Principle of the plate loading tests
4.1 MJARDEVI, LINKOPING Test field
The test field is located in western Linkoping,just across the road from the Swedish Geotechnical Institute and Linkoping University. The area has been developed in the late 80's and the 90's in connection with the construction of Mjardevi Science Park. The soil profile is dominated by silty soils but is very heterogeneous with alternating layers of silt and clay and many infusions of clay lenses and coarser objects, the latter increasing with depth. The weathered crust in the upper part of the profile is about 2 m thick and this and a somewhat softer layer about 1 m thick lying just below consist of more clayey soil. The ground water level is located about 1 m below the ground surface and may vary with season from the ground surface to 1.8 m below. The pore water pressure is approximately hydrostatic from the ground water level. The silty soil profile is estimated to be 16 to 17 m thick and is followed by coarser soil. Only the upper parts of the profile down to about 20 m depth have been investigated, the depth of the various investigations depending on the ability of the various methods to penetrate this type of soil.
First investigation
The soil conditions at the test site, which is relatively large and comprises half a block with five buildings, Fig. 4.1.1, were first investigated in 1988 before construction of the buildings, (Ottoson and Bergdahl 1988). The first investiga
tions were aimed at determining the type of soil, its stiffness and the possibilities for foundation on spread footings, and also to investigate the depth of penetration in dynamic probing and thereby the necessary length of driven piles if these were required. The investigations were performed with a local non-standard type of static total pressure sounding, dynamic probing according to the Swedish HfA method, pore pressure measurements at two levels and disturbed sampling by screw auger. The non-standard static total pressure sounding method basically uses the same principle as the ordinary Swedish static total pressure sounding
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" C
100 m
Fig. 4.1.1 Plan of buildings in the test area.
method, i. e. cp 22 mm rods, a total pushing force of 10 kN and rotation of the rods with the maximum force applied when this force alone is insufficient to penetrate the soil. The difference is that the ordinary square tip with 1000 mm2 cross section and a slip coupling, which enables separation of tip resistance and rod friction, is replaced by a twisted cp 25 mm screw-shaped tip normally used in the weight sounding test. The Swedish HfA dynamic probing test uses a 63.5 kg hammer with a free fall of 0.5 m and a cp 45 mm conical tip with apex angle 90° and a mantle length of 90 mm. The penetration resistance is registered as the number of blows required for every 0.2 m penetration. The rod friction is estimated by measuring the torque required to rotate the rods and the net penetration resistance is then calculated. From experience, this method yields approximately the same results as theSPT-test, N20 HfA. Net "" N30 SPT' (Bergdahl and Ottosson 1988).
Investigations and load tets in silty soils 33
According to the results from these penetration tests, the silty soil generally appears to be very loose down to 3.5 m depth. It then becomes varyingly loose to medium stiff down to about 10 m depth and then mainly loose at further depths down to stop in penetration. The variation in the results was considerable, particularly below 3.5 m depth, Figs. 4.1.2 - 3. Penetration stop was obtained at widely varying levels. The static total pressure soundings stopped at depths varying from 5 to 20 m, in all cases but one at less than 15 m, and the cause was generally judged to be a large object such as a cobble or boulder. The dynamic probing tests generally penetrated somewhat further, but initial attempts to penetrate past what were believed to be large objects down to a firm bottom soon
Total penetration force, kN
5 10
5
' '
\ II II
.,.,
Rotation E
.s:::.
c.
Ql 100
15
- Estimated average curve
Fig. 4.1.2 Variation in results from static total pressure soundings at Mjardevi.
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:, < (D V> rt ~- o· :::, V> p.) :::, 0.. 0 p.) 0.. ~ r;: 3·
~-
~ Q, V, v.l (J\111.:::111.::: HfA j_ t 1
106 543 120 m + tip lost
IJ I '+ +0.00 HfA +0.00 HfA +0.00 /IIEIIIE Ill.Ell/= IIIEIIIE IIIE'IIIE 111=111:= 111.=111.=
L
59rr
52 63' 6
' ' I I I I I 60/0 1020304050 Blows/0.2 m 178 140
r
1.)-LI I' I I I so;o 1020304050123 1020304050 Blows/0.2 m Blows/0.2 m m rod + tip lost 1020304050 Blows/0.2 Fig. 4.1.3 Results of dynamic probing tests at Mjardevi. 7 m rodsresulted in breakage of the rods and loss of the tips together with a certain number of rods. In all cases, the encounteling of large objects was judged to be the reason for the stop in penetration.
The classification of the disturbed samples showed that the soil in the upper 7 .5 m consists mainly of silt but that it contains numerous thinner layers of clay. The dry crust on top, particularly its upper part, consists of clay. Also the softest layer just below the crust contained so much clay that it appeared possible to take relatively undisturbed samples with the Swedish standard piston sampler. This was attempt
ed in three boreholes where samples were taken at 2.5 m depth and in two holes attempts were also made to take undisturbed samples at deeper levels where more clayey soil had been found.
