Assessment of in situ properties of municipal solid waste with a large-diameter borehole method

Abstract

A Municipal Solid Waste Borehole Assessment (MBA) was developed to assess in situ geotechnical properties of municipal solid waste (MSW) during the boring of gas extraction well construction. A Large-Diameter Borehole Caliper (LDBC) was lowered into the borehole to measure the diameter and record the condition of the wall by time-lapse video photography. The results indicated that the borehole experienced significant radial compression with depth following completion. Radial compressions amounted to approximately 7.5% at 9.14 m, 10% at 21.3 m and 11% at 27.4 m below ground surface. The bulk modulus was estimated by using the captured volumetric strains and reported lateral earth coefficients, and the results showed that it increases with increasing depth. For MSW, the bulk modulus increased up to 13.4 MPa in a linear trend with depth. The unit weights of MSW were obtained using three diameter readings from LDBC, auger barrel outside diameter and outer cutting bit outside diameter. The results showed that the diameter based on outer cutting bit yielded realistic unit weights (5.08–9.68 kN m^–3) due to unrealistic calculated saturations by other two assumed diameters. The borehole assessment with LDBC was shown to be an efficient and valuable means for characterising MSW and effectively designing gas extraction wells. The research provided a means to assess the waste mass with accuracy at great depths by directly observing and measuring borehole condition.

Keywords

Municipal solid waste landfills borehole field test bulk modulus unit weight waste management

Highlights

A Large-Diameter Borehole Caliper was developed to assess in situ municipal solid waste (MSW) properties.

The borehole experiences significant radial compression (>10%) at 30.5 m depth.

The bulk modulus of MSW increased linearly with depth up to 13.4 MPa.

MSW gross unit weights except cover soils were ranged from 5.08 to 9.68 kN m^–3.

Introduction

For efficient management of municipal solid waste (MSW) landfills, it is important to characterise MSW properties. The mechanical and geotechnical (static and dynamic) properties of MSW are mostly uncertain and hard to generalise in situ (Alidoust et al., 2021; Kavazanjian et al., 2001; Mousavi et al., 2020; Zekkos et al., 2010). Previous in situ studies to characterise MSW have mostly involved the exhumation of waste using test pits which limits the study of waste to relatively shallow burial depths and characterisation primarily focused on the classification of waste constituents as well as unit weight and moisture content properties related to the follow-on disturbed testing of MSW (Gabr and Valero, 1995; Hossain et al., 2009; Karimpour-Fard et al., 2011; Reddy et al., 2009, 2013; Xu et al., 2014).

MSW has been studied in depth using conventional and relatively small borehole diameters (<0.4 m) as well, where downhole geophysics was performed on the waste mass (Chen et al., 2009; Machado et al., 2010; Matasović et al., 2010). The completion of small-diameter boreholes to deep depth is difficult due to the random existence of waste obstructions. As a result, the work often ends with auger refusal or drill stem breakage forcing borehole abandonment and relocation (Hartwell, 2015). Extraction of undisturbed samples using these methods has not been feasible. Accordingly, conventional geotechnical drilling and sampling of MSW to significant depths has typically provided limitedly useful information. Therefore, many researchers (Feng et al., 2017; Kavazanjian et al., 1995; Reddy et al., 2009a; Zekkos et al., 2005, 2010) conducted large-diameter boreholes to characterise MSW properties. However, there is limited assessment of the borehole condition in terms of (1) borehole compression, (2) diameter measurement and (3) segregation of the MSW and soil layers to eliminate the randomised effect of intermediate soil covers in these studies. It is important to investigate changes of MSW properties with depth properly due to the compression and biodegradation of landfilled MSW. An understanding of these behaviours can be achieved from direct observation of the waste profile exposed on the borehole wall.

The objective of this study is to develop the Municipal Solid Waste Borehole Assessment (MBA) methodology to evaluate the MSW cuttings that are extracted from the boring operation and observe and record the condition of the borehole. In this study, an experimental methodology has been developed for characterising the index properties, including unit weights and bulk modulus of the MSW encountered in gas collection and control system (GCCS) designs. This process includes the development and deployment of a Large-Diameter Borehole Caliper (LDBC), downhole and compression measurement, calipering and photography of the borehole, and sampling of MSW. With the continuous photos, MSW was visually analysed. The results of LDBC data analysis, borehole caliper and compression measurements, and soil layer analysis are provided to evaluate the MBA methodology. Through this method, the in situ MSW properties can be determined more accurately and effectively.

