Development of geophysical investigation for verifying treatment efficiency of underground cavities Hasan A. Kamal* Kuwait Institute for Scientific Research, Infrastructure Risk and Reliability Program, Environment and Urban Development Division, Kuwait Mahmoud F. Taha, Hassan J. Karam Kuwait Institute for Scientific Research, Building and Energy Technologies Department, Kuwait * Corresponding Author: hkamal@kisr.edu.kw Abstract Several sinkholes were detected in a residential area located close to the coastal side on the north east corner of the Arabian Peninsula. The area consists of around 2,500 residential houses. Sudden ground surface subsidence occurred with different sizes varied to a maximum of 31 m in depth due to underground cavities. A decision was made by the government to evacuate the affected area. Comprehensive investigation studies were conducted for understanding the causes of the sinkhole occurrence in addition to recommend treatment measures for the affected areas. The results of the investigation studies revealed that the geological profile is made of sandy overburden soil uncomfortably overlaying Karst limestone bedrock. The cause of the sinkholes was attributed to the dissolution of the limestone bedrock and the subsequent raveling of the overburden soil cover. Microgravity technique was used in the geophysical investigation to detect underground deep cavities. The method focused on the affected area close to the occurred subsidence. The validity of the microgravity technique was confirmed after applying a drilling program in the affected area for detecting underground cavities. Treatment measures were applied in a pilot area for minimizing the risk of ground surface subsidence. The treatment included filling the underground deep voids and cavities in the bedrock layer with cement mortar and fractured rocks with cement paste. After treatment application, another geophysical investigation was carried out in the same treated area in order to record the actual changes in the negative anomalies by comparing the investigation results, before and after treatment. In this paper, the nature of the case under study is described and the causes of the sinkholes are presented. The results of the microgravity technique are presented and its efficiency is proved for treatment of underground cavities. Keywords: geophysical; microgravity; subsidence; sinkhole; cavity 1. INTRODUCTION A residential area was developed in a desert terrain located in the northeast coastal side of the Arabian Peninsula consisting of approximately 2,500 housing units including community services. After eight years of construction, four major ground subsidence occurred in the form of sinkholes which caused destruction of property and threatened human lives. An immediate action was taken by the government to evacuate the affected area considering this area as a high risk [1](I. Al-Rifaiy, 1990). After fifteen years and within the same area, four new sinkholes were observed during a new investigation program. Upon the occurrence of the sinkholes, several investigation programs were conducted and the results revealed that the geological profile is made of sandy overburden soil uncomfortably overlaying Karst limestone bedrock. The cause of the sinkholes was attributed to the dissolution of the limestone bedrock and the subsequent raveling of the overburden soil cover. The affected area is characterized by a typical desert environment with gentle ground slopes from about 270 m above sea level in the extreme south-western corner of the country towards the lowlands in the northeast [2, 3](D. Budon & A. Al-Sharhan, 1968; A. Mukhopadhyaya, 1996). Morocco Rabat / November 23-25, 2011 1
As part of the investigation programs, geophysical survey was carried out to investigate the existence of underground cavities and their lateral extent. The microgravity technique (gravimetric testing) was used as it proved dependable, non-destructive and risk free. The survey results were represented in contour maps delineating anomalies varying from negative to positive values. The negative values interpreted as low density subsurface layers and the possibility of existence of cavities. As the contour numbers increase into the positive numbers, the possibility of cavities existence is reduced. The geotechnical investigation was also carried out in the investigation program by drilling boreholes to depths reaching the limestone bedrock layer, up to the depth of 85 m. The geological profile of the area consists of 31 m to 39 m of overburden comprising of dense to very dense sand, which is underlain by limestone bedrock formation [4](H. Kamal et al., 2007). The soil-rock interface contains dark chert nodules and sandstone. The groundwater table was close to the soil-rock interface level. From the drilling records, the cavities were encountered at several depths at the limestone formation. 