6585845010

29

Czechoslovakia

sockets, steel rode, and so on. This taak was not achieved auccessfully.

Higher dead load of the whole eąuipment haa usually caused nigher point pressure on the sampling bit snd an easier fail-ure of the soil around the tip. Conse-ąuently there was uncertainty in the initial break of the bit in the soils below the borehole bottom. Other difrer-ent types of winches and the uncertainty in brake forces led to loss of energy during the so called free fali of the drop hammer. Even after three types of automatically working jawa for drop ham-mer locking had been developed, the re-suita were still distorted by errors.

Some of them were caused further when testing boreholes of different diameters and by the disturbance of soils under the casing shoe or by improper cleaning of the hole bottom. Special problems have arisen when testing below water table.

It was found out that by using SPT meth-ods in cased boreholes there was too much to be standardised in order to obtain results accurate enough to exclu-de other testing methods. Therefore further use of SPT methods was directed mostły to the determination of sand den-sity and cohesive soil compactness when tests were carried out directly at the bottom of test pits or on the surface of subsoil in the excavations. For these purposes SPT samplers with pin guided drop hammers were used. A relatively good relation was achieved between SPT and piąte loading tests in case the den-sity or compactness of soil deposits was rather high. In relatively weak or looae soils the total weight of the SPT eąuipment with the standard energy was too high and the distinguiahability was too Iow. In some cases the SPT eąuipment penetrated the soils by its own weight. On the other hand when SPT eąuipment was used on the surface of weathered bedrock and the reąuired number of blows exceed-ed 50, the apparatus was many times dam-aged either on the spoon bit or in the threads below the impact-cap.

As standard penetr8tion tests were directed to at an eyaluation of the densi-ty of granular soils, the problem of the influence of different moisture content was studied (Drozd, 1964). It has been found that different moisture contents in fine-grained soils of the same densi-ty can cause great differencies in the resulting blow number. This number de-pends on the effective stress between grains. It can be either increased owing to the apparent cohesive strength due to the capillary forces and water under-pressure at dilating dense soils or, on the other hand, decreased owing to the uplift when testing below water table and owing to the increase of neutral water pressure in loose and medium dense soils. The same phenomena were observed with the underpressure or overpressure of air captured in the yoids of relati-vely Iow permeable soils. These soils could be ąuite dry.

Further it was found out that the great decrease of blow number due to the liq-uefaction of loose water-saturated sand is not caused by the impact which cre-ated a certain seismic effect but just by ordinary perforation of the test tool through the soil and by local change of volume. This shows that the decrease in penetration resistance can be observed even at static penetration tests.

Certainly all these results have led to a certain decrease of the first enthusi-astic wave for using penetration methods and their widespread use. It became elear that próbably morę promising results could be obtained either by intro-duction of static penetration methods or of methods for which some manufacturer could assure greater production of a cheap and simple penetration eąuipment.