Arab J Geosci (2011) 4:1131–1150 DOI 10.1007/s12517-010-0124-3
ORIGINAL PAPER
Estimation of near-surface geotechnical parameters using seismic measurements at the proposed KACST expansion site, Riyadh, KSA Hashim Almalki & Abdel-Khalek El-Werr & Kamal Abdel-Rahman
Received: 12 December 2009 / Accepted: 17 January 2010 / Published online: 18 May 2010 # Saudi Society for Geosciences 2010
Abstract Eight shallow seismic refraction profiles were conducted at the proposed KACST expansion site, northwest of Riyadh, to estimate the near-surface geotechnical parameters for construction purposes. Both compressional (P) and shear (S) waves were acquired, processed, and interpreted using “time-term” technique which is a combination of linear least squares and delay time analysis to invert the first arrivals for a velocity section. The most important geotechnical near-surface parameters such as stress ratio, Poisson’s ratio, material index, concentration index, N value, and foundation material-bearing capacity are calculated. The results of these seismic measurements were compared with the results of borehole report in the project area in terms of number of layers, the lithological content, thicknesses, and N values of rock quality designation. A good matching between the results was observed particularly at the sites of boreholes. Keywords Seismic refraction . Geotechnical . Near surface
H. Almalki (*) King Abdul Aziz City for Science and Technology, Riyadh, Saudi Arabia e-mail:
[email protected] A.-K. El-Werr Geophysics Department, Faculty of Science, Ain Shams University, Cairo, Egypt e-mail:
[email protected] K. Abdel-Rahman National research Institute for Astronomy and Geophysics, Helwan, Cairo, Egypt e-mail:
[email protected]
Introduction Seismic refraction method is widely used in exploration geophysics and subsurface investigations (Musgrave 1967; Hobson 1970; Green 1974; Mooney 1976; Palmer 1986). In seismic refraction survey, the necessary condition for critical refraction is that the seismic velocities of different layers should be higher than those of the overlying layers. The first arrival travel times are detected on a seismogram, and if there are no hidden layers (such as low-velocity zones or thin beds), the interpretation is straightforward. The primary applications of seismic refraction are for determining the depth to bedrock and bedrock structure. Due to the dependence of seismic velocity on the elasticity and density of the material through which the energy is passing, seismic refraction surveys provide a measure of material strengths and can consequently be used as an aid in assessing rippability and rock quality. From the engineering point of view, soils are defined as the material overlying the bedrock produced by rock weathering. It is unconsolidated material of the earth’s crust used to build upon or used as a construction material. The seismic method has emerged as a powerful tool in computing the elastic moduli from which their elastic deformation can be estimated (Stumpel et al. 1984; Davis and Taylor 1979). The technique has been successfully applied for mapping depth to the base of backfilled quarries, depth of landfills, thickness of overburden, and the topography of groundwater aquifers. The ultimate goal of the present study was to establish the ability of seismic measurement to tangibly delineate the shallow subsurface layering as well as determine the following shallow soil engineering parameters: stress ratio, Poisson’s ratio, material index, concentration index, N value, and foundation material-bearing capacity.
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Geologic setting The study area lies within the framework of King Abdulaziz City for Science and Technology (KACST), Riyadh as shown in Fig. 1. Riyadh area is found between latitudes 24° N and 25° N and longitudes 46°30′ and 48° E. It is located in the eastern part of the Najd province in east-central Saudi Arabia. Generally, Riyadh area is underlain by (Phanerozoic) Mesozoic to Cenozoic sedimentary rocks of the Arabian shelf and covered to a large extent by Quaternary deposits. According to Vaslet et al. (1991), Jurassic to Early Cretaceous rocks (Fig. 2) cropping out in Riyadh area and assigned to the newly defined Diriyah supergroup consist of: 1. The informal Byradah group, comprising Late Permian to Triassic deposits, mostly crops out to the west of the quadrangle boundaries. 2. The informal Shaqra group, of Jurassic age, the Middle to Late Jurassic part of which is represented in the quadrangle. 3. The Thamama group comprising Early Cretaceous rocks. In Riyadh quadrangle, the exposed sediments are represented by Middle Jurassic Dhruma Formation at the base and Late Pleistocene to Holocene surfacial deposits Fig. 1 Map of study area with location of seismic lines and boreholes
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(recent lacustrine deposits, Aeolian sand deposits, and fluviatile deposits).