2. APPLIED TREATMENT MEASURES The work is carried out in a pilot area with a total surface area of around 62,000 m 2. The treatment measure is based on injection of cementatious grout material from the ground surface to the underground cavities which required drilling of boreholes up to the cavities level. The main purpose of the application of the treatment measures is to eliminate the possibilities of collapses of the upper surface of the Karst cavities of the limestone bedrock by filling up of the uppermost cavities in the rock formation with grout. By closing the bedrock voids and preventing soil raveling from the upper layers, the thick overburden of dense sand will assure a sufficient ground support for all structures above ground [5](H. Kamal et al., 2006). Boreholes are drilled during the investigation programs to identify the causes of these sinkholes in the form of grids as exploratory and grouting boreholes. Since all boreholes are similar in scope, the boreholes are used for grout filling. Control holes are drilled immediately after completing the grout injection work. The grout holes employed rapid drilling techniques without core recovery for reaching the required depth, Figure 1. The applied grout treatment measures consisted of two grouting methods. The first method is to use injection technique for filling the underground cavities and voids in the bedrock by a thick cement mortar grout pumped through heavy duty pipes. The up-stage method is used starting injection from the cavity bottom as shown in Figure 2. The grout used in this method is a stable cement mortar grout containing fine aggregates (sand). Injection method from the ground surface is used with low pressures for proper filling of the cavities. This treatment is not intended to densify the rock or to improve its strength, but to fill up the existed voids and cavities and to prevent migration of sand from the overburden layer into the limestone bedrock. The second grout method is called permeation grouting which is used in locations where open cavities are missing in the highly pervious uppermost layers of the bedrock. Permeation grouting is used for filling open fissure and small Karst channels in the limestone bedrock injected into the rock mass under pressures using packers as shown in Figure 3. The permeation grout consists of cement-water mix without aggregates, using additives such as bentonite and/or plasticizers. 3. MICROGRAVITY SURVEY 3.1. Basic Theory The principal of any gravity survey is to identify areas of contrasting density by collecting surface measurements of the Earth s gravitational field. Gravity measurements are based upon Newton s Law of gravitation as M M r 1 2 F g = G (1) 2 Morocco Rabat / November 23-25, 2011 2
where F g is the force of gravity between two objects; G is the universal gravitational constant, equal to 6.673 x 10-11 N-m 2 /kg 2 ; M 1 and M 2 are the masses of the two objects; r is the distance between the two objects. To focus on the gravitational effect of the earth, it is convenient to divide Eq. (1) by the mass of the test object, M 1, to obtain the gravitational attraction per unit mass expressed as F GM g = (2) M g 2 = 2 1 r where quantity g represents the gravitational acceleration of a freely falling object when earth mass is substituted for M 2. The unit of gravitational acceleration is Gal and the commonly used unit in regional gravity survey is milligal. Figure 1. Drilling of a deep vertical borehole. Figure 2. Injection of cement grout into an underground cavity. Morocco Rabat / November 23-25, 2011 3
Figure 3. Pumping of permeation grouting into open fissure and small Karst channels in the limestone bedrock. The microgravity method involves measuring the very small differences in gravitational force field. The interpretation depends on determining the relative variation from one point to another within the surveyed area. Each observed value is a sum of the effects of: latitude, elevation, topography, earth tide effects, and lateral changes in the density distribution in the subsurface. Separate from the geological effects, these effects are related to the position of the measurement on the earth surface, sun and moon effect, instrument drift, shape of the earth, and local variation in topography. Only the vertical components of the gravitational acceleration are measured in the exploration field work. After removing the effects, any remaining variation is a function of the geology under investigation. The microgravity data is produced as full color residual anomaly maps, where the mass deficiencies indicated by negative residual anomalies. 