Optimal control time evaluation for “dry DSM” soil-cement composites

Soil improvements with hydraulic binders are a widespread practice in foundation works. They vary depending on the mixing method (jet grouting hydraulic, deep soil mixing -mechanical), medium type (wet/water, dry/air) and binder type (cement, lime, fly ash or mixtures). The produced component’s strength changes in time thus its control should change in time as well. The paper presents the results of laboratory testing of an organic soil component mixed in dry method. The process of samples preparation and testing methodology of compressive strength and stiffness is described. Volatility of the parameters in time is considered. On the basis of the results, recommendation for optimal quality control time and its methodology for soil-cement components might be proposed.


Introduction to dry Deep Soil Mixing
Deep soil mixing is a ground improvement method which leads to enhancing a number of soil geotechnical properties. Cement, blast furnace slag, quick lime, fly ash or gypsum are additives used in this method. Binder is mixed with a soil at least several meters into the ground. Deep soil mixing technologies have been developed all over the world since 1970s. Polish experience [1], [2], [3] is linked mainly with the DSM wet technology. Due to popularization of the DSM technology, researches on limitation in its use for organic soil [4], [5], [6], [7] have been conducted. Interesting remarks on dry soil mixing in highly organic soils can be found in reference [8]. Special equipment is used for this purpose, depending on soil mixing type: columns installation (columns are formed one by one in different patterns and mutual position), mass stabilization (mixing unit with a mixing drum) (Fig. 1). Deep Soil Mixing is commonly used as an enhancement for embankment, excavations, foundation of houses and light warehouses, but also as geohydrological barrier or land reclamation/remediation of contaminated areas [9], [10], [11], [12]. There are no restrictions about the depth of treatment in this type of strengthening. For mass soil stabilization (Fig. 1), either wet mixing method or dry mixing method is used. Therefore, the binder can be delivered as slurry or in a dry form with compressed air as a medium. In both methods water plays the key role in binder hydration. In the wet method, water is delivered as a grout component and in the dry method -binder is mixed with high-water content soil to obtain necessary parameters.
Due to the use of special equipped excavator the depth of treatment is limited to about 6 m. Dry deep soil mixing was invented in Sweden in 1960s and became a very popular method of soil stabilization in Scandinavian countries, as well as in Japan. It is applicable in organic soils with water content exceeding 60% [8] and commonly accepted that lower water content does not allow carrying successful hydration process of binder.
For dry soil mixing, a number of binders are used. Cement is in general used as a binder for most soil conditions. Lime is suitable for inorganic soil, as is clay and slit clay. Lime provides initial dewatering effect and the increase of PH, but stabilization effect can be poor, as humic acids inhibit strengthening reaction [8]. Slag and ashes are used as secondary products in the binder. They need to be mixed with cement or lime. With their use the significant improvement of permeability is observed which results in decrease of water and chemicals penetration inside the stabilized mass [8]. Moreover it is an environmental friendly solution as both materials are an industrial waste and can be reused for a good purpose. The mixture of slag and cement is suitable (with some restrictions) for peats and organic soils.

Laboratory test
Initial laboratory tests are almost mandatory in case of dry mixing. This is due to high sensitivity of the mix parameter to various parameters described in the next section.

Objectives and scope of the research
The objectives of the research were to investigate:  the optimal time of improved soil Quality Control after its implementation [13],  the influence of cement amount on compression strength in a function of curing time [14],  the relation between strain changes and time of curing,  time influence on the accuracy of the archived results,  profitability of deep soil mixing in soils with water content below 60%. The research was conducted according to recommendations given in [15], [16] on the population of 148 samples with amount of cement -CEM IIIA 32,5 -vary between 120 kg/m 3 -230 kg/m 3 . The peats for laboratory test come from Oł awa -city in Lover Silesia province, Poland. Peats were characterized by the content of organic component between 5.80% -8.28% -measured in combustion process. The natural moisture varied from 37.4% up to 56.2 %. The uniaxial compressive test of the samples was carried out at Wroclaw University of Science and Technology and was a base for further considerations.

