Recycled gypsum for slope stabilisation

Transcription

1 Plasterboard technical report/ final Recycled gypsum for slope stabilisation Results of trials to determine the working parameters for the use of recycled gypsum derived from waste plasterboard for soil slope stabilisation. Project code: PBD015 ISBN: [Add reference] Research date: January to August 2007 Date: November 2007

2 WRAP works in partnership to encourage and enable businesses and consumers to be more efficient in their use of materials and recycle more things more often. This helps to minimise landfill, reduce carbon emissions and improve our environment. Written by: Dr David Lawson Front cover photography: Soil erosion measurements WRAP and STRI believe the content of this report to be correct as at the date of writing. However, factors such as prices, levels of recycled content and regulatory requirements are subject to change and users of the report should check with their suppliers to confirm the current situation. In addition, care should be taken in using any of the cost information provided as it is based upon numerous project-specific assumptions (such as scale, location, tender context, etc.). The report does not claim to be exhaustive, nor does it claim to cover all relevant products and specifications available on the market. While steps have been taken to ensure accuracy, WRAP cannot accept responsibility or be held liable to any person for any loss or damage arising out of or in connection with this information being inaccurate, incomplete or misleading. It is the responsibility of the potential user of a material or product to consult with the supplier or manufacturer and ascertain whether a particular product will satisfy their specific requirements. The listing or featuring of a particular product or company does not constitute an endorsement by WRAP and WRAP cannot guarantee the performance of individual products or materials. For more detail, please refer to WRAP s Terms & Conditions on its web site:

3 Context WRAP WRAP (Waste & Resources Action Programme) works in partnership to encourage and enable businesses and consumers to be more efficient in their use of materials and recycle more things more often. This helps to minimise landfill, reduce carbon emissions and improve our environment. Established as a not-for-profit company in 2000, WRAP is backed by Government funding from Defra and the devolved administrations in Scotland, Wales and Northern Ireland. WRAP and plasterboard Through its Construction Programme, WRAP is helping the construction industry cut costs and increase efficiency through the better use of materials. Plasterboard is used extensively in the construction and refurbishment of buildings as a lining for walls and ceilings, and for forming structures such as partitions. Plasterboard waste can arise on construction sites for a number of reasons, including wasteful design, off-cuts from its installation, damaged boards, and over-ordering. It is estimated that over 300,000 tonnes per year of waste plasterboard is produced on construction sites. It can also arise from strip-out activities during refurbishment and demolition projects; the waste arisings from this source are significantly higher. In total it is estimated that over one million tonnes of waste plasterboard are produced each year from construction and demolition activities. Most of this waste is currently disposed to landfill, even though it can be easily recycled. WRAP receives funding from Defra through the Business Resource Efficiency and Waste (BREW) programme to divert plasterboard waste from landfill by working to overcome the barriers to plasterboard recycling. Additional funding is also received from the devolved administrations in Scotland, Wales and Northern Ireland. WRAP is working to overcome these barriers through the following key areas: plasterboard waste minimisation; site waste management; segregation and collection of plasterboard waste; development of infrastructure, including waste logistics and recycling capacity; market development for materials from plasterboard recycling recycled gypsum and reclaimed paper; education, awareness and behavioural change; and informing and influencing legislation, regulations and policy. More information on WRAP s work can be found at Recycled gypsum for slope stabilisation 1

