chapter5

CHAPTER 5

INTRODUCTION TO DESIGN

5.1	Basic Principle of Design
5.2	Design Loads
5.3	Soil Parameters
5.4	Facing Allowable Stresses

5.1 BASIC PRINCIPLE OF DESIGN

The following analyses have to be conducted in designing geosynthetic-reinforced soil retaining walls (GRS-RW):

1. Internal Stability Analysis

2. Stress Analysis of Facing

3. External Stability Analysis

4. Settlement Analysis of Foundation

In the design of GRS-RW, the internal and external stability analyses are conducted by assuming a failure surface and calculating for the ultimate resisting forces. The limit equilibrium approach is conducted to ensure that the system satisfies a prescribed factor of safety. Therefore, the deformation under service conditions is not accounted for. In the RRR method of construction, geosynthetic layers are closely spaced such that the deformation of GRS-RW is minimized. This restraining effect has been confirmed through field measurements.

The stiffness of geosynthetic reinforcement is considered small and a large deformation will result if the system is designed using its ultimate tensile strength. The soil might have reached its residual states at the ultimate strength of geosynthetic reinforcement. Thus, the design has to be conducted by allowing a minimum deformation of the geosynthetics.

1. In the internal stability analysis, failure of reinforced soil (through the toe or slope) due to the breakage and pullout of geosynthetic reinforcement is considered.

2. The facing stress analysis is conducted to ensure that it has an adequate stability to resist earth pressure exerted by the external load and dead weight of the soil.

3. The external stability analysis provides an adequate safety of the GRS-RW system against failure through its base.

4. The magnitude of settlement and settlement rate are calculated so that their effects on the wall facing and geosynthetic reinforcement may be investigated.

5.2 DESIGN LOADS

1. The design loads are decided based on the application, size, and type of structure; climatic, environmental, and construction conditions, among other factors.

2. The loads are considered for the following conditions:

a) Dead Load

b) Live Load (if applicable)

c) Earthquake

d) Loads present during Construction (if applicable)

The following loads are considered in designing GRS-RW. They are decided based on several conditions:

1. Dead-Load Conditions:

a) Dead weight of reinforced soil

The dead weight of reinforced soil is calculated as the product of the bulk unit weight g _t and total volume of backfill soil (excluding pavement). g _t is determined from in-situ measurement, but can normally be estimated using the values given in Table 5.1.

Table 5.1 Unit Weight of Backfill Soil

Type of Soil	Unit Weight, g _t (kN/m³)
Gravel	20
Sandy Soil	19

In the external stability analysis, the dead weight of facing is included.

b) Live load

A surcharge of 10 kPa is used to account for highway constructed above the GRS-RW (see Figure 5.1).

c) Dead weight of other structures attached to GRS-RW

The deadweight of sound wall and other affiliated structures are considered.

2. Live-Load Condition

a) Wind load

The specifications of the Japan Highway Public Corporation consider the lateral pressure acted by the wind to be 1.5 kPa.

b) Impact load

The impact load is considered with reference to the Guardrail Design Guidelines of the Japan Highway Public Corporation (Details are not given in this Manual).

3. Earthquake Loading

The effect of earthquake is considered by applying inertia force to the backfill soil and facing according to the type of structure, regional location and ground conditions (so-called modified seismic coefficient method).

For GRS-RW used in highway application, a standard horizontal seismic coefficient of k _o = 0.15 is used in design. The design seismic coefficient is then decided by applying the regional and ground correction factors.

4. Rainfall

The effect of rainfall is normally neglected. This is because the drainage design has been conducted to allow for a rapid runoff during heavy rainfall. The shear strength of the soil used in design is based on a value smaller than the actual shear strength. This reduced value should have compensated for possible strength reduction during rainfall.

The stability of GRS-RW during rainfall has been confirmed through full-scale field tests.

The Highway Earthwork Specifications do not require seismic design for a retaining wall less than 8 m high. However, seismic design is required for GRS-RW even if it is of height less than 8 m. The GRS-RW is designed to resist loading under the following conditions, but typically for dead- load and earthquake conditions.

a) Dead-load conditions: dead weight

b) Live-load conditions: dead weight + live load

c) Earthquake conditions: dead weight + earthquake load

d) During constructions: earthquake load is typically excluded.

The load during construction, including that of roller compactor (20 kN) and dump truck (20 kN), may be satisfied by specifying a minimum geosynthetic reinforcement length (L ³ 35% wall height or 1.5 m, whichever the greater). A specific design is not required.

Loading beyond the above-mentioned magnitude would require additional considerations.

5.3 SOIL PARAMETERS

The parameters of backfill and foundation soils used for design are decided based on the results of in-situ soil investigation.

1. Soil Parameters - Backfill

The soil parameters and representative design values are given in Table 5.2. Note that these values are subject to the conditions that the soils are well compacted and the drainage facilities are properly designed.

