Footings of Residential Buildings , Erected on Clay Soils with Organic Content

In the proposed article the author examines issues determining physical and mechanical soils with organic matter content. Conditional definitions of organic soils depending on the value of the relative organic content matter are determined. The biogenic soils are examined using three groups peat, blocked soils and sapropels. Depending on the relative content of organic substances, the author compares various peat types. The division of the soils with organic content is presented. A definition of sapropels is presented. The problem of complexity of determination of the amount of organic substances by calcination of samples is described. The current methods used to determine the amount of organic substances currently in Russia isexamined. The paper reviews physical and mechanical properties of organic soils. The on-site test-plate data is compared with laboratory test data. The problems of sapropel tests are described. The results of experimental studies, conducted at a construction site in the new suburban district of Moscow are shown.


Introduction
Swamp and boggy lands occupy large areas worldwide.Such areas in Canada occupy 170 mln ha, and 150 mln ha in Russia.This situation is an issue for railroads and highways, passing through boggy lands, and large construction sites, consisting of soils rich in organic matter.
The last 70 years in Russia saw investigations of such soil properties as well as measurement of the properties of clay soils, containing organic inclusions, in relation to the applied methods and subsoil-footing interaction analyses.
Organic matter content analysis is mandatory for sand and sand-silt tests in Russia.Relative organic matter content is evaluated as organic matter content in the the total sample mass ratio.By the relative organic matter content the soils are defined as "soils with organic matter admixture" if the organic matter content varies between 0,03-0,1.Such soils in Russia in geotechnical survey belong to the group of biogenic soils, classified as peats, peaty soils and sapropels.Soil, containing 50% and more of organic matter are classified as peats.It is assumed that organic matter resulted from swamp plants decomposition in high moisture content environment and the shortage of oxygen.
Depending on the relative organic matter content the peat varieties are classified as lowdegraded, if their organic matter degradation is below 20%, medium degraded with degradation rate of 20-45% and highly degraded with the degradation rate over 45%.
Peat-containing soils include sandy and silty clay soils, containing 10-50% mass of organic content.The peaty soils are classified as low peat content (relative organic content is 0.1-0.25),medium content (relative organic content rate is 0.25-0.40)and high peat content (with relative organic matter content over).
Sapropels include the fresh-water silt, formed up due to degradation of organic plant residues at the bottom of stagnant water bodies.Usually sapropels contain over 10% of organic matter.The sapropel structure changes by compaction, porosity decreases, and such soils are called saprocols.Sapropels are classified by their organic matter content as mineral ones (0,10-0,30); medium mineral content (0,30-0,50) and low mineral content (more than 0,50) ones.

Methods
Determination of organic matter content is an important issue.It often happens that the organic matter content was determined by ignition of the samples at 600-900 ˚C.The loss of mass due to ignition indicated the organic content.But the new research showed that both organic.fine clay and silt particles as well as salts burn out.Therefore, in Russia the organic matter content is currently measured by carbon content (method of I.V.Tyurin, modified by Simakov).
Due to the fact that thick sapropel strata (saprocols) are seldom used as subsoil for residential buildings higher than 35 stories (high-rise building construction) a necessity emerged to re-investigate specific physical and mechanical properties of such soils.Therefore, the authors collected and analyzed archive and published material from various geological organizations (deformation and strength parameters of sapropels).
Conventionally, all sapropels are classified as lacustrine, swamp and lacustrine-swamp ones mainly buried under the layer of other soils.
The analysis of the investigations, carried out in 1950-1990s in Belorussia as well as in north-east areas of Russia demonstrated that such sapropel deformation modulus was assumed as 2-14 MPa and up to 0,05-0,1 MPa due to change of vertical pressures on samples.All researches established that the more mineral particles are present in the sapropels the greater is the sapropel strength.
The experiment investigations were carried out in the new neighborhood of Moscow i.e., Luberetsky aeration sites.We performed these investigations for the project engineers and the customer.
As per survey data on sites of neighborhoods #13 and 17 the buildings' subsoils contain thick sapropel strata which ought to be better called saprocols.
The site survey included both laboratory and field tests.According to survey data from drilled holes up to 42 m deep, CPT, in situ plate tests, laboratory tests, the 17 to 25 storey high-rise site subsoil features the following soil strata.
Contemporary swamp deposits occur below a medium thickness stratum, represented by two layers of different color: black to light-brown, having different consistency of soft plastic to hard, 4.6-8.1 m thick.Organic content in various samples amounts to 10-21%.
Sapropels, with color varying from black and dark-grey to light-brown, are 6.0-9.6 m thick.
These layers are underlain by alluvial sands, saturated, dense, containing carbonate coarse grain debris.
Below are limestones, defined as Upper Carbon.This site was investigated by CPT Fugro and Pika-17 continuous down-pushing to determine the soil strata homogeneity in general.The CPT tests measured qc and fs.CPTs were performed near test holes, drilled earlier, in order to specify the probe resistance.
At the site soil properties were also investigated by test plate loading.The tests in the holes were performed with Kashirsky 600 cm2 area plate.
The undisturbed soil samples (monoliths) taken from the test hole (undisturbed) were at least 108 mm dia and at least 20 mm high.These samples were brought to the laboratory, where soil design parameters were investigated, required for the structures' subsoil strength and stability analyses.E.g., stiff clay with 5.3-7.9organic content had filtration consolidation ratio CHν=0,008 cm2/min while its undrained shear resistance was CHu =29 kPa.
Hard clays with organic matter content up to 40% consolidations ratio 40% had filtration consolidation ratio CHν =0.018 cm2/min while the undrained shear resistance CHu =18 kPa.
Clay soils, having low organic content, related to the sapropel (saprocol) stratum, were investigated in most detail.The filtration consolidation ratio of this soil was CHν =0,0011cm2/mi while its undrained shear resistance was CHu =7,8 kPa.
The sapropel, compacted to porosity ratio 1.3 and less, are called sapprocols.Determination of sapropel design parameters for the project engineers to analyze footing bearing capacity and settlement is a complicated problem.
The data analysis from the tests, performed to investigate sapropel propertis in Belorussia, in Leningrad and in Moscow region, demonstrated that the sapropel deformation parameters, measured in laboratory compression tests, was essentially (times 6-18) different from the 500 cm2 and 3000 cm2 plate test data.
Therefore, in order to carry out experiments on "Nekrasovka" site in Moscow, 3 test sites were set up at the construction site of block 5 in neighborhood 13 (A, B) and at the block 6B site in the same neighborhood.
Test pits were excavated on the test sites, penetrating the peat-containing soil strate down to the boundary of the sapropel stratum.Soil samples were taken from the sapropel stratum by the method of cutting ring.The samples were preserved (in wax or latex) and delivered to the laboratory where soil strength parameters (φ, c), compressibility (E) and permeability ratio in vertical and horizontal directions were measured.
From these samples cut from sapropel strata smaller samples we cut out for triaxial testing.

