Stress history and stress conditions

The soil in the sections can generally be assumed to have consolidated for the self-weight of the soil layers. These can be assumed to have had a thickness corresponding to an almost horizontal ground surface level with the surrounding non-eroded and non-excavated ground. The pore water pressures at this initial stage may be assumed to have been higher than at present. The groundwater level particularly in the coarser sand and silt layers close to the river has then been lowered successively as the river has eroded its channel down through the soil layers. The present groundwater situation can only have prevailed since the excavations were performed about 15 years ago. Any significant consolidation for increased effective stresses resulting from the lowered groundwater level after the excavation can only have occurred in superficial layers directly behind the upper crest above the excavated area, i.e. at Point S3 in Section A and Point S11 in Section C. It is more difficult to estimate how the water pressures in the bottom layers have varied during the simultaneous land-elevation and erosion process. However, it may be assumed that the present pressure conditions here represent the lowest pressures so far and that they have in principle prevailed for a long time.

A large number of oedometer tests have been performed. The results of the tests in Section A are presented separately for each borehole since the soil conditions here vary somewhat, Fig. 31.

The results at Points S13 and S2 show that the soil has consolidated for the assumed maximum effective overburden pressure and has also obtained a slight overconsolidation in relation to this. Because of the later unloading, the soil in these points is now overconsolidated or heavily overconsolidated in relation to the present stress condition.

The results at Point S3 just behind the upper crest in principle also show that the soil is slightly overconsolidated in relation to the assumed maximum effective overburden pressure. However, in a zone around the level +7 metres, the test results show almost normally consolidated conditions. This zone coincides with the zone where both the relation w_N/w_L and the sensitivity are elevated. The oedometer tests on the two lowest levels also show normally consolidated conditions, but this is assumed to be related to disturbance of the samples, which contained clay mixed with coarser material.

The results at Point S4 generally show a normally consolidated clay. This is in spite of the fact that the estimated maximum effective stresses here are somewhat lower

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Fig. 31. Evaluated preconsolidation pressures compared to assumed maximum effective overburden pressure in Section A.

a) Point S13 below the river b) Point S2 in the excavated area

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Present effective overburden pressure = estimated effectice overburden pressure before excavation

c)

d)

Fig. 31. Evaluated preconsolidation pressures compared to assumed maximum effective overburden pressure in Section A.

c) Point S3 behind the upper crest d) Point S4, 50 metres behind the crest

because of a thinner sand and silt layer and higher pore pressures. The clay at this point is different from that at the other points in other respects too and has a considerably higher sensitivity and w_N /w_L relation.

A compilation of all results shows that the preconsolidation in the main part of the investigated soil mass is similar and corresponds to an overconsolidation ratio of about 1.15 in relation to the assumed previous maximum effective overburden pressures, Fig. 32. Exceptions are Point S4 and a small depth interval at Point S3, which show normally consolidated conditions for the prevailing stresses. It is possible that a small previous overconsolidation here has been broken down by different circumstances, such as leaching.

The results of the CPT tests also show that the soil behind the excavated area is almost normally consolidated for the assumed maximum previous effective overburden pressure and that the values at Point S4 are lower than at the other points. The results also show that the evaluation method that was proposed in SGI Information No. 15 (Larsson 1991), strongly overestimates the preconsolidation in homogeneous overconsolidated soil and that this increases with increasing overconsolidation ratio. pressure in points S2, S3 and S13 - " - -"- in point S4

OCR = 1.15

Fig. 32. Compilation of evaluated preconsolidation pressures from CRS tests in Section A.

In the evaluation method proposed in SGI Information No. 15, the evaluated preconsolidation pressure is corrected with respect to the overconsolidation ratio.

