B.5 Testing
B.5.1 General
The method of testing utilised has to be adapted to the purpose of deep mixing. Thus, for settlement reduction, the elastic modulus value is of main interest, while for improvement of stability and elimination of the risk of failure, the strength of the columns is of main interest. As regards immobilisation and/or confinement of waste deposits or polluted soil and containment, overlapping and low permeability of the columns are the determining factors.
B.5.2 Laboratory testing
B.5.2.1 General
Laboratory testing represents one of the means used for analysing the possibilities of treating the actual soil and checking the result of deep mixing. It includes on one hand laboratory mixed soil samples and on the other hand samples taken at various depths in the columns installed.
B.5.2.2 Laboratory mixed samples
The laboratory mixed samples offer a possibility to study which quantity of binder, which type of binder, or combination binder/filler/admixture, which binder factor and water/binder ratio that are required to stabilise the soil for the intended purpose.
For the laboratory investigation of soil/binder samples, reference is given to the following procedures included in the Design Guide from [6]:
- 1) laboratory procedure for preparation and storing of test samples of soil stabilised by binders for Deep Mixing, Column applications;
- 2) laboratory procedure for preparation and storing of test samples of soil (especially peat) stabilised by lime and cement-type materials for mass stabilisation applications.
NOTE Laboratory procedure for preparation and storing of test samples of soil for Japanese dry and wet mixing methods have been standardised by the Japanese Geotechnical Society.
The correlation between the strength properties of laboratory mixed samples and the corresponding properties under field conditions is very uncertain. If extensive experience is available of the correlation between the strength characteristics of laboratory mixed samples and the corresponding characteristics of columns installed in soil of equal geological origin as the laboratory mixed samples, a conservative correlation coefficient can be applied. The same type of mixing tool, binder and binder content should be used as in the reference object.
B.5.2.3 Core sample
Core samples can be taken by means of a rotary core drilling equipment. Core samples can be used to study deformation characteristics, strength and uniformity. Choice of coring technique and core diameter is highly dependent on the treated soil strength and grading. Triple tube samplers are recommended for columns in soft soils. The number of cores depends on the size and/or complexity of the project. A minimum of at least three core borings are recommended in a construction work. The sampling should extend to full depth of treatment. Fundamentally, the rate of strength gain in dry mixing and wet mixing is different. It is affected by the moisture content and hydration characteristics of the binders. Temperature is of significant influence for the strength increase. Temperature increase in the ground by the hydration effect of the binder is affected by various parameters such as binder type, binder factor/content and treated soil volume. Sample disturbance may be a significant concern and influence the sample characteristics. Core sampling should be supplemented with other test methods as listed below.
The strength characteristics and the elastic modulus, Ecol, of the samples are normally determined by unconfined compression tests. However, the results thus obtained will be affected by the existence of cracks in the samples. If cracks can be observed, triaxial testing is preferable (see prEN 1997-2).
The compression modulus Mcol of the samples is determined by oedometer tests (see prEN 1997-2). For assessing the settlement behaviour of the stabilised soil, the elastic modulus of the column is more representative than the œdometer modulus. The use of the œdometer modulus in settlement analysis instead of the elastic modulus of the column leads to an underestimation of the long-term settlement [1].
Hydraulic conductivity tests require special equipments built for the purpose, as no standard apparatus exists. However, the permeability can be estimated by back-calculation from the value of the coefficient of consolidation determined by œdometer tests.
B.5.2.4 Wet grab samples
Wet grab sampling is used in the European wet method. Wet grab samples are taken prior to initial set of the treated soil. They are extracted from critical depths of the columns with a suitable sampling tool, usually one per 500 m3 of treated soil volume or one per day. The samples are obtained by lowering an empty wet grab sampling device to the sample depth, capturing the fluid sample, closing the wet grab sampling device, and bringing the sample to ground surface where the material is processed and placed into cylinders for testing. The samples are cured at a prescribed temperature in standard size sample mould, cylinders or cubes. Testing of the samples, as described above, is normally performed after 7 days and 28 days of curing. Curing conditions of the treated soil in-situ on the one hand and of the wet grab soil sample on the other, are different and influence the strength and the rate of strength increase.
