depend- ing on the peat mass depth from 0.03 m for the upper layer to 0.1 m for the lower. The value of b in the writers' tests varies from 2.4 to 2.6 and, for all practical purposes, it can be considered to be constant for the peats in the investigated region and equal to 2.5.

Based on the etstablished relation, the final settlement s of the surcharged peat layer, whose thickness is Ehi, can be determined as the sum of the settlements of the individual layers

phi i=l ~ , i + bp

The modulus o f i n i t i a l d e f o r m a t i o n Es, i o f each l a y e r c o r r e s p o n d s t o t h e c o e f f i c i e n t a , and i t can be d e t e r m i n e d d i r e c t l y from t h e p e a t c o m p r e s s i o n t e s t s , by r e c o n s t r u c t i n g t h e com- p r e s s i o n c u r v e in t h e c o o r d i n a t e s p/X - X. The i n t e r c e p t on t h e a x i s o f o r d i n a t e s i s numer i -

234

H o,. kPa

~0 r - " ~ - -- -- -

0 - - - 50 lO0 I50 200 250 ,TO0~, ~ yS

..~ ~ ~ \2 ° ~ :z0 ~ '~ " ° ~ - . . o _ _ . . _ o . . . ~ T o -

5O

Fig. 2. Graph of layer compaction of peat mass with time under hydraulic- fill layer (site No. 8). 1-4) Deep marks.

0,3

~0 P, ~ kPa O' I0 20 30 Fig. 3. Variation of relative compres- sibility of upper and lower layers of peat mass for increase in the load. I) Upper layer; 2) lower layer.

cally equal to E s.

A similar approach to the deciphering of the compression curve was worked out previously by Korolev [4].

At the experimental site, also the increase and dissipation of the pore pressure both with depth in the hydraulic-fill base and with time were studied. During the hydraulic- fill placement of the weakly filtering sand on the water-saturated peat soil, in the last- mentioned soil the total pore pressureup increased sharply as a result of rise of the ground water level and decrease in the porosity of the peat mass during its compaction: Up =

I ! u + u, in which u is the pore-water pressure caused by changes in the groundwater leve~, and u is the excess pore pressure developed as a result of decrease in the peat porosity after compaction.

The measurement of Up by the pore piezometers, as well as of the groundwater level by the radioisotope instruments made it possible to trace their variation and in the final analysis to determine u in the peat layer.

Figure 4 shows graphs of variation of the pore pressure with depth of the peat mass at test site No. 8 at different times. The cycle of measurements carried out immediately after the hydraulic-fill placement of the sand layer showed that up had increased sharply. The value of u at the initial instant of time amounted to 0.7-0.8 p, which was due to the high peat porosity (e = 18-24) and the quick load application (the 3.6-m-high sand layer was placed by the hydraulic-fill method in 7 h). At the interface with the filtering layer, the value of u at the same instant of time was also high (0.4-0.5 p), which was caused by the weak filtering capacity of thehydraulic-f~ll layer. During the first three months after completion of the hydraulic-fill placement of the material, the dissipation of the pore pressure

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o kp, o kP% , ke,

I,\ , " ' r "

Fig. 4. Pore pressure variation with peat mass depth and time. a) After transfer of sand surcharge to peat; b) after months; c) after 300 days.

~ O

0 50 100 150 ;tOO 260 t, days Fig. 5. Dissipation characteristics of pore pressure coefficient 8 in peat with time under sand surcharge.

took place at an intense rate, and subsequently this process was slowed down and continued for a considerable period. In the writers' experiments, under compaction of the peat by means of the weakly filtering sands, the value of u after 300 days from the start of loading amounted to 0.1 p.

A sufficiently clear representation of the variation of u is given by an experimental investigation of the dissipation of the pore pressure coefficient 8 = u/p with time (Fig. 5). It was found that for loading of the peat mass by means of the weakly filtering sand, the greater part of the settlement takes place during the period of seepage consolidation.

The investigation of the nature of the compaction of the peat mass with time showed that it occurred most intensely in the first 3-4 months after the hydraulic-fill placement time. The settlement in this period reached 80-90% of the final. However, not all the layers of the peat mass were uniformly compacted with time as a result of the heterogeneity of its structural and seepage properties with depth. In the writers' experiments, the top layer consolidation (H = 1 m) ended practically in the course of i month, whereas the underlying layer continued to be deformed during the entire observation period.

The peat mass compaction duration was determined not only by the seepage properties and creep of the peat, but also by the compacting pressure from the hydraulic-fill layer, which increased with time. After completion of the hydraulic-fill placement of the material, as a result of watering of the hydraulic-fill layer, the sand is in a suspended state, and for this reason only a part of the pressure from the hydraulic-fill sand mass is transferred to the underlying layer. As time elapses, the water is expelled from the hydraulic-fill layer and the suspending action of the water disappears, which leads to increase in the compacting pressure. In the hydraulic-fill layer, consisting of fine and silty sands, the water discharge process takes place over a long period of time in contrast with the filtering sands, in which the groundwater level drops in the course of one week. In the writers' tests, the changes in the density and water content of the hydraulic-fill sand with depth were recorded by PPGR-I and VPGR-I radioisotope instruments [5]. The observations showed that after 300 days in the hydraulic-fill layer, 3.6 m high, the groundwater level dropped 2.2 m, which amounts to about 90% of the final load pf, which is the pressure from the hydraulic-fill mass, determined for the natural groundwater level, taking into account the settlement of the compacted peat layer. The nature of the variation, with time, of the load Pt caused by the hydraulic-fill sand can be described by the exponential equation

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= mf [l- e-w+a)], (2)

in which d is a coefficient which characterizes the initial load applied immediately after the hydraulic filling; c, rate of transfer of the load from the hydraulic-fill sand mass, with a dimension which is the reciprocal of time; and t, time which elapses after completion of the hydraulic filling.