The latter samples provided more precise information on the stratification of the soil and showed that also the soil in the more clayey parts of the profile below the dry crust consists of varved/layered partly clayey silt and alternating varves/
layers of clay and silt. The clay also appears as small lenses in the silt. The unit weight of the soil is about 1.95 t/m3 and the natural water content varies between 20 and 42 % depending on the clay content. A number of specimens were prepared for oedometer tests on the most clayey parts of the samples and constant rate of strain oedometer tests and incrementally loaded tests were performed, both types in accordance with the relevant Swedish standard. Indications of a preconsolida
tion pressure could be observed only from the results of two of the constant rate of strain tests. Both specimens were from 2.5 m depth and the indicated precon
solidation pressures were 110 and 204 kPa respectively. The preconsolidation pressure can vary rapidly with depth just below the dry crust and the penetration test results indicate that the minimum strength value may be found about half a metre further down. Nevertheless, the results from the oedometer tests indicate a certain overconsolidation in the soil. The evaluated moduli from the oedometer tests show that the minimum value of the oedometer modulus would be around 2.7 MPa.
Pressuremeter tests
An attempt was then made to obtain better values of the compressibility of the soil, which would also be more representative for a larger volume of the heterogeneous mass, by using pressuremeter tests. The pressuremeter tests were performed with Menard type equipment and the pre-drilled holes were made by use of a so-called
"bentonite screw". The bentonite screw is a screw auger with a hollow stem and hollow rods through which a bentonite suspension is pumped down when the screw
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is withdrawn. This technique had previously been found to work very well in preparing pre-drilled holes for pressuremeter tests in sand, (Bergdahl et al. 1984).
At Mjiirdevi, however, pre-drilling proved to be very difficult because of the heterogeneity of the soil and the embedded coarse objects. The results of the following pressuremeter tests also indicated that the soil had been heavily disturbed. The measured volume-pressure-creep curves were erratic. In many tests, the limit pressures and yield pressures could not be evaluated because the initial size of the cavity in the pre-drilled hole was too large in relation to the possible expansion of the pressuremeter probe and the evaluated moduli were very low. Several attempts were made to achieve good pre-drilled holes and tests were performed at four different points. A total of twelve tests were carried out but, according to the criteria for the relations between the pressuremeter modulus and the net limit pressure presented by Baguelin et al. (1978), the soil in all tests but one at a shallow depth in the lower part of the crust was to be considered as remoulded.
Raft foundation with pre-loading
The results from these investigations showed that, even if the softness of the soil estimated from the results of the penetration tests and the very low measured moduli in the pressuremeter tests could both be considered exaggerated, a foundation on spread footings would require very large dimensions of the footings and still involve possible problems because of the risk of relatively large and particularly uneven settlements. The results of the investigations also showed that there would be a considerable risk of breaking and loosing driven pre cast piles because of the embedded coarse objects in the soil. Furthermore, it was anticipated that a considerable amount of pile testing and re-driving would be required because of the risk of "false stops" in this silty soil. It was therefore considered more prudent to use raft foundations for the buildings in the area. Also in this case, there was a risk of uneven settlements and a scheme for pre-loading was designed.
The largest buildings were to be 4-storey buildings with a ground plan in the form of an H. The loads from the buildings were concentrated to the outer walls and to a row of pillars along the centre line of the connecting central part of the building.
The contact pressure on the ground in these parts was calculated to be 50 kPa and the pressure in the areas inside was estimated to be 25 kPa. The pre-loading was intended to correspond to this load plus a possible ground water lowering of 1 m and a certain overload. The pre-loading consisted of earth fills with heights and shapes modulated to closely model this loading situation, Fig. 4.1.4. Also the other buildings with other shapes and heights were pre-loaded in a similar fashion.
Investigations and load tets in silty soils 37
~ I:I : I : I : I : ·;,.; i : I : I : I :I : I ~~
lli I I I : I I I I I Jlml: I I I : I I I : I g~
Fig. 4.1.4 Layout of pre-loading fill for the H-shaped 4-storey buildings and photo of a fill under construction.
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The fills were constructed in a sequence in such a way that the site for one building was first pre-loaded and the load was allowed to act until the primary settlement process had been finished, whereupon the masses in the fill were moved to the next site and so on. Before the fills were put in place, a number of settlement gauges and horizontal settlement hoses were placed on the ground. The settlements of these gauges and hoses were recorded throughout the period of pre-loading.
The settlements beneath the fills were roughly estimated to become about 0.1 m.
It was difficult to predict the time for consolidation more accurately but this was estimated to be a couple of months. The fills were to be rather high , about 5 m, and in order to assure that no stability problems would occur, piezometers were installed below the fills and the filling operations were to be halted in case excessive pore pressures developed.
In the actual pre-loadings, in which the filling up was performed in about 10 days, most of the settlements occurred during the time for load application and the settlements had evened out to consist only of long term creep settlements after about 20 days. No significant excess pore pressures developed during the construc
tion of the fills. In most cases, the piezometers showed maximum increases between O and 5 kPa. In one case, an increase in pore pressure of 20 kPa was recorded, but this may be assumed to have been very local in a more clayey portion of the soil mass or a measuring error.
