ROAD CONSTRUCTION WITH RRP-SPECIAL
The scientific background: the physical-chemical reactions
associated with the processing of highly cohesive soils with RRP

How RRP function as an ion exchanger and how does it act upon silt and colloidal particles during the treatment?
The specific reactions between water and soil particles will be examined here in detail. In general, in soil mechanics, it is usual to draw a distinction between two phenomena of water: static water and water in motion.

The latter in particular (whose motion is caused by penetration or by the action of gravity) greatly helps to accelerate many reactions initiated by the treatment w/h RRP.  Static water, though it does not move under the action gravity, is nevertheless not to be regarded as completely motionless. Generally speaking, the motion caused by osmotic forces or molecular movement is very slight, but over a long period of time considerable masses of water may nevertheless be transported as a result of this - either as liquid or as gas (evapora-tion). Static water remaining in the soil can be sub-divided into four categories differing from one another chiefly in the order of magnitude of the force with which they adhere to the soil particles: 1. Chemical water, incorporated in the crystal structure of the soil minerals. 2. Absorbed water, which is held on the surfaces of the soil particles. 3. Water which is held by surface tension at the points of contact of the soil particles. 4. Capillary water in the pores between the soil particles.

Normally the finest colloidal particles of soils are negatively charged. The enveloping film of absorbed water contains a sufficient number of positive charged metal ions  such as sodium, potassium, aluminum and magnesium- which ensure charge equalization with the respect to the electrically negative soil ion. Absorbed or hygroscopic water is, as already stated, mainly responsible for the swelling and shrinking properties of soils. A soil particle comprising only chemically combined water cannot swell, i.e., it cannot alter its structural density. Only the film of absorbed water adhering firmly to the particle surface can expand in volume as a result of further water absorption when the soil is wetted.

This effect is more particularly prominent in fine-grained soils, such as clay. Since this absorbed water is held in a "stable" form

on the clay particles, thickening of this water film will involve a  displacement of the centers of the particles toward one another with the overall effect that the volume of the mass of soil increase.  Therefore, in order to achieve the densest possible packing of the clay particles and to obviate the undesirable swelling and shrinking behavior of such soil, it is necessary merely to reduce the thick-ness of this water film (which, as already pointed out, is held very firmly to the particles) or break the film. The only possibly way to do this economi-cally and permanently is by ion exchange.

Because of its electrokinetic properties the RRP solution acts upon the positive and the negative charges of the soil particles.

The effects of this action are threefold:: 1. The film of absorbed water is greatly reduced and in fact entirely broken. 2. The soil particles acquire a tendency to agglome-rate. 3. As a result of the relative movement, the surface area is reduced and less absorbed water can be held thereby, so that this in turn reduces the swelling capacity. Moreover, these three factors facilitate compaction of the soil or indeed make it in fact possible.

In bringing about this phenomenon, the positive charges of the hydronium ion or of the negatively charged hydroxyl ion will normally combine with the positively charged metal ions in the water adhering to the surface of the particles. Because of the effect of R RP in reducing the electric charge of the water molecule there is sufficient negative char-ge to exert adequate pressure on the positively charged metal ions in the absorbed water film. As a result of this the existing electrostatic potential barrier is broken.
When this reaction occurs, the metal ions migrate into the free water, which can be washed out or removed by evaporation. Thus the film of absorbed water enveloping the particles is reduced. The particles thereby lose their swelling capacity, and the soil as a whole acquires a friable structure. This is an irreversible process.

Chemical water:
This water which is incorporated in the crystal structure and thus chemically combined with the soil minerals forms only a very minor proportion of the water in the soil. It cannot be expelled from it even by drying at above 1 100C. From the techni-cal construction point of view this water is to be regarded as an integral constituent of the soil itself and of no effect, i.e., it can be ignored.

Absorbed water:
Water adhering to the surfaces of the soil particles can be partly, but not entirely, driven out by drying in a oven. When soil dried in this way is allowed to cool, it will re-absorb water in amounts depending on the humidity of the ambient air.

Water held by surface tension:
Most of the water retained in soils is derived from water which is held by surface tension at the points
of contact between particles or which otherwise can move as pore water or as free water in the capillaries and larger voids.

