Water Proofing of Foundation Wall (IBC)

At first we will learn about ground water control according to IBC. Where it is found that ground water table is at equal or more than 152mm below lowest level of floor (bottom level), the floor and foundation wall as well need not to waterproofed i.e. damp proofing is enough according to recommended material and method of IBC.

When not found at desired elevation, proper control system can be adopted to lower ground water level according to following discussion:

Design of ground water control system to lower the water table should be based on standard and approved principles of civil engineering considering at least following:

• Permeability of soil

• Rate of entrance of water to designed drainage system

• Expected and maintained capacity of the pumps

• Head of operation of pumps

• Rated discharge capacity of disposal area In the ground water control system.

Again walls that will be treated or constructed should be of concrete/masonry; should be strong enough to withstand expected hydrostatic pressures, along with other lateral pressures/loads that are active in the walls.

Here hydrostatic pressure is considered as waterproofing term, when hydrostatic pressure condition dominate in the field and design of ground water control not include or not enough to lower expected elevation (as discussed above).

We know due to imperfection of concrete, formwork sealing and improper mixed or proportioned concrete, the surface of concrete member have some defects. We have discussed many posts about such imperfection of concrete work. However, our concern is here surface preparation; we are not discussing much about imperfection.

Before applying water proofing materials on surface of concrete walls, the imperfection like recesses or holes appeared after removal of formwork or any types of cracks (if not structural cracks) should sealed using bituminous materials or other accepted materials or method also can be used in this purposes.

In case of masonry walls, the exterior walls under ground level should be covered with plaster; may be of cement plaster or with other suitable material. The recommended thickness should be equal or more than 9.5 mm (3/8 inch) when mortar of Portland cement is used.

The plaster should used to masonry footing too. Now think about waterproofing, this should be used from top or above of maximum level of ground water table to bottom of wall. The IBC recommended to keep waterproofing more than 12” above of ground water table. The rest portion of wall should be damp proofed according to IBC as discussed in previous post.

Water proofing material (IBC)

Should consists of two ply hot mopped felts, equal or more than 6 mil polyvinyl chloride, polymer modified asphalt of 40 mil, 6 mil thick polyethylene or another approved materials or methods that is capable to bridge non-structural cracks. Sealing and lapping of joints in membrane should be provided according to installation instruction of manufactures.

Geologic and Geotechnical Investigation for Seismic Design of Foundation

Dear reader we have already discussed about seismic site classification and seismic design criteria according to international building code. We know seismic design categories are category A,B,C,D,E and F. At first we will learn about investigations required for foundations that support structures assigned to seismic design categories C to F.

The structures that are assigned to the seismic design category C, D, E and F according to section 1613, IBC 2009, geotechnical investigation should include determination of seismic and geologic hazards as follows:

1. Stability of slope of foundation is located in relevant site condition.

2. Liquefaction potential; dear reader we have discussed many posts about liquefaction hazard, analysis of hazard and some mitigation against this natural phenomenon.

3. Differential settlement

4. Displacement in surface due to lateral spreading or faulting.

Relation between faulting and occurrence of earthquake, fault geometry and paleoseismology have already discussed in previous posts.

These requirements are for seismic design category C to F; but when the structure is assigned to the category D to F, that is D, E or F, some additional geotechnical investigations are required; that is for category C only above four investigations are enough. But for category D,E and F more information is required which include

-Lateral earth pressure

-Soil strength reduction

-Recommended mitigation measure

We need to include following additional information to design a foundation supporting structures assigned to these categories:

1. Lateral pressure on retaining wall or foundation walls as a consequence of earthquake motion. There have many record of failure of earth retaining work during excavation and foundation construction work.

2. Liquefaction potential and strength loss of corresponding soils for

–PGA

–Magnitude

–Source characteristics

This parameter must be consistent with design seismic ground motions.

In calculation of peak ground acceleration (PGA), soil amplification factors are derived based on site specific studies. (Reference ASCE7, chap-21). 

Alternatively PGA can be taken as S_DS /2.5, 

where S_DS=Maximum considered seismic spectral response accelerations (for short period, ref: 1613.5.4 IBC-2009).

3. With assessment of liquefaction potential, consequences of liquefaction and subsequent strength loss should be included. Which includes:

–Differential settlement

–Lateral movement

–Lateral loading on foundation

–Reduction in bearing capacity of foundation soil

–Increased lateral pressure on earth retaining wall

–Floating potential of buried foundation element or structures.

4. At last, how to mitigate the consequences of above phenomenon? This discussion of measure to mitigate should include at least –Ground stabilization –Selection of proper foundation depth and types as well –Selection of proper structural system; the objective is to accommodate expected forces and subsequent displacement –Requirement of combination above measures and appropriate way to account in designing foundation and structures.

