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Use of Rigid Foundation System on Expansive Soils

Dafalla, M. A.1, Shamrani, M.A.2, Puppala, A.J.3 and Ali, H.E.4

1Asst. Professor, 2 Professor, 3 Professor-Visiting, 4 Graduate Student

BRCES, Civil Engineering Department, King Saud University, Riyadh, Saudi Arabia, [email protected]

ABSTRACT Expansive soils pose a major maintenance nightmare for geotechnical practicing engineers in the semi-arid and arid zones of the world. The introduction of rigid substructure is a costly approach but can be made practical and financially tolerable for soils where expansion is characterized within medium to low problematic classes. This paper outlines justifications and design concepts for a rigid substructure foundation of a two story concrete frame structure. The project was constructed in a district in Saudi Arabia where many buildings experienced serious damage due to expansive soil problems. Upheaval forces likely to act against the proposed structure were determined and used in the finite element analysis and design of an appropriate rigid substructure design. Plots of moments and forces were determined and critical sections were pointed out. Methods and stages of construction were monitored and a baseline vertical movement was established for future verifications. The advantages and limitations of this system are also discussed. INTRODUCTION Saudi Arabia is a country located within a semi-arid region where drying and wetting is common and frequent. Expansive soil problems became of great concern following the introduction of concrete frame structures in residential buildings following the oil boom in the late 1970s. The problems of expansive soils do not show up immediately. Instead, cracks start to develop following rainy seasons and an accumulation of leaking water from domestic use. Damages became common after a few years. The foundations used to support the superstructure were generally flexible consisting of isolated pads, short columns and ground beams. The walls were made of concrete blocks. Upward movement due to expansive soils was found to twist the flexible substructure foundation and cause serious damage to brick walls and some concrete structural members such as ground beams and on grade slabs. The damage mainly appeared in the form of severe diagonal cracks in brick walls and concrete members. Examples of typical damage are shown in Fig 1 and Fig 2, respectively.

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Fig 1: Distortion and twist caused by expansive soils in Al Ghatt Region.

Fig 2: Foundation cracks caused by expansive soils in Al Ghatt Region.

The practice to introduce a rigid massive foundation support in this region was normally opposed due to the high cost of the foundation being unacceptable to local home owners. This work is aimed at providing a rigid design support, which will compensate for the expansive ground subsurface. The geotechnical parameters are determined and integrated with structural analysis using a finite element program to arrive at the minimum dimensions of the rigid slab support system. The proposed rigid support foundation used for this analysis is an inverted T section strip foundation. The system is similar in appearance to waffle slab design or hollow raft foundation. Fig (3) shows a three-dimensional view of a typical building with a rigid substructure foundation studied in this research.

Fig 3: 3D Model for a two storey building of 9 panels 6m x 6m spans



GEOTECHNICAL PROPERTIES The rigid design approach given in this publication is suggested for the region of Al Ghatt in Saudi Arabia. Al Ghatt is a populated town located 270 km to the Northwest of Riyadh at latitude 26º 32´ 42´´ N and longitude 43º 45´ 42´´ E. The town is situated within a large wadi with surrounding high hills. The subsurface formation at Al Ghatt is generally overlain by quaternary silt, sand and gravel layer followed by greyish brown to olive green silty to clayey shale. The shale belongs to the Durma formation which is made of accumulation of mud deposits (Dhowian et al., 1990). The thickness of the overburden layer is variable across the town being very thin close to the hilly zone and increasing towards the wadi center. The geotechnical properties of the silty shale and clayey shale have been investigated by several researchers and geotechnical firms in Saudi Arabia. Generalized soil profile for the area is shown in Fig. 1 and Table 1 tabulates results of laboratory tests. Table 1. Geotechnical properties of Al Ghatt clay Property Site data General Area Ranges LL 60 40-70 PL 28 15-35 PI 32 20-40 S.G. 2.79-2.8 2.7-2.8 Free swell 7.2 0.1-10 Swelling Pressure 273 kPa 40-1000 Swell Index Cs 0.098 0.01-0.2 Percent clay 50 30-60