MBA method

MBA procedure

The opportunity to access MSW at depth comes from frequent harvesting of biogas generated in MSW landfills. The regular need for GCCS borings represents a low-cost opportunity to gain access that would otherwise be very expensive. In effect, there is no additional cost to the GCCS project when in situ MBA testing is performed.

MBA was performed to test and refine the proposed experimental protocol regarding the development of downhole data, collection of cuttings and evaluation of MSW properties. The borehole was sampled on approximate 3 m segments to assure a reasonable degree of precision (±1%) in determining the segment mass and volume. Segment cuttings were placed in individual stockpiles, covered in plastic sheeting and labelled for processing. A total of 698 kg of MSW were obtained for laboratory testing from the nine segment stockpiles. The average drum contained 77.6 kg of MSW that represented 6% of the MSW ultimately determined to be present in nine segment drill cutting stockpiles.

Testing site

In May 2014, a GCCS borehole was completed at the Bluff Road Landfill (BRL) in Nebraska, USA, for Gas Extraction Well 14-060 (GEW 14-060). The borehole was completed by the drilling contractor using a hydraulic rotary type and IMT International A125 drill rig, and equipped with a 0.812-m OD (outside diameter) paddle bucket auger (Figure 1(c)). The contractor selected the 0.813-mm bucket auger with the following characteristics: inner row of cutting teeth ID (inside diameter) = 0.7239 m, barrel ID = 0.7493 m, barrel OD = 0.762 m, outer row of cutting teeth OD = 0.8047 m and overall bucket length from bottom of cutting teeth to outside top = 1.60 m.

Table 1.

MSW wet and dry unit weights by segment and diameter methodology.

Segment	Wet weight (kg)			Volume (m³)			Wet unit weight (kN m^–3)	Dry unit weight (kN m^–3)
Segment	Soil	MSW	Total	Soil	MSW	Total	γ_{w MSW}	γ_{d MSW}
Incremental volume computed using LDBC readings
1	972	669	1641	0.48	1.03	1.51	6.36	4.80
2	950	1548	2498	0.49	1.41	1.90	10.78	8.27
3	1520	732	2252	0.76	0.59	1.35	12.21	9.58
4	695	1419	2113	0.34	0.99	1.34	13.98	10.18
5	276	1184	1460	0.16	0.95	1.10	12.28	9.73
6	804	1401	2205	0.40	0.97	1.36	14.25	11.33
7	518	1442	1960	0.26	0.88	1.14	16.02	13.15
8	833	1389	2222	0.41	0.77	1.18	17.74	12.75
9	1170	1247	2417	0.61	0.68	1.29	17.96	14.38
Volume used based on auger barrel OD (outside diameter) 0.762 m
1	785	855	1641	0.39	1.00	1.39	8.37	6.32
2	968	1529	2498	0.49	1.42	1.91	10.59	8.13
3	1459	792	2252	0.73	0.59	1.31	13.26	10.40
4	726	1387	2113	0.36	1.07	1.43	12.66	9.22
5	300	1159	1460	0.17	1.04	1.21	10.93	8.67
6	864	1341	2205	0.43	1.00	1.43	13.10	10.42
7	593	1366	1960	0.29	1.01	1.31	13.24	10.87
8	915	1307	2222	0.45	0.90	1.35	14.25	10.25
9	1270	1147	2417	0.67	0.73	1.39	15.43	12.35
Volume is based on auger barrel outer cutting bit OD (outside diameter) 0.804 m
1	875	766	1642	0.43	1.12	1.55	6.72	5.08
2	1080	1419	2499	0.55	1.58	2.13	8.81	6.77
3	1626	626	2252	0.81	0.65	1.46	9.38	7.37
4	809	1305	2114	0.40	1.20	1.60	10.68	7.78
5	334	1125	1461	0.19	1.16	1.35	9.52	7.55
6	963	1242	2205	0.48	1.12	1.60	10.90	8.66
7	661	1299	1960	0.33	1.13	1.46	11.29	9.27
8	1020	1202	2223	0.50	1.00	1.50	11.77	8.46
9	1415	1002	2418	0.74	0.81	1.55	12.10	9.68

MSW: municipal solid waste; LDBC: Large-Diameter Borehole Caliper.

The selected borehole offered access to a thick waste profile, which included multiple phases of waste placement. The completion depth was approximately 27.4 m below the ground surface and ~3 m above the top of the primary liner. The rate of waste fill was estimated at approximately 1- to 1.5-year intervals of each lift until 2005. The age of waste placed in the first insulating layer (Lift 0) at the time the borehole was completed was around 22.3 years. The waste lift thicknesses were reported to average 4.6 m in initial compacted thickness.

