IX. FEASIBILITY STUDY OF FIELD APPLICATION OF ELECTRO-OSMOTIC GROUTING – AN EXAMPLE 164
9.4 Post Treatment Bearing Capacity 172
During earthquake, the soil treated by electro-osmotic grouting will not liquefy.
While untreated soils around the treated zone will liquefy. All the building loads will be taken by the treated zone. The dimension of the grouted zone shall be sized so that the treated zone can take all the building loads and transfer it to the dense sand below during strong earthquakes.
9.4.1 Foundation Design Loads
The design loads of the grouted soil shall comply with the original design of the
terms of ASCE 7-02, American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures.
Assume the slab of the first floor is directly seated on grade soil, and no loads from the first floor will be transferred to the wall footings. Design loads of the strip wall foundation include roof live loads (snow load, wind load, etc.), roof dead loads (weight of corrugated metal roof), second floor dead loads (weight of concrete slab), second floor live loads, weight of wall, and weight of the concrete footing. According to ASCE 7-95, the loads listed in Table 9.3 can be used in the design of the strip footings. These loads will be used in the grouted soil bearing capacity design, too.
Table 9.3 Foundation Design Loads
Load Type Magnitude Unit
Roof Live Loads 1.8 KN/m2
Roof Dead Loads 1.0 KN/m2
Second Floor Dead Loads 4.0 KN/m2
Second Floor Live Loads 6.2 KN/m2
Weight of Wall 3.5 KN/m2
Weight of Concrete Footing 50.0 KN/m
Figure 9.5 shows the typical tributary area of the each wall. From the figure, the tributary area of interior wall No.2 is bigger than any other walls and its load condition is the most critical The total live loads on the foundation of interior wall No. 2 is:
(Roof live load + Second Floor Live Load) x 4.5 m = 34.0 kN/m
Assume the height of the building above grade is 6 meters. The total dead loads on the foundation is:
(Roof dead load + Second floor dead Load) x 4.5 m
+ Weight of wall + Weight of footing = 93.5 kN/m.
Exterior Wall No.1 60 m
8.5 m
20 m
3.0 m 8.5 m
4.5 m 4.5 m Interior Wall
No.1
Interior Wall No.2
Exterior Wall No.2
Figure 9.5 Typical Tributary Area of Wall Footings
9.4.2 Post-Treatment Soil Compression Strength
Generally say, one-shot injection can save construction time and cost. Two shot injection may promote higher soil strength, but it involves doubled construction time and much higher cost. It is recommended that two shot technique not considered in practical construction unless absolute necessary.
The hospital building is a low-rise building and the load on the foundation is relatively light. It does not require that the treated foundation soil to be of very high strength. One shot injection method is selected for this site, and colloidal silica and
grout mix is used as the grouting material.
CaCl2
The strength of treated soil can be obtained from compression triaxial test results.
From Figure 6.11, Undrained Triaxial Compression Strength of Colloidal Silica Treated Specimens, the minimum peak shear strength of the colloidal silica treated specimens is 600 kPa (Sample Ludox-002). In this design, 600 kPa will be used as the design compression strength of the treated soil.
9.4.3 Design Requirement of Grouted Zone
As shown in Figure 9.1, the height of grouted zone is the distance from bottom of footing to top of clay layer, or 1.5m. Two zones are treated under each strip footing. The two grouted zones are of the same width. In current design, the grouted zone are treated as a plain concrete wall and be designed in accordance to the American Concrete Institute ACI 318-99 and ACI 318R-99, Building Code Requirements for Structural Concrete and Commentary. Specifications of Chapter 22 of the ACI code, Structural Plain Concrete, is used in the strength design of the grouted zones.
Section 22.6.5 is the Empirical Design Method for Structural Plain Concrete Walls. According to this section, axially loaded structural plain concrete walls of solid rectangular cross section shall be permitted to be designed by Equation 9-1.
u
nw P
P ≥
φ (9-1)
Where Pnw is the nominal axial load strength computed by Equation 9-3, φ is strength reduction factor, Pu is the factored axial load, as given by Equation 9-2,
Load Live Load
Dead
Pu =1.4⋅ +1.7⋅ (9-2)
In this project, Pu = 1.4(93.5)+1.7(34.0) = 188.7 kN/m.
Nominal axial load, Pnw, is defined as
] 32 ) ( 1 [ ' 45 .
0 2
h A l
f
Pnw c g c
− ⋅
⋅
⋅
⋅
= (9-3)
Where ' specified compressive strength of concrete, here it is the specified strength of the treated soil, or 600 kPa; is the gross section area of unit length of wall; h is the thickness of each treated zone; and l
fc
Ag
c is the depth of treated area.
As shown in Figure 9.2, dimensions of each of the two grouted zones are: h=0.6m, lc=1.5m. Total capacity of the two grouted zones is:
' 3756 . 0 )) 5) . 0 32
5 . ( 1 1 ( 1 6 . 0 ' ( 45 . 0 7 . 0 2
2 Pnw fc 2 = ⋅ fc
− ⋅
⋅
⋅
⋅
⋅
⋅
⋅
=
⋅φ .
Substitute fc'=600 kPa, total capacity of the two grouted zone is 2⋅φPnw= 225.5 kN/m, which is higher than the required strength of 188.7 kN/m. The strength of the grouted zone is high enough to take all the live loads and dead loads from the exterior footing.