After Dafalla, M. A. and AL-Shamrani, M.A.(2008). The light structures including asphalt pavements, boundary walls are the most affected by the distress caused by expansive subsoil. Single and two storey buildings also suffered from the expansive subsoil to variable degrees. The main factors contributing to the amount of damage include but are not limited to the following: foundation depth, foundation type, stress level applied by the superstructure and subsurface soil conditions. The first author led a team from the King Saud University, Riyadh, Saudi Arabia in February 2008 and conducted a general survey of residential dwelling damages in the area. Failures due to expansive soil problems took many forms. Uplift and twisting boundary walls for the residential structure is a very common problem. This type of failure is reported frequently in Al Ghat and was also associated with water leakage or waste water disposal within the foundation surroundings. Crack or failure intensity was found mild to the east of the town, where the thickness of the overburden top soil is in excess of 1-m and very severe to the west part where foundations are placed right on the expansive shale formation. Failures due to expansive soil are classified by the authors as in Table 2.



Table 2. Classification of Failures in Al Ghatt region Class of failure

Description Occurrence

Mild Distortion in the form of partial uplifting with minor cracks seen only in close vicinity (1 to 2 mm)

East of Al Ghatt

Medium Diagonal cracks in the order of 2mm to 5mm. Minor wall tilting

East of AlGhatt

Severe Displacement and disintegration of structural units. Cracks in the order of 5mm to 10mm. Tilt observed.

West of Al Ghatt

Very severe Displacement and disintegration of structural units. Cracks in the order of centimeters. Major tilt and mis-alignment

West of Al Ghatt

After Dafalla and AL-Shamrani (2008) FOUNDATION SYSTEM AND DIMENSIONS Several methods were adopted in geotechnical practice to reduce or eliminate the risk of heave in light structures and pavements. These include treatment of the near foundation soil using lime and cement, use of soil replacement technique, use of piers and piles with or without bottom enlargement and many other approaches. The effectiveness of these methods is not necessary similar or applicable for different geographical regions due to varying environmental conditions and mineral composition.

Reviews on foundation and expansive soil made by Chen (1988), Nelson and Miller (1997), Dhowian et al (1990) and Abduljawad (1991) provide useful information that enables design engineers to reduce risk when supporting structures on expansive soils. One of the approaches of handling this problem is to consider a rigid substructure foundation system. The rigid foundation design can take different forms from a massive thick raft to rigid frames or stiffened concrete members. In this research study, an inverted T section with dimensions as shown in Fig 5 is selected. The strip foundation grid supporting the superstructure is presented in Fig 4.



Fig 4: Rigid frame substructure foundation with swelling pressure loads assumed at the exterior corners

Fig 5: Foundation dimension selected for the structure under study.

MODELING APPROACH FOLLOWED The approach followed in the modeling the proposed type of rigid foundation is based on assuming swelling pressure to act at the exterior parts of the structure as they impose the minimum stress on the ground. Inundation is normally started at the edge of the structure as water flowing from the neighborhood or leaking from the utilities is close to the edges. Interior column loads impose higher loads and are unlikely to produce critical bending or shear stresses even when the soil inundated is limited to the central part of the structure. The design loads are based on the British Standards CP 8110. Analysis and computations were carried out using a finite element program. The case assuming dead and expansive load combination was found to give the maximum bending moment and maximum shear force close to the first interior support. RESULTS AND DISCUSSION The geotechnical investigation for the site was carried out by a local geotechnical consulting agency. The swelling pressure tests carried out on several samples revealed values in the order of 200 kPa. This value is chosen as the design swell pressure value for this project. It must be noted that swelling pressures obtained in a one-dimensional oedometer apparatus can be three times higher than the actual swell pressure in the field. Al-Shamrani and Dhowian (2003) suggested the use of triaxial loading condition in test measurements to allow for the confinement effect or lateral restraint. They found that the compressibility value determined from oedometer measurements was about 3.6 times the value calculated from the field data, whereas based on the results of triaxial swell tests, the ratio of the laboratory to the field value was about 1.3.



