CHAPTER 13

CONSTRUCTION CONTROL

13.1

General

13.2

Backfill

13.3

Geosynthetics

13.4

Facing

 

13.1 GENERAL

The construction control ensures that GRS-RW project can be accomplished safely and smoothly within the planned time framework.

Construction control confirms the use of materials according to the design and specifications, to conduct model construction and to follow the specified construction procedure so that GRS-RW will be completed safely and smoothly.

Therefore, the planning, design and progress are compared from time to time during construction. The difference, if any, should be remediated or minimized as much as possible.

 

13.2 BACKFILL

The backfill soil should be compacted well so that the GRS-RW will be stable against external loads without exhibiting differential settlement.

The backfill soil control is managed through selection of appropriate backfill soil and thickness of soil layer based on the quality control or construction method control.

The are two suggested specifications for backfill soil control: Highway Earthwork Construction Specifications and Design Guidelines, Both published by the Japan Highway Public Corporation.

I. Highway Earthwork Construction Specifications, JHPC

The compaction control may be done by quality control or construction method control. These two approaches are different in certain aspects. The characteristics of individual approach should be understood, together with the project, such as its scale, soil conditions, and other site conditions, so that appropriate control method can be selected.

1. Control by Quality

a) Maximum dry density and optimum water content

The ratio between field maximum dry density and standard density obtained from compaction test (degree of compaction) should be compared based on the optimum water content to investigate if it is within a specified range. For dynamic compaction, methods I and II are available. Method I is suitable for backfill soil or subgrade whereas method II is for road foundation.

The field density is measured by sand replacement method (JIS A 1214). The nuclear method (RI device) is also used.

For quality control of road structure, the following values are normally used:

Degree of compaction: Method I, maximum dry density greater than 90%.

Water content during construction: water content on the wet side that gives optimum water content and 90% degree of compaction

b) Void ratio of air or degree of saturation based on construction water content

The strength of compacted soil and deformation characteristics should be kept within the range of design criteria. The void ratio of air and degree of saturation are determined from the following equations:

  (13.1)

  (13.2)

where

Va

: air void ratio (%)

Sr

: degree of saturation (%)

r d

: dry density

r w

: density of water

Gs

: specific gravity of soil

 

The void ratio of air is controlled to be less than 15%. Based on degree of saturation, the soil is judged to be appropriate or inappropriate. The upper limit of water content is related to the trafficability and the lower limit should satisfy the required design mechanical properties of soil.

This method does not require compaction test, but the density of soil has to be known. If the method using maximum dry density is less applicable, this method may be used.

c) Strength and deformation characteristics of soil

The strength and deformation characteristics of compacted soil are determined using, for example, SPT resistance, CBR, bearing capacity coefficients, etc.

These control parameters have the advantages of having strong correlation with performance of GRS-RW. The strength of soil after percolation of water is, however, not obtained. Thus, the method is applicable to good quality sand and gravel. This method is not recommended for other types of soil. It may be combined with the method using the void ratio of air or degree of saturation.

2. Control by Construction Method

This method is controlled through the items such as the type of compactor, number of blows, thickness of soil spreading, etc. The field compaction test is conducted to confirm the quality of backfill soil.

 

II. Design Guidelines, JHPC

1. Control by Quality

This approach is applicable to sandy soil, based on the void ratio of air. For the soil whose void ratio is difficult to be controlled, the density ratio (Dc) or a specially regulated value (Ds) is used. For soft and weak rocks that crush during compaction, the air void ratio is kept below 13%. If the air void ratio cannot be kept below this specified value, Dc or Ds may be used to control compaction after consulting with the supervisor.

a) General

The RI (radio isotope) device (nuclear method) is used for control. For small-scale construction of less than 30,000 m3, the sand replacement method may be used.

 

b) RI device

Only an approved RI device should be used. The use of RI device should be reported to the supervisor.

c) Test specimen

For the test specimen, physical properties of material, compaction characteristics, strength characteristics, etc., should be investigated to determine if the particular material is suitable for construction. The information will also be used to decide the construction and control methods.

d) Standard test and routine control test

The standard test is conducted to set the basis of quality control of construction. The routine tests are conducted to check if the constructed earth structure meets the standard quality as specified.

If the results of standard test differ from the properties obtained from material tests, i.e., a different type of material is indicated, the material tests should be repeated.

If the routine test does not meet the standard quality, construction has to be redone. The causes of failure to meet the quality should be identified so that it will not be repeated.

e) Model construction

The "model construction" is conducted prior to actual construction. Table 12.1 is referred to on the types of compactors used and the different components of structure. The main points in model construction are summarized below:

- to decide the standard construction method (number of spreading, thickness of spreading, type of compactor, number of passes, method of quality control, value of control parameters)

- to decide number of machineries

- to clarify precautions related to construction control

For control using sand replacement method

a) The sand replacement method is used in small-scale project, typically less than 30,000 m3.

b) Density measurement using sand replacement method (refer to JIS A 1214)

c) ~ e) same as above.

 

2. Control by construction method

a) General

This method is applicable to rock mass that cannot be controlled through quality. For rock mass, model construction is performed. Based on surface settlement, the type of machinery and number of passes are decided.

b) Material Testing

The physical properties of rock are investigated to see if construction method may be used.

c) Model construction

- to be conducted as early as possible before an actual construction

- the scale of model construction is decided based on discussion with the supervisor. The standard size is 30 cm along the direction where the machine moves and 20 cm wide (width of running per machinery = 10 cm, 2 machineries are used).

- the standard number of passes are 2, 4, 6, 8, 10, and 16.

- the thickness of construction is selected to be between 1 to 1.5 times the largest diameter. Sometimes, the large ones are removed.

- Two types of machineries are used and the results are compared.

- The settlement is measured after each pass.

d) Control value

The type of machinery is selected based on results of model construction. From the relationships between settlement and number of passes, the number of passes is selected to be larger than the point of curvature shown in the graph.

 

e) Routine quality control

The area of construction and the number of passes corresponding to the thickness of spreading obtained in a model construction are used to estimate the total time of compaction. The time required for compaction is compared with the time record. The recorded time should not be less than the required compaction time.

 

13.3 GEOSYNTHETICS

The geosynthetic should posses adequate long-term strength to render stability to the reinforced soil. Therefore, precautions are required to prevent geosynthetic from rupture or degradation during construction.

The geosynthetic is connected to the concrete facing, It functions with the soil as an integrated structure over long-term period.

To avoid rupture and strength degradation, the geosynthetics should not be exposed directly to the sun or to be bent.

During construction, the geosynthetics should not be damaged by gravel or soil that have sharp edges.

 

13.4 FACING

The concrete facing is connected to the geosynthetic so that reinforced soil constructed with the RRR method performs as an integrated structure. Controls are needed to ensure that relative settlement between the backfill soil and foundation would not occur.

In the RRR method of construction, no form is used during construction of concrete facing so that the wrapped-around geosynthetic will be connected directly to the facing and function as an integrated body.

The backfill soil should be well compacted so that relative settlement will not occur.

If consolidation settlement of facing foundation is anticipated, construction time should be adjusted until the foundation exhibits adequate stability.

The instrumentation may be conducted for construction control and post-construction monitoring of GRS-RW.

 

  

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