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Characteristics of soil-peat-rubble mixtures by mercury intrusion porosimetry

A b s t r a c t.The effect of cyclic changes of temperature and the addition of organic matter on pore properties of model urban soil was investigated.Mixtures composed from a loessial soil with 10–50%w/w of rubble and 6%w/w of peat were studied.The mixtures were moistend to 25%of their field water capacity and subjected to cyclic changes of temperature.Before and after the cycles granulometric composition,organic matter content and mercury intrusion porosimetry studies were performed.Samples of the model urban soil before freezing-thawing cycles had lower values of the structural parameters than after cyclic changing of temperature.Those hanging concerned first of all the total cumu-lative volume and pore size distributions.The observed changes depended on the properties of added building materials,which changed to a greater extent under the cycles'change of temperature than the soils themselves.

K e y w o r d s:urban soil,mercury intrusion porosimetry,gra-nulometric composition,organic matter,rubble

INTRODUCTION

Structure is one of the most of dynamic elements of the soil environment.It determines the physical,chemical and biological properties of soils [12,14].Many studies have pointed out the important role of organic matter,pH,minera-logical composition and many others in the formation and stabilization of soil aggregates [4,13,15].Organic matter (peats)is frequently added to city-soils to improve their fertility.The structural properties of urban soils in the cities are particularly affected by building activity.Building ma-terials inserted into the soil horizon markedly alter the structure of these soils,first of all in superficial layers.After the end of building activities,rubble is usually left on the building site.This rubble contains large or small pieces of brick,concrete,foam concrete and mortar.The role of these

components of urban soils on soil structure is less well understood.

Humidity and temperature are the main elements of the climate that determines the physicochemistry and mecha-nical properties of soils.Low temperature and capillary wa-ter destroy soil aggregates and increase specific surfaces of soils [6,11].Freezing and thawing of water cause mixing of soils layers within all horizons [1,3,19].All these effects significantly influence soil structure.

The soil structure can be characterised by its porosity [5,8–10,18].Many soil properties,e.g.,transport of water and gases in soil or penetration by roots,depend on soil pore volume,radius and bulk density.

The aim of this work was to study the influence of cyclic changes of temperature and the addition of organic matter on physicochemical properties of model urban soils obtained by enrichment of a soil with various doses of a rubble.

MATERIALS AND METHODS

Artificial urban soils composed by mixing loessial soil (Elizówka-Lublin)with various amounts of a rubble con-taining equal w/w proportions of 1mm sieved materials:brick,concrete,foam concrete and mortar were studied.Soil material from 0–20and 20–40cm depth (A and B horizons)was used.The amount of the rubble in the mixtures was 10,20,30,40and 50w/w%.

The mixtures were moistened to 25%of their moisture and subjected to six cyclic changes of temperature.Each cycle consisted of a one-week treatment at 30°C following by one week at minus 35°C.

Next the samples were subjected to porosimetric (Carlo Erba Mercury Porosimeter 2000)and granulometry (areo-metric method)analysis.Before porosimetric measurements,

Int.Agrophysics,2002,16,177–181

Characteristics of soil-peat-rubble mixtures by mercury intrusion porosimetry**

M.Hajnos and G.Bowanko*

Insitute of Agrophysics,Polish Academy of Science,Do?wiadczalna 4,P.O.Box 201,20-290Lublin 27,Poland

Received February 11,2002;accepted March 26,2002

2002Institute of Agrophysics,Polish Academy of Sciences

*Corresponding author’s e-mail:gbowanko@demeter.ipan.lublin.pl **This work was partly supported by the State Committee for Scientific Research,Poland under the Grant No.6P06H 06420.

I N T E R N A I O N A L

A g r o h y i c w w w .i p a n .l u b l i n .p l /i n t -g o p h y s c

the samples were oven-dried at105°C and then outgassed in a vacuum to remove physically adsorbed water.The range of mercury pressure applied allowed us to study pores with equivalent radii ranging from3.7to7500nm.The pore radii were calculated using the Washburn equation[7,16].

The surface tension and the contact angle of mercury were assumed to be48.9J m–2and141.3,https://www.wendangku.net/doc/f08373427.html,ing cylindrical pore model bulk density the surface area,average pore radius and total porosity were calculated[7,17].

Granulometic analysis was performed using the aero-metric method of Bouyoucos modified by Cassagrande and Pruszy?ski.For the latter analysis,the soil samples were dispersed using a0.5%Calgon(sodium metahexaphos-phate)water solution.

