Emergency Trench Shoring and Rescue: A Simplified Method for Calculating Lateral Earth Pressures

Table 2

Coulomb earth-pressures versus the total shoring resistance of the T-L method and the C-80 method. Bold values denote a failure condition where the expected resistance is less than the calculated earth-pressure force.

Trench depth	Friction angle, ϕ	Ground slope, i	Lateral earth-pressure coefficient, ka	Bulk unit weight		Coulomb earth-pressure resultant, Qh^1		Total shoring resistance
								RT-L^2		RC-80^3		RC-60^4		Rrt^5
	Degree	Degree		pcf	kN/m^3	kips	kN	kips	kN	kips	kN	kips	kN	kips	kN
4 ft (1.2 m)	20	0	0.49	120	18.9	1.9	8.4	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
	30	0	0.33	125	19.6	1.3	5.9	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
	40	0	0.22	130	20.4	0.9	4.1	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
	20	12	0.60	120	18.9	2.3	10.2	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
	30	12	0.38	125	19.6	1.5	6.8	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
	40	12	0.24	130	20.4	1.0	4.4	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
	30	30	0.75	125	19.6	3.0	13.3	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
	40	30	0.32	130	20.4	1.3	5.9	3.3	15.0	6.3	27.9	5.0	22.2	2.1	9.5
8 ft (2.4 m)	20	0	0.49	120	18.9	7.5	33.5	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
	30	0	0.33	125	19.6	5.3	23.5	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
	40	0	0.22	130	20.4	3.7	16.3	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
	20	12	0.60	120	18.9	9.2	41.0	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
	30	12	0.38	125	19.6	6.1	27.0	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
	40	12	0.24	130	20.4	4.0	17.8	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
	30	30	0.75	125	19.6	12.0	53.4	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
	40	30	0.32	130	20.4	5.3	23.7	12.1	55.0	22.8	101.3	17.7	78.6	8.5	38.0
12 ft (3.7 m)	20	0	0.49	120	18.9	16.9	75.3	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
	30	0	0.33	125	19.6	11.9	52.8	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
	40	0	0.22	130	20.4	8.2	36.6	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
	20	12	0.60	120	18.9	20.7	92.2	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
	30	12	0.38	125	19.6	13.7	60.9	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
	40	12	0.24	130	20.4	9.0	40.0	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
	30	30	0.75	125	19.6	27.0	120.1	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
	40	30	0.32	130	20.4	12.0	53.3	28.1	127.5	49.5	220.3	38.0	169.1	19.2	85.4
16 ft (4.9 m)	20	0	0.49	120	18.9	30.1	133.9	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
	30	0	0.33	125	19.6	21.1	93.9	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
	40	0	0.22	130	20.4	14.6	65.1	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
	20	12	0.60	120	18.9	36.9	164.0	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
	30	12	0.38	125	19.6	24.3	108.2	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
	40	12	0.24	130	20.4	16.0	71.1	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
	30	30	0.75	125	19.6	48.0	213.5	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
	40	30	0.32	130	20.4	21.3	94.7	48.4	219.9	86.5	384.9	66.0	293.8	34.1	151.8
20 ft (6.1 m)	20	0	0.49	120	18.9	47.0	209.2	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2
	30	0	0.33	125	19.6	33.0	146.8	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2
	40	0	0.22	130	20.4	22.9	101.8	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2
	20	12	0.60	120	18.9	57.6	256.2	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2
	30	12	0.38	125	19.6	38.0	169.0	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2
	40	12	0.24	130	20.4	25.0	111.0	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2
	30	30	0.75	125	19.6	75.0	333.6	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2
	40	30	0.32	130	20.4	33.3	148.0	77.0	349.9	133.8	595.0	101.8	452.7	53.3	237.2

¹Coulomb resultant is calculated for the width of a standard shoring panel: 4 ft (1.2 m). ²R_T-L is calculated at the maximum L to be comparable to Coulomb earth-pressure calculations. ³The C-80 method requires that the top of the shoring support the same pressure as the bottom of the shoring, i.e., a uniform pressure distribution based on the maximum lateral earth pressure, thereby multiplying the earth-pressure by a minimum factor of approximately 2.0. ⁴C-60 material is explained in Martinette [1] and in OSHA [3] where the c = 60 in Equation (2). OSHA [3] explicitly states that this material should not be used as the “worst case” soil. ⁵The “rule-of-thumb,” Equation (2), is commonly used by first responders based on Martinette [1]. Bold values denote a failure scenario.