The value of c can be calculated if the rate of descent of the groundwater level in the hydraulic-fill layer is known. The intensity of the discharge of the groundwater from the hydraulic-fill sand is determined from the geometric parameters of the hydraullc-fill pattern, the coefficient of permeability k~ , and the intensity of the discharge of the pore water from the compacted layer. The groundwater level for the time t can be determined from the equation

HInA+Q~ , (3) ~ = l n a + k S i t A ,

in which H is the hydraulic-fill layer height; £ is the half-width of the hydraulic-fill pattern; n is the hydraulic-fill sand porosity: i is the hydraulic gradient along the seepage path; A is the width of the investigated fill (for the plane problem, A = i); Qz is the discharge of pore water from the compacted layer

S kp o.8~etatgt (4) Q~ = y~ h.p '

i s a c o e f f i c i e n t which depends on t h e s eepage p r o p e r t i e s o f t h e s u r c h a r g i n g and compacted s o i l s [ 6 ] ; Tw, u n i t w e i g h t o f w a t e r ; and hp, t h i c k n e s s o f t h e compacted p e a t l a y e r .

I n t h e a n a l y s i s o f t h e s e t t l e m e n t o f t h e p e a t mass w i t h t i m e , i t i s n e c e s s a r y t o t a k e i n - t o a c c o u n t t h e d i s s i m i l a r c o n s o l i d a t i o n o f i t s l a y e r s , which can be r e p r e s e n t e d as t h e sum o f t h e s e t t l e m e n t s s 1 o f t h e upper ( a c t i v e ) l a y e r and s2 o f t h e u n d e r l y i n g l a y e r s : s t = s 1 + s 2.

The s e t t l e m e n t o f t h e upper l a y e r f rom t h e f i e l d e x p e r i m e n t d a t a ends in t h e c o u r s e o f one month, and i t can be c o n s i d e r e d t o be i n s t a n t a n e o u s . The u n d e r l y i n g p e a t l a y e r s were compacted f o r a s u f f i c i e n t l y l ong p e r i o d , and t h e i r c o n s o l i d a t i o n can be d e s c r i b e d by t h e e q u a t i o n

s~ = %sf , (5)

in which Qz is the degree of consolidation, which with sufficient accuracy for engineering calculations is determined from the equation

Q~= pz(% +2.s~) (6) pf (Es+ 2,5 Pz)

The effective pressure Pz is calculated on the basis of the solution of the differential equation of one-dimensional consolidation taking into account the seepage properties of the surcharging layer and the change in the load with time:

Pz= Pf [1 - -e - (g+et ) ] (1--kAT~ -r") , (7)

in which T v is a time factor; A and m are coefficients which characterize the rate of compaction of the peat (for the peats in the region, A = 0.i, m = 0.8); and k is a correction coefficient which depends on the seepage properties of the hydraulic-fill and compacted soils [63.

The coefficient of consolidation is determined by Taylor's method. To take into account the effect of the permeability of the surcharging weakly filtering sand layer, it is recom- mended that it be multiplied by the coefficient k, which can be found from a special table prepared by the writers.

Conclusions

i. Consolidation of a surcharging peat mass with depth takes place nonuniformly. The most intensely deformed is the top layer, whose compaction is completed 30 days after the start of loading and amounts to more than 50% of the total settlement of the entire peat mass.

237

2. The ~rans~er, to the compacted base, of the load from the hydraullc-fill weakly filtarin E sand mass takes place over a fairly lon8 period of time as a result of slow dis- chafes of the water from the sludge and a related 8radual descent of the Eroundwater level in the hydraulic-fill layer (after 300 days the load had increased from 0.5 to 0.9 p).

3. The proposed method of analysis of consolidation of peat surcharged by weakly filtering sands takes into account the relation between the values of k D of the hydraulic-fill base layers, the slowln8 down of the application of the load from the %eaRly filtering layer, and the different layer deformabilltles of the compacted peat mass.

LITERATURE CITED

1. L. S. Amaryan, Strength and Deformability of Peat Soils [in Russian], Nedra, Moscow (1969).

2. N. P. Kovalenko, A. D. Khudyakov, and V. S. Gorelikov, Preconstruction Compaction of Peat Deposits [in Russian], Severo-Zapadnoe Knizhnoe Izd-vo, Arkhangelsk (1971).

3. M. A. Shaposhnikov, Geotechnical Investigations of Marsh Soils for Construction [in Russian], Stroiizdat, Leningrad (1977).

4. A. S. Korolev, "Determination of compression properties of clays and peats by semi- graphical method," Osn. Fundam. Mekh. Gruntov, No. 6 (1965).

5. I. V. Lavrov , P. A. Konovalov, and I. N. Kulebyakin, "Use of radioisotope methods for investigation of hydraulic-fill bases," Osn., Fundam. Mekh. Gruntov, No. 6 (1983).

6. P. A. Konovalov and I. N. Kulebyakin, "Effect of permeability of surcharge layer on peat consolidation," in: Eighth European Conference on Soil Mechanics and Foundation Engineer- ing [in English], Vol. 2, Helsinki (1983).

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Compaction of peat masses by weakly filtering soil surcharges