The method was proposed on the basis of the investigations in Swedish clays made by Larsson and Mulabdic (1991) combined with results from Norway and the United Kingdom presented in literature. According to that compilation, the preconsolidation pressure could be evaluated from

)

where σ´_c = preconsolidation pressure q_T = total tip resistance σ_v0 = total overburden pressure w_L = liquid limit

OCR= overconsolidation ratio

However, the Swedish investigations in principle comprised normally consolidated and only slightly overconsolidated clays with an average overconsolidation ratio of about 1.3, and there was no national experience of overconsolidated soils. The correction for overconsolidation was introduced mainly to take the fissured nature of the overconsolidated dry crusts into account. Later investigations in clay till have shown that this method strongly overestimates the preconsolidation pressure in this type of soil (Larsson 2000). New experience from Canadian clays also shows that no correction should be made for overconsolidation in the evaluation of preconsolidation pressure in this way (Demers and Leroueil 2002). Without correction for overconsolidation the evaluation is simplified to

L

When the results in Section A in Torp are evaluated in this way, a relatively good correlation is obtained with the results from the oedometer tests regardless of where in the slope the borehole is located and what overconsolidation ratio the clay has, Fig. 33. No correction for overconsolidation ratio should thus be made in this homogeneous type of clay in which the state of consolidation varies from normally consolidated to heavily overconsolidated and the overconsolidation is a result of a real unloading.

The soil conditions in Section C were very similar in the different boreholes. The results of all oedometer tests have therefore been compiled and show very similar conditions in the section regarding preconsolidation as well, Fig. 34. The results scatter somewhat, but the trends in all test points show that the clay has consolidated for the assumed maximum effective overburden pressures and has become a small overconsolidation ratio of about 1.15 in relation to these. The results of tests on samples taken at great depths at Point S9 fall out of this general picture, but these samples were taken in the zone with coarser soil and embedded sand and silt layers.

The samples can therefore be assumed to be significantly disturbed. Also the samples from the clay below this zone may be assumed to be partly disturbed because of infusions of coarser material. Undisturbed samples of a very good quality have been taken with the Swedish standard piston sampler from considerably greater depths in other locations (e.g. Claesson 2003), but this has then been done in more homogeneous and high-plastic clays.

The clay under the river and at the riverbanks has become heavily overconsolidated because of the erosion. The overconsolidation has also increased at Points S8 and S9 because of the excavation. At Point S11, the effective stresses may be assumed to have increased somewhat because of the lowered groundwater level. However,

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Fig. 33. Evaluated preconsolidation pressures from CPT tests in Section A.

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Fig. 34. Evaluated preconsolidation pressures from CRS test in Section C.

this has not entailed that the preconsolidation pressure has been exceeded. Whether or not the stress increase has nevertheless brought any extra consolidation and increase in preconsolidation pressure because of creep effects cannot be inferred from the test results.

The stress and consolidation conditions are thus similar along the investigated area except for the part behind the upper crest in Section A where the clay has obtained different properties, which are probably a result of leaching.

Similar preconsolidation pressures to those evaluated from the oedometer tests were evaluated from the results of the dilatometer test. More strictly, it is the overconsolidation ratio that is evaluated from the dilatometer tests and in the evaluation method employed this ratio can never be less than 1.0 (Larsson 1989).

This limitation is to some extent reflected in the results from greater depths, Fig. 35.

A partly revised method for the evaluation of the overconsolidation ratio for dilatometer tests in overconsolidated soil is presented further on in the report (Chapter 3.8), but this does not affect the evaluation in the normally or only slightly overconsolidated soil at Point S11.

The preconsolidation pressures evaluated from the CPT tests showed the same overestimation of the preconsolidation pressure in zones with homogeneous overconsolidated clay as in Section A when a correction for overconsolidation ratio was applied. When this correction was omitted, concordant results were obtained which were similar to those obtained from the oedometer tests, even if the values in general were somewhat too low, Fig. 36. However, the empirical evaluation is very sensitive to the given liquid limit and the results lay within the normal band of scatter. The general recommendation, see e.g. SGI Information No. 15, is therefore that the evaluation from CPT tests should be used only in combination with results from oedometer tests. The oedometer tests are then used to determine the value of the preconsolidation pressure on a number of levels and the evaluation from the CPT tests is primarily used as a support for the estimated distribution of the preconsolidation pressure with depth.

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Fig. 35. Evaluated preconsolidation pressures from dilatometer tests at Point S11 in Section C.

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Fig. 36. Evaluated preconsolidation pressures from the CPT tests in Section C.

2.7.4 Shear strength