4 Executive summary Over one million tonnes of waste plasterboard are estimated to be produced each year in the UK from construction and demolition activities. Most of this waste is currently sent to landfill, even though it can easily be recycled. WRAP is working to divert plasterboard waste from landfill by seeking to overcome the barriers to plasterboard recycling. One area of its work is to develop markets for the materials from plasterboard recycling (recycled gypsum and reclaimed paper). This project evaluated the use of waste plasterboard as a source of gypsum as an additive to two soil types to benefit slope stabilisation. The project was led by the Sports Turf Research Institute (STRI) and also involved a plasterboard recycler, Plasterboard Recycling UK Ltd (PBRUK) This work arose from a trial carried out in 2006 which showed that recycled gypsum applied to a sandy loam soil on a 25 slope could significantly reduce surface soil erosion. Even where the slope was vegetated, the effect was still evident. It was decided to extend the trials to investigate a number of factors that could influence the effectiveness of the material in reducing surface soil erosion. Trial design and results A number of trials were carried out from January 2007 to August 2007 at STRI s trials grounds in West Yorkshire to further investigate the ability of recycled gypsum to reduce soil erosion from slopes. The factors investigated were: rate of application; slope gradient; soil type interaction with conventional slope stabilisation mesh systems; and any effect on vegetation establishment. The results showed that soil erosion could be reduced by applying recycled gypsum at 2, 4 and 10 kg/m 2. However, at the highest of these rates there could be problems with gypsum being washed down-slope if not intimately integrated with the soil. Of particular note was the ability of the recycled gypsum to significantly reduce the slippage of clay soil on a 32 slope to a distance of 120 mm compared with a distance of 215 mm for untreated soil. This effect was also observed when the material was applied in conjunction with slope stabilisation materials. The mechanism for this effect is uncertain. No deleterious effect on vegetation establishment from recycled gypsum application was found. Conclusions The application of recycled gypsum has been successfully demonstrated to benefit slope stabilisation. The optimum application rate within the constraints of the trial was 2 4 kg/m 2. At the highest rate tested (10 kg/m 2 ), there was no significant increase in the ability to reduce soil surface erosion or slippage. The application of recycled gypsum was found to have a beneficial effect on slope stabilisation both on its own and in conjunction with geosynthetic meshes. Recommendations Further work is recommended to: measure the amount of erosion/slippage after the recycled gypsum is mixed to the full depth of topsoil; test the stability of the different soil types after the recycled gypsum has been applied; carry out standard soil engineering tests on soil treated with gypsum; investigate the optimum method for combining gypsum treatment with conventional erosion control products; measure the effect of gypsum application on the moisture content of the soil; and analyse the future market potential that could realistically be attained if recycled gypsum was used in the application. Recycled gypsum for slope stabilisation 2

5 An operational trial should provide these results. This could be embarked on with organisations such as the Highways Agency and Network Rail, and their landscaping providers. Recycled gypsum for slope stabilisation 3

6 Contents 1.0 Introduction Project partners Source of recycled gypsum Overview of trial design Trial methodology Main trial Measuring surface soil erosion Vegetated trial Mesh trial Vegetation establishment trial Results from the main trial Effects of recycled gypsum application on surface soil erosion slope slope slope Overall effect Soil slippage Snow cover Weed cover Discussion Results from the vegetated trial Discussion Results from the mesh trial Discussion Results from the vegetation establishment trial Discussion Business case Conclusions and recommendations Recommendations for further research Appendix A Weekly precipitation Recycled gypsum for slope stabilisation 4