Borehole data Twenty-four exploratory borings have been drilled at the proposed site (which may be greater than the study site) during the period from 19 to 22 July 2007 (Soil & Foundation Co. Ltd.). The purpose of this geotechnical investigation was to determine the depth and characteristics of the different subsurface formations, the required foundation depth, the most suitable and economical type of foundation, and allowable bearing capacity and expected settlements under the proposed structure (Table 1). Standard penetration test (SPT) and laboratory measurement for rock core sample which is known as rock quality designation (RQD) were conducted from the surface and thereafter at 1.5-M depth intervals with the use of a 140-1b hammer to recover sample in the split-spoon sampler. Coring in rock was performed by using double-tube core barrel fitted with diamond bits. The borings were drilled with depths varying from 8.0 to 12.0 M from the existing ground levels (Table 2). According to the results of the boring data, the proposed site is underlain mainly by granular soil and limestone rock formation. The surface layer is composed of dense to very dense dry to damp silty sand with gravels. This layer has a
89 84 100 100 100 33 40 47 38 43 50 100 100 100 35 40 46 86 100 100 100 100 81 89 100 75 100 100 32 36 48 85 100 100 100 100 100 87 85 100
RQD N—values (rock quality designation), SPT N—values (standard penetration test)
86 85 100 100 100 75 84 90 79 82 100 75 88 100 81 100 100 75 83 100 84 100 100 74 80 100 100 100 70 100 100 78 75 85 74 84 100 100 100 4.5 6 7.5 9 10.5 12
70 81 100
65 22 24 100 19 26 48 65 77 100 17 28 45 59 74 40 62 70 45 70 77 100 20 25 46 70 79 44 66 72 40 59 75 45 66 74 47 70 68 42 57 65 37 60 68 41 55 70 44 60 77 48 57 70 39 58 69 45 60 68 40 52 65 45 58 72 42 55 62
B-11 B-10 B-9 B-8 B-7 B-6 B-5 B-4 B-3 B-2 B-1
SPT and RQD “N” values
0 1.5 3
B-23 B-22 B-21 B-20 B-19 B-18 B-17 B-16 B-15 B-14 B-13
Eight seismic refraction profiles are conducted in the study area for the generation of both P- and S-waves as shown in Fig. 1. P-wave data are acquired at every channel in a
Depth (m)
Data acquisition
Table 1 N value from boring results (Soil and Foundation Co. Ltd. 2007)
maximum thickness of about 7.4 m from the ground surface (Table 3). The second layer is composed mainly of very dense limestone with a thickness up to 6.5 m. The recommended type of foundations should be placed at a depth of at least −3.0 m below the original ground level. The borehole report for the area: The surface layer of brown, dense to very dense, dry to damp silty sand with gravel. This layer has thickness ranging from 0.3 to 5.5 m. The second layer is completely weathered limestone. This layer has an average thickness of 6.5 m, while the third layer is composed of creamy moderately weathered, fractured limestone rock. The rock quality designation N values for the first layer range from 17 to 22. By comparing the results of boreholes with that of the present study, it could be concluded that there is a good matching in thicknesses for both first and second layers and for N values where it ranges from 9 to 27 from the present study.
B-12
Fig. 2 Lithostratigraphic column of the central Saudi Arabian Mesozoic (after Vaslet et al. 1991)
47 66 78
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0.8 – 7.2 8 0.5 – 7.5 8 1.5 6.5 – 8 0.3 – 7.7 8 6.7 5.3 – 12 7.3 0.7 – 8 SM silty sand with gravel, CWL completely weathered limestone, ROCK limestone interbeded with thin layer of silty sand
7.4 0.6 – 8 0.3 – 7.7 8 6 2 – 8 6 2 – 8 6.4 1.6 – 8 7.3 4.7 – 12 6.5 1.5 – 8 6.5 1.5 – 8
B-21 B-20 B-19 B-18 B-17 B-16 B-15 B-14 B-13 B-12 B-11 B-10 B-9
7.3 0.7 – 8 6.8 1.2 – 8 6.7 1.3 – 8 6.5 1.5 – 8 7.3 4.7 – 12 6 2 – 8 7 1 – 8 6.5 1.5 – 8 7 5 – 12
The collected waveforms have been subjected to a sequence of processing steps started by the elevation correction to the ground datum. Then, the displayed waveforms have been filtered through the application of Fuzz filter that is a fivepoint Parzen (triangular) smoothing filter that attenuates high-frequency noise. Low- and high-cut filters have been applied for all traces, while notch filters are applied to eliminate electromagnetic noise coupled from AC power lines. Baseline correction is carried out for all of the recorded traces. After the aforementioned corrections have
SM CWL ROCK TOTAL
Data processing
B-8
separate file. But we have selected only seven shot points distributed all over the profile as shown in Fig. 3. The intergeophone spacing was 2 m for all profiles. Generation of Pwaves was carried out along profiles using hydraulic weight drop as a source of energy. These waves have been recorded throughout repeated impacts on a metal striker plate and vertical geophones (Fig. 4a), while shear wave generation is conducted along the same eight profiles of Pwaves. Also, the inter-geophone spacing was 2 m for all profiles. Kobayashi method has been applied for creating and recording of S-waves. In this method, a wooden plate of 3 m long, 30 cm wide, and 20 cm thick was held in firm contact with the earth’s surface by the weight of a heavy vehicle. S-waves are acquired by hitting one end of the plate horizontally using a sledgehammer. Then, the plate is struck on the opposite side to obtain the opposite (reverse) polarity that adds the SH-wave effects (Sheriff and Geldart 1986). S-wave records are usually displayed in the same manner as P-waves (Fig. 4b). The horizontal geophones are used only for recording S-waves where the shot point lies at the midpoint of the profiles. A total of five stacks were made per each S-wave shot location and three stacks per each P-wave. Both waves (P and S) were recorded using 14 Hz.