3.2. Geophysical Investigation Results - Before Treatment Application Before applying the treatment measures to the area under study, a geophysical survey is carried out using the microgravity method. The survey emphasized on the highest risk area specified within the pilot area and other selected locations in the affected residential area. Around 1000 microgravity stations are measured with a mesh of 10x10 m reduced to 5x5 m in critical areas. Residual maps have been calculated considering that the negative anomalies representing a subsurface mass deficit resting within both the shallow clastic formation and the upper part of the bedrock limestone underneath which is located at about 40 m deep. This mass deficit can be attributed to cavities, weathered soil at shallow depths or porous coarse sands. The residual map for the area is shown in Figure 4, which detects the extent and distribution of underground cavities. 3.3. Geophysical Investigation Results - After Treatment Application After applying the treatment measures, another geophysical survey is carried out in the treated area using the same microgravity method for recording the actual changes in the negative anomalies by comparing the investigation results, before and after treatment. In this survey, 500 microgravity points is measured with a mesh of 10x10 m reduced to 5x5 m in critical areas. Figure 5 shows the residual map for area representing gravity positive changes that can be considered an effect of applying the treatment measures. Morocco Rabat / November 23-25, 2011 4
INVACO2: International Seminar, INNOVATION & VALORIZATION IN CIVIL ENGINEERING & CONSTRUCTION MATERIALS N : 2O-160 Hasan A. K., Kuwait Institute for Scientific Research, Kuwait Figure 4. Residual map for the pilot area, before treatment application. Figure 5. Residual map for the pilot area, after treatment application. 4. DISCUSSION AND CONCLUSIONS For the area under study, treatment measures are applied that included filling the underground deep voids in the bedrock layer with cement mortar and fractured rocks with cement paste using cavity filling and permeation grouts. The main aim is to minimize the risk of ground surface subsidence. An evaluation program is conducted for assessing the efficiency of the implemented treatment measure. Filling of the Karst cavities and channels is confirmed as layers of the used grout is found in sections of the control boreholes, Figure 6. Morocco Rabat / November 23-25, 2011 5
Two geophysical investigations are carried out and compared, before and after treatment application, to verify the efficiency of the applied treatment measures. The same method and professional experiences are used for the purpose of accurate comparison. Residual maps represent the density variation within the clastic soil sediments and the underlying limestone bedrock in terms of low positive and negative anomalies depending upon the geological conditions. The residual map for the area after treatment application, Figure 5, shows gravity positive changes which can be considered as a consequence the applied treatment measures. However, these gravity changes have not eliminated the negative anomalies of the gravity field in the study area. It seems that part of the negative mass is compensated by the geotechnical treatment, while there are still rock volumes with very low density, may be contributed to unfilled cavities, fracture zones, loose sands and/or permeable channels in the bedrock layer and the overburden sands. Acknowledgments Figure 6. Grout were found in the section of the controls boreholes. The authors appreciate the support of the Public Authority of Housing Welfare (PAHW) of Kuwait and Kuwait Institute for Scientific Research (KISR) for this research project. References [1] Al-Rifaiy, I., 1990. Land Subsidence in the Al-Dhahar Residential Area in Kuwait: A Case History Study. Quarterly Journal of Engineering Geology, London, (23), 337-346. [2] Burdon, D., and Al Sharhan, A., 1968. The Problem of Paleokarstic Dammam Limestone Aquifer in Kuwait. Journal of Hydrology, (6), 385-404. [3] Mukhopadhyaya, A., Al-Sulaimi, J., Al-Awadi, E., and Al-Ruwaih, F., 1996. An overview of the tertiary geology and hydrology of the northern part of the Arabian gulf region with special reference to Kuwait. Earth Science Review, (40), 259-295. [4] Kamal, H., El-Hawary, M., Abdullah, W., Abduljaleel, A., Taha, M., Karam, H., Abdul-Salam, S., Al-Sanad, S., Abbas, M., Al-Shatti, F., Al-Elaj, M., Al-Arbied, A., and Al-Furaih, R., 2007. Preparation of Tender Documents, Supervision of Implementation and Evaluation of Treatment Measures of the Pilot Area of Al-Dhahar-Phase II, Final Report, KISR, Kuwait. [5] Kamal, H., Taha M. and Karam H., 2006. Implementation of Treatment Measures for Sinkholes Occurrence in the State of Kuwait, The Second International Conference on Problematic Soils, Selangor, Malaysia 3-5, December. Morocco Rabat / November 23-25, 2011 6