Determination of Bearing Capacity of the Soil Using Cross-Hole Seismic Survey for a Water Treatment Plant, Thi Qar, Southern Iraq

Abstract


Introduction
These reliable techniques define different properties that are used in a variety of applications such as engineering, construction and hydrology (Sharma, 1986).Seismic methods involve the recording of seismic waves (naturally occurring or generated) to detect subsurface interfaces and the measurement of seismic velocity changes with depth (Telford et al., 1990).The method of measuring seismic velocities through wells can provide detailed information about the lithic properties, geological structure, and their durability in the event that engineering projects are carried out in them (Mikhail and Dorokhin, 2019).This method includes releasing seismic waves at regular intervals and depths from one well to another (one of which is a transmitter of the waves, and the other receiving them at the same depth) and deducing the velocity of the waves that travel between them.The aim of the cross hole method is to research about cavities, fault and the changing in horizontal layers (Tezcani et al., 2009).It is worth noting, that fewer wells are needed to obtain a continuous subsurface image, and thus reduce the total costs and obtain a large coverage of the physical properties of soils or rocks.
The aims of the present study are: • Estimation of the permissible bearing capacity and soil compaction for the purpose of designing the most secure and economical foundation .• Determination of the soil profile section, ground water, subsoil geometrical, physical, and chemical properties up to a predetermined depth .• Verify the validity of the site as a basis for supporting the proposed structure .

Location of the Study Area
The study area is located in southern Iraq between the two governorates of Nasiriyah -Basrah, and it is 400 km from the capital Baghdad (Fig. 1), which is 105 km west of Basra, and 70 km east of Nasiriyah, This field is bounded from the south and southeast by the Al-Luhais field, which is 30 km away.

Geological and Tectonic Setting
The study area is located within the subzone Zubair in the Mesopotamian basin of the unstable shelf (Jassim and Goff, 2006), The study area was affected in various degrees by the orogeny of the Alpine mountains, that occurred during the Tertiary period.The direction of the axis of the folds and sedimentary basins was parallel to the direction of the alpine movement, which is northwest -southeast (Al-Mashhadani, 1984) based on the tectonic divisions of Buday and Jassim (1987).Structures must be designed to withstand the forces generated during earthquake shaking.Although the structures are designed according to international building code (IBC 2006), the seismic design standards are taken from the Uniform Building Code (UBC 1997) which included research into seismic and hazardous areas in Iraq.Tectonically, Iraq is located in a relatively active seismic zone on the tectonical active northern and eastern borders of the Arabian plate (Al-Sinawi, 2006).

Instruments and Field Work
Seismography WZG-12 is a geophysical device that records the arrival time of seismic waves (Fig. 2).The seismic source of the survey is a hammer, an electric spark or an explosion, and it must produce enough energy to penetrate the subsurface layers and thus explore the subsurface waves.In addition, this examination is used for prospecting in many fields, including hydrology and problems with soils under electrical stations, railways, roads, and bridges (Von Ketelhodt et al., 2018).
Prior to the field work, a reconnaissance trip was conducted on 10 July 2018 in the study area (Luhais and Suba oil field) by the Multidisciplinary Consulting Services Office of Dhi Qar University to review the area, where the engineering facility is to be established.The nature of the area and its topography were identified.The locations of the wells were selected (the sites were chosen by the beneficiary) to be drilled.All the positive and negative points facing the cross-hole geophysical survey and the geotechnical engineering examinations of the soil were recorded, which includes, boreholes standard penetration test (SPT), collection of soil samples (disturbed and undisturbed), and recording of groundwater table (GWT).

Cross-Hole Survey Through Wells
A seismic survey was conducted through the Cross-Hole wells using a seismograph (WZG-12) on 25 July 2018.The source and the recipient were separated inside the wells with a specific distance is 3 meter between two wells by the geophysicist and according to the objective of the study area.Cross-Hole seismic refraction surveys were carried out in the study area.The dummy probe power source was used inside one of the wells and the receiver was Triaxial Geophone Receivers (Von Ketelhodt et al., 2018).

Field Work Procedures
The basic principle in the cross-hole survey is the use of the initial arrival times taken for the distance between the wells.The wave velocities were found by using the general law of velocity depending on the time and distance between the two wells.The cross-hole method, also called the well-to-well detonation method.It is possible to obtain data from many receiver wells at various horizontal depths within the wells, while the energy source is generated inside a single well for the same depth in receivers (Davis and Schultheiss, 1980).This method is considered one of the important and widely used geophysical methods in the fields of civil engineering as it gives a description of the soil and the subsurface layers (Ibragimov et al., 2016).Furthermore, data processing aims to get a greater signal ratio, while attempting to reduce noise using software to allow accurate seismic data interpretation.In addition, noise obtained from a variety of sources in the study area, including movement from the fast traffic on the street near the wells and vibrations from the operation of heavy machinery near the work area (Sharma, 1986).