Laboratory tests

The stockpiles were transferred to the University of Nebraska–Lincoln Geotechnical Laboratory to be tested for geotechnical index properties, including moisture content, unit weights and specific gravity. The moisture contents were measured using a dry gravimetric basis on the samples of each barrel consisting of an average of 8.76% of total weights. Specific gravity was calculated having the dry volume of each constituent based on known specific values of every constituent category and borehole geometric dimensions. The total weights (wet and dry) of soils were calculated with the assumption that each soil layer within the segments has similar moisture content and unit weight properties as the samples obtained from the cutting stockpile. Full laboratory methods and results are described in Hartwell et al. (2021).

Preparation of LDBC

Borehole characterisation included confirmation of borehole depth, measuring the borehole caliper and photographically recording the boring depth. A device developed by the first author called the Large-Diameter Borehole caliper (Figure 1(a)) was lowered into the borehole to measure the diameter of the borehole at (approximate minimum 0.3 m) intervals.

The LDBC consists of a three-wheeled, expandable leg truss assembly spaced equally radially on a vertical borehole axis. The triple truss assembly would keep the device centred in the borehole when lowered into the large-diameter borehole.

The device is able to measure the diameter of the borehole versus depth. The position of the wheel and the ends of the expandable truss struts vary proportionally with the diameter, relative to the position of the strut end on the vertical borehole axis. With the bottom wheeled strut fixed to the bottom of the standard measurement bar, the difference between the end of the upper strut and the measurement standard index is a measure of the borehole diameter as represented by the three equally spaced wheel contact points. Figure 1(b) shows the LDBC during the calibration process inside a jig.

A horizontal metal reference edge was used that was fastened to the safety framework to precisely note the engineer tape depths to the nearest 0.3 mm. The measurement from standard depth tape (A₂) and the index tape (A₁) was recorded along with the elapsed time that was measured with a stopwatch. The readings were recorded to the nearest ±0.3 mm.

Downhole photography

LDBC was accommodated with photographic and lighting equipment which allowed the collection of visual information on the condition of the side of the borehole to facilitate the selection of in situ potential test locations for another downhole equipment. Three GoPro™ Hero 3+ video cameras were mounted to the central vertical stem with three LED battery powered flood lights (ProMaster™ LED 120, 94 mm × 81 mm, 51 Lux @3.048 m) (Figure 1(b)). The camera lenses were offset radially from the borehole vertical axis about 89 mm, which resulted in their being positioned 244 to 328 mm from the borehole wall. With a photo interval of 10 seconds and an average duration at each depth of 15.91 seconds, each depth record station was assured of having a reasonably representative still photographic record.

Calculation of MSW unit weight

MSW unit weight is an important property for landfill engineering, which is required for slope stability analysis, geomembrane puncture, vertical stress and landfill capacity estimation (Kavazanjian et al., 1995; Yu et al., 2011; Zekkos et al., 2006).

With the demarcation of buried compacted soil cover layers along the borehole profile, the volume of each layer (V_{soil i}) within each segment (i) can be determined. When subtracted from the total borehole segment volume (V_{Total i}), the segment volume of (V_{MSW i}) is obtained

V_{Total i} = V_{MSW i} + V_{Soil i}

(1)

MSW and soil layer segment volumes were a simple calculation of constant cross-sectional area (A_x) times the layer thickness (T_y) for the auger barrel and auger bit

\begin{array}{l} V_{Total barrel i} = A_{barrel} \times [\sum^{} T_{MSW} i + \sum^{} T_{soil} i] \\ A_{barrel} = π {(\frac{d_{barrel OD}}{2})}^{2} and d_{barrel OD} = 76.2 cm \end{array}

(2)

and

\begin{array}{l} V_{Total bit i} = A_{bit} \times [\sum^{} T_{MSW} i + \sum^{} T_{soil} i] \\ A_{bit} = π {(\frac{d_{bit OD}}{2})}^{2} and d_{barrel OD} = 88.44 cm \end{array}

(3)

Borehole segment volume attributed to the diameter measurements recorded by the LDBC was determined under the assumption that the side of the borehole was straight between readings, thereby allowing the use of the formula for the volume of a frustum of a right circular cone (Selby and Weast, 1967). The volume of the segment and the individual MSW and soil layers were the summation of a series of stacked frustums. Frustum positions within the borehole were determined based on the observed depth of the soil layers. The frustum end areas were computed based upon the straight-line interpolation between LDBC diameter recording depths. The LDBC-based frustum volume for a length of borehole between caliper readings (n) within segment i is given in the following equation