Samples preparation
The samples were prepared as a mixture of peat and cement in various proportions. The mixing process was carried out with an electric stirrer under constant rotational speed. Mixing process lasted around eight minutes. Placing soil-cement mixture into the cubic forms was divided into four layers under constant pressure of 18 kPa. The free surface was carefully flattened to obtain an even surface for uniaxial test. The cubic forms of internal dimensions 150×150×150 mm were used. First, they were cleaned and covered with a thin layer of oil to guarantee easiness of samples removal. The samples were stored in laboratory conditionsconstant temperature 18-20C. They were unformed 5 days after the preparation and stored in special containers.

Uniaxial compression test
The uniaxial compression test was chosen, because in most of cases the DSM columns are designed and created under compression rather than by way of bending or tension. The test was conducted at Wroclaw University of Science and Technology using Compression Test Machine. During the investigation, the sample was under compressive forcelinearly increasing as a result of upper pad constant movement in the lower pad directionuntil the moment of the sample's destruction. During the test, several parameters were registered on the basis of the type of the chart of stress in a function of strain which was proposed. As the basis for stress/strain calculation, the following equations (1) and (2) were used: It is easily observed that the column sample failure shape corresponds to columnar failure model rather than cone failure model, as it was for soil-cement samples tested at Wroclaw University of Science and Technology by Karpisz and Jaworski [13]. The elastic modulus E1 was determined for a linear part of stress-strain curve -at approximately 50% of compressive strength fc.
The slope of the diagram varies at the initial fragment due to the fact that pads did not fit the samples precisely, and thus the compressive force was not transferred through the whole accessible area. Fig. 9. Marking of the elastic modulus E 1 at 50% of f c .   Within a series (constant amount of cement), the value of strain almost does not change between 28 th -84 th day of curing. It might be observed that for higher cement content, lower values of strain were registered. That leads to a straightforward conclusion: the main aspect that affects strain values is cement amount. Besides its influence on strain value, the decrease of strain volatility is noticeable. The results are ambiguous. Hence, the proposed conclusions are not fully objective. The diagram (Fig. 13) has a horizontal trend. The dependence of elastic modulus on strength in time varies from 70 to 120. The formula given in work [4] for DSM laboratory test is as follows:

Research program results
Theroposed value exceeds the values obtained in this research. The given formula was accurate for the tested group of samples of organic soil mixed by the wet method and for certain cement types. For current research (peats, CEM III, dry mixing), the dependence between elastic modulus and compressive strength can be formulated as:

Conclusions
Control of soil improvement works quality is a basic factor for the reliability of foundations. Both, implemented testing technologies [17] and supervision quality (human factor) [18] play a decisive role, especially when the product (soil cement composite) is not uniform and the variability of parameters may be time dependent due to internal corrosion process. The two main factors that affect the results are cement amount and soil compaction. Cement content below 200kg/m 3 should be taken out of consideration for DSM Dry technology in case of soils which were considered (I om~6 -8%, w n =38-56%). None significant growth of the compressive strength in time appeared. Higher amount of binder influences the strain decrease; it improves column stiffens, but still to the lower level than in the case of wet mixing. Even local changes of soil compaction can highly affect soilcement composite's properties, thus precise geological studies should be carried on. During the stabilization process, the mixing and compression parameters should be carefully observed. They vary depending on the rotation speed, feed speed, blades number and compressed air stream. It is a common practice to enhance compressive strength by overloading the stabilized area with additional surcharge right after mixing. Laboratory tests confirm the impact of pre-compression on the achieved results in a time scale. It also speeds up the settlement process which is the main issue in organic, highly compressible soils. The research indicates that in order to capture all of the changes of soil-cement composite, optimal time for quality control is around 28 th , 56 th and 84 th day of curing. Finally, despite the general perception, in the DSM Dry technology it is possible to considerably improve parameters of soil with the natural moisture content around 40%.