7 1.0 Introduction WRAP is working to divert plasterboard waste from landfill by seeking to overcome the barriers to plasterboard recycling. One area of its work is to develop markets for the materials from plasterboard recycling (recycled gypsum and reclaimed paper). This project evaluated the use of waste plasterboard as a source of gypsum as an additive to soil types to benefit slope stabilisation. The potential for surface soil erosion from slopes during road engineering projects is most critical prior to the establishment of vegetation on embankments and cuttings. Such erosion can cause problems for overall stability of the slope. The erosion is extended to the formation of gullies. Mesh and materials produced from plastic, jute and other materials are currently laid on the slope surface prior to vegetation seeding in order to reduce soil surface erosion. Under severe rainfall conditions, however, erosion incidents do occur even on established, grassed slopes and particularly if coverage is poor. This can cause significant problems with regard to soil wash onto road surfaces, with consequent effects on road surface drainage. Gypsum (calcium sulphate) is often used for the purpose of soil stabilisation and the creation of soil structure. The calcium present causes soil clay particles to aggregate into stable structures. Unstructured clay particles will disperse and cause blockage of pores within the soil structure, thus significantly reducing drainage capacity. For this reason, gypsum is often applied to soils that have been inundated with sea water. The high concentration of sodium in sea water causes soil clay particles to disperse and de-structure the soil. No evidence was found of the use of gypsum to stabilise soil slopes. However, it was considered that gypsum might be able to perform a similar role in the stabilisation of soil slopes in particular the recycled gypsum derived from waste plasterboard. As well as the stabilisation factors mentioned above, the material would physically prevent the movement of soil particles down slope. 1.1 Project partners The project was led by the Sports Turf Research Institute (STRI) and also involved a plasterboard recycler, Plasterboard Recycling UK Ltd. STRI is the independent market leader in turfgrass research and agronomy. It is the UK's national centre for consultancy in sports and amenity turf, and a recognised world centre for research. Its consultancy team advises at over 2,000 turf facilities including golf, soccer, rugby, cricket, racing, bowling and tennis. David Lawson is a Senior Research Officer at STRI ( He has over 20 years experience in soil chemistry, plant nutrition and fertiliser use on sports and amenity turf. He has supervised and undertaken over 150 commercially or grant-funded trials ranging from the acidification of irrigation water to the use of compost materials in amenity and sports turf maintenance. He has also carried out a number of consultancy roles for WRAP and other organisations such as English Heritage. David also provides a supervisory role within STRI s Soil Testing Laboratory. Plasterboard Recycling UK Ltd (PBRUK) ( was founded in February 2004 to provide a costeffective and environmentally sound alternative to the disposal of waste plasterboard. PBRUK collects and processes clean, uncontaminated plasterboard waste from construction projects across the UK. The material is processed into its constituent parts of gypsum and paper. The recovered gypsum is utilised in a number of industries including cement manufacture, agriculture and horticulture, and as absorbent granules. The recovered paper is composted. 1.2 Source of recycled gypsum The recycled gypsum used in the trials was obtained from the PBRUK site at Edmonton in north London. The material was the company s standard end product from the processing of waste plasterboard from both construction and demolition activities. According to PBRUK, the recycled gypsum contained less than 1% paper. 1.3 Overview of trial design During 2006, a preliminary trial was established at the trials ground of STRI at Bingley in West Yorkshire. Recycled plasterboard was applied at rates up to 4.0 kg/m 2 to a sandy loam soil at a 25 o slope. Surface erosion was measured by the insertion of troughs at the down-slope end of the plots. Prior to grass being established, this application rate reduced soil loss by over 50% relative to untreated soil. Even when grass was established, Recycled gypsum for slope stabilisation 5

8 the gypsum material reduced soil run-off by 48%. More details of this trial are available from WRAP (Project PBD ). It was decided to undertake a more extensive series of trials to further evaluate the use of recycled gypsum for slope stabilisation. In 2007, trials were established to measure the effect of: a wider range of gypsum application rates; soil type sandy loam or clay loam; varying slope gradient; gypsum rate on soil erosion after vegetation has been removed; combining gypsum treatments with soil stabilisation mesh material; and gypsum treatments on the establishment of Highways Agency grass mixture on sandy or clay loam textured soil. The first three factors were examined within one multi-factorial trial the main trial (see section 2.1). The other three factors were investigated in the vegetated, mesh and vegetation establishment trials respectively (see sections ). 2.0 Trial methodology All the trials were carried out at STRI trial grounds at Bingley in West Yorkshire. The weekly precipitation measured at the trials ground is shown in Appendix A. 2.1 Main trial The main trial investigated a range of gypsum application rates along with three slope gradients on two soil types (Table 1). Table 1 Design of main trial Parameter Slope gradient Soil type Values 20, 25 and 32 Clay loam Sandy loam Gypsum addition rate 0, 2, 4, and 10 kg/m 2 The soils were established in 2,200 mm long 500 mm wide 150 mm deep plastic trays with drainage holes in the base. Soil was laid to a depth of 120 mm. The recycled gypsum was applied by raking it into the soil in the trays to a depth of approximately 20 mm. There were two replicates for each soil gypsum combination, randomised within two blocks on each slope. This gave 16 combined soil/gypsum treatments on each slope. The trays were placed on the appropriate slopes on 30 January Soil erosion measurements were made every two weeks starting on 6 February. On 31 May, an estimate of ground cover by weeds was made visually and using a hand-held Spectrum 1000 chlorophyll meter supplied by Spectrum Technologies plc ( The spectrometer measures the amount of light reflected by chlorophyll in plant material; the greater the reading obtained, the greater the density of live vegetation. On 5 June, the trays were sprayed with glyphosate weedkiller to kill off the grass and broadleaved weeds that had established. The estimate of ground cover using the chlorophyll meter was repeated on 20 August. Recycled gypsum for slope stabilisation 6