V_{LDBC fn i} = [\frac{π t_{n}}{3} {[{(\frac{D_{n 1}}{2})}^{2} + {(\frac{D_{n 2}}{2})}^{2} + ({(\frac{D_{n 1}}{2})}^{2} \times {(\frac{D_{n 2}}{2})}^{2})]}^{2}]

(4)

where $t_{n}$ is the thickness of the portion of the segment of interest, $D_{n 1}$ is the diameter of the top at the top of the frustum in portion n and $D_{n 2}$ is the diameter at the bottom of the frustum in portion n. The borehole segment frustum volume attributed to the LDBC measurements is therefore

V_{Total LDBC i} = \sum^{} V_{LDBC MSW fn i} + \sum^{} V_{LDBC soil fn i}

(5)

After determining the wet and dry unit weights of MSW by segment using the measured weights and calculated volumes, the volume of MSW solids (V_{MSW s i}) can be computed using the following formula

V_{MSW s i} = \frac{γ_{d MSW}}{G_{MSW i} \times γ_{w}}

(6)

where G_{MSW i} is the segment-aggregated MSW-specific gravity and $γ_{w}$ is the unit weight of water. Once the volume of MSW solids is determined, the volume of voids is known since

V_{MSW e i} = 1 - V_{MSW s i}

(7)

With this it follows that the void ratio (e) of MSW segment (e_{MSW i}) and degree of saturation (S_{MSW i}) can be determined

e_{MSW i} = \frac{V_{MSW e i}}{V_{MSW s i}}

(8)

S_{MSW i} = \frac{V_{MSW m i}}{V_{MSW e i}}

(9)

where V_{MSW m i} is the volume of moisture using the aggregated MSW moisture content.

Results and analysis

Borehole compression and caliper measurements

Borehole radial compression measurement is useful for gas collection system design and instalment. The maximum and minimum borehole diameter computed from the recorded data was 0.966 and 0.664 m at the ground surface and at a depth of 20 m, respectively. The average borehole diameter was 0.741 m.

The borehole caliper data shown in Figure 2(a) as a plot of borehole depth versus borehole radius (r_LDBC) is obtained from the LDBC. Also, the average borehole radius for each 1.52 m vertical interval beginning at ground surface, the auger barrel outside radius (r_barrel) and the outer cutting bit radius (r_bit) are shown.

Figure 2(a) reveals that the borehole caliper measurements show a borehole with consistently smaller radii than expected when compared with the nominal auger barrel diameter. The observed radial compression for MSW and soil layers is shown in Figure 2(b) for the 1.52-m interval radius (r) average versus depth (d).

Given the measurement of the borehole caliper by the LDBC, the variation in borehole radii with depth could be explained with the assumption that the borehole was cut to the diameter of the auger barrel outer cutting bits (d_bit = 0.8047 m). Figure 2(a) shows three of the borehole radius measurements (with the incremental radii, outer cutting teeth OD and barrel OD) that exceeded the outside diameter of the auger barrel, outer cutting teeth. The outer cutting teeth radius (r_bit) was 0.4023 m. The two largest recorded radius measurements were at the ground surface and at a depth of 0.884 m. These were the result of auger barrel over-reaming that took place due to the repeated auger barrel insertion. The auger barrel was inserted into the boring 23 times to complete the 27.92 m borehole. The third borehole radius exceedance was recorded at a depth of 3.35 m where the radius was 0.4028 m.

Twenty-six of the borehole radius measurements (21.7% of the total) exceeded the outside diameter of the auger barrel of 0.762 m (r_barrel = 0.381 m). Of these measurements, the first 20 exceedances occurred in the top 9.1 m of the borehole. The deepest exceedance occurred at a depth of 26.49 m. Based on these observations, it appears a reasonable assertion that the r_bit (not the r_barrel OD or the incremental radius measured by the LDBC) represents the reference radius to which the borehole was cut during the drilling operation.

Based upon the visual evidence of an increasingly, intact borehole wall condition with depth, the radius exceedances below the uppermost 9.1 m of the borehole were not the result of auger over-reaming or barrel wandering off plumb due to obstructions, or the excessive pulling of reinforcing waste tendrils from the formation. Rather the measured radii (r_LDBC) indicate borehole squeezing below a depth of about 3.66 m, where the condition of the borehole becomes relatively free of waste reinforcing tendril downhole draping.

