CHAPTER 4

MATERIALS

4.1

Geosynthetics

4.2

Facing

4.3

Backfill Soil

 

4.1 GEOSYNTHETICS

The geosynthetics used to construct GRS-RW should posses adequate tensile strength and long-term durability, such as resistance to environmental conditions and creep.

The geosynthetics are installed to increase the stability of soil structure by acting as tensile reinforcements. Thus, they should have adequate factors of safety under different loading conditions (dead load, live load and earthquake). By considering the variations of geosynthetic materials, the allowable design rupture strength under different loading conditions is denoted as Tk. This standard strength is reduced further by considering installation damage, creep, and other factors, as discussed below:

1. Design Rupture Strength, Tk

Considering the variation in material properties, the rupture strength used in design is obtained from the following equation:

(4.1)

where

Tave

: average value of rupture strength (kN/m)

(if failure strain exceeds 15%, the tensile stress at 15% is adopted)

s X

: standard deviation

a

: a coefficient (commonly taken as 0.67)

 

2. Reduction factor considering deterioration in alkaline environment, a 1

In the RRR method of constructing GRS-RW, the geosynthetic layers are wrapped and keep in direct contact with the concrete facing. Thus, a reduction factor is used considering its deterioration under alkaline environment. The freshly placed concrete has a pH as high as 12, but it neutralizes with time. The portion of concrete that is in direct contact with geosynthetic layers is expected to neutralize within a year or two. a 1 is obtained as the ratio of the strength of specimen after having been submerged in a pH12 solution for 700 days to the strength obtained from a standard test.

3. Reduction factor considering installation damage, a 2

There is a high possibility that the geosynthetic will be damaged during construction when gravel soils are used as backfill. a 2 is adopted to account for possible strength reduction due to construction damage. For the geogrid, it is defined as the ratio of the number of intact threads after having been subject to some specified construction procedures to the total number of threads tested.

4. Reduction factor considering creep, a 3

Under normal loading conditions, there is a potential for creep to develop and leading to a large deformation of geosynthetic. The strength of geosynthetic has to be reduced by a factor to represent a stress level where creep will not be a concern. It is defined by the ratio of the strength at which creep rate does not exceed 3.5´ 10-5 per hour, for a period of 500 hours, to the standard test strength.

5. Reduction factor considering impact loading, a 4

For extreme loading, such as that subject to several cycles of earthquake loading, the potential of strength reduction is account for by using this factor. The impact loading strength is obtained as the strength that does not lead to failure or which does not exceed 15% axial strain under three load repetitions at a frequency of less than 1 Hz, 24 hours after applying a load of magnitude 30% the standard test strength. The reduction factor is obtained as the ratio of the impact test strength to that of the standard test.

6. Reduction factor considering repeated loading, a 5

A reduction factor has to be applied to the strength considering repeated loading. It is obtained as the ratio of cyclic strength to that of the standard test. The cyclic strength is obtained after loading a specimen to 1.5 million cycles at a frequency of 20 Hz, with an amplitude of 16 kN (100 kPa) superimposing a sustained load of 16 kN (100 kPa).

 

7. Design rupture strength after applying reduction factors

Under different loading conditions, with consideration to the strength reduction by various factors, the design rupture strength of a geosynthetic is obtained from the following equations:

Dead load:

  (4.2)

Live load:

  (4.3)

Earthquake load:

  (4.4)

where

Ta

: recommended strength of the manufacturer (commonly 80% of Tk)

Tk

: standard design rupture strength

a 1

: reduction factor for deterioration under alkaline environment

a 2

: reduction factor for installation damage

a 3

: reduction factor for creep

a 4

: reduction factor for impact load

a 5

: reduction factor for repeated loading

The live load is considered when the wind and impact loads will be a concern. It is not required in most designs.

As a reference, for a geogrid commonly used for GRS-RW construction in Japan, made of vinylon and coated with polyvinyl chloride, the following reduction factors are obtained: a 1= 0.98, a 2= 0.93, a 3= 0.7, a 4= 0.9, a 5= 0.8. In a series of tests conducted at the Japan Railway Technical Research Institute, the following factors are obtained for some commonly used geosynthetics under different loading conditions: a j= 0.5 - 0.6, a i = 0.65 - 0.95, a e = 0.65 - 0.95.