Calculating the Velocity of the Cross-Hole Seismic Waves
Velocities of P-Wave and S-Wave are calculated based on the arrival times of the waves between the source and receiving wells for each depth gradually, according to the depth chosen for the study (in the present study, the vertical distance is 1 meter) (Stokoe and Santamarina, 2000).The velocities of seismic waves were calculated starting from 1 m to 10 m as depth (Table 1).The following equations were used to calculate the velocity. (1) (2) (3) where: Vp = velocity of primary waves; Vs = velocity of secondary waves Tp = arrival time of primary waves; Ts = arrival time of secondary waves X = distance between source and receiver Table 1.The arrival times, velocities of primary, secondary waves, and the distance between wells for the Cross-Hole seismic survey of the study area between BH1 and BH2.Table 1 indicates the arrival time and velocity of primary and secondary seismic waves for the cross-hole survey in the study area.It was noted that the primary wave velocities ranged between (262-535 m/s), and secondary wave velocities ranged between (153-293 m/s) for all depths from the surface to 10 meters.It is worth noting that the velocities of the primary and secondary waves are very large due to the presence of a cement layer at a depth of 1 m.Moreover, it was noticed very clearly, that the velocities of the primary and secondary waves decrease at the depths (4 and 5 meters) compared to the depths above and down.This is not normal, where primary wave velocities ranged between (262-284 m/sec), and secondary wave velocities ranged between (153 -172 m/sec).Hence, there is a change in velocity at these depths, which indicates a change in the type of lithology or the physical properties of this particular soil.It can also be noted that, the velocities of seismic waves, in general, are of a low value because the study area soils are of medium density and hardness, therefore the velocities of the waves are affected accordingly.

The Relationship Between Wave Velocities and Depths
The relationship between the velocities of primary and secondary seismic waves and depths is very important, in addition to the relationship of arrival times of waves with depths (Fig. 3).This relationship gives the change or anomaly clearly, when plotted on the X and Y axes, and thus this change or anomaly indicates the presence of weakness in the soil of study area (Ertsalov et al., 2015).

Fig.3. Relationship between the velocities with depths
It can be noticed the relationship between the velocity of seismic waves with depth, since the velocity of waves from the surface to 3 meters, a natural gradual increase was recorded.But there is a clear change in the speed of the primary and secondary waves at depths 4 and 5 meters (Fig. 3) .This was observed in laboratory examinations of samples taken from wells drilled in the study area, or there was anomalies with low elasticity modulus were present (SIR, 2018).Primary waves have significant velocity at a depth of 1 m due to the presence of very solid soil (a mass of cement) at this depth (stiff soil)

Results of the Elastic Modulus of Layers
The elastic and geotechnical properties of the soil are calculated based on the velocities of the primary and secondary waves, the ratio between them and the density.These characteristics are critical for determining the values of the change in elasticity module or the physical properties of soil layers at different depths between seismic wells (in this study the elastic properties were calculated per 1 meter) (SIR, 2018) as in Table 2.
Table 2.The values of elasticity coefficients calculated according to the cross-hole between wells drilled in the study area.
Soil elasticity modules are very important to reflect the nature of the soil, that give a clear idea to the engineer for using the appropriate type of foundations, as well as the nature and loads of the engineering structure to be built on the surface of the geophysical and engineering studied area (Maslak et al., 2013).It is noticed that, the values of the elasticity modules are clearly proportional to the velocity values, as they decreased at depths of 4 and 5 m, and this is another evidence of weak soil at these depths (Table 2, Fig. 4 a and b).For example, the values of the poisson's ratio ranged between 0.21-0.24at depths of 4 and 5 m respectively.The groundwater level increased, which led to a decrease in the velocity of the secondary wave, which inevitably affected the values of the poisson's ratio.These depths, according to laboratory tests at the well (BH1 and BH2), consisted a layer of brown, gray, red, and white soil to sandy silty clay soil with amount of white spots, and medium consistency.Therefore, when the layer type differed, all the elasticity coefficients (Young modulus, poisson's ratio, volumetric modulus and shear modulus) differed with it.It is worth noting that the constant groundwater level at two wells (BH1 and BH2) was 1.2 m and (BH3) was 1 m.In addition, dynamic elastic modulus is associated with seismic wave's velocities by these relations.So, the researcher will use these equations for the research (Davis and Schultheiss, 1980).Where : Density and it is measured by gm/cm 3 .Also, the values of the Bulk modulus are proportional to primary wave velocity, while the values of the Young modulus are proportional to secondary wave velocity.Thus, when the shear modulus and the Young modulus decrease, this indicates the presence of moisture content, water saturation, or weakness within this layer.It would depend on the velocity of the shear waves more than others, because they have a greater amplitude and a lower frequency, which give more information about the layer in terms of shear elastic module and the Young modulus (Bezrodny et al., 2016).Depending on the velocities of seismic waves (S-waves and P-waves), the geotechnical characteristics were calculated between the wells used in the Cross-hole survey, as shown in Table 3.These results are very important for the purpose of knowing the engineering properties of the soil between the two wells (BH1 and BH2) for each selected depth ( Bezrodny et al., 2015) .
According to the relation of the bearing capacity values with hardness, it was noted that the bearing capacity at 2 and 3 m is greater than 5 tons / m 2 .This refers that the soil was medium stiff for the depths 4 and 5 m, where the bearing capacity was less than 5 tons/m 2 , which means that the type of the soil is soft.Moreover, the depths 6, 7, 8, 9 and 10 m had a greater bearing capacity more than 10 tons/m 2 , indicating that the soil is a stiff type to very stiff.