MSW and soil radial compression

The amount of squeezing can be of importance to the design of downhole experimental devices to accommodate this potential restriction and is also significant and relevant to establishing the landfill gas capture capacity of GEWs of varying nominal diameters. The borehole compression factor has not been considered in the design of GEWs up to this time. Power functions of both the individual data points and the 1.52-m average depth segments indicate a reasonable correlation with the outside diameter of the outer row of auger barrel cutting bits. The predicted borehole radius at a depth of 0.3 m varies from 0.408 to 0.414 m for individual and 1.52 m intervals, respectively. The borehole radius (r) expressed in the form of r = ar^b, where constant ‘a’ varies from 0.40 to 0.42 m and exponent ‘b’ varies from 0.034 to 0.05. As shown in Figure 2(a), the correlation coefficient varies from 0.34 to 0.73 for the individual and 1.52 m interval data, respectively. There is a trend of decreasing borehole radius (r_LDBC) with increasing depth. The data as well as the power trends indicate the borehole squeezed shut after the borehole was initially cut through. During the time that elapsed between the completion of GEW 14-060 and the onset of borehole logging using the LDBC, the borehole underwent radial compression. The borehole compression was referenced to the initial borehole diameter cut by the outer edge of the cutting teeth. In this case, the reference borehole radius was 0.4023 m. The observed radial compression for MSW and soil layers is shown in Figure 2(b) for the 1.52-m interval radius (r) average versus depth (d).

The development of a large-diameter downhole testing device must be able to not only pass by obstructions created by draped and dangling waste tendrils but also radial compression, which will likely exceed 10% in borings of 30.5 m or more. GEW 14-060 radial compression amounted to ~7.5% at 9.14 m, ~10% at 21.3 m and ~11% at 27.4 m.

LDBC data recorded at depths where soil layers were observed were separated from the remainder of the data set and the two populations were evaluated separately. First, the soil layers appear to experience less squeezing than the MSW layers. Second, there appears to be a bottom effect that reduces the amount of squeezing within ~2 diameters of the bottom of the borehole.

A first-order rate equation (FORE) analysis was performed as described by Handy (2002). In this analysis, the measured quantity (Y) that is believed to approach an ultimate value (Y_u) is the percentage of radial squeezing [(Δr/r_bit)100]. The result of this analysis indicates that the maximum amount of radial compression that would be experienced with depth is –14.3% and –10.4% for MSW and soil, respectively. In this case, the Y_u had maximum correlation coefficients (R²) of 0.49 and 0.36, respectively, at the noted percentage of Δr/r_bit. These Y_u_’s resulted in a minimum borehole radius of 0.345 and 0.361 m for MSW and soil, respectively.

Downhole photography

The photographic record allowed the detailed mapping of the borehole wall to a depth of about 27.27 m. This photographic mapping was used to observe the general condition of the borehole sidewall. After a review of the time-lapse video photography, the borehole was shown to be relatively plumb, straight and free of significant obstructions.

Downhole photography (samples shown in supplementary figure) revealed that the borehole condition was very ragged near the surface with considerable waste reinforcing tendrils visibly protruding into the borehole. These reinforcing tendrils draped down the side of the borehole masking the actual sidewall over much of the perimeter. In the top 2.44 m, red, green, yellow and blue waste fragments were noted, but below that depth, the amount of waste with any colour other than grey or brown was limited to relatively small fragments, and the colour of waste was predominantly white, blue and green. The decrease in size of exposed colour fragments (and all waste reinforcing tendrils) with depth may be related to the efficiency of the shearing action of the auger barrel cutting teeth. The reason for the observed absence in bright fragment colours may be due to the presence of an anaerobic environment and the progress of associated biodegradation and/or due to the trimmed reinforcing tendrils being smeared with moist soil and waste as the auger barrel is repeatedly inserted and withdrawn. Qualitative evaluation of the borehole photography shows the reinforcing tendrils to be most prevalent in the upper 4.57 m of the borehole. The length and abundance of draping reinforcement diminished considerably with depth such that below a depth of 7.62 m, the borehole wall was relatively clean and free of draping waste reinforcing tendrils. Below a depth of 15.24 m, the condition of the borehole was trimmed to clean and uniform condition with very little draping tendrils.

While the still photos obtained from the time-lapse video photography showed the potential of free moisture based on the sheen of some of the exposed surfaces, confirmation of the presence of free leachate was not possible. The video recording proved more elucidating, where leachate droplets were observed at 10.64 and 10.94 m in segment 4, at 17.04 m in segment 6, at 23.41 m in segment 8 and at 25.85, 26.18 and 26.79 m in segment 9. In each case, the leachate appeared to be perched on a compacted soil layer.

