Abstract of Special Research Report (SRR-No.14)

National Institute of Occupational Safety and Health, Japan

Prevention of Trench Failure Accidents in Exacavation Works


Katsunori OGAWA

: The accident associated with earth failures (e.g. trench failures and slope failures) is one of the three major accidents in construction work. These accidents occasionally involve serious accident in which more than 3 workers are injured. The numbers of accident in construction work have been gradually decreasing, yet they still account more than 1000 accidents in every year. Among them, trench failures and slope failures are major causes of accidents that account for 6-8% of accidents in construction work.
    In recent years, construction work has been increasing in number because of development of infrastructures as water words, etc. In addition, excavation work is very common to construction works since most of construction works have to be performed on the ground. For these reasons, hazardous situations due to both trench failures and slope failures during excavation work have been especially increasing. Considering these situations, it is necessary to establish the effective countermeasures for preventing trench failure and slope failure.
    According to the investigation of fatal accidents caused by earth failures, almost 40% of these accidents occurred during trench excavation work. So, this report focused on prevention of trench failure accidents.
    Main objectives of this research project are to understand the situation of accident in trench excavation works, to clarify the characteristics of ground vibration induced by construction machines, to investigate the effects of such vibration on geotechnical characteristics of soil and to understand of failure mechanism of trench.
     For this purpose, comprehensive research work have been carried out as follows:
  (1) Analysis of fatal accidents caused by trench failures.
  (2) Field test to measure the ground vibration and earth pressure induced by construction work.
  (3) Effects of cyclic loading due to construction machinery on geotechnical characteristics of soils.
  (4) Deformation and failure behaviors of trench in geotechnical centrifuge model tests.

Analysis of the Fatal Accident Caused by Trench Failure

Yasuo TOYOSAWA, Noriyuki HORII and Satoshi TAMATE

: Trench failures cause a considerable number of deaths and injuries in Japan. For the purpose of establishing countermeasures, it is necessary to understand the basic information about fatal accidents due to trench failure. Little attention has been given to the fatal accidents due to earth failure, especially on the relations between the failure patterns and cause of death.
    90 fatal accidents in trenches were examined with close examination of the situations surrounding these accidents which occurred during the period 1986 to 1988 in Japan.
    The examination revealed the following characteristics of the fatal accidents due to trench failure :
  (1) Concerning the cause of death, suffocation accounted for 31%, pressure 21%, fractures 17% and the rupture of internal organs 13%. In each case, each worker who died experienced soil pressure on their chest or abdomen.
  (2) With regards to the buried portion, about 60% of the workers who died were not entirely buried. The percentage of the workers who were buried entirely accounted for 31%.
  (3) Trench failures could be divided into four types. They are ; [1]Type-1 Failure due to surface slip, [2]Type-2 Failure by toppling, [3]Type-3 Failure by sliding or rotation and [4]Type-4 Failure by falls or block failure.
  (4) Failure occurred as a clod or block of earth. The clod or block of earth kept its shape until it hit the worker. Because of the narrow space in trench, the worker could not escape and was trapped between the clod of earth and the trench wall.
  (5) 85% of the fatal accidents were related to back-fill. In other words, fatal accidents occurred where the soil has been disturbed due to earlier earthwork.
  (6) Nearly 80% of the accidents involving small-scale trench failure occurred when the height of the trenches was less than 3m. 23% of the accidents happened in trenches of less than 2m depth.
  (7) Nearly 30% of the workers died during the work for installing and dismantling trench supports. A safety procedure of installing and dismantling trench supports that dose not require workers in the trench needs to be developed.

Field Tests to Measure the Ground Vibration and Earth Pressure Induced by Trench Excavation Work

Satoshi TAMATE, Noriyuki HORII and Yasuo TOYOSAWA

: A lot of slope failure accidents have been occurring in every year during the carrying out of trench excavation work. Some of these accidents were caused by vibration induced by the movement of construction machinery such as bulldozers and backhoes. These vibration may have been the cause of the decrease in ground foundation strength. Generally, slope stability in trench excavation is calculated using the static state conditions. To estimate foundation strength during excavation, it is necessary to make sure the influence of vibration caused by the movement of construction machinery and pile driving work and the earth pressure change acting on sheet pile with excavation. Therefore, field tests were carried out to investigate all these factors at the RIIS testing field.
    A backhoe and a bulldozer were used for tests. In order to measure the vibration, acceleration sensors were set up on the ground surface, at a depth of 1m, 2m and 3m. These tests were performed under the conditions of low and high speed passing in addition to the different distance between acceleration sensors position and passageway for each construction machinery. Regarding the pile driving tests, sheet piles were driven by use of a crane and a vibro-hammer. Ground vibrating acceleration were measured with driving of sheet piles. Fourier analyses were performed to investigate the frequency characteristics of transmitted ground vibration. In order to measure the earth pressure, three earth pressure sensors were used. These sensors were attached on the surface of one sheet pile.
    The results from field tests are summarized below ;
  (1) The predominant frequency of vibration induced by the movement of construction machinery was distributed from about 20 to 30 Hz.
  (2) The vibrating acceleration caused by the movement of construction machinery was about 10 gal on ground surface and 3 gal at a depth of 3m.
  (3) The predominant frequency of vibration induced by the pile driving work was distributed from about 10 to 20 Hz.
  (4) The vibrating acceleration on the ground surface caused by the pile driving work tended to be large in case driven depth of the sheet piles were shallow. The horizontal component of the vibrating acceleration tended to be small in comparison to the vertical component with a increase of ground depth.
  (5) The earth pressure acting on sheet piles decreased with excavation.