B-7
20 26 35 40 48
B-6
17 24 32 36 46
B-5
22 28 38 43 50
B-4
51 61 72 81 90 99 100 100
Avg.
B-3
37 52 62 70 75 85 100 100
Min.
B-2
100 70 79 100 100 100 100 100
Max.
B-1
Avg.
Subsurface formation thicknesses (m)
Min.
Soil
Max.
B-22
RQD N values
Table 3 Subsurface formation thicknesses at the investigated area from borehole data (Soil and Foundation Co. Ltd. 2007)
SPT N values
B-23
Table 2 Estimated SPT and RQD “N” values at the area of interest (Soil and Foundation Co. Ltd. 2007)
7.3 4.7 – 12
Arab J Geosci (2011) 4:1131–1150 B-24
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Fig. 3 Layout of P-wave profile no.1
been applied, the data are subjected to the following steps using SeisImager Software Package: 1. First break-picking of the digitized seismic waveforms (shot records or set of seismograms) for all channels
Fig. 4 Samples of the acquired raw data. a P-wave of the left far-offset shot point. b Right side S-wave mid-point shot
along all profiles for both P- and S-waves as shown in Fig. 5a. 2. After the first break-picking has finished for all channels of the seven shot records from seven shot points at each profile, seven travel time–distance (T–D)
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Fig. 5 a P-wave first break-picking of profile 1. b P-wave travel time curve of profile 1. c Final velocity depth model of profile 1
curves are established along each profile for both Pwave and S-wave as shown in Fig. 5b. 3. Layer assignment procedure for every layer of P-wave profiles on each T–D curve has been achieved. 4. Finally, the depth model for the detected layers under each profile has been constructed representing the final step in this processing scheme as shown in Fig. 5c. The inversion algorithm uses the delay time method (Pakiser
and Black 1957) to obtain the depth model, which is then trimmed up by a series of ray tracing and model adjustment iterations that seek to minimize the discrepancies between the field measured arrival times and the corresponding times traced through the 2.5D crosssectional depth model. For direct arrivals through layer 1, velocity of the surface layer (V1) is computed by dividing the direct distance (DD) from each shot point
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(SP) for each geophone (Geo) by the corresponding arrival time. These individual velocities are averaged for each shot point (Avg. V1). Refracted arrivals for layers beneath layer 1 are computed by two methods: (1) Regression, in which a straight line is fitted by least square best-fit line to the arrival times representing the velocity layer and (2) the Hobson–Overton method (Scott 1973), which is a least-square adaptation of the plot of differences method (Redpath 1973a, b). (1) Regression velocities (V) and intercept times (Ti) are computed for refracted arrivals at a group of geophones (Geos) to the left or to the right of a shot point. Average velocity (Avg. V) and average intercept time (Avg. Ti) are displayed along with the number of points used in the regression. Average velocities are computed by taking the reciprocals of the weighted averages of the slopes of the regression lines.