It appears that when subjected to the shear action of the auger barrel cutting teeth above a depth of 3.05 m, the reinforcing waste tendrils failed almost exclusively within the waste mass at the bond with adjoining waste. Below a depth of 17.77 m, the shearing action of the cutting teeth produced failure in the reinforcing material itself at the edge of the borehole. Between these depths, the bond between the reinforcement and the adjoining waste strengthened and the frequency of shear failure through the reinforcing material itself became increasingly prevalent. Based on these depths, the approximate total vertical stress (σ_v) (Hartwell et al., 2021) at which the reinforcing tendrils predominantly shear along the bond with the adjoining waste is about 35.9 kPa. The σ_v at which the reinforcing tendrils predominantly shear through the reinforcing materials is about 227.4 kPa. The σ_v depends on which bore-hole diameter hypotheses is employed to calculate the γ_{MSW w}. The variation in σ_v versus depth using these differing diameters is shown in Figure 3.

Bulk modulus of MSW and soil

Landfills are required to conduct stability analysis since large displacements and transient conditions in landfills can result in slope instability. Therefore, stiffness properties including shear modulus and bulk modulus are important parameters that designers can use as input parameters to evaluate landfill stability performances (Kavazanjian et al., 1996; Matasović et al., 1995; Sahadewa et al., 2015). It appears that the borehole had potentially reached an active earth pressure condition by the time the LDBC diameter measurement was recorded. The radial displacement measurements allow us to estimate the bulk modulus of MSW and soil material through the borehole depth. The bulk modulus is defined as the ratio of mean effective stress and volumetric strain of material. Lateral earth pressure data (K₀) is required to calculate the horizontal stress before estimating the bulk modulus (K_b). Although there is scarce information on the lateral earth pressure of MSW, Dixon and Jones (2005) measured the earth pressure coefficient in a landfill using pairs of pressure cells. The results appear to suggest that lateral earth pressure coefficient increases following a power function trend up to around a depth of 4 m and shows a relatively constant value below 4.0 m

K_{0} = 0.536 d^{0.15} when d \leq 4.0 m R^{2} = 0.38

(10)

K_{0} \approx 0.78 when d > 4.0 m

Figure 4(a) shows the results presented by Dixon and Jones (2005) excluding an outlier data (K₀ larger than 1).

The bulk modulus of MSW and soil was calculated using the mean effective stress at each depth. The vertical stress in the same borehole was calculated by measuring the weight of each segment cut and it was related to the depth by the following equation of Hartwell et al. (2021; Figure 3)

σ_{v} = 9.3814 . d^{1.09581}

(11)

where $σ_{v}$ is the vertical stress in kilopascals and d is the depth in metres. Figure 4(b) and (c) shows the estimated trend of bulk modulus of MSW and soil with depth excluding outliers. The bulk modulus of MSW appears to fall between two linear functions. The estimated bulk modulus of MSW can be written as

K_{b} = a \times d

(12)

where a is between 0.2 and 0.55, and d is the depth in metres. According to Figure 4(b), the bulk modulus of MSW and soil appears to be within a similar range, while the bulk modulus increase appears to follow a power function trend according to

K_{b} = 0.965 . d^{0.675}

(13)

There is no structural liner in place to restrict the inward motion of the soil or MSW layer. The elastoplastic material continues to strain inward until tangential stress builds sufficiently to form a compression arch to restrict further inward relaxation movement creating a plastic zone.

Given the observation that the radial borehole compression of MSW exceeded that of the adjoining soil layers, we can conclude that the MSW layer inward movement tended to drag the soil layer interface with it or at least impart stress onto the soil that would tend to cause inward movement. The opposite is also true, in that the soil would tend to impede the inward movement of the adjoining waste layer. Furthermore, since the horizontal strength of MSW is lower than the horizontal strength of the soil layer, the failure or zone of horizontal slippage should be positioned at the soil/MSW interface or within the waste mass but not within the re-compacted soil cover layer.

Results of MSW unit weights

The total wet weight and dry weight of soil within each segment can be computed. Subtracting the soil segment wet weights from the gross cutting segment wet weights yields the wet unit weight of MSW. Using the aggregated moisture content for each segment allows the computation of the dry unit weight of MSW in each segment. This would not be achievable without eliminating the adverse impact of random buried soil layers throughout the waste column by the developed methodology. This shows the necessity of a complete borehole analysis to find accurate results versus depth. Table 1 shows the resulting wet and dry unit weights of each segment for MSW layers using the three borehole diameter hypotheses.

Figure 5(a) and (b) shows the wet and dry unit weights of MSW at each segment using three different assumed outer diameters where all show an increase of unit weights with depth following a power function. The results show that the unit weight values and trends can considerably differ when different borehole radius is used, and therefore careful consideration of the borehole radius is required to determine which diameter is to be chosen as the realistic diameter.