Capillary water:
This is water lodged in the pores between the soil particles; it can be partly or entirely removed by seepage, evaporation, or water extraction in suitable equipment.
The most difficult problem is raised by absorbed water which adheres to the whole surface of each soil particle and practically forms part thereof. This film of water enveloping the particles, which ultimate-ly governs the expansion and shrinkage of collo1dal soil constituents, cannot be completely eliminated by purely mechanical methods. However, by means of temperature effects, addition or removal of water or mechanical pressure it is possible to vary the amount of water held in this manner. Such varia-tions are attended by swelling or shrinkage. This provides an ideal point of operation for RRP. To obtain a better understanding of this, the prin-ciple on which the action of RRP is based will be explained. In this context the electrostatic cha-racteristics of soil particles will also have to be con-sidered.
As a result of a lowering of the dipole moment of the water molecule there occurs dissociation into a hydroxyl (-) and a hydrogen (+) ion. The hydroxyl ion in turn dissociates into oxygen and hydrogen, while the hydrogen atom of the hydroxyl is trans-formed into a hydronium ion. The latter can, in the nascent state, accept or part with positive or nega-tive charges, according to circumstances.

The shrinkage-time diagram clearly shows a kind of saw-tooth pattern with the "teeth" diminishing to zero in course of time. It thus appears that when water is added after shrinkage has occurred, the shrinkage decreases to an amount corresponding to the amount of capillary water that has emerged. If the soil is allowed to dry again, so that water evaporates from it, the shrinkage that will then occur will never be quite so great as it was pre-viously. This accounts for the fact that surfaces treated with RRP solution and left uncovered will always increase in stability over a prolonged period of time.

The most notable properties of RRP and their effects are:
1. Reduction of the dipole moment: this has a water-repellent effect in the individual soil particles and at the same time reduces then swelling capacity.
2. The electrokinetic phenomenon: this causes stabilization of the soil particles.
As a result, the soil acquires higher shearing strength, its computability is significantly improved (density is a function of the force applied) and its permeability is considerably increased in compari-son with untreated soil. The effectiveness of the processing can be verified by means of suitable tests (CBR test, cylinder compression test).
In general, the soil particles align themselves parallel to one another, as a result of the formation of an electrical "cushioning" which causes a sliding effect that takes place in the horizontal molecular struc-ture (the molecular constitution of graphite would be comparable to this).
Broadly speaking, a soil of colloidal character has a structure comparable to a house of cards. Because of this, the soil can contain fairly large voids which are filled either with water or with air. During treatment with RRP these voids must in any case be filled with pore water (derived from the static water) or otherwise the saturation of the soil will bring about the flow effect already referred to. Only in this way can ion exchange by higher-valiancy cations take place and the dipole moment of the soil par-ticles be reduced.

When the reaction has occurred, less water can accumulate in the soil than was originally possible. As a result, the swelling capacity is reduced, the internal moisture of the soil is reduced, and comple-te compaction to zero content of air-tilled voids becomes possible because of the space that has be-come available from the expelled pore water. Subsequent additions of water cannot reverse this process, once the latter has been accomplished (the swelling capacity is destroyed and the shearing strength increased). According to the laws of physics the above-mentioned reactions should also be able to take place in soil layers in which no "movement" of water occurs in consequence of penetration into the pores, since static water, too, can serve as a medium for ion transport. The rate at which the reaction takes place in static water is very difficult to determine, as other electrocinetic variables are also involved. The numerous factors include the resistance of the potential barriers (which varies according to the type of soil), specific pore water requirement of the soil particles (naturally striving to achieve saturation), particle size, surface area and pH of the soil. The hydrogen ions which are liberated in the dis-sociation of the water molecules can once again react with free hydroxyl ions and form water along with gaseous hydrogen H2. It is important to note, that the moisture content of the soil affects the surface tension and is thus a factor affecting compaction.

It should furthermore be pointed out that dry soil is poorly suited for compaction only because of the surface tension of the water contained in it. This is the reason why a certain total quantity of R RP so-lution is necessary for processing the area of ground in question. This is important, for if less than the total required quantity of solution is applied, its penetration into the ground will be adversely affected These two phenomena (gas and water formation; surface tension) can be reduced by an

increase in moisture content. If the forces involved are reduced as a result of in-creased moisture content, the RRP solution can penetrate more easily into the capillary structure of the soil and the ion exchange process can take place more rapidly. The water released in conse-quence thereof can either seep away or be expelled by the kneading action of, for instance, a sheepsfoot roller and then evaporate at the surface. RRP therefore creates favorable conditions for com-paction qy changing the zeta potential of the clay and silt particles. The zeta potential (electrokinetic potential) de-creases with increasing concentration of the ions of opposite charge from the RRP solution. The cations and anions are liberated from the diffuse double layer, which reduces the swelling properties of the soil.