a
where V is the desired refractor velocity, Δxi is the difference between distances to geophone i from the two opposing shot points (one on each end of a spread of geophones), Δti is the corresponding difference in arrival times, and n is the number of geophones over which the summations are made. The computed velocity (V), the average difference in delay times at the two shot points (TdSP), the overall standard error for the group of geophones (Std Err Overall), and the four highest errors (Erri) at the geophone numbers (Geo) have been obtained. In case more than one pair of shot points are used in the velocity computation, the
Amplitude in arbitrary unit
0
25
Time (msec)
Normalized Trace Stacking
Frequency Filter
50
75
100
125
Raw
Filtered
Stacked
Power Spectrum of Trace #1 LP
LowPass Cut Frequency
Power Spectrum of Trace #32 LP
140 Hz
LowPass Cut Frequency
400 Hz
b
140 Hz
Fig. 6 a The effect of reverse polarity of one side shot and stack on trace no. 2 in line 1. b Low-pass frequency filter (the lower left panel is the frequency spectrum of trace no. 1, and the lower right panel is the spectrum of trace no. 32)
(2) Hobson–Overton velocities are computed if there are reciprocal arrivals from two opposing shot points at two or more geophones by the following formula: P P ΔX 2 ð Δxi Þ2=n P P i P V ¼P ð1Þ Δxi Δti ð Δxi Þð Δti Þ=n
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Table 4 Ranges of concentration index and stress ratio correspondent to the soil competent degree (after Abd El-Rahman 1989)
Concentration index, Ci Stress ratio, Si
Weak
Fair
Good
Incompetent
Fairly competent
Competent
Very soft
Soft
3.5–4.0 0.7–0.61
4.0–4.5 0.61–0.52
average for all pairs is computed, weighted by number of points used to compute the velocity for each pair. Formulas used for computing the parameters above are: X X TdSP ¼ Δti Δxi =V =n ð2Þ Erri ¼ Δti Δxi =V TdSP StdðErrOverall Þ ¼
ð3Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X Err2i =n
ð4Þ
In some cases, during the acquiring of shear waves, some of the vertical component (SV) of the shear wave energy could convert into compressional wave which results in a P-wave arrival—or a delayed P-wave arrival. Moreover, if the geophone is completely blind for compressional waves, some of the converted P-waves are converted back to shear waves which produce a pre-shear wave arrival. Those arrivals are easily distinguishable and are completely different from shear wave arrival. The first case is a pure P-wave arrival which will have the same polarity in the two-side shot, while the second one is like the shear wave arrival, but it has very low amplitude. Different techniques were applied to avoid and/or solve such problems. The most common and powerful method is to reverse the polarity of one side shot and stack it with the other side shot. Such stacking should eliminate any P-wave arrival and will emphasize shear wave arrival in the same time. However, since the two shots are not symmetric, there is always a residual of P-wave, and in fact, this technique will enhance shear wave arrival. Moreover, it is a nature of the hammer as a source to produce pulses of different power, and hence, traces of different amplitudes will be Table 5 Poisson’s ratio (σ), rigidity (µ), and material index (Mi) correspondent to different material (after Birch 1966; Gassman 1973; Tatham 1982; Sheriff and Geldart 1986) Rock sample
σ
μ
Mi
Liquids Perfect elastic rocks Very hard indurated rocks
0.5 0.25 0.00
0 μ=λ λ=0
−1 0 +1
Fairly compacted
Moderate compacted
Compacted
4.5–5.0 0.52–0.43
5.0–5.5 0.43–0.34
5.5–6.0 0.34–0.25
recorded (Fig. 6a). Therefore, adaptations were introduced to this method. Prior to stacking, low-pass filter was applied to the data. The cutoff frequency is gradually decreased as we go far from the shot point (Fig. 6b). This is due to the fact that the Earth acts like a low-pass filter, so near the shot, the source waveform is composed of a high-frequency band, and far from the shot, it will have a much lower frequency band. The first and the last traces in the spread have a cutoff frequency of 140 Hz, while the traces at 62 and 64 m have a cutoff frequency of 400 Hz. Then, the corresponding traces in the two side shots are normalized to maximum amplitude limit. After stacking, the resulting amplitude is compared with the amplitude of the two side blows. The stacked trace amplitude should be greater than the individual traces—if the two signals are of the same polarity. If it is not the case, then the stacked sample amplitude is set to zero. This amplitude examining process will continue until the amplitude of the stacked sample is greater than the individual samples by a minimum of noise level. This noise level was introduced because the condition will usually hold for random noise. Geotechnical parameters No construction material has more variable engineering and physical parameters than the ground’s soil. These parameters vary both laterally and vertically, and often, the variations are strong (Bowles 1982). In order to evaluate the competence of the subsurface for construction, some of the shallow soil geotechnical parameters were calculated. Several parameters are calculated: the concentration index (Ci), the material index (V), N value, Poisson’s ratio, the Table 6 Soil description with respect to Poisson’s ratio (σ) and material index (Mi) (after Birch 1966; Gassman 1973; Tatham 1982; Sheriff and Geldart 1986) Soil description Incompetent Fairly to parameters to slightly moderately competent competent
Competent Very high materials competent materials
Poisson’s ratio (σ) Material index (Mi)
0.4–0.49
0.35–0.27
0.25–0.16
0.12–0.03
−0.5 to −1
−0.5 to 0.0
0.0–0.5