9 2.1.1 Measuring surface soil erosion To measure surface soil erosion, a polythene lip was inserted under the soil at the bottom end of the tray. This lip fed into a plastic trough in which was placed a 0.6-metre length of filter paper to collect any soil from natural run-off. The collection system is shown in Figure 1. Soil was collected over the whole period of the trial, with individual measurements being made at two weekly intervals. The collected soil was dried at 80 C for 16 hours before being weighed. Figure 1 Surface erosion measurement system showing collection paper in trough 2.2 Vegetated trial This trial investigated the effect of a range of gypsum application rates on soil erosion from sparse grass vegetation cover. In this trial, the slope gradient and soil type were kept constant but the gypsum addition rate was varied (Table 2). Table 2 Design of vegetated trial Parameter Slope gradient Soil type Values 32 Sandy loam Gypsum addition rate 0, 2, 4 and 10 kg/m 2 The soil was placed in trays as for the main trial. An amenity turf, which had previously been physically worn to reduce surface grass cover, was established in each tray on 4 January The gypsum was then applied and lightly rolled into the surface. The trays were placed on the slope on 8 January. On 26 May, the trays were sprayed with a glyphosate weedkiller to kill off the dense grass cover that had established Soil erosion measurements were made as for the main trial. 2.3 Mesh trial This trial investigated the interaction of gypsum application and commercial soil protection mesh materials on erosion from a clay soil. In this trial, the slope gradient and soil type were kept constant while the gypsum addition rate was varied (Table 3) on a soil surface covered by two mesh materials supplied by Tenax UK Limited ( and unprotected (control). To test whether there would be any difficulty in applying recycled gypsum to soils covered by an erosion protection mesh, only the two higher rates used in the main trial were applied. Recycled gypsum for slope stabilisation 7

10 Table 3 Design of mesh trial Parameter Slope gradient Soil type Values 25 Clay loam Gypsum addition rate 0, 4 and 10 kg/m 2 Mesh Protection * For specifications, see Control (no protection) Tenax Multimat 100 plastic mesh* Tenax HiSlope 400 coir mesh* The mesh protection materials were laid on the surface and topped off with 20 mm depth of soil as recommended by Tenax UK. The gypsum material was then cultivated into the top 10 mm of soil on top of the reinforcement mesh. This work was carried out on 1 February On 5 June, the trays were sprayed with a glyphosate weedkiller to kill off grass and broadleaved weeds that had established. Soil erosion measurements were made as for the main trial. 2.4 Vegetation establishment trial This trial investigated the effect of applying recycled gypsum on the establishment of a grass/clover sward growing on a clay loam or sandy loam soil. This trial was carried out using two soil types with differing equivalent gypsum addition rates (Table 4). Table 4 Design of vegetation establishment trial Parameter Slope gradient Soil type Values Not applicable Clay loam, Sandy loam Gypsum addition rate (equivalent)* 0, 2, 4 and 10 kg/m 2 * Because the trays are much smaller than in the other trials, this is a pro rata value. The trial was carried out in small plastic trays (300 mm long 400 mm wide 90 mm deep). The gypsum was raked into the top 20 mm of soil surface. Seed mixture (3 g DLF Pro 05) was raked into each tray on 15 February The trays were placed in a glasshouse to ensure the soil temperature was adequate for seed germination. Irrigation was applied equally to all trays. The trays were placed outside (horizontally) on 9 May. Measurements were made of ground cover every two weeks both by subjective assessment and according to chlorophyll light absorbance as measured using the hand-held Spectrum 1000 chlorophyll meter used in the main trial (see Section 2.1). 3.0 Results from the main trial 3.1 Effects of recycled gypsum application on surface soil erosion Figures 2 to 4 show the results of the surface soil erosion measurements obtained from the main trial slope On the lowest slope (20 ), no consistent trends were observed over the period of the trial (Figure 2). Recycled gypsum for slope stabilisation 8

