Mtodo Mononobe Okabe Okabe (1926) y Mononobe y Matsuo (1929), desarrollaron las bases para un anlisis pseudoesttico que evala las presiones ssmicas que desarrollan los suelos sobre muros de contencin. Este mtodo es conocido como el mtodo Mononobe Okabe. El mtodo considera la aplicacin de aceleraciones pseudoestticas, tanto en direccin horizontal como vertical, que actan en la cua activa considerada en el mtodo de Coulomb. Las magnitudes de estas fuerzas pseudoestticas, horizontales y verticales, estn relacionadas con la masa de la cua, ecuaciones 2.17 a 2.20. donde: ah = aceleracin horizontal, av = aceleracin vertical, kh = coeficiente ssmico horizontal, kv = coeficiente ssmico vertical, Fh = Fuerza pseudoesttica horizontal, Fv = Fuerza pseudoesttica vertical, g = aceleracin de la gravedad, Wi = Peso de la cua. El mtodo Mononobe Okabe puede ser adaptado para su utilizacin en estabilidad de taludes considerando a las fuerzas pseudoestticas, calculadas mediante las ecuaciones 2.19 y 2.20, al considerar el peso de la dovela en estudio dentro del equilibrio de fuerzas y/o momentos que emplee el mtodo utilizado para el clculo del factor de seguridad. Consolidacin de suelos Se denomina consolidacin de un suelo a un proceso de reduccin de volumen de los suelos finos cohesivos (arcillas y limos plsticos), provocado por la actuacin de solicitaciones (cargas) sobre su masa y que ocurre en el transcurso de un tiempo generalmente largo. Producen asientos, es decir, hundimientos verticales, en las construcciones que pueden llegar a romper si se producen con gran amplitud. Desecacin Una desecacin en suelos es el proceso de formacin de grietas poligonales en el suelo compacto al perder el agua y la humedad en perodos de sequa. Las grietas de desecacin afectan mayormente a los terrenos arcillosos. La contraccin de estos suelos puede dar lugar a la formacin de grietas de hasta 4 cm de anchura y ms de 1

m de profundidad. La accin profunda de los cambios de temperatura y otros agentes atmosfricos, pueden ser entonces la causa de deslaves ulteriores. Tambin representan un papel en la formacin de suelos poligonales y en los fenmenos de reptacin. Precarga El mtodo de precarga consiste en someter al terreno a una presin aplicada en la superficie antes de colocar la carga estructural, con la finalidad de aumentar la densidad del mismo, disminuir los asientos que pueden originar posteriormente las construcciones y, en consecuencia, aumentar la capacidad portante. La aplicacin de la precarga suele realizarse mediante rellenos de tierra o mediante la colocacin de tanques de agua sobre la superficie. Estos sistemas se suelen utilizar para acelerar la consolidacin de terrenos cohesivos blandos, aunque tambin se aplican para la mejora de rellenos, limos orgnicos e inorgnicos, turbas, etc. Se requiere un estudio minucioso del terreno que se va a consolidar para poder predecir la magnitud de los asientos y el tiempo de consolidacin. Para ejecutar este mtodo de compactacin, se extiende sobre el terreno que se desea compactar una carga con un peso que tiene un valor de 1 2 veces el de las cargas que transmitir al terreno el edificio que se va a construir. La consolidacin puede hacerse por zonas, utilizando las tierras existentes, que son transportadas de uno a otro sitio, o en su caso, trasladando los tanques de agua. A este sistema se le suele denominar mtodo de la duna mvil.

T4.4.2 Assessment of liquefaction potential combining field data and laboratory

A methodology that combines field and laboratory data has been developed and widely

used in the standard design practice in Japan (Port and Harbour Research Institute, 1997;

Yamazaki et al., 1998; Ministry of 'Transport, Japan, 1999). In this procedure, the evaluation

of liquefaction potential is performed in two steps. The first step uses the grain size

distribution of ground and counts the number of strokes of the standard penetration test

(SPT N-value) of a soil layer. If the results of this first step are located near the border

between the conditions of liquefaction and non-liquefaction, then the first step can be

supplemented with the second step, cyclic triaxial tests.

(1) distribucin de tamao de grano y N - valor SPT (primera etapa)

Initially, a soil is classified according to the areas of gram size distribution shown in Fig. 33.

