Assessment and retrofitting of deteriorated light structure founded on expansive soil

ABSTRACT

Structure damages are becoming very common when constructed in arid and semi-arid areas on problematic soil. This may be attributed to the inadequate geotechnical investigation prior to construction. The problem is most effective in case of light weight structures constructed on arid problematic soils without any special considerations for these types of soil, and the problems start to arise with the entrance of structure to the service and the seepage of water to the founded soil. In this paper, a case study for a villa in a gated compound; just after 6 months from construction, the walls of this light structure suffered severe cracks. A careful inspection and geotechnical site investigation showed that the structure’s foundations have been subjected to differential movement at its western part, due to percolation of water into bed of high potential swelling soil. In addition, the stiffness of the basement columns was very low. Some consultants recommended to demolish the villa, nevertheless, an intensive structural analysis taking into consideration conditions of the concrete skeleton had led to select proper structure’s remedial measurements to stop the heave and shrinkage problems, and to improve the structure’s stiffness. The retrofitting measures included removing the expansive clayey layer which was of limited thickness and area; modifying the foundation system, and replacing the basement columns with inadequate strength with new ones. The measurements of the monitoring process during and after finishing repair processes had shown no movements, which reflects the efficiency of implemented retrofitting techniques.

KEYWORDS:

• Deterioration
• expansive soil
• repair techniques
• foundation damage
• differntial movement
• soil-structure-interaction

Introduction

Expansive clayey soils experience volume changes as a result of moisture changes leading to differential movements below the structure’s foundation. When a structure is constructed on such a soil, it applies an upward pressure on the foundation upon inundation, termed as swelling pressure. If the foundation transfers a downward stress which is smaller than the swelling pressure, the foundation moves upwards. These movements tend to affect the walls, and possibly the skeleton, and eventually destabilize the whole structure. Light structures, such as single or double storey buildings, which generally transmit smaller stresses to the soil than the swelling pressure are greatly subjected to damage [1,2].

Soil scientists recognize that shrink-swell behavior of expansive soils can be predicted by examining a combination of physical, chemical, and mineralogical soil properties; however, no one property accurately predicts shrink-swell potential. Often, most expansive soils are clayey with high content of smectite minerals [3]. The swelling potential of expansive soils mainly depends upon the properties of soil, environmental factors and stress conditions.

Differential movements redistribute the structural loads causing concentration of loads on portions of the foundation and large changes in moments and shear forces in the structure not previously accounted for in standard design practice [4,5]. The damages are due to design faults, cheap construction materials, poor workmanship, leakage from sewage and water pipes seepage, climatic condition and swelling behavior of expansive soils. Understanding the causes of building damage will significantly contribute to the proper selection of effective repair technique which results in prolonged service life of buildings [6–8].

This study is concerned with a unit of R.C. light weight structure within a compound at El Sheikh Zayed, Giza government. This building suffered severe wall cracks, which may affect the stability of the building. To understand the causes of the problem, a comprehensive structural-geotechnical assessment program was designed. The methodology of approach to investigation, tests conducted remedial measures and their implementation and other relevant aspects are presented and discussed in the subsequent sections.

Methodology of problem assessment

Building description and visual inspection

The structure under study is an R.C. building which consists of a basement + ground floor + first floor. The structural system is R.C. isolated footings resting on P.C. isolated footings and connected by smells, columns, beams and the ceilings are of hollow blocks and ribs slab type. The boundary basement walls are of R.C. type; however, some of these walls are not connected with the ceiling or with openings. The cracks started after about 6 months of the building construction. The owner had modified the location of the basement bath, which was taken into consideration during the assessment of the nature of the problem.

During the periodical site visits, the increase in the size of the wall cracks was observed; the following most obvious notes along the axes, as an example, are shown in Figure 1.

Figure 1. The ground floor plane of the building under study.

– Relative movement along axes (6) &; (5), and the crak pattern indicates vertical movement for the building at axis (6) with higher rate at axes (B & C).

– Inclined cracks between axes (5 &6) along axis (C) in the wall of the first floor. This did not appear in the basement and ground floor because no walls along axis (C) exist in these floors.

– Heave of basement floor was observed between axes (5&6) towards axis (5).

– Various cracks were observed on the exterior and interior walls as shown in photos of Figure 2.

Figure 2. Examples for the severe cracks pattern on the exterior and interior walls.

The pattern of the cracks runs from the corner toward the adjacent opening or along the walls with v-shaped, wider at the top than the lower part, this pattern reveals that the subsoil had yielded increase in volume change, which is reflected in the form of basement floor heave and wall cracks.