11 Figure 2 Effect of recycled gypsum application on soil loss, 20 slope* Gypsum Weight dry soil (g) o slope 0 kg/m Clay soil 2 kg/m 2 4 kg/m 2 10 kg/m Weight dry soil (g) Sandy soil Feb 6 Mar 21 Mar 4 Apr 23 Apr 15 May 30 May 18 Jun 3 Jul 19 Jul 2 Aug 15 Aug 0.0 Date * Numerical values show SED (standard error of the difference) values for slope/soil/gypsum interaction values where P< slope Some general trends were apparent on the medium slope (25 ) (Figure 3). On the majority of assessment dates, the amount of soil washed from the sandy soil was greatest where no gypsum was applied and least where gypsum was applied at a rate of 10 kg/m 2. On 15 May, the effects were statistically significant. The amounts of material eroded from the clay soil were generally less than in the sandy soil. For the earlier period from February to April, there was a general trend for more soil to be lost from the control trays than those including gypsum. The effect was not as consistent after this period. Recycled gypsum for slope stabilisation 9

12 Figure 3 Effect of recycled gypsum application on soil loss, 25 slope* Weight dry soil (g) o slope Clay soil Gypsum 0 kg/m 2 2 kg/m 2 4 kg/m 2 10 kg/m Weight dry soil (g) Sandy soil 22 Feb 6 Mar 21 Mar 4 Apr 23 Apr 15 May 30 May 18 Jun 3 Jul 19 Jul 2 Aug 15 Aug 0.0 Date * Numerical values show SED values for slope/soil/gypsum interactions where P< slope For the greatest gradient (32 ), no significant trends were observed for the clay soil and the amounts of material lost were relatively small (Figure 4). On 15 May on the sandy soil, the amount of soil lost was significantly greater in the control than from all gypsum treatments. On most assessment dates for the period from mid-may to August, the amounts of material lost were greatest in the untreated soil. Recycled gypsum for slope stabilisation 10

13 Figure 4 Effect of recycled gypsum application on soil loss, 32 slope* Weight dry soil (g) o slope Clay soil Gypsum 0 kg/m 2 2 kg/m 2 4 kg/m 2 10 kg/m Sandy soil X Data 22 Feb 6 Mar 21 Mar 4 Apr 23 Apr 15 May 30 May 18 Jun 3 Jul 19 Jul 2 Aug 15 Aug Weight dry soil (g) Date * Numerical values show SED values for slope/soil/gypsum interactions where P< Overall effect Statistically significant effects were obtained on 4 April, 15 May and 19 July when the soil erosion values were averaged over all slopes and soils. The total erosion levels across both sandy and clay based soils are shown in Table 5. Recycled gypsum for slope stabilisation 11