An example for computation analysis output shows the maximum bending moment and shear force diagrams, which are presented for a 200 kPa swell pressure case. It can be noticed that the section subjected to the highest moment is at the first interior support, computed as 1224 kN-m. The section subjected to the maximum shear force is also noted at the first interior support and computed as 707 kN. Fig 6: Bending moment and shear force diagram for a 200 kPa swelling pressure. The critical bending moments and shear forces were computed for a range of swelling pressure values assumed for subsoil surrounding the rigid foundation system. Table 3 presents these resulting values for each swelling pressure. A linear relation is plotted for both bending moment and shear force. In the absence of swelling, it can be seen that the bending moment at the critical section does not exceed 290 kN-m and the shear force does not exceed 315 kN.



Table 3: Values of Bending Moment and Shear Force at critical section due to Swelling Pressure.

Critical Bending Moment Versus Swell Pressure

y = 4.67x + 290

0

400

800

1200

1600

2000

2400

2800

3200

3600

0 100 200 300 400 500 600 700Swell Pressure (kPa)

Criti

cal

Mom

ent (

kN.m

)

Fig 7: Critical bending moment versus swell pressure

Critical Shear Force Versus Swell Pressure

y = 1.9586x + 315.33

0

200

400

600

800

1000

1200

1400

1600

0 100 200 300 400 500 600 700Swell Pressure (kPa)

Crit

ical

She

ar F

orce

(kN

)

Fig 8: Critical shear force versus swell pressure.

DESIGN METHODOLOGY Based on the computation carried out for the rigid T- inverted strip foundation a section can be designed to withstand critical bending moment and shear force, both generated as a result of the swelling pressure of the subsoil. It is suggested not to add any further safety factor as the supplied value from the geotechnical practice is normally based on conservative testing approach. It is always necessary to consult the geotechnical engineer to verify the testing method used to obtain the swelling pressure. The test result shall be reduced and corrected for confinement, if carried out in a one dimensional oedometer. A reduction factor of 3 is reliable. This work is aimed at establishing a design procedure to become a routine when dealing with areas known of swelling problems. The rigid design methodology (RDM) for expansive soil presented in this paper can be further developed to accommodate and refine several variables that affect the design and assumptions made. The methodology followed for

Swelling Pressure (kPa)

Critical Bending Moment (kN.m)

Critical Shear Force (kN.m)

100 757 511 200 1224 707 300 1691 903 400 2158 1099 500 2625 1295 600 3092 1490



design of foundation at this site assumed a T-section with constant width and steel reinforcement but varying section height. The section height that satisfies the critical bending moment and shear force requirements is selected for the design. The approach can be followed for other sites of different swelling pressures and the appropriate section height can be obtained based on the geotechnical input and subsurface soil conditions. The grid of the structure can also be variable and in this case computations need to be carried out following the same steps to arrive at the exact section required. Table(4): Allowable Swell Pressure for Different Concrete Section depths.

Note: (fy & fyv=420N/mm2, fcu =25N/mm2, Steel used 20Ф16(As=.00402m2) & 2Ф10@100 links (Asv= 0.001571m2)) From the Table 4, it can be mentioned that a section depth of 1.2 m will be sufficient to resist both bending stresses and shear forces induced by a swelling pressure of 200 kPa. As can be seen from the table above, the design can be suitable for various swelling pressures but at high values the required design depth will involve extra cost resulting from deep excavation and cost of construction material to fill up the frame below finished grade level. Therefore this system is appropriate to areas with low to medium swelling where swelling pressure is less than 300 kPa. The additional costs in substructure is justified when compared to maintenance cost involved in flexible designs such as isolated footings or flexible raft foundations used in local practice in typical areas.

H (m)

th (m)

B (m)

tb (m)

Area (m2)

% steel

MomentCapacity

(kN.m)

Shear Capacity

(kN)

Allowable Swell Pressure due to moment capacity (kPa)

Allowable Swell

Pressure due to shear capacity

(kPa)

0.8 0.4 1.2 0.4 0.64 0.63 642 719.7 75 207 0.9 0.4 1.2 0.4 0.68 0.59 789 741.2 107 218 1 0.4 1.2 0.4 0.72 0.56 936 761.4 138 228