Results
Some results are given below.Because there exists a viewpoint that soil deformation parameters, determined by 600 cm 2 test plate are essentially different form similar parameters, obtained by 3000 cm 2 and 5000 cm 2 plate tests, we also tested deformability of sapropel soils with 3000 cm 2 plates (on test site #2).
In situ soil plate tests photos are shown below:

Discussion
Analysis of laboratory and in-situ test data on sapropel deformability enabled the following conclusions: 1.The deformation modulus of the tested sapropels, determined by data from laboratory tests, are times 8-17 less than the value of the plate test modulus; 2. It was established that in the course of sapropel compression test, stabilization of test plate settlements both in the hole and in the subsoil under large diameter (3000 cm2) plates is much greater than in ordinary clay soils of plastic consistency.This fact ought to be taken into account during plate tests on sapropels.
3. Comparison of 600 cm2 plate test data with that of 3000 cm2 plate test data demonstrated that the value of the respective deformation modulus under test plates of greater diameter is times 2,3 -4,3 greater than that of 600 cm2 one.The up-to-date computer analysis, applied to determine a building virtual stability, possible tilt in the course of operation, installation of deep footings and different piles as well as for the design of a "bore wall" it is necessary to know the values of soil strength parameters.
Sapropel strength parameters were conducted in laboratory in direct shear devices on 60 cm2 area samples (Gidroproject design) and in triaxial apparatus on 3.6 cm dia samples.Direct shear tests were done according to the conventional method, however, there occurred uncertainty in registrating the sample shear moment.Usually mineral samples are sheared or split at 3 mm slider movement.This is the criterion for identifying soil strength parameters.Unfortunately, there was no shear failure in sapropel samples taken from test site #1 (Building #5, Aprt.13(A, B) in Nekrasovka, Moscow) neither at 3 mm, 5 mm or more just the sample front portion crushed.
Similar problems emerged during the tests sapropel samples, taken for the same pits.The sample in a triaxial apparatus deformed, following the load growth, no "split", however, occurred.
Similarly, to the test techniques for soft saturated clay soils it is necessary to indicate what strength parameters correspond to vertical deformations.E.g. if vertical deformation is 5% then φ =20˚ and c=0.01 MPa.
There was an attempt to determine the sample stress and strain state in a triaxial apparatus by measuring pore pressure in the lower plate.
Unfortunately, in the cases of partly saturated soil this method failed.However, when the clay sample was water saturated (humidity rate over 0.93) the sample collapse moment was registered as sharp pore pressure rise.
There are currently several schemes in Russia for determining soil strength parameters.These are consolidated drained test, non-consolidated and non-drained test, etc.
According to investigation, conducted on 16 sapropel samples, tested in triaxial and direct shear "Gidroproekt" devices the closest test results were obtained after investigation of sapropel strength parameters in triaxial compression devices with measured pore pressure and results of one-plane shear devices by fast shear method (Maslov).
For projects on sapropels it is necessary to consider that chemically they contain pore water, aggressive to steel and concrete.
Therefore, it is recommended to use either hydro-isolated piles or those made of special concrete.
A large raft footing is the most applied design solution for civil structures, sitting on a 1.5-3.0m thick sand layer.The layer shall be filled with maximally compacted coarse or medium coarse sand.This work was financially supported by Ministry of Education and Science of the Russian Federation (#NSh-3492.2018.8).