Effects of Cyclic Loadings Due to the Construction Machinery
on Geotechnical Characteristics of Soils

Noriyuki HORII, Yasuo TOYOSAWA and Satoshi TAMATE

: When clay layers are subjected to transient loadings such as induced by an earthquake or traffic, excess pore water pressure are generated. Due to the generally low permeability of clays, such pore water pressures persist for some time after the cyclic events. An understanding of the behaviour of soils subjected to subsequent undrained loading is necessary not only for the safety design of structures but also for the safety of the working environment of construction workers under such conditions.
    The proper modelling of such soil behaviour requires, first of all, a better insight into the actual cyclic loading imposed on the soil and, secondly, an understanding of the effect of such cyclic loading on the subsequent undrained shear characteristics of soil. Towards this ultimate objective, the characteristics of the loading imposed by construction machinery on the soil during a construction process were measured on site and discussed in the separate paper in this Reports. In this study, laboratory tests have been carried out to examine the effect of cyclic loading on subsequent undrained shear behaviour.
    Samples of kaolin clay were first subjected to uniform cyclic loading with frequencies varying between 0.1 Hz and 10 Hz and with single strain amplitude varying between 0.1% and 0.4%. The subsequent response of the clay under monotonic loading in triaxial apparatus was examined and the results of some of the laboratory measurements are also discussed in this report.
    Main results obtained in this study are summarized as follows ;
  (1) For frequencies up to about 1 Hz, the accumulated excess pore water pressure measured is the fully developed value, but for frequency of 10 Hz, a long waiting time is required for the full development of the accumulated excess pore water pressure.
  (2) Within the range of strain amplitude investigated, there seems to exist a threshold amplitude strain level of about 0.2%, below which the accumulated mean excess pore water increases very slowly with εcyc but above which the changes are appreciable.
  (3) Below the threshold strain amplitude, the fully developed excess pore water pressure reduces with increasing loading frequency.
  (4) The equivalent stiffness of soil decreases with repeated loading cycles but the rate of degradation seems to reduce with increasing strain amplitude.
  (5) previous cyclic loading history leads to a reduction in undrained shear strength as well as in the undrained stiffness at large strain levels.

Deformation and Failure Characteristics of Trench Excavation in Centrifuge Tests

Yasuo TOYOSAWA, Noriyuki HORII and Satoshi TAMATE

: Accidents due to trench failure frequently occur on construction sites. Statistical analysis shows that more than 90% of fatal accidents during trench excavation in Japan, occurred as a result of collapse of un-supported or insufficiently supported trench walls.
    To study un-supported trench deformation characteristics and failure mechanisms due to excavation, centrifuge model tests on preconsolidated kaolin were undertaken.
    Two types of model test were carried out using centrifuge. In the first series of test, trench excavation was simulated by the in-flight draining away of a heavy liquid from a trench at the predetermined acceleration. In the second series, the acceleration increased until failure occurred in the model vertical cut.
    The progress of deformation during excavation was analyzed from the deformation of targets on the model measured from a sequence of photographs taken at intervals of about 0.3 seconds until the model collapsed. From the calculated strains the development of γmax up to collapse was evaluated.
     Based on the results of centrifuge tests simulating excavation, the following conclusions were obtained. :
  (1) The similar circular arc failure surfaces were observed in all vertical cuts (SLV test series), irrespective of the strength of the model ground. On the other hand, for trench excavation (TRE test series), both wedge type failure and circular arc failure were observed.
  (2) Photographic measurements showed that prior to failure, the strains were concentrated around the toe in vertical cuts (SLV tests). However, in trench excavation (TRE tests), the strains were observed over a large area behind the trench wall.
  (3) For both trench and vertical cuts, at the onset of failure, the strains were concentrated around the toe of the slopes and the shear band appeared in this area first.
  (4) Using the strength parameters from triaxial compression tests on K0 specimens taking into account the effect of strain rate, the simplified Bishop's method based on a circular failure surface gave excellent agreement with the observed failure mode in the centrifuge tests.

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