Standard Penetration Test
This type of test is commonly, used for field testing in geotechnical engineering.It is a measure of soil hardness and its conditions (Table 4).On-site standard test results for wells BH2, BH1 and BH3 (Fig. 5 a, b and c), it can be observed the values of standard penetration N (N is the average of SPT value), which gradually increase with depth in the three wells.It started from a depth of 4 m, and gradually increased to a depth of 15 m.This reinforces our conclusion, that the shallow depths from the surface to a depth of 6 m is weak soil with less hardness (SIR, 2018).

Calculation of the Geotechnical Properties of the Soil
These tests demonstrate soil engineering behavior, thus, seismic waves are mainly, affected by the type of the soil.However, they play a role in giving a clear view of the textural soil layers in the site as in geophysical methods (Fig. 6).Laboratory tests were conducted for soil samples taken from wells BH1, BH2 and BH3.

Chemical Tests
The chemical tests of soil wells included sulfate, gypsum, organic contents, and dissolved solids (Table 5).From the above table, it can be seen that, the value of SO3 varies in the three wells between 1.98% and 2.48% for soil, and the percentage of organic matter in soil samples varies between 1.4% and 1.92%.The content of gypsum and soluble solids ranged between 4.25-5.33%,3.37 -9.86 % respectively.From the aforementioned geophysical and geotechnical results depending on the weight of the engineering structure and the type of foundations used the weights were large, it was assumed that the foundations of the type (Deep pile) were used.as well as the engineering study was presented Sulfate Resisting Portland cement, Sulphate (for foundations and all) suggestion for use soil Concrete.

Conclusions
The seismic cross-hole method is characterized by high worthiness in measuring the velocity of direct waves through the soil layers, where the velocity of primary and secondary waves are measured along the horizontal line between the source well and the receiving well.Determining the zone of weakness or thin layer at depths 4 and 5 meters based on a seismic cross-hole survey, and the results of laboratory tests.Therefore, it is preferable to improve the specifications of the soil at these depths by tamping and vibrators, or the deep pile foundation should be used.Through the results of the measured geotechnical properties, such as Poisson's ratio, material modulus, concentration coefficient, lateral ground pressure coefficient, internal friction angle, and maximum load capacity; the nature of the soil, in general, was identified, and therefore, the type and depth of foundations are selected accordingly.It is preferable to stay away from depths 4 to 5 meters if the engineering structure has weights that exceed the loading capacity of the soil at the mentioned depths.The increase in the proportions of chemicals in the soil such as; Gypsum, SO3, TSS, and ORG, favors the use of Sulfate Resisting Portland cement for the foundations and all concrete in contact with the soil.The values of the material modulus, concentration coefficient, and friction angle (Ø, I c, I m) were within their ranges, indicating a clear decrease at the weak and loose depths, while their values increased relatively, at the more solid depths.
The results of the particle size analysis of the soil components showed that they vary in the proportions of their volume components.In general, they are divided into two layers, the first is silty clayey sand, and the second layer is sand.The proportion of sand is higher compared to clay and silt.The maximum load capacity values give a wide range, because it depends on shear waves (Vs) only, which reflects the ability of this wave to be sensitive to changes and its importance in engineering studies.It is also observed that, these values converge in their meaning with the values of standard penetration, which enhances the importance of measuring this characteristic.It was noted that the bearing capacity values increase with depth, especially below the depth of 4 m, which records an agreement with the values of standard penetration, except for some weak depths, which indicated a decrease in its carrying capacity, especially at shallow depths.

Recommendations
It can be chosen the appropriate foundations for the construction of the engineering facility based on its weight, loading capacity of the soil, and according to the results that appeared.It is preferable to use other geophysical methods to increase the confirmation of the results accuracy for the soils of the study area, such as; Seismic down-hole or Seismic up-hole methods.

Fig. 1 .
Fig.1.Location map of the study area

Fig. 4 .
Fig.4.a and b, relationship between elasticity modules with depths angle of internal friction; Ko = Coefficient of lateral earth pressure at rest.(Ic) = Concentration index; (Im) = Material index; (qu) = Ultimate bearing capacity.

Fig. 6 .
Fig.6.A cross-section of the lithology of the three wells in the study area, BH1, BH2 and BH3.

Table 3 .
Geotechnical properties computed based on cross-hole results.

Table 4 .
The results of SPT of soil for the study area (BH1, BH2 and BH3).

Table 5 .
The chemical tests of soil in the study area