14 Table 5 Overall effect of recycled gypsum application on soil erosion (g dry soil) on dates when effect significant (P<0.05) Date Gypsum application rate (kg/m 2 ) SED (P<0.05) 4 April May July Soil slippage Slippage is the movement of soil in the upper end of the tray downwards towards the bottom of the tray. It was measured in terms of the distance along the 2.2-metre length of the trays from the top. During February after some particularly heavy rainfall (see Figure A1 in Appendix A), a mass movement of soil to the full depth (150 mm) within the trays was noted, particularly on the 32 slope. Measurements were made of the extent of slippage on 23 February. Further measurements were made on 10 April and 22 June after further heavy rainfall periods. The results were significant and pronounced only on the steepest of the three slopes with the clay soil (Figure 5). The inclusion of gypsum gave rise to highly significant effects even at the lowest application rate of 2 kg/m 2. The slippage of the untreated soil by 215 mm was reduced to 145 mm by the application of 2 kg/m 2 recycled gypsum and 120 mm by the application of 10 kg/m 2. Figure 5 Effect of gypsum application on clay soil slippage, 32 slope* o slope Clay soil 14.0 Gypsum 0 kg/m 2 2 kg/m 2 4 kg/m 2 10 kg/m 2 Soil slippage (mm) Feb 10 April 22 June Date * Numerical values show SED values between means (P<0.05). A photograph taken at the end of the trial shows a close-up of the slippage that took place on clay soil to which no recycled gypsum had been applied (Figure 6). Recycled gypsum for slope stabilisation 12

15 Figure 6 Close-up of soil slippage on steep slope showing slippage of untreated clay soil 3.3 Snow cover Of some interest was an effect observed in early February after a recent snowfall when it was noted that snow coverage varied between treatments. A light covering of snow fell in the early morning of 8 February, equivalent at most to 0.8 mm of rainfall. This was included in the weekly precipitation data (see Appendix A). A visual assessment of percentage snow cover (i.e. the percentage of the soil surface in the tray covered with snow) was made on 8 February. Table 6 shows the results averaged over the three slope gradients. The thickness of the snow was not assessed as it was only a thin covering. Table 6 Effect of gypsum application on snow cover as assessed on 8 February 2007 Clay soil Sandy soil Gypsum application rate (kg/m 2 ) Snow cover (%) Gypsum application rate (kg/m 2 ) Snow cover (%) SED (P<0.05) 3.5 SED (P<0.05) 3.5 Overall, more snow coverage was apparent on the clay soil although the amount of snow cover increased with increasing gypsum application rate on both soils. The effect was much more pronounced with the sandy soil where the snow coverage of 15% on the untreated soil rose to 89% with an application of 4 kg/m 2 recycled Recycled gypsum for slope stabilisation 13

16 gypsum. Although not specifically measured, it was likely that presence of the recycled gypsum led to a greater reduction in the soil temperature, resulting in less snow melt. 3.4 Weed cover Figure 7 shows the results of the assessment of weed cover averaged over all three slope gradients. Weed invasion was greater in the clay soil and was reduced by increasing amounts of recycled gypsum application. This may have been due to the gypsum reducing water penetration of the soil, thus creating less hospitable conditions for weed establishment. It is likely that the establishment of weeds on the soils restricted soil erosion and did so to the greatest extent in the control treatments. This would therefore confound the effects of the gypsum application. For this reason, the weeds were killed off with glyphosate weedkiller on 5 June. Figure 7 Effect of gypsum application on vegetation ingress as measured by chlorophyll reflectance (averaged over all gradients)* 160 Spectrum value (0 to 999) Clay soil 0 kg/m 2 2 kg/m 2 4 kg/m 2 10 kg/m May 20 Aug Spectrum value (0 to 999) Sandy soil May 20 Aug Date * SED values are shown for each measurement date. 3.5 Discussion As would be expected, greater surface soil erosion was found in the sandy soil and at the highest slope gradient. There was a general trend for the gypsum application rates of 2 and 4 kg/m 2 to reduce erosion. However, there were some occasions where the highest application rate of 10 kg/m 2 appeared to cause more material to be eroded. This was due to gypsum material washing off the surface due to lack of soil incorporation. It is likely that the reduction in overall soil slippage caused by all gypsum treatments was a result of some effect on the soil water balance. This effect on soil water may have also had an effect on the reduced weed establishment with gypsum application and, indirectly, the higher snow cover observed at higher gypsum application rates. Recycled gypsum for slope stabilisation 14