Some considerations on testing the capillarity of soils
Geological investigations have shown that the capillary rise of water in various types of soil remains within certain limits. For example, the capillary rise for the following soils is given as follows:

1. Sand    2.0 -   0.6 mm      3 -   10 cm                 0.6 -   0.2 mm    10 -   30 cm                  0.2 -   0.1 mm    30 - 100 cm                 0.1 - 0.06 mm    30 - 100 cm

2.  Silt       0.06 -   0.02 mm       1 - 3- m                  0.02 - 0.006 mm       3 - 10 m                 0.006 - 0.002 mm    10 - 30 m 3. Clay     0.002                       30 - (300) m

Therefore it is assumed here that the capillary rise of such magnitudes, as indicated in the above table, can infact be measured. If this were so, it would mean that all the research so far carried out with RRP in connection with capillarity would be in-correct, because in all these experiments the only soil specimens made for testing were cubes about 10cm in height, so that any capillary rise in excess of 10cm could not be measured with such specimens anyway
Now if RRP so alters the properties of cohesive soils - and in this context we refer to the report of Dipl.-lng. Schwarz, Lutzelsachsen - that the indi-vidual soil colloids are no longer able +0 absorb water in the form of pore water and the principal effect consists merely in that RRP-processed cohesive soils can be treated or compacted as if they were cohesion less soils, then the capillary rise must perforce adjust itself to the altered soil material.

For perfect functioning of the processing solution the minimum requirement is that the soil should have optimum water content; a slightly higher water content will intensify the reaction; but on no account must the amount of water in the soil approach the saturation limit, for this will result in a reduction in penetrating power and in the effectiveness of the process. A further snag that arises if the soil water content reaches saturation is that the surface of the ground becomes sealed off by the original swelling effect.

General method for processing of clay and silt soils with RRP as a chemical-physical agent Since the main property of RRP is that it gives rise to ion exchange and since ion exchange is not possible in the absence of water, the absolute pre-condition is that the soil to be processed should possess a certain water content  which, ideally, should be a little in excess of the optimum content. Subsequently added water will weaken the inter-granular forces and almost entirely destroy surface tension, i.e., RRP can infiltrate more easily or re-duce the absorbed water film by surrounding the soil particles and thus penetrating completely into the capillary structure. Since not all the particles are immediately reached by infiltration, it is advisable to wait some time so as to enable ion ex-change to take place also by osmotic water move-ment, with due regard to the three-dimensional rod-like structure of the medium. At the same time the gases evolved in the chemical reaction should have an opportunity to escape freely into the atmosphere. These processes should in fact take place parallel to each other.
Under weather conditions characterized by inten-sive evaporation the treatment should, at times of strong sunshine, preferably be carried in the early morning or late afternoon in order to minimize water losses. It may, in such circumstances, be necessary to water the soil daily during the interval between the last processing pass with RRP and compacting it.

Working with RRP-Special:
 

1. Remove topsoil 2. Grade the subgrade to specified level 3. Spray RRP 4. Apply optimum compacting (rolling) 5. Install the surface

First remove the topsoil and Grade the subgrade to specified level.The RRP solution can be applied from a tank waggon.
Now for the next stage in our procedure for using RRP. After application of the processing solution, the soil needs water sprayed onto it to enable the RRP to penetrate to sufficient depth.
It will be necessary either to wait for rain (20 - 30 mm rainfall or to spray the soil with water (20 - 30 litres per m2) in severa passes. Of course, the natural moisture content of the soil should be taken into account. Gases which are formed during the process must be able to escape; the soil should therefore be kept open.
To give the processing solution sufficient time to penetrate deep into the ground, a waiting period of two weeks should be allowed. Under extreme soil conditions a longer interval will be necessary. Then the subgrade is graded to finished levels, as illustrated in the photograph on this page. Adequate discharge of rainwater from the ground should be ensured.
Next, the soil is compacted to optimum density. The most favourable time at which rolling should be carried out can be de-termined by measuring the Proctor density
On completion of rolling, the main operations involved in the RRP process have been comple-ted. The soil is now strong and frost-resistant. No further base courses are needed.
Of course, soils processed with RRP should always be covered by a surfacing, i.e., a top layer or wearing course, since RRP stabilised material is primarily intended as a base giving sup-port to a surfacing. Depending on the job in question, water-bound, bituminous or other types of surfacing can be laid direct on the stabilised subgra-de. If a waterbound surfacing of cohesive screened stripped top material is employed, it will likewise have to be treated with RRP. We consider it ad-visable to test the bearing capa-city of the processed subgrade qefore installing the surfacing (plate bearing test).
 

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World Exclusive Coordinator for RRP Reynold’s Road Packer - Germany
Pasea Estate
Road Town
British Virgin Islands
Email: office18@ch-non-food.com