Controlled Low-Strength Material to Support Foundation (IBC)

Dear reader this is relatively new term in this blog, controlled low-strength material; this are used to establish structural fill/backfill of trench. In United States there have widespread use of it as foundation backfill. The special attention is for backfilling for utility and storm drainage on highway projects.

Dear reader our concern is here foundation backfill, we will have just introductory idea about controlled low-strength material. This will be expressed as CSLM for convenient of discussion.

This is produced by mixing cement, aggregate, fly ash and essentially water. So we can take it as concrete; the difference is very low strength which is at best 1200 psi and mostly less than that value. We know ordinary concrete have strength around 3000 psi.

Due to very low strength, we will not choose it as supporting material for building, bridges etc. rather we will go for backfilling as discussed earlier. The important parameter is it has greater flowability than normal concrete.

International building code provides us some geotechnical investigation required for using this materials as foundation backfilling material. Dear reader, yes, we are talking about shallow foundation and following are the requirements:

1. Specification to prepare site before placing controlled low strength material should be provided in geotechnical investigation.


2. Specification for CLSM


3. Field or laboratory test methods for determining bearing capacity, in other word compressive strength of CLSM


4. Test methods to approve CLSM in field


5. Frequency and number of in-situ tests, to determine acceptance according to Item 4 (above).

Structural Phenomena except Cracking in Black Cotton Soil

Other structural phenomena associated with black cotton soil except cracking are 

•Gilgais

•Slickensides

•Structural sphenoid aggregate

•Self mulching in surface soils.

All of the phenomena stated above are also attributed to shrinkage and swelling of soil under moisture change; but actual mechanism is not understood clearly. 

The soil movement under moisture variation is not found in only vertical or horizontal planes. This will produce sphenoid aggregate or wedge-shape aggregate and due to movement past each other, the peds becomes polished resulting slickensides.

The development of gilgais is the most interesting phenomena associated with black cotton soils. Gilgais is topographic phenomena where alternate depression and mounds occur at soil surface. The intermediate areas between them are called a shelf.

Many mechanism and many forms of gilgais were described considering uneven swelling & shrinkage of soil. The forms are

• Round or nomal

• Mehen hole

• Lattice

• Linear

• Tank

• Stony

The gilgais are formed by repeated swelling cycles of black cotton soil followed by subsequent shrinkage when moisture is lost. Soil becomes cracked and the cracks are filled with loose materials. When the soil mass swells under next rewetting cycle, the pressure in soil cannot be relieved by cracking which exerts forces sideway and results mounds.

The depressions hold water and make soil wetter and suffer more swelling and subsequent mounds and obviously more shrinkage under drying.

The cracks allow water to penetrate more deep into the soil mass leading more swelling and subsequent shrinkage. The increasing swelling and shrinkage results repeated depressions and mounds.

Thus regular heterogeneity occurs which made the mounds generally more alkaline than depression (shelf); but very few data is available for both shelf and mounds.

How is Specimen of Collapsible Soil Collected?

Dear reader we are discussing here only collapse test. We know it is very difficult to predict behavior of collapsible soil and our attempt is to collect specimen of collapsible soil such that it represent actual situation in the field. In this regard, undisturbed specimen is essentially required.

Dear reader we know collapsible soil may be from natural soil or filled material. If our site is situated on the ground that may have collapsible soil and the soil type is filled (uncontrolled) material; we can prepare specimen by compacted filled material to filed density and filed moisture content as well.

We have already learned about block sample; which is the best method to collect undisturbed specimens. Trimmed block sample can be collected alternatively extruded directly from sampler into confining ring.

Specimen requirement:

Specimen Dia=2.5” (6.4 cm)

Specimen height=1.0” (2.5 cm)

The trimming is done very quickly to minimize possibility of alter moisture content of soil specimen.

The water content, in most case, in collapsible soil is low and to keep in field condition trimming and storing should not be done in high humid environment. Trimming typically done in confining ring having thickness of specimen is equal to height of confining ring.

Fill and Grading of Foundation Site's Requirements in Flood Hazard Areas, IBC

All over the world there have many destruction related to natural disaster like flood. With the different flood control measures, we tried to control flood but in many cases we cannot control flood and we have establish areas under flood hazard based on flood hazard map with another supporting data.

The flood hazard map should include at least following-

• Flood boundary map

• Flood way map

• Flood insurance rate map

• Areas of special flood hazard

The federal emergency management agency provides this data and map as well.