17 4.0 Results from the vegetated trial Figure 8 shows the soil erosion measurement results from the vegetated trial. There were initial trends for more material to be lost from the gypsum-treated soils. This was caused by gypsum being washed off the surface and included in soil weights; it was not possible to cultivate the recycled gypsum material into the vegetated surface and it was prone to washing off. Figure 8 Effect of gypsum application on surface soil erosion from part vegetated slope* 3.0 Gypsum 29 Jan 14 Feb 26 Feb 21 Mar 4 Apr 23 Apr 15 May 30 May 18 Jun 3 Jul 19 July 2 Aug 15 Aug Weight dry soil (g) kg/m 2 2 kg/m 2 4 kg/m 2 10 kg/m Date * Numerical values show SED values where differences significant at P< Discussion There was no significant reduction in soil erosion from gypsum application. This was almost certainly because it was not possible to incorporate the material with the underlying soil. This was due to the vegetation cover at the time of application. Recycled gypsum for slope stabilisation 15

18 5.0 Results from the mesh trial Recycled gypsum was applied at two rates (plus a control) to soils covered by industry standard coir and plastic meshes. These meshes are shown in Figure 9. Figure 9 Plastic mesh on left and coir mesh on right The results of the surface erosion measurements (Figure 10) show a relatively large erosion episode measured on 4 April. On the plastic mesh, there was a significant reduction in the amount of soil eroded by the addition of recycled gypsum at 4 or 10 kg/m 2. During June and July, however, there was a relatively high rate of erosion (although not statistically significant) from the highest gypsum application. This was almost certainly due to gypsum being washed from the surface. Recycled gypsum for slope stabilisation 16

19 Figure 10 Effect of application of recycled gypsum on soil surface erosion Weight dry soil (g) Plastic * 0.35 * 1.46 Gypsum 0 kg/m 2 4 kg/m 2 10 kg/m 2 Weight dry soil (g) Coir Control 12 Mar 21 Mar 4 Apr 23 Apr 10 May 30 May 18 Jun 2 Jul 19 Jul 2 Aug 15 Aug Weight dry soil (g) Date There were no significant effects at the combined reinforcement/gypsum level. An asterisk indicates significant effects from the gypsum rates (averaged over all reinforcement treatments); the SED is shown. Figure 11 shows the amount of overall soil slippage in the trays. On 10 April and 22 June, the overall effects of the reinforcement treatments were significantly different (P<0.05), with SED values of 3.5 and 3.4 respectively. The main significant effects were due to the presence of the coir or plastic reinforcement. However, there was also a trend for a reduction in soil slippage from the application of recycled gypsum, particularly at the 4 kg/m 2 rate. Recycled gypsum for slope stabilisation 17

20 Figure 11 Effects of gypsum incorporation on soil slippage Slippage (mm) Feb 0 kg/m 2 4 kg/m 2 10 kg/m 2 0 Slippage (mm) April Plastic Coir Control Slippage (mm) June Plastic Coir Control Plastic Coir Control Reinforcement 5.1 Discussion No major trends in soil surface erosion were observed due to either the presence of the reinforcement materials or the application of recycled gypsum. However, the reinforcement materials had a significant effect in reducing soil slippage. Addition of recycled gypsum at 4 kg/m 2 appeared to provide an additive effect, though this was not evident at the highest application rate of 10 kg/m 2. This may have been in part due to the difficulty in incorporating the material at this application rate. Recycled gypsum for slope stabilisation 18

21 6.0 Results from the vegetation establishment trial Figure 12 shows the results from the establishment trial with regard to vegetation ground cover as measured by the chlorophyll spectrometer. Neither the clay nor the sandy loam soil types demonstrated any major establishment effect from the application of recycled gypsum. Of note was the difference in species composition between the soils. From May onwards, the ground cover in the sandy soil was dominated by white clover. In the clay treatments, grass species dominated cover throughout the trial period. Figure 12 Effect of gypsum incorporation on live vegetation cover* Spectrum reading (0 to 999) Clay soil Control 2 kg/m 2 4 kg/m 2 10 kg/m 2 Sandy soil 12 Mar 23 Mar 5 Apr 18 Apr 4 May 16 May 30 May 18 Jun 4 Jul 17 Jul 31 July 18 Aug Spectrum reading (0 to 999) Date * Numerical values show SED values where differences significant at P< Discussion There was no major deleterious effect on vegetation establishment from the application of recycled gypsum. This indicated that the material could be applied to provide soil stabilisation with vegetation subsequently established by seeding. Recycled gypsum for slope stabilisation 19