The fills were moved about 30 days after their construction was started. The settlements in the points located beneath the highest portions of the fills at this time ranged from 31 to 74 mm and were randomly distributed. The same pattern appeared for all the fills and in four similar pre-loaded areas in the test field the average settlements ranged from 40 to 53, mm with an average of the total of 47 mm. These values refer to settlements after 1 month and they may be extrapolated to correspond to about 55 to 75 mm and 66 mm respectively after 10 years by using the Schmertmann (1970) time factor.
After the pre-loading, the buildings were constructed, Fig. 4.1.5. Only very small settlements occurred during construction and no settlement problems have been reported afterwards.
The method of raft foundation with pre-loading has been adopted for most of the surrounding area, both at Mjardevi Science Park and in the adjacent university area, and is now used on a more routine basis. The same fill material is being moved about in the area and placed well in advance on sites for planned buildings. This
Investigations and load tets in silty soils 39
Fig 4.1.5 Building under construction.
pre-loading is normally performed without special settlement observations or pore pressure measurements. At the same time, problems with loss of piles have been reported whenever driving of pre-cast piles has been attempted. In one such case, more than 50 m of drilling rods and a large number of probe tips were lost in the dynamic probing to predetermine the depth to end bearing strata. In the following piling operation, also a large number of the piles were damaged.
Dilatometer tests
Shortly after the first series of investigations, and when the pre-loading operations were already in progress, the Institute acquired its first flat dilatometer. The equipment was tested in a number of test fields with different soil conditions, among them the site at Mjardevi. Being the only available equipment, the dilatometer was handled very carefully and was pushed down with a hand operated drill rig until it encountered hard resistance against penetration. In spite of this, the dilatometer was found to penetrate to about the same depths as the previous static soundings, i.e. until it hit a large enough object embedded in the soil. No attempts were made to force it past such objects and tests were only made at a few points in order to gain experience of how the equipment worked in this type of soil.
The results at first appeared to be very eITatic and the soil classification based on existing charts in general yielded coarser material than had been established from
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the sampling operations, particularly in the upper levels of the profile. In the light of what expired from the gathered results from the investigations in all the test fields, a new classification chart was developed based on a material index corrected for overconsolidation ratio, (Larsson 1990). Using this chart, the soil classification for most of the profile agreed more closely with the actual soil conditions. However, at several levels the material index became very low and actually fell below the lower limits for the clay region in previous classification charts. The gathered experience shows that this is typical for the types of silty clays or clays with silt layers which become severely remoulded in all types of soundings and at insertion of in situ test equipment, and in which it is often very difficult to obtain undisturbed samples. Very low values of the material index may also be found in organic soils, but in this particular case this possibility could be ruled out directly based on geological considerations, Figs. 4.1.6 - 7.
Because of the disturbance, very low values of most other parameters, such as undrained shear strength, overconsolidation ratio and particularly compression modulus, were evaluated in the zones where the soil could be assumed to have been more or less remoulded at the start of the test. However, since these zones are easily identified, it is possible to make a better estimate using the overall picture from the less affected zones, together with empirical relations. The measurements in all zones not obviously disturbed indicated a certain overconsolidation with an estimated overconsolidation ratio of 1.5 or higher. In overconsolidated cohesive soils, the modulus in the overconsolidated stress range is often estimated as a direct function of the undrained shear strength. From the dilatometer tests, a value of the undrained shear strength is estimated and this value has been found to be less affected by disturbance at insertion of the dilatometer than the other parameters.
Rules for how the moduli in overconsolidated clayey soils can be estimated from dilatometer tests in this way were presented by Larsson (1990). When these rules were applied to the results from Mjiirdevi, they were found to yield approximately the same moduli as the ordinary interpretation in undisturbed layers and higher values in the disturbed zones, Fig. 4.1.8. A check against the results from the oedometer tests also indicated that the empirically estimated values were in the right range. The procedure for evaluating the dilatometer tests in this and other difficult soil profiles is described in further detail in Chapter 6.
Investigations and load tets in silty soils 41
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Compression modulus, MPa
0 10 20 30 40
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Fig. 4.1.8 Estimated moduli from dilatometer tests at Mjardevi.
CPT tests and weight sounding tests.
Recently, in connection with the current project, supplementary CPT tests and weight sounding tests have been performed. The weight sounding tests were performed at two points and indicated a heterogeneous soil profile with a weaker layer between 2 and 4 m depth. The ordinary tests both stopped at 8-9 m depth. One of the soundings could be advanced further after using blows to pass large objects at this level and then again at 12 m depth until it finally had to be stopped at 14 m depth, Fig. 4.1.9.
The CPT tests were also performed at two points and the cone penetrated down to 9 and 11 m respectively until the tests had to be terminated, Fig. 4.1.10. For both weight sounding tests and for CPT tests, the ability to penetrate in this type of soil was thus about the same as for the other types of soundings and push-in equipment,
Investigations and load tets in silty soils 45
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