The content of organic matter in the samples was mea-sured by dichromate oxidation using Tyurin method[16].

RESULTS AND DISCUSSION

The results are shown in Tables1–3and Figs1–3.Addi-tion of a rubble altered soil granulometric composition is shown in Table1.The fraction of coarse>1mm particles (sand fraction)increased which caused a decrease in finer particles content,simply by their dilution.This indicates that the soil after rubble addition can become more easily permeable for water and over dried.However,with the addition of the peat and after wetting-drying cycles the fractions of silt and clay increased,counteracting the above changes.

Figures1and2show examples of the pore volume versus intrusion pressure for the soil,peat and rubble,before and after cyclic changes in temperature.The rubble material, as well as the peat,had a very high porosity.Therefore their addition to the soil caused the overall increase in the pore volume in the whole pore range,which is illustrated in Fig.

3.Cyclic changes of temperature altered the porosity of the samples.These changes were similar in mixtures containing soil from both A and B horizons,therefore only the curves for the A horizon are depicted.

As seen in Table2,the pore volumes of the samples before cyclic changes of temperature ranged between 101.16mm3g–1(soil,A horizon)and175.21mm3g–1,(soil

178M.HAJNOS and G.BOWANKO

Rubble (%)

A horizon

B horizon

Sand Silt Clay Sand Silt Clay

(%)

Initial soil-rubble mixtures

0 10 20 30 40 505

21 Initial soil-rubble mixtures after addition of peat

0 10 20 30 40 5026

20 Soil-peat-rubble mixtures after cyclic changing of temperature

0 10 20 30 40 5011

Sand(1–0.1mm),Silt(0.1–0.02mm),and Clay(<0.02mm).

T a b l e1.Granulometric composition of model urban soils before and after cyclic changes of temperature

CHARACTERISTICS OF SOIL-PEAT-RUBBLE MIXTURES 179

Fig.3.Exemplary pore size distribution functions of model urban soils as affected by cyclic changes of temperature.Abbreviations:I –initial A horizon of soil;II–soil +50%rubble;III –soil +50%rubble +peat;IV–mixture III after cyclic changes of temperature.

Fig.1.Exemplary pore size distribution functions of model urban soils before cyclic changes of

temperature.

Fig.2.Exemplary pore size distribution functions of model urban soils after cyclic changes of temperature.

B horizon–50%of rubble).After cyclic changes of tem-perature these values generally decreased ranging from 86.77mm3g–1(soil A horizon–10%of rubble)to144.07 mm3g–1(soil B horizon–30%of rubble).The total porosity of the materials before applying temperature cycles ranged between18.6and30.48%and after the cycles these values decreased ranging between16.74and26.51%(the same soils as above).Most probably large aggregates were dama-ged due to the temperature and humidity effect,which lead to the decrease in overall porosity and increase of fractions of smaller pores.

In the soil and the model mixtures which were not sub-jected to cyclic changes of temperature,the radius of domi-nant pores was around1to3mm.After these cycles a slight

T a b l e2.Pore properties of model urban soil after and before cyclic changes of temperature

I–Initial soil-rubble mixtures,II–Initial soil-rubble mixtures after addition of peat,III–Soil-peat-rubble mixtures after cyclic changing of temperature.

T a b l https://www.wendangku.net/doc/f08373427.html,anic matter content(%)in model urban soils before and after cyclic changes of temperature

shift in dominant pore radii towards larger sizes(1.5–3mm) occurred.These changes in pore size distribution indicates a rearrangement of the samples stucture.Simultaneously,the effect of the temperature changes may involve dehydrata-tion of soil colloids which can lead to stronger aggregation and an accompanied increase of large and medium-size pores which,despite the drop in pore volumes,may lead to some shift in dominant pore radii.

The content of organic matter after addition of the peat increased from0.35to4%,however after each temperature cycle this subsequently decreased(Table3).This might be due to the intensive mineralization of organic matter in well aerated samples.This unfavourable process may be connec-ted with laboratory conditions.In natural environments the organic matter decrease is frequently restored by turnover of residues of living organisms.

CONCLUSIONS

As a result of cyclic changes of temperature a subsequ-ent decrease in the content of organic matter was observed in model urban soils.Additions of peat and rubble to the soil caused an increase of the total cumulative volume of pores in all pore size range accompanied by an increase of the re-lative amounts of medium and large pores.Freezing-thawing cycles decreased pore volumes of the soil-rubble-peat mixtures and caused a further increase of the dominant pore range.

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