1.1 0.4 1.2 0.4 0.76 0.53 1083 780.5 170 238 1.2 0.4 1.2 0.4 0.8 0.50 1229 798.8 201 247 1.3 0.4 1.2 0.4 0.84 0.48 1376 816.4 233 256 1.4 0.4 1.2 0.4 0.88 0.46 1523 833.4 264 265 1.5 0.4 1.2 0.4 0.92 0.44 1670 849.8 296 273 1.6 0.4 1.2 0.4 0.96 0.42 1817 865.8 327 281 1.7 0.4 1.2 0.4 1 0.40 1964 881.3 358 289 1.8 0.4 1.2 0.4 1.04 0.39 2111 896.4 390 297 1.9 0.4 1.2 0.4 1.08 0.37 2258 911.2 421 304 2 0.4 1.2 0.4 1.12 0.36 2405 925.7 453 312

2.1 0.4 1.2 0.4 1.16 0.35 2552 939.9 484 319 2.2 0.4 1.2 0.4 1.2 0.34 2699 953.8 516 326 2.3 0.4 1.2 0.4 1.24 0.32 2846 967.5 547 333



CASE STUDY EXAMPLE The design of the substructure foundation using the RDM method was utilized for a site in a central region in Saudi Arabia. The court complex building at Alghatt was constructed in the year 2007 by AL Artawiya Contracting Company (Saudi Arabia based firm). The whole site was excavated to the required depth and structural fill was used for a thickness of 1.5m carried out in layers of 20 cm compacted to a density not less than 95% of maximum dry density of the modified proctor test. The fill is recommended to reduce the intensity of swelling and increase the overburden pressure on the swelling shale. Figures 9, 10, 11 and 12 show the construction stages of the project in which this rigid foundation system was used. Minor adjustments were made to suit the project requirements with the design section remained unchanged. In order to monitor the future state of the structure, it was the suggested by the authors to have a base line measurements for the vertical movement at several points of the structure and some reference points. This data will be presented in a future publication on the performance of structures designed using this approach. Fig 9: Site grading and backfilling

Fig 10: Forms set for concrete pouring.

Fig 11: Steel reinforcement put in place.

Fig 12: Bottom part of T-section casted.

CONCLUSION The distortion is normally observed in the light structures due to the high flexibility of the frames and substructure foundations. This is not tolerated by the brick walls normally used to fill up the panels in concrete frame structures. Severe cracks



can be shown when twist or movement takes place. Therefore the use of Rigid Design Methodology approach is expected to give less flexible support and reduce the chances of cracks and damage. The design approach given in this paper is easy to apply and can be revised to consider variables not applicable in particular sites. The rigid design given in this publication is recommended for sites indicating a swelling pressure of less than 300 kPa. Further work is required to establish a system which will allow for different concrete material properties and different reinforcement arrangements. Current on-going research is exploring the potential use of the designed system in the field conditions. REFERENCES Abduljawad, S. N. (1991). "Characteristics and Chemical Treatment of Expansive Clay in Al Qatif, Saudi Arabia." Engineering Geology Journal, Volume 30 Elsevier Publisher pp 143-158. Al-Shamrani, M. A. and Dhowian A. W (2003)."Experimental Study of Lateral Restraint Effects on the Potential Heave. " Engineering Geology 69 (2003) pp. 63–81. Al-Sheraida, F. M. (2008). The Effect of Chemical Addition on Expansive soil Under Dry condition. Submitted in partial fulfilment of the requirement for the degree of Bachelor of Science in Civil Engineering, King Saud University, Riyadh, Saudi Arabia. BS CP 8110 (1997)." Structural use of concrete :British Standards." Code of Practice for reinforced concrete design.. Al Muhandis, Nizar Kurdi. Geotechnical report (2006). "Court Complext Building Site" Geotechnical report,AMNK, Riyadh, Saudi Arabia No. 093/02/2006. Chen, F.H. (1988). "Foundationon Expansive soil." Elsevier Scientific Publishing Company, Amsterdam. Dafalla, M. A. and AL-Shamrani, M.A.(2008). "Performance –based solutions for foundations on expansive soils-Al Ghatt region, Saudi Arabia." .GEO- CHIANGMAI Conference - Thailand Dhowian A.W, Erol, A. O. and Youssef A. (1990). "Evaluation of Expansive soils and foundation methodology in the Kingdom of Saudi Arabia." Published by the General Directorate Research Grants Programs- King Abdulaziz City for Science and Technology. Nelson, J and Miller D.J (1997). "Expansive Soils: Problems and Practice in Foundation and Pavement Engineering." Published by Wiley Professional.



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