Excavation and subsoil conditions

To investigate the subsoil conditions, the basement slab was removed and the sand fill between the footings was excavated, then three open pits were excavated with depths in the range of 5.00 to 7.00 m from the sand fill, whereas outside the building a borehole with depth of 15.00 m was drilled below the garden level. Undisturbed and disturbed samples were taken to determine the physical and mechanical properties of the soil stratifications at the site.

The main soil stratification below the foundation level is as follows:

Upper sandy clayey layer

Layer of yellowish gray, weak-cemented clayey medium to coarse Sand, thickness of about 0.25 m.

Silty clay layer

Layer of very stiff gray, Tafla Silty Clay (Swelling soil), with thickness of 2.00 to 3.00 m.

Lower sandy layer

Layer of yellow, cemented medium to coarse sand, with increase of depth becomes uncemented medium to coarse sand to the end of excavation or drilled depth.

Ground water table

No groundwater level was encountered down to depth of about 10.00 m. However, it was observed that relatively high moisture content within the upper layers close to the eastern side of the building, which reveals that the water seepage has taken place at this side.

Properties of silty clay layer

Physical and index properties and swelling characteristics were determined on the soils by following relevant procedures. Soil classification (USCS), Atterberg limits ASTMD 423, D424, and D4318. Gain size analysis ASTMD422, Swelling characteristics [9]. The test results are given in Table 1.

Table 1. Properties of swelling clay layer.

Property	Data
Bulk Unit weight (kN/m^3)	19–20
Moisture content at wet state (%)	10–20
Moisture content at natural state (%)	4.0–6.0
Liquid limit (wl) (%)	92–105
Plastic limit (wp) (%)	30–38
Plasticity index (%)	62–67
Free Swell (F.S.) (%)	150–250
Swelling Pressure (kN/m^2)	2500
Clay percent (%)	55
Silt percent (%)	32
Sand percent (%)	17

To determine the swelling pressure, undisturbed samples were extracted from an open pit at its natural moisture content, away from the wetted zone of the expansive layer. The swelling pressure was carried out using the one-dimensional oedometer test following the pre-swelled method as recommended by the Egyptian code of Practice [9]. The measured value reflects the high swelling potential of the swelling soil as classified by [10]. However, it is worth noting that studies carried out by [Mosleh and Al Mhaidib, 1999]], [11], to compare the results of oedometer test and field heave had shown that about one-third of the volume change is reflected as surface heave. Accordingly, to convert the potential volume change to the expected heave in field, a factor of about one-third is to be applied to the heave measurements obtained from the oedometer tests. As the heave reflects the swelling potential of the expansive layer, the field swelling pressure can be roughly assumed to be one-third of that estimated from the oedometer test. In this case study, the induced swelling pressure of about 800 kPa is much higher than the stress exerted by the light structure at the foundation level.

From the previous survey of subsoil conditions, it is clear that, because of inadequate geotechnical investigation prior to construction, the building was constructed on arid problematic soil, without any special considerations for this type of soil. The problematic soil under foundation level beneath 0.25 m of sandy soil is a highly potential expansive soil with thickness of about 2.00 to 3.00 m.

Assessment of R.C. Skeleton conditions

The rigidity of the structure Skeleton plays an important role to minimize the destructive effect of differential heave that may be developed due to founding on expansive soil. The R.C. skeleton elements of the building were firstly visually inspected, and then the concrete strength was evaluated. Inspection of the foundation system had shown that the boundary smells along axis (6) yielded cracks along with rust of steel, which may be due to cheap construction materials, poor workmanship.

To evaluate the effect of cracks on the structural safety of the building, the quality of the concrete strength of the skeleton elements was checked. The concrete elements were tested using destructive and nondestructive test methods. The results indicated that the strengths of the majority of the footings and columns under consideration were extremely low, namely, less than 50% of the design strength, Figure 3.

Figure 3. The foundation system and the cracks in concrete of boundary smells along axis (6).

Retrofitting techniques

Understanding the causes of building damage will significantly contribute to the proper selection of effective repair technique that results in prolonged service life of buildings [8,12].

Retrofitting or repair involve five basic steps: (a) finding the deterioration, (b) determining the causes, (c) evaluating the strength of the existing structure, (d) evaluating the need for repair, and (e) selecting and implementing a repair procedure [13].

In spite of a report prepared by the consultant who advised to demolish the building, yet, based on intensive geotechnical and structure and economic analyses it was decided to repair the structure [14].

Searching for the source of water leakage had shown that when the owner changed the location of the basement bathroom, the old sewage pipe toward the main manhole outside the building was removed, while its entrance opening in the manhole was left as is without plugging, that was the reason for seepage from the main manhole through the sand toward the swelling clayey layer.

The basic retrofitting concept is based on the following observations:

1. Stop leakage from the main manhole by clogging the discharge old hole; check all the other sewerage system, and inlet water supply pipes.