B.5.3 Field testing
B.5.3.1 Field trial tests
Because of the uncertainty regarding the applicability of the column characteristics determined in the laboratory, in-situ tests are required. One of the most important issues, namely to investigate the uniformity of the columns, can be fulfilled by some type of sounding, or by core boring as mentioned above, and/or by lifting up whole columns. Determination of the mechanical and hydraulic conductivity properties of the columns require special equipments. A field trial test for this purpose usually comprises two to three column installations with varied binder content.
Another important aspect of field-testing is to determine the criteria for the construction control of deep mixing. The construction control values may include penetration and retrieval rate of the mixing tool, rotation speed and torque of the mixing tool, overlapping width and rate of delivery of binder/slurry. When a column has to be founded in a firm bearing stratum, the torque and/or the change of penetration resistance are measured to establish the critical construction control values.
B.5.3.2 Direct determination of mechanical properties
Pressuremeter tests (see prEN 1997-2) can serve as a basis for determination of the shear strength and the compressibility of the column. The tests require preboring of a hole in the column into which the pressuremeter can be inserted.
Geophysical tests serve as a basis for determination of the properties of the treated soil under dynamic action and can be used for investigation of the integrity of the columns and also for indirect determination of the deformation modulus and strength. However, the interpretation of results from geophysical tests is still at the research stage.
B.5.3.3 Investigation of uniformity and indirect determination of mechanical properties
CPT tests, representing conventional cone penetration tests are used for determination of the strength parameters and the continuity of the column. The CPT method has potential limitations compared to the column penetration test in terms of maintaining verticality. Due to the size of its point, the CPT is also only testing a limited proportion of the column volume. Stepwise preboring is often necessary to keep the cone test inside the column.
Static/dynamic penetration tests, which are a combination of penetration and hammering test, are useful for treated soil with unconfined compressive strength ≤ 4 MPa.
Column penetration (see Figure B.2) test is carried out using a probe that is pressed down into the centre of the column at a speed of about 20 mm/s and with continuous registration of the penetration resistance. The probe is equipped with two opposite vanes. The method can normally be used on columns with a maximum length of 8 m and with unconfined compressive strength < 300 kPa. In the case of longer columns the probe may end up in the soil outside of the column. This can be avoided by preboring a vertical hole in the centre of the column. Preboring should be made without percussion. By the use of preboring, the column penetration test can be used for columns with maximum unconfined compressive strength of 600 kPa to 700 kPa to a depth of 20 m to 25 m.
By the reverse column penetration test the uniformity of the column can be determined along its whole length. In this test, a probe, fitted with vanes equal to those used in the column penetration test, is attached to a wire rope placed below the bottom of the column while it is being constructed. The wire rope, which should have strength of at least 150 kN, runs through the whole column up to the ground surface. The strength of the column is obtained by measuring the resistance obtained when drawing the probe up to the ground surface. The withdrawal should take place at a speed of about 20 mm/s. The choice of vane type should be the same as suggested for the column penetration test. As indicated, the method can be used as a measure of the variability with depth of the strength of the column rather than as a direct measure of the shear strength. The method is presently still under development.
B.5.3.4 Hydraulic conductivity tests
Pressure-permeameter tests are used in a similar way as the pressuremeter and can serve as a basis for determination of the permeability of the column in radial direction.
Various types of field tests can be used to assess the hydraulic properties in the field. However, no standard equipment exists for determination of the permeability.
Key
- 1 Supra wire 1/2 inch
- 2 Tube, dy = 36 mm threaded on easing
- 3 Wedge for supra wire
- 4 Internally threaded casing on sounding rod
- 5 Supra wire 1/2 inch