Table 2 shows the void ratio and saturation of MSW layers within each segment using the three borehole diameter hypotheses. Results from Table 2 indicate that realistic results can only be achieved if the borehole diameter that produced the segment stockpile is the outer diameter of the outer cutting teeth (D_bit) (because of unrealistic saturation values of the other two assumed diameters). Therefore, the outer cutting bit diameter is considered the initial diameter that existed at the time the auger was advanced past any depth as well as throughout the completion of the borehole. This outer diameter was considered when analysing the unit weight trends with depth which are presented by Hartwell et al. (2021).

Table 2.

Aggregated MSW void ratio and degree of saturation by borehole methodology.

Segment	Segment mid-depth (m)	σ_v (kPa)	G_i _MSW	V_MSW (m³)	V_e (m³)	V_m (m³)	e	S (%)
Incremental volume computed using LDBC readings
1	1.52	23.4	1.661	0.295	0.705	0.159	2.393	22.6
2	5.15	61.2	1.700	0.496	0.504	0.255	1.016	50.6
3	8.69	114.7	1.703	0.573	0.427	0.267	0.744	62.6
4	11.70	149.2	1.552	0.669	0.331	0.388	0.495	117.3
5	14.60	178.6	1.736	0.572	0.428	0.259	0.748	60.6
6	17.50	207.3	1.633	0.707	0.293	0.297	0.414	101.6
7	20.18	257.8	1.454	0.922	0.078	0.293	0.084	376.2
8	23.41	293.3	1.556	0.835	0.165	0.508	0.197	309.0
9	26.40	337.0	1.725	0.850	0.150	0.365	0.177	243.1
Volume used based on auger barrel OD
1	1.52	23.4	1.661	0.388	0.612	0.210	1.576	34.2
2	5.15	61.2	1.700	0.488	0.512	0.251	1.050	48.9
3	8.69	114.7	1.703	0.623	0.377	0.290	0.605	77.0
4	11.70	149.2	1.552	0.606	0.394	0.352	0.651	89.2
5	14.60	178.6	1.736	0.510	0.490	0.231	0.962	47.1
6	17.50	207.3	1.633	0.651	0.349	0.274	0.536	78.4
7	20.18	257.8	1.454	0.762	0.238	0.242	0.312	101.9
8	23.41	293.3	1.556	0.671	0.329	0.409	0.489	124.4
9	26.40	337.0	1.725	0.730	0.270	0.314	0.370	116.3
Volume is based on auger barrel outer cutting bit OD
1	1.52	23.4	1.661	0.312	0.688	0.168	2.208	24.4
2	5.15	61.2	1.700	0.406	0.594	0.209	1.463	35.1
3	8.69	114.7	1.703	0.441	0.559	0.206	1.267	36.8
4	11.70	149.2	1.552	0.511	0.489	0.297	0.957	60.6
5	14.60	178.6	1.736	0.444	0.556	0.201	1.254	36.2
6	17.50	207.3	1.633	0.541	0.459	0.228	0.848	49.6
7	20.18	257.8	1.454	0.650	0.350	0.207	0.538	59.0
8	23.41	293.3	1.556	0.554	0.446	0.337	0.804	75.7
9	26.40	337.0	1.725	0.572	0.428	0.246	0.748	57.5

MSW: municipal solid waste; LDBC: Large-Diameter Borehole caliper.

Soil layers

One of the most useful developments resulting from the use of the LDBC for the MBA was to precisely demark buried soil layers. Because of this demarcation, the scalability of the sidewall photography and the relatively undisturbed condition of the large blocks of soil recovered from the MSW stockpiles, significant geotechnical testing and classification of both soil and MSW became possible.

Nineteen distinct soil layers were encountered in GEW 14-060 borehole. The total soil layer thickness for the borehole was 8.72 m compared to the total MSW thickness of 19.2 m. This produced an MSW to soil ratio of 2.2 to 1. The average soil layer thickness was 0.414 m with a range of 0.061 to 1.22 m.

The remaining soil layers appear to be buried daily cover layers that were placed upon the working faces as they progressed across each of the 10 waste lifts, which form the entire thickness of fill at the GEW 14-060 location. The total thickness of the soil layers appearing to be the working face daily cover layer at this location was 3.63 m (maximum = 0.454 m and minimum = 0.134 m). The average thickness was 0.302 m. By comparison, the 19 MSW layers separating the soil layers were on average 0.96 m thick, with a range of 0.079 to 2.91 m.