In some cases, design flood elevations in flood hazard areas are not mentioned or sometimes floodway also not designated; in such cases building official have authority to ask to submit following data from applicant:

1. From sources like state, federal or other sources, acceptable design flood elevation or floodway data.

2. Submit the same according to accepted hydraulic/hydrologic engineering practices applicable to determine special flood hazard areas and registered professional should be employed in documenting this.

Flood hazard areas, established in these procedures, IBC provided some requirements to approve filling and establishing grading in these area. This code allows grading and filling only when:

a. Fill is provided, compacted and established slope to minimize slumping, shifting and erosion while rise & fall or subside of flood water; when required wave action is also encountered. 

b. In floodways, proposed grading and filling, or any of them will not produce in any rise in flood level while design flood is appeared in flood hazard areas.

 
Now who is authorized to ensure this? As per code the demonstration should be approved by hydraulic and hydrologic analysis executed by registered design engineer/professional following standard and established engineering practice.

c. When structure is located in flood hazard areas and also action of high velocity wave is exist in respective areas, fills will be placed or conducted in such way that water and also waves toward foundation and structures are diverted effectively.

d. Dear reader in last part we have discussed about action to be taken when flood elevation or floodways are not designated. When design flood elevation are know, specified in flood hazard map but flood ways are not defined, the filling or grading can be done only when cumulative result of encroachment of propose flood hazard in combination of encroachment of expected and existing flood hazard area, will not rise design flood elevation by more than 1 foot anywhere around the fill.

Grading Around Building Foundation Site (IBC)

Dear reader we know surrounding condition of foundation also influences foundation performance. Dear reader here we will discuss about site grading requirements around foundation according to international building code.

There have specific guideline about slopes immediate adjacent to foundation; the ground should have slope away from foundation equal or more than 1 vertical to 20 horizontal i.e. a slope of five percent for a distance of minimum 10 feet which is measured normal to face of wall.

Now we have to consider about physical obstruction and boundary lines that prohibit measuring 10 feet distance discussed above. An approved method alternative to measuring perpendicular to foundation wall/ wall has to use; IBC suggested us to check 5% slope flowing water directing away from foundation.

In this purpose we can use swales; which should have minimum slope of 2 percent when they are observed within 10 feet of building foundation. When there have impervious surfaces around foundation within 10 ft of building foundation a minimum slope of 2% away from foundation is recommended in IBC.

But considering climatic condition or also of soil too, the ground slope away from foundation is permitted to reduce. The recommended slope is equal or more than 1 vertical to 48 horizontal i.e. 2% slope.

However any procedure used to determine ground level around the building foundation should consider additional settlement encountered for backfill.

Dimensions Requirements for Deep Foundation (IBC)

International building code provides some requirements about dimensions of deep foundation. The requirements are classifies as precast and cast-in-situ or grouted-in-situ deep foundation.

Precast deep foundations

Minimum dimension in lateral direction of precast concrete element of deep foundation should be 203 mm (8 inch). Square element should have chamfered corner.

Cast-in-situ or grout-in-situ deep foundation

Dimension requirements of cast-in-situ and grouted-in-situ deep foundation are classified based on cased and uncased deep foundation.

Cased deep foundation

Nominal diameter of foundation elements of cast-in-situ deep foundation having permanent casing should be equal or more than 203 mm(8 inch).

Uncased deep foundation

Cast-in-situ foundation elements not having permanent casing should have diameter at least 305 mm (12 inch). The length of elements should be limited to 30 times average diameter.

The element of deep foundation should be designed and supervised during installation by registered engineer having professional knowledge in geotechnical engineering and especially on deep foundations.

The foundation engineer should provide a report about design stating that foundation elements have installed complying approved construction documents.

Micropiles 

The outside diameter of micropile should not exceed 305 mm (12 inch). Minimum diameter is as discussed above.

Recommended Concrete Cover for Foundation (IBC)

Dear reader we know concrete need cover to avoid exposure of its reinforcements. The purposes are corrosion control, fire protection and of manifold. Dear reader we will learn here about concrete cover for foundation element according to international building code (2009).

Concrete may be prestressed or nonprestressed, for both cover is essential for reinforcement. The recommended concrete covers are as below:

When longitudinal reinforcing bars have spacing less than 38 mm (1.5 inches), they should be considered as bundled bars. The concrete cover should have specification provided in ACI 318 (Section 77.4).

Dear reader the spacing we are talking about are obviously clear distance. The concrete cover stated above should be measured from outermost point in surface of reinforcing steel to concrete surface.

Concrete is sometimes placed in permanent/ temporary casing or in a mandrel. In measuring concrete cover inside surface of casing/mandrel is considered as concrete surface.