The strength as discussed above is for the load applied to the machine direction of a geosynthetic and based on the geosynthetic material alone. When used with the soil, the stress transfer is affected by the soil-geosynthetic interaction. For applications other than above-mentioned conditions, the geosynthetic has to be selected considering the type of soil, aperture, direction of loading, friction coefficient, among others.

The reduction factors discussed above are for typical types of construction, environment and loading conditions. For unusual constructions, such as the use of cement-treated soil or acidic soil, and GRS-RW without a facing or if the facing is not installed over an extended period of time, special consideration to the reduction factors is required. Since the performance of reinforced soil depends on the interaction between the geosynthetic and soil, it is highly recommended that pullout test to be conducted. This should be referred to the Design Manual of the Public Works Research Institute, Ministry of Construction.

4.2 FACING

The facing should exhibit adequate flexural rigidity that contributes to additional long-term stability of the GRS-RW system.

 

The quality of concrete and steel reinforcement used in the facing construction should follow that of the Highway Bridge Specifications. They are elaborated in Section 5.4.

 

 

4.3 BACKFILL SOIL

The soil used as backfill in the GRS-RW should be easy to compact, stable against external loads, and should not deform largely after construction.

1. Requirements

The use of on-site soil is more economic than importing soil. The backfill soil should be compacted and /or stabilized to meet the following requirements in the long run.

a) The stability should not be affected by traffic load, earthquake, rainfall, etc.

b) Large and/or differential settlement should not occur

The traffic load acting on the subgrade of pavement can be extremely large, thus the materials used in the subgrade should posses required stiffness.

 

 

2. Recommended Conditions

The soil used in the subgrade of pavement, satisfying above-listed requirements, includes well-graded sand, gravel or coarse soil of little fine contents. The sandy on-site soil may be used for road construction without problems. However, if the soil is susceptible to volume change due to penetration of water, the use should be careful examined.

If a planar geosynthetic sheet is used, a large frictional force is mobilized between the reinforcement and soil. With a non-woven geotextile having drainage capability, the high-water content cohesive soil may be used. However, the case histories of GRS-RW constructed using cohesive soil is still very limited at this stage. The use of cohesive soil in GRS-RW construction would be supported by research and additional case histories.

The following types of soil should not be used as backfill:

a) expansive soil and rock, such as bentonite

b) rock that absorbs water and fractured easily, such as mud stone

c) highly organic soil or soil exhibiting large compressibility

d) frozen soil

For mud stone, if the results of slacking test (specified by the Japan Highway Public Corporation) give less than 50% slacking ratio, it may be used subject to a proper construction control (see Chapter 13).

The rocks or rock-like materials may damage the geosynthetic during construction. Only geosynthetics having strong resistant against abrasion or geosynthetics whose configuration allows it to interlock effectively with rock may be used. The use should be accompanied by applying a larger reduction factor against construction damage and with additional precautions during construction. The rock of diameter greater than 10 cm is not recommended for construction in the reinforced soil zone.

 

3. Problematic Soils

The sandy soils from volcanic ash (such as shirasu) and decomposed granite (masado) are stable under natural states, but they may not be suitable as backfill material.

Note: These two types of soil are typical to the Japanese geological conditions. Thus they are not elaborated in this translated version of Manual. There are different types of problematic soil in each country that the engineers should be aware of.

 

4. Soil Stabilization

If the soils do not satisfy the requirements as stated in Section 1, they may have to be improved. The following items, a) to d) are to be considered before selecting for methods of soil treatment. If lime/fly ash and cement are to be used for treatment, possible effect of alkalinity and heat on the geosynthetic should be considered.

a) Cost - Check if import soil will lead to a lower cost compared to that of soil stabilization. The cost of import soil as well as the cost involved in disposing on-site soil should be compared with the cost of soil treatment (material cost, construction, etc.).

b) Environmental conditions - The environmental issues and transportation routes related to disposal of on-site soil should be considered. Cement and lime have an initial pH as high as 10. Although it will neutralize gradually, the impact on environment during the initial stage should be account for.

c) Schedule - The transportation of soil, capability of construction equipment, required working days, etc., should be considered.

d) Construction conditions - The conditions and volume of soil are among items to be considered.

 

5. Methods of soil treatment

The methods include:

a) Stabilization using cement

b) Stabilization using lime/fly ash

c) Stabilization using geosynthetic materials

In addition, the soil may be improved by mixing with good quality soil.

 

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