2. Some elements of the foundation system suffered cracks and were with poor concrete quality. On the other hand, the columns were also with strength much less than the required minimum design value, therefore, replacing the basement foundation system and the columns was an essential requirement to improve the structural safety of the structure.

3. The site investigation indicated that the thickness of the expansive layer is limited with thickness of about 2.00 to 3.00 m. and cover about 60% of the building’s area within axes (4) through (6). Due to the inclination of the swelling soil layer, it is worth mentioning that it is extremely difficult to control the seepage of water from outside the villa that consequently affects the superstructure. Therefore, to permanently terminate the problem and its future circumstances, it was decided to remove the swelling soil under each footing and modify the system to be with new R.C. isolated footings resting on plain concrete piers supported on the deep clean sand layer following the swelling soil. This option was decided based on the concept of replacing the basement columns with new adequate ones, and to be only applied for the area where the swelling deposit layer exists.

The building is an R/C skeleton type building, the ceiling is hollow blocks slab type, supported on isolated footings, and the retrofitting process was implemented as follows:

First step: removing selected columns and their footings

1. To check the structural safety of the building during the retrofitting process, the building had been monitored with the aid of surveying prisms supported all around the building at the mid-height of the first floor walls.

2. The ceilings of the basement, ground and first floor around the columns were supported by steel screw jacks and wooden plates, taking into consideration the wooden plates to be perpendicular to the ribs of the hollow block ceiling.

3. All the walls connected with the columns under repair were first removed.

4. The steel beams supporting the columns to be demolished were supported by frames of steel I-Beams and hydraulic Jacks as shown in photos of Figure 4. The load of the column was first counteracted by the hydraulic jack supported on the outer frame I-beams, then steel plates were inserted between the horizontal supporting I-beams and the inside vertical frame I-beams. The counter load of the hydraulic jack was just little higher than the estimated column own weight and part of the adjacent wall that was not beared by the screw jack. Then, the hydraulic jack was released after ensuring that the beam was exactly loaded by the interior steel frame.

   Figure 4. Photo supporting the column beams using I-beam steel frames.

   

5. The concrete column was removed and the steel bars were cut leaving the upper 1.00 m length to be welded with the new column steel, the process of demolishing the building columns was applied for a certain column, and column’s footing was then removed as shown in photo (b) of Figure 4.

6. Finally, the soil beneath the removed footings was excavated until totally removing the swelling soil stratum and reaching the sandy soil.

Second step: construction of new footings and new columns

1. The plain concrete pier was casted, and after hardening, foam sheets were placed around the side surfaces of the pier, and the space between the swelling clayey soil and the pier was filled with slightly compacted clean sand as shown in photo (a) of Fig. 5(5). The reason for placing the foam sheets and surrounding sand is to absorb the lateral swelling pressure of the expansive soil upon inundation.

   Figure 5. The new footing P.C. pier, and casting of the new columns.

   

2. The steel cage of the new designed footing was placed over the P.C. pier, and the concrete was casted.

3. The footing dwells and the upper left old column steel bars were welded to the new column steel bars, then the column concrete was casted as illustrated in photo (b) of Figure 5.

4. The previous steps were applied to the next column leaving two old columns in between, and so on.

5. All columns were connected by newly casted drop beams to increase the building stiffness.

The process of the building’s retrofitting took about 6 months, during which and after the completion of retrofitting works, the measurements of the monitoring process had shown no movements, which reflected the efficiency of the implemented retrofitting techniques.

Summary and conclusions

In spite of the report prepared by a consultant who advised to demolish the building, extensive exerted geotechnical, structural and economic analyses lead to the decision of repairing the structure rather than demolishing it. From the present case study, the following conclusions are drawn as follows:

1. Because of inadequate geotechnical investigation prior to construction, the building was constructed on arid problematic soil, without special precautions for founding on this type of soil.

2. The extensive geotechnical investigation showed that the structural foundation system was partially constructed on highly potential swelling clayey soil following a thin layer of sandy soil.

3. Due to leakage of sewage water, continuous widening of the wall cracks persisted due to differential heave of the expansive soil.

4. The stiffness of the building’s Skeleton plays an important role in minimizing the effect of differential heave on the structural safety of the building.

5. Examining the concrete strength of the structural elements; especially the foundation system and basement columns had shown that the strengths of the majority of elements were extremely low, namely, less than 50% of the design strength.

6. Retrofitting of the building was based on the total removal of the high potential swelling soil, being with limited thickness and covering only 60% of the building area. In addition, modifying the foundation system to R.C. footings resting on P.C. piers, isolated from the surrounding expansive soil by foam sheets and clean sand.

Disclosure statement

No potential conflict of interest was reported by the author(s).