22 7.0 Business case The trial results indicate that application of recycled gypsum will aid the stability of soils on slopes, both on clay and sandy textured material. It is likely that such a capability would be made in addition to conventional slope stabilisation techniques. On more shallow slopes where geo-matting would not be necessary, however, the material will help to prevent soil erosion even where vegetation is to be established. This reduces the amount of soil reaching road surfaces and reduces the potential for siltation of drains. The only commercial product that could provide similar advantages is the binder materials used in hydroseeding work on slopes. The cost of such treatment application would be per m 2, including material and application. An application of recycled gypsum applied at 4 kg/m 2 would cost 0.16 per m 2 for the material. The cost of application would depend on the method used. It may be possible to use a hydroseeding system, which would cost approximately 0.20 per m 2. The total cost of applying recycled gypsum using a hydroseeding system would therefore be 0.36 per m 2 at an application rate of 4 kg/m 2. Alternatively, the recycled gypsum material could be mixed mechanically with the top layer of soil on the slope surface at the same time as seeding. This would cost approximately 0.60 per m Conclusions and recommendations The application of recycled gypsum has been successfully shown to benefit slope stabilisation. The results would be applicable to any part of the UK. There would be little or no temperature effect. The results of these trials indicate that the application of recycled gypsum at between 2 and 4 kg/m 2 aids the stability of soils on slopes both on clay and sandy textured material. At the highest rate tested (10 kg/m 2 ), there was no significant increase in the ability to reduce soil surface erosion or slippage. The preliminary erosion trial in 2006 indicated that an application rate of 4 kg/m 2 was more effective than lower rates at reducing erosion and this is the recommended application rate. The main aim of the trial was to investigate the effect of the incorporation of recycled gypsum on surface soil erosion from slopes. However, the results have highlighted other properties of gypsum application that may, in fact, be more important for the longer term stabilisation of soil slopes. The principal conclusions from the results are as follows. At the higher slope gradients (25 and 32 ), there was evidence of reduced surface erosion following the application of recycled gypsum on the sand textured soil. The recycled gypsum needs to be well incorporated into the soil otherwise the material itself will be washed off the surface. A recycled gypsum application rate of 4 kg/m 2 was optimum for reducing erosion and for ease of soil incorporation. Surface application of gypsum reduced down-slope soil movement on clay soil to a depth of 120 mm compared with 215 mm for untreated soil. The mechanism is uncertain, but may be due to changes to water penetration in the soil. This requires further investigation. Gypsum application has no deleterious effect on the establishment of a recommended grass and clover seed mixture for highway use. Gypsum application reduces weed invasion. Recycled gypsum for slope stabilisation 20

23 8.1 Recommendations for further research Although significant effects on surface soil erosion and soil slippage from gypsum application were observed in the trials, uncertainty about the mechanism of these effects remains. Although it is likely that surface binding occurred, there may have been other factors at work such as soil water content. To provide further information on the effect of applying recycled gypsum, the following research is recommended: Measure the amount of erosion/slippage after the recycled gypsum is mixed to the full depth of topsoil. Test the stability of the different soil types after the recycled gypsum has been applied. Carry out soil engineering tests on soil treated with gypsum. Investigate the optimum method for combining gypsum treatment with conventional erosion control products. Measure the effect of gypsum application on the moisture content of the soil Analyse the future market potential that could realistically be attained if recycled gypsum was used in the application. In addition, the potential for carrying out scaled up to a field level on a constructed embankment should be investigated. Potential users of this application include the Highways Agency and Network Rail. It is possible that an operational trial could be embarked on with them and their landscaping providers. Recycled gypsum for slope stabilisation 21

24 Appendix A Weekly precipitation Figure A1 Weekly rainfall Recycled gypsum for slope stabilisation 22

25 Printed on xx% recycled content paper