Figure 5(c) shows the wet unit weight of the combined soil and MSW layers (gross wet segment unit weight) versus borehole depth of each segment midpoint for each of the three borehole radius methods. The regression coefficients for the auger barrel OD and auger bit OD methods yield relatively poor correlation (R² < 0.5 except for the r_LDBC data where R² ~ 0.76), but these correlations improved when soil and MSW layers were examined separately. There is a trend of increasing wet gross unit weight with depth, which seems intuitive since the MSW consolidates as waste fill is added to the cell over time.

Figure 5(d) shows a comparison of the calculated dry, wet and gross unit weights by the auger bit with lower boundary proposed by Kavazanjian et al. (1995), and gross unit weights reported by Matasović and Kavazanjian (1998) and Zekkos et al. (2006). The total gross MSW unit weight segment data for this study lie approximately centred within the range of the cited studies, well to the right of the Kavazanjian et al. (1995) lower unit weight limit. The data set approximately parallels the Kavazanjian limit but is strongly randomised by the buried soil layers encountered at random intervals throughout the borehole. The position of the auger bit wet MSW unit weight closely parallels and is about 0.94 kN m^–3 higher than the Kavazanjian limit line, which is approximately linear at depths shallower than 30.48 m. The MSW wet weight data from this study have had the random soil layers removed, and exhibit a strong relationship with vertical stress, which is correlated to depth. Considering the meaning of the Kavazanjian line as being a lower limit of the unit weight population, it is reasonable to assert that its position (given sufficiently large data population of landfill profiles with varying numbers and thicknesses of buried soil layers) would be proximal to an MSW profile that excludes any soil layer and only contains a nominal moisture content that is representative of the moisture content as received at the working face. The dry MSW unit weight also closely parallels to the Kavazanjian lower limit, but it lies below the limit.

Conclusion

This study used a large-diameter borehole intended for GEW to develop geotechnical information on the buried MSW. The developed methodology, called the MBA, tests the cuttings obtained from the large-diameter gas borehole, and then documents the properties of the borehole. Also, the borehole was observed and recorded under the in situ condition of the borehole using a device called Large-Diameter Borehole Caliper.

Based on the measurements and observation, the borehole experienced significant radial compression with depth following completion. The radial compression amounted to ~7.5% at 9.14 m, ~10% at 21.3 m and ~11% at 27.4 m. Review of the 1.52-m interval caliper data shows two trends that are of potential interest. First, the soil layers appear to experience less squeezing than the MSW layers. Second, there appears to be a bottom effect that reduces the amount of squeezing within ~2 diameters of the bottom of the borehole. A FORE analysis of the MSW and soil layer radial compression indicates that the maximum amount of compression with depth is –14.3% and –10.4% for MSW and soil, respectively. The radial compression of the borehole and the measurement of the diameter by the LDBC also allowed the estimation of MSW’s bulk modulus by translating the radial strains to volumetric strains. For MSW, the bulk modulus increased up to 13.4 MPa in a linear trend. The bulk modulus estimations can be helpful in landfill designs where dynamic properties of MSW are used for slope stability analysis.

The MBA technique allowed the determination of gross segment (MSW and soil combined) unit weight versus depth as well as MSW and soil only unit weights. Unit weights and other MSW properties were obtained using three assumed borehole diameters to calculate the gas well capture radius. The calculated unit weights of MSW ranged from 4.8 to 14.38, 6.32 to 12.35 and 5.08 to 9.68 using LDBC readings, auger barrel outside diameter and outer cutting bit outside diameter, respectively. The results showed that only one of the outermost diameters (outer diameter of the outer cutting teeth (D_bit)) used to calculate the unit weights can be realistic. Furthermore, since the entire length of the borehole is collected as cuttings and weights were obtained, the vertical stress within the borehole became known along its entire depth. This is important because it allows the researcher to eliminate the adverse impact of the random positioning of buried soil layers within the waste column, to obtain a more precise relationship of geotechnical properties with vertical stress rather than depth. As a result of the ability to fully characterise MSW with depth, these large-diameter gas boreholes offer excellent, economic opportunities for exploring in situ characteristics of MSW to a great depth. Even though the obtained MSW geotechnical index properties showed relatively good agreement with other existing reported results, the potential exists to expand the testing cases to be generalised or extrapolated to other landfills.

Supplemental Material

sj-docx-1-wmr-10.1177_0734242X211057012 – Supplemental material for Assessment of in situ properties of municipal solid waste with a large-diameter borehole method

Supplemental material, sj-docx-1-wmr-10.1177_0734242X211057012 for Assessment of in situ properties of municipal solid waste with a large-diameter borehole method by John Hartwell, M Sina Mousavi, Jongwan Eun and Shannon Bartelt-Hunt in Waste Management & Research

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Jongwan Eun

Supplemental material

Supplemental material for this article is available online.

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