T4 is considered not liquefiable soil if the grain size distribution is out of the liquefaction

zone at the possibility figure T4.33. With floor distribution curve grain size falling within the

liquefaction zone possibility, liquefaction potential is evaluated based on the following

procedure.

a) Equivalent N-value

An equivalent N-value, 65, is calculated by correcting for the effective overburden

pressure of 65 kPa using the following expression

65 =31 0.018(

65)

0.0041( 65) + 1.0

(T4.25)

Soil with low coefficient of uniformity Uc

Fig. T4.33. Grain-size distribution of soil having the possibility of liquefaction (Ministry of

Transport, Japan, 1999).

where 65 = measured SPT N-value of a soil layer (obtained with typical energy efficiency

in Japan), = effective overburden pressure of a soil layer (kPa), calculated with respect

to the ground surface elevation at the time of the standard penetration test.

b) Equivalent acceleration

An equivalent acceleration, , is estimated by the following equation

= 0.7

where = maximum shear stress in the layer obtained from site response analysis,

= effective overburden pressure calculated, this time, with respect to the ground surface

elevation at the time of earthquake.

c) Liquefaction potential assessment for clean sands

For clean sands (i.e. soils with fines content Fc less than 5%), liquefaction potential is

determined by using the chart in Fig. T4.34. The zone to which a soil layer belongs is

determined from the equivalent N value, 65 , and the equivalent acceleration, .

Liquefaction potential is assessed as follows:

- Zone I has a very high possibility of liquefaction. Decide that liquefaction will

occur. Cyclic triaxial tests are not required.

- Zone II has a high possibility of liquefaction. Decide either to determine that

liquefaction will occur, or to conduct further evaluation based on cyclic triaxial

tests.

Equivalent acceleration aeq (gal)

Fig. T4.34. Liquefaction potential assessment based on equivalent acceleration and

equivalent N- values (Ministry of Transport, Japan, 1999).

(T4.26)

- Zone III has a low possibility of liquefaction. Decide either to determine that that

liquefaction will not occur, or to conduct further evaluation based on cyclic

triaxial tests. When it is necessary to allow a significant safety margin for a

structure, decide either to determine that liquefaction will occur, or to conduct

further evaluation based on cyclic triaxial tests.

- Zone IV has a very low possibility of liquefaction. Decide that liquefaction will not

occur. Cyclic triaxial tests are not required.

d) Correction for silty or plastic soils

For a soil with fines content higher than 5%, the SPT N value is corrected according to the

following three cases:

Case 1: The plasticity index < 10 or the fines content 5 < < 15%.

An equivalent V-value obtained from Eqn. (T4.25) is corrected through the following

equation:

65 = 65/

where: 65 = a corrected equivalent N-value, = a correction factor based on the fines

content (Fig. T4.35). Using the corrected equivalent V-value, the liquefaction potential is

evaluated in the same manner as c) by referring to Fig. T4.34.

Case 2: The plasticity index 10 < < 20, and the fines content > 15%. Two

corrected equivalent N-values are calculated as follows:

65 = 65/0.5

65 = +

= 8 + 0.45( 10)

(T4.27)

(T4.30)

(T4.29)

(T4.28)

Fig. T4.35. Reduction factor for critical SPT N- value based on the fines content (Ministry of

Transport, Japan, 1999).

The two corrected equivalent IV-values are plotted in Fig. T4.34 against the equivalent

acceleration, and the zone to which a soil layer belongs is determined as follows:

- When 65 is in Zone I, liquefaction potential is evaluated by Zone I.

- When 65 is in Zone II, liquefaction potential is evaluated by Zone II.

- When 65 is in Zone III or IV, and when NZ is in Zone I, II, or III liquefaction

potential is evaluated by Zone III.

- When 65 is in Zone HI or IV, and when NZ is in Zone IV, liquefaction potential is

evaluated by Zone IV.

Case 3: The plasticity index >20, and the fines content > 15%.

A corrected equivalent N-value is calculated using Eqns. (T4.29) and (T4.30), and then

liquefaction potential is evaluated using Fig. T4.34.

2) Cyclic triaxial test (2nd step)

When the liquefaction potential cannot be determined from the grain size distribution and

SPT A-value (i.e. for zones II and III), it is evaluated based on undrained cyclic triaxial tests

using undisturbed soil samples. Based on the relationship between the cyclic stress ratio

and number of cycles to cause liquefaction in the laboratory tests, the cyclic resistance

(1/) 1=20 for use in the liquefaction potential assessment is obtained by reading off

the cyclic stress ratio at 20 cycles of loading (Fig. T4.36). Using this cyclic resistance, the in-

situ liquefaction resistance is obtained using the following equation:

=0.9

(1 + 20)

3(1/) 1=20

(T4.31)

Fig. T4.36. Correction of laboratory liquefaction resistance for in-situ condition (Ministry of

Transport, Japan, 1999).

In this equation, the following corrections are applied

- Stress correction: The stress correction for in-situ (0) conditions from the triaxial

test condition (isotropic consolidation with mean confining pressure of ().

- Input motion correction: The correction for irregularity of earthquake motion (e.g,

impact type/vibration type) from the harmonic motion in the cyclic tri- axial test.

The correction factors are given as

Impact type input motion = 0.55

Vibration type input motion = 0.7

The seismic stress ratio = / is calculated through site response analysis. The

liquefaction potential (safety factor) is given as

=

(T4.32)

If < 1.0, the soil layer is expected to liquefy.

Laboratory data on pore-water pressure increase can be also obtained and used for design

of remediation methods against liquefaction (see Technical Commentary 6).