The internal stability analysis of GRS-RW is considered similar to the earth pressure calculation of the conventional retaining wall. Thus, the values given in Table 5.2 are similar to those used in the conventional retaining wall design.

Table 5.2 Strength of Backfill Soil

Type of Soil	Internal Friction Angle (^o)	Cohesion
Gravel	35	-
Sandy Soil*	30	-

*: for clean sand, the friction angle of gravel may be used.

2. Soil Parameters - Foundation

The parameters of foundation soil are decided based on in-situ soil investigation.

For large-scale construction, construction with specific conditions, important structures, construction on soft or clayey foundation, detailed soil investigation is required with a careful determination of soil parameters.

The parameters of foundation soil, c and f , are determined from the SPT N-value, unconfined compression and triaxial compression test results, among others. They are normally based on the Highway Earthwork Specifications, as shown in Table 5.3.

Table 5.3 In-Situ Soil Investigation and Soil Parameters for Design

(a) Laboratory Testing

External Load:

Design Parameters	Soil Testing¹
Unit weight, g	test to determine unit weight of Soil
Shear strength parameters, c and f	triaxial test, unconfined test, etc.
Earth pressure coefficients, K_H and K_V	soil classification tests (estimated based on soil type)

Bearing Capacity of Foundation:

Design Parameters	Soil Testing¹
Shear strength parameters, c and f	triaxial test, unconfined test, standard penetration test (estimated from N-value), etc.
Allowable bearing capacity, q_a	soil classification tests (estimated based on soil type)

Stability Analysis:

Design Parameters	Soil Testing¹
Unit weight, g	test to determine unit weight of Soil
Shear strength parameters, c and f	triaxial test, unconfined test, etc.

Consolidation Settlement:

Design Parameters

Soil Testing²

Compression index, C_c

Coefficient of consolidation, C_v

Coefficient of compressibility, m_v

test to determine natural waster content, liquid limit test, consolidation test

(b) In-Situ Testing:

Bearing capacity of Foundation, Stability Analysis:

Design Parameters	Soil Testing³
Lateral coefficient of soil reaction, K	In-situ test to determine K (pile foundation), estimated from results of unconfined test, triaxial test, SPT
Allowable bearing capacity, q_a	plate loading test (shallow foundation)

In-situ tests are conducted at an interval of 40 to 50 m.

1: The laboratory tests are to be conducted using undisturbed soil samples. For sophisticated ground conditions, soil strength profile may be obtained through soundings.

2. Location of underground water and ground elevation are required for all structures.

3. The tests are conducted in stages. If a rapid variation of soil strata is detected between any two points of investigation, the test has to be done in between the points.

5.4 FACING ALLOWABLE STRESSES

The allowable-stress concept is used to design reinforced concrete members. This is decided based on the strength of concrete and steel reinforcement by considering different factors and loading conditions.

The standard value of allowable stresses is referred to the Highway Earthwork Specifications and Highway Bridge Design Specifications, as given below:

1. Steel Reinforcement

The steel reinforcements are in accordance with the JISG 3112 standard. The standard allowable tensile strength for each type of reinforcement is given in Table 5.4.

Table 5.4 Allowable Stress of Steel Reinforcement (MPa)

Type of Steel Reinforcement	SR235	SD295A	SD295B	SD345
Standard Allowable Tensile Strength (based on yield strength)	140	180	180	180

2. Concrete (Reinforced Concrete)

a) The standard allowable stress of concrete (in the reinforced concrete) is decided based on the design value of standard strength, s _ck:

b) The standard allowable flexural-compressive strength (with axial load) is given in Table 5.5.

Table 5.5 Allowable Compressive Stress and Shear Stress of Concrete (MPa)

Design Value of Standard Allowable Strength, s _ck	21	24	27	30
Allowable Flexural-Compressive Stress	7	8	9	10
Allowable Shear Stress	0.85	0.9	0.95	1.0

c) For the portion of concrete without shear reinforcements, the allowable shear stresses are given in Table 5.6.

Table 5.6 Allowable Shear Stress of Concrete (MPa)

Design Value of Standard Allowable Strength, s _ck	21	24	27	30
For Shear Stress sustained entirely by Concrete	0.36	0.39	0.42	0.45

3. Concrete (Unreinforced Concrete)

a) The standard allowable stress of concrete (unreinforced concrete) is decided based on the design value of standard strength, s _ck:

b) The standard allowable flexural-compressive strength (with eccentric axial load), s _ca, is determined from Equation 5.1.

(5.1)

c) The standard allowable flexural-tensile stress, s _ta, is determined from Equation 5.2.

(5.2)

Where s _{ta £}0.3 MPa.

The design strength of geosynthetic is discussed